Measurement cell

Abstract

A measurement cell (114) for inspection of a fluid (110) comprises an inlet (118), an outlet (122) and at least one flow channel (124) that extends in a first direction from the inlet to the outlet. The cross-section of the flow channel varies, in a plane normal to the first direction, to define or comprise an inspection zone (120). The inspection zone has an elongate cross-section in a plane normal to the first direction. The measurement cell may be incorporated in a system for measuring a parameter of the fluid, such as homogeneity of density or aeration. An acoustic transducer (130) is disposed along the inspection zone. Examples include clamshell casing portions, plug and socket, sacrificial insert, high pressure measurement cell. Also claimed is an insert suitable for a measurement cell, for example directly coupled to a mould, including housing alignment and mould location features.

Description

The present disclosure relates to a measurement cell, a system and kit of parts incorporating the measurement cell, associated methods, and an insert for a measurement cell. It is desirable to be able to determine parameters (e.g. density, aeration, homogeneity, mix ratio) of a fluid (e.g. a material mixture comprising a plurality of mixed components) for use in, for example, manufacturing processes. Homogeneity is the degree to which a mixture is uniform throughout, meaning that there are no visible or measurable differences between different parts of the mixture. The homogeneity of a material mixture is important in some manufacturing processes because, if the mixture is insufficiently homogeneous (in other words, if the mixture is heterogeneous), the quality of the product produced by the process may be inadequate. Several techniques for determining parameters of a fluid are known. For example, where the fluid is a material mixture, the mixture can be visually inspected to identify any visible differences, such as colour variations, particle size differences, or layering. Visual inspection is inaccurate, and cannot be used to determine the parameter of internal regions of the mixture that are not visible. As another example, samples can be taken from different parts of a fluid and analysed for the presence of different components or impurities. Analytical techniques such as chromatography, spectrophotometry, or microscopy can be used to detect and quantify the presence of (for example) different materials within the mixture. Such sample-and-analysis techniques are slow, and cannot provide real-time information on the parameters of the mixture, or fluid more generally. Other techniques for determining homogeneity, among other parameters of the fluid (e.g. the presence of air bubbles) include measurement of the rheological properties, dielectric constant or impedance of a material mixture. These techniques typically involve placing a probe in contact with the mixture and, therefore, require the probe to be cleaned to ensure accuracy. Known methods and systems for determining one or more parameters of a fluid have a number of associated drawbacks. Measurement, or determination, of the parameter is susceptible to transient properties of the fluid (e.g. any turbulence of the flow, stagnant regions etc.), material interference by the surrounding volume (e.g. mixing chamber, where appropriate), and other interference. Accurate determination of the parameter (e.g. homogeneity) of the fluid is therefore difficult, if at all possible. For the above reasons, it is desirable to determine one or more parameters of a fluid in-situ, in a reliable and non-invasive manner. There exists a need to overcome the disadvantages associated with existing systems and methods, whether mentioned in this document or otherwise. According to a first aspect of the invention there is provided a measurement cell for inspection of a fluid, comprising: an inlet disposed proximate a first side of the measurement cell; an outlet disposed proximate a second side of the measurement cell; and at least one flow channel that extends in a first direction from the inlet to the outlet, the at least one flow channel having a cross-section, in a plane normal to the first direction, that varies along the flow channel to define an inspection zone; wherein the at least one flow channel comprises the inspection zone, the inspection zone having an elongate cross-section in a plane normal to the first direction. The measurement cell may otherwise be described as a test cell. The measurement cell being for inspection of a fluid may otherwise be described as enabling the measurement of a parameter of the fluid, or enabling the inspection of the fluid. Parameters include a level of aeration of the fluid (e.g. the presence of air bubbles) and the density of the fluid. In some embodiments the fluid may be a material mixture (e.g. a mixture of a plurality of components). In some embodiments the fluid may have particle In particular, the material mixture may be a resin mixture. Where the fluid is a material mixture, the measurement cell may be described as being for sampling a material mixture. In some embodiments the fluid may have particles suspended in it. Advantageously, the measurement cell can be used to determine a parameter of the material mixture, such as a homogeneity of the mixture. The second side may oppose the first side. Elongate means a major dimension is larger than the perpendicular (e.g. major width) dimension. For example, where the crosssection is rectangular, the major dimension is (one of) the long sides of the rectangle, and the perpendicular dimension is the length of one of the short sides of the rectangle. Where the cross-section is elliptical, for example, the major dimension is the greatest straight-line distance between edges of the ellipse (e.g. a major extent of the ellipse). The perpendicular dimension is the (minor) dimension of the ellipse. Put another way, if the ellipse were to be bound by a rectangle, the major dimension is the long side of the rectangle, the perpendicular (minor) dimension is the short side of the rectangle. The major dimension is preferably at least twice as large as the perpendicular dimension. The inlet may be defined by a fluid connector. Alternatively, the inlet may be integrally formed with a measurement cell (e.g. an aperture defined therein). The first side of the measurement cell may be a side of the measurement cell which is normal to a major dimension of the measurement cell. Put another way, the measurement cell may be generally elongate, and the first side defines one end side of that elongate geometry. The inlet may be disposed at the first side of the measurement cell. The outlet may be defined by a fluid connector. Alternatively, the outlet may be integrally formed with the measurement cell, such as an aperture thereof. The outlet may be disposed proximate a second side, or at a second side, of the measurement cell. The second side may oppose the first side. In such cases the inlet and outlet may be disposed at opposite sides of the cell, and in particular at opposite sides of a major dimension of the measurement cell. At least one flow channel extending in a first direction from the inlet to the outlet may otherwise be described as the first direction defining a flow direction of the fluid that passes through the flow channel. The flow channel may be described as being defined by an overall volume which is an interior of the measurement cell. The first direction may be continuous and straight where only a single flow channel is incorporated. Alternatively, the flow channel may incorporate a change of direction, particularly in instances where a plurality of flow channels are incorporated. At least one flow channel having a cross-section in a plane normal to the first direction may be described as a shape of the fluid as it passes along the flow channel. The cross-section varying along the flow channel may be continuous in nature or may be discrete (e.g. step changes in the geometry). The inspection zone may otherwise be described as a constricted zone or a constricted region. The inspection zone having an elongate cross-section in a plane normal to the first direction may be described as a narrowing of the flow channel in the inspection zone. The elongate cross section of the inspection zone may be quadrangular (e.g. rectangular) or may incorporate one or more arcs (e.g. be generally elliptical). The inspection zone may have a cross-sectional area that differs from the inlet and / or outlet of the flow channel. The inlet and outlet of the measurement cell may be configured for connection to a fluid line, e.g. a hose. In contrast, the inspection zone may have a different configuration. The inspection zone may be configured for sensing (e.g. ultrasonic sensing). The inspection zone may have a different cross-sectional area to each of the inlet and outlet. The different (elongate) cross-sectional area of the inspection zone may be described as being adapted for sensing (e.g. obtaining measurements associated with the fluid). This may enable more precise signals to be obtained using one or more sensing apparatuses, such as ultrasonic transducers, on fluid in the inspection zone. This, in turn, may facilitate the more accurate determination of a parameter of the fluid. Advantageously, the measurement cell provides an environment in which the fluid can be sampled in an accurate and reliable manner. Provision of the measurement cell overcomes issues where, for example, if the fluid were to be sampled in a mixing volume, where conditions of the fluid (e.g. turbulence and / or stagnant regions) or geometries of the volume could otherwise lead to it being difficult to reliably sense or sample the fluid. The measurement cell provides an environment in which a laminar flow of the fluid can pass and be reliably sampled as the fluid passes through the measurement cell. The measurement cell also provides an environment in which signal quality, from an adjacent acoustic transducer, for example, can be improved (e.g. to provide a reliable determination of a parameter of the fluid). Further advantageously, the measurement cell can be readily incorporated in-line in an existing manufacturing line without the need for significant changes to other components. The inspection zone may be defined by two parallel surfaces. The two parallel surfaces of the inspection zone are preferably the longest sides (e.g. major sides) of the elongate cross section. For example, the inspection zone may have a generally rectangular section. Advantageously, by providing the two parallel surfaces which define the inspection zone, flow through the inspection zone is effectively flattened. A more accurate and reliable inspection of the fluid, and determination of a parameter of the fluid, can thus occur. The cross-sectional area of the at least one flow channel may be substantially constant, or reduces slightly, in the inspection zone. The cross-sectional area of the flow channel in the inspection zone being substantially constant may be described as the cross-sectional area of the flow channel varying by less than around 2%. The cross-sectional area of the flow channel being substantially constant encompasses the cross-sectional area of the flow channel being constant. The cross-sectional area of the at least one flow channel in the inspection zone reducing slightly encompasses the cross-sectional area reducing by up to about 10% between a first, upstream end of the inspection zone and a second, downstream end of the inspection zone. Advantageously, by maintaining a substantially constant cross-sectional area, the pressure of the fluid remains generally constant through the inspection zone. This reduces the risk of interference as the fluid is sampled, resulting in a more reliable parameter determination. Advantageously, where the cross-sectional area of the flow channel reduces slightly, it can be ensured that the flow channel is full of fluid and, again, the inspection, and determination of the parameter, occurs reliably and accurately. The at least one flow channel may comprise first and second flow channels, wherein the first flow channel comprises a first inspection zone, and the second flow channel comprises a second inspection zone. Each of the first and second flow channels may extend between the inlet and the inlet. Alternatively, the first and second flow channels may extend between respective first and second inlets and outlets. Advantageously, by providing a plurality of flow channels, a higher flow rate of fluid through the measurement cell can be maintained whilst the fluid is still sampled. The measurement cell may further comprise a transducer mount disposed along the inspection zone. The transducer mount may comprise a pad (e.g. a flat surface). A predetermined distance preferably extends between the transducer mount and the inspection zone. The transducer mount being disposed along the inspection zone may otherwise be described as the transducer mount overlapping, or being angled towards, the inspection zone. The transducer mount may be described as being disposed adjacent the inspection zone. Advantageously, providing the transducer mount along the inspection zone facilitates the determination of a parameter of the fluid in the inspection zone when a transducer, or other sensing apparatus, is mounted thereto. The transducer mount may be disposed parallel with one of the two parallel surfaces. The transducer mount being disposed parallel with one of the two parallel surfaces may otherwise be described as the transducer having an axis which is directed through one of the two parallel surfaces. Described another way, the transducer mount is positioned so that a transducer mounted thereto is directed towards at least one of the two parallel surfaces (e.g. towards the inspection zone). The transducer mount is preferably a surface which is, itself, parallel to the two parallel surfaces. The transducer mount may be described as disposed adjacent one of the two parallel surfaces. Advantageously, providing the transducer mount parallel with one of the two parallel surfaces means that the fluid can be readily sampled as it passes through the inspection zone, and that a reliable and accurate reading can be determined. A first transducer mount may be disposed, along the first flow channel, parallel with one of the two parallel surfaces that define the first inspection zone; and wherein a second transducer mount may be disposed, along the second flow channel, parallel with one of two parallel surfaces that define the second inspection zone. Advantageously, providing a respective transducer mount parallel with a parallel surface of each of the first and second flow channels, adjacent the corresponding inspection zone, means that fluid passing through the two flow channels can be readily sampled. The measurement cell may comprise first and second portions, the first and second portions being separable from one another. The measurement cell may be described as defining an enclosure. Providing the first and second portions, separable from other another, advantageously exposes an internal volume (e.g. a chamber), defined by the measurement cell. The provision of separable first and second portions is desirable for reasons of being able to clean the measurement cell between uses or to separate the first and second portions where one or more of the first and second portions is only designed for single use. The first and second portions may be different materials. First and second major sides of the elongate cross-section of the inspection zone may be defined by the first and second portions respectively. Described another way, a first major side of the elongate cross-section may be defined by the first portion. A second major side of the elongate cross-section of the inspection zone may be defined by the second portion. The first and second portions may therefore be described as collectively defining the cross-section of the inspection zone. Advantageously, this arrangement facilitates the cleaning of the inspection zone between uses. The first and second portions may comprise casing portions, the casing portions being separable about a split line. The first and second casing portions may otherwise be described as a clamshell design. The casing portions being separable about a split line may otherwise be described as the casing portions being engageable with one another about the split line. This arrangement is advantageous for reasons of being able to readily clean the measurement cell, specifically the inspection zone thereof, and is also readily manufactured by exposing internal surfaces of the measurement cell. The inlet and / or outlet may be defined by a fluid connector. The first portion may comprise a housing, and the second portion may comprise an insert, wherein the insert is at least partially received in the housing. The insert being at least partially received in the housing may encompass the insert being entirely received by the housing. The insert and housing may be described as a plug and socket arrangement. One or more alignment features of the insert may be for alignment of the insert with the housing. The insert may be described as being plugged into the housing. Advantageously, the insertion of the insert into the housing means the insert can be readily removed from the housing for cleaning and / or replacement. The insert may be a sacrificial insert for connection to a mould. The insert being a sacrificial insert is intended to mean that the insert is at least detachable from the housing. The insert being for connection to a mould may otherwise be described as the insert remaining connected to the mould even after a moulding process owing to the passage of fluid (e.g. a material mixture) across the insert and into the mould. In such instances, the insert is preferably detached from the mould during a finishing process (having already been detached from the housing of the measurement cell during a demoulding process). A new insert can then be placed in the housing of the measurement cell. Advantageously, a sacrificial insert means that the measurement cell can be used with a moulding process that uses a curable material mixture, such as resin. The insert may be single use. That said, in other embodiments the insert may be reusable (e.g. through a cleaning process). The cleaning process may comprise soaking the insert in a solvent bath to dissolve any resin remaining thereon. The insert may comprise one or more location features for alignment of the measurement cell with respect to a mould. The one or more location features may comprise projections (e.g. in the form of pins) and / or recesses. Alignment of the measurement cell with respect to the mould advantageously means that the insert, and so the measurement cell, can be correctly positioned, easily, with respect to the mould. According to a second aspect of the invention there is provided a system for measuring a parameter of a fluid, comprising: a measurement cell, comprising: an inlet disposed proximate a first side of the measurement cell; an outlet disposed proximate a second side of the measurement cell and at least one flow channel that extends in a first direction from the inlet to the outlet, the at least one flow channel having a cross-section, in a plane normal to the first direction, that varies along the flow channel; wherein the at least one flow channel comprises an inspection zone, the inspection zone having an elongate cross-section in a plane normal to the first direction; a first acoustic transducer disposed along the inspection zone of the at least one flow channel, the first acoustic transducer being configured to produce a first signal indicative of an acoustic wave transmitted through the fluid in the inspection zone and received at the first acoustic transducer; and a processor configured to: receive the first signal from the first acoustic transducer; and process the first signal to determine the parameter of the fluid within the at least one flow channel. The first acoustic transducer may be an ultrasonic transducer. The first acoustic transducer may operate in pulse-echo mode (e.g. in which the acoustic transducer omits an acoustic wave, and subsequently receives the reflected acoustic wave back) or may operate in a pitch-catch mode (e.g. in which the first acoustic transducer is a receiving transducer and a further transducer is configured to omit an acoustic wave into fluid and towards the first acoustic transducer). The acoustic transducer(s) may be an ultrasonic transducer. The acoustic transducer(s) may operate above the upper audible limit of human hearing, typically above 20 KHz. For example, the acoustic transducer(s) may operate in a frequency range of between around 20 kHz and around 20 GHz. The acoustic waves may be ultrasonic waves, which may be in the frequency range of between around 20 kHz and around 20 GHz. Processing the first signal to determine the parameter of the fluid may comprise comparing the first signal with a further signal to ascertain homogeneity of the fluid between the two different locations. Processing the signal to determine the parameter of the fluid may comprise comparing the first signal to a computational model (e.g. to determine whether or not the parameter matches an expected parameter predicted by the computational model). Advantageously, the system can be used to determine a parameter of a fluid passing through the measurement cell in an accurate and reliable manner. The measurement cell may comprise first and second flow channels, wherein the first flow channel comprises a first inspection zone, and the second flow channel comprises a second inspection zone; the first acoustic transducer may be disposed along the first inspection zone, the first acoustic transducer being configured to produce a first signal indicative of an acoustic wave transmitted through the fluid in the first inspection zone and received at the first acoustic transducer; and a second acoustic transducer may be disposed along the second inspection zone, the second acoustic transducer being configured to produce a second signal indicative of an acoustic wave transmitted through the fluid in the second inspection zone and received at the second acoustic transducer; and the processor may be configured to: receive the first signal from the first acoustic transducer and the second signal from the second acoustic transducer; and process the first signal and the second signal to determine the parameter of the fluid within the first and second flow channels. The second acoustic transducer may be an ultrasonic transducer. Like the first acoustic transducer, the second acoustic transducer may be configured to operate in either pulseecho or pitch-catch mode. In pitch-catch mode the second acoustic transducer is the receiving transducer, and a further acoustic transducer is the omitting transducer. Processing the signal to determine the parameter of the fluid may comprise comparing the first and second signals to ascertain homogeneity of the fluid between the two different flow channels. Processing the signal to determine the parameter of the fluid may comprise comparing the first and second signals to a computational model (e.g. to determine whether or not the parameter(s) matches an expected parameter predicted by the computational model). In other embodiments, the measurement cell may comprise a single flow channel comprising a plurality of inspection zones. A corresponding plurality of sensing apparatuses may be disposed along respective inspection zones (e.g. one sensing apparatus for each inspection zone). One or more of the acoustic transducers may be configured to: produce a signal indicative of an acoustic wave transmitted through the fluid at the corresponding inspection zone; and / or emit an acoustic wave into the fluid. Described another way, one or more of the acoustic transducers are configured to operate in pulse-echo or pitch-catch mode. Where the one or more acoustic transducers operate in pitch-catch mode, the first and second acoustic transducers are the receiving transducers, and further transducers are used to omit a (respective) acoustic wave. The acoustic transducers are preferably configured to operate in pulse echo mode so that the total number of transducers used in the system can be reduced. The system may be for measuring a parameter of a material mixture, further comprising: a volume for receiving, and mixing, components of the material mixture coupled to the inlet of the measurement cell; and a mould for forming a moulded product coupled to the outlet of the measurement cell. The volume for receiving and mixing components of the material mixture may otherwise be described as a mixing volume or mixing tank. The mould for forming a moulded product may be described as a mould for supporting and shaping a material mixture during the moulding process. The volume may be directly coupled to the inlet of the measurement cell, or one or more other components may interpose the volume and the measurement cell. The mould may be directly coupled to the outlet of the measurement cell (e.g. where the measurement cell comprises a sacrificial insert) or, alternatively, one or more other components may interpose the mould and the measurement cell. Advantageously, providing the measurement cell in the aforementioned position advantageously means that the homogeneity of the material mixture, downstream of the volume, can be determined as the material mixture is directed into the mould. The processor may be located remotely from the first acoustic transducer and the measurement cell. The processor may be configured to communicate with the first and / or second acoustic processors (where appropriate) via a communication network. The network may be a local area network (LAN) or a wide area network (WAN). The WAN may use the internet. In one particular implementation, the processor may be a cloudbased processor. Alternatively, the processor may be a local processor. In other words, the processor may be co-located with the first and / or second acoustic transducers (where appropriate) and the measurement cell. In this example, the processor may be directly connected to the first and / or second acoustic transducers, e.g., by electrical cables or wires, without using a communication network. According to a third aspect of the invention there is provided a kit of parts for measuring a parameter of a fluid, the kit of parts comprising: a measurement cell, comprising: an inlet disposed proximate a first side of the measurement cell; an outlet disposed proximate a second side of the measurement cell; and at least one flow channel that extends in a first direction from the inlet to the outlet, the at least one flow channel having a cross-section, in a plane normal to the first direction, that varies along the flow channel; wherein the at least one flow channel comprises an inspection zone, the inspection zone having an elongate cross-section in a plane normal to the first direction; a first acoustic transducer disposed along the inspection zone of the at least one flow channel, the first acoustic transducer being configured to produce a first signal indicative of an acoustic wave transmitted through the fluid in the inspection zone and received at the first acoustic transducer; and a processor connectable to the first acoustic transducer, the processor being configured to: receive the first signal from the first acoustic transducer; and process the first signal to determine the parameter of the fluid within the at least one flow channel. According to a fourth aspect of the invention there is provided a method of measuring a parameter of a fluid, comprising: causing a first acoustic transducer, disposed along the first inspection zone of the first flow channel of the system according to the third aspect of the invention, to produce a first signal indicative of an acoustic wave transmitted through the fluid, in the first inspection zone, and received at the first acoustic transducer; and processing the first signal to determine the parameter of the fluid. The method of measuring a parameter of a fluid may comprise a method of measuring a parameter of a material mixture, ora material mixture sample. As previously described, the acoustic transducer may operate in a pulse-echo or pitch-catch mode. The method may further comprise causing the first acoustic transducer to emit an acoustic wave into the fluid. Put another way, the first acoustic transducer may be operated in a pulse-echo mode. The first acoustic transducer may be one of a pair of acoustic transducers operated in pitch-catch mode. In such instances, a further (e.g. second) acoustic transducer may be caused to emit an acoustic wave into the fluid (said acoustic wave being received by the first acoustic transducer). The method may further comprise directing at least some of the fluid through the first inspection zone of the first flow channel. Processing the signal to determine the parameter of the fluid may comprise comparing the signal to a computational model to ascertain whether the parameter is an expected value. The method disclosed herein has several advantages over known techniques. First, the disclosed method can be performed in situ in real time (unlike sampling-and-analysis techniques). Second, the disclosed method can be performed non-invasively, e.g., without placing a probe in contact with the fluid (unlike techniques involving the measurement of rheological properties, dielectric constant or impedance). This, in turn, avoids or simplifies cleaning, reduces the risk of contamination, and reduces the risk of the fluid leaking. Third, the disclosed method can be performed by retrofitting a measurement cell, and acoustic sensors, to existing manufacturing equipment, without the need to modify the equipment significantly and without the need to change the manufacturing process performed by the manufacturing equipment. The method disclosed herein can be used in a variety of manufacturing processes to help ensure that a fluid used in, or produced by, the process is acceptable. This can improve the quality of products produced by the manufacturing process. The disclosed method finds application in mix, meter and dispense (MMD) composite manufacture, although many other uses of the method are described herein. The fluid may be a material mixture. The material mixture may be a polymer material mixture. The material mixture may be a composite material mixture. Advantageously, measuring a parameter of a fluid using the measurement cell provides an accurate and reliable parameter. The method may further comprise: causing a second acoustic transducer, disposed along the second inspection zone of the second flow channel of the system according to claim 17, to produce a second signal indicative of an acoustic wave transmitted through the fluid, in the second inspection zone, and received at the second acoustic transducer; and processing the second signal to determine the parameter of the fluid. The processing step preferably comprises processing a signal to determine a parameter of the fluid in the first inspection zone, and processing the second signal to determine the parameter of the fluid in the second inspection zone. Processing the first and second signals may comprise comparing the signals to determine the homogeneity of the fluids in the first and second inspection zones. Where the method is a method of measuring a parameter of a material mixture, the determined parameter is preferably the homogeneity of the two material mixture samples in the first and second inspection zones. In other embodiments the other parameter determined may be a level of aeration in the fluid or a density of the fluid. According to a fifth aspect of the invention there is provided an insert for a measurement cell according to the first aspect of the invention. Advantageously, the insert is removable from the housing to provide an insert that can be cleaned between uses. According to an sixth aspect of the invention there is provided an insert for a measurement cell, comprising: a flat surface extending at least partway along an extent of the insert; an alignment feature for alignment of the insert with a housing; and one or more location features for alignment of the insert with respect to a mould. The insert may be described as sacrificial (e.g. for single use). The first flat surface may be one of two parallel surfaces that defines an inspection zone. The first flat surface may extend along a majority of an extent (e.g. axial length) of the insert. The alignment feature may be one or more adept portion, a tapered surface or a ramped portion. One or more location features may comprise a plurality of location features. The one or more location features may comprise projections and / or recesses. This certainly may be described as being receivable by a housing. The insert may be described as being culpable to a mould. After a moulding process is complete, the insert may be destructively detached from the mould and disposed of. Optional features of any aspect of the invention may be combined with an optional feature of another aspect of the invention, where appropriate. Brief Description of the Drawings Embodiments of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a system for measuring a parameter of a material mixture, comprising a measurement cell in accordance with an embodiment of the invention; Figure 2 is a magnified view of the measurement cell, acoustic transducer and (local) controller of the system shown in Figure 1; Figure 3 is a flowchart schematically indicating a method in accordance with an embodiment of the invention and which can be carried out in connection with the system shown in Figures 1 and 2; Figure 4 is a perspective view of a measurement cell according to an embodiment of the invention; Figure 5 is a perspective view of the measurement cell of Figure 4 provided with various features omitted; Figure 6 is a plan view of a first portion of the measurement cell shown in Figures 4 and 5; Figure 7 is a cross-section side view of the measurement cell of Figures 4 to 6 through a first flow channel; Figures 8 to 10 are cross-section views taken about a plane through different axial positions along the measurement cell of Figures 4 to 7; Figure 11 is a perspective view of a measurement cell according to another embodiment of the invention; Figure 12 is an alternative perspective view of the measurement cell of Figure 11; Figure 13 is a cross-section side view of the measurement cell of Figures 11 and 12; Figure 14 is a cross-section plan view of the measurement cell of Figures 11 and 12; Figure 15 is a perspective view of the cross-section shown in Figure 14; Figures 16 to 19 are cross-section views taken about a plane through different positions along the measurement cell of Figures 11 to 15; Figure 20 is a perspective view of the housing of the measurement cell of Figures 11 to 19 in isolation; Figure 21 is a perspective view of the insert of the measurement cell of Figures 11 to 19 in isolation; Figure 22 is an outlet end view of the insert of Figure 21; and Figure 23 is an inlet end view of the insert of Figures 21 and 22. Figure 1 is a schematic illustration of an example system 100 for measuring a parameter of a fluid. In the illustrated example the fluid is a material mixture 110, but in other embodiments a single component fluid may otherwise be used. The system comprises a mixing chamber 101, which defines an internal volume 102. The mixing chamber 101 comprises two or more (in this case, two) inlets 103, 104. The mixing chamber 101 further comprises one or more (in this case, one) outlets 105. The inlets 103, 104 and outlets are in fluid communication with the internal volume 102 of the mixing chamber 101. In embodiments where the fluid is not a material mixture, but is instead a single component fluid, it will be appreciated that the mixing chamber 101 and associated multiple inlets may be omitted. A material mixture 110, which has two or more components, is formed within the internal volume 102 of the mixing chamber 101. More specifically, a first component 111 of the material mixture 110 enters the volume 102 through the first inlet 103, and a second component 112 of the material mixture 110 enters the volume 102 through the second inlet 104. The first and second components 111, 112 are injected, or otherwise caused to flow, into the internal volume 102 of the mixing chamber 101. For example, the volume 102 may be at a lower pressure than a source of the first and second components 111, 112 such that the pressure difference causes the components to flow into the volume 102. If the material mixture 110 has more than two components, then each component may enter the volume 102 through additional respective inlets (not shown in Figure 1). Once inside the volume 102, the first and second components 111, 112 mix with one another to form the material mixture 110. The first and second components 111, 112 may mix through diffusion. Optionally, a mechanical agitator (not shown in Figure 1) may be provided within the volume 102 to assist mixing. The first and second components 111, 112 are fluids, and either may be a solid (e.g., a powder), a liquid or a gas. The material mixture 110 is also a fluid, and is typically a liquid when in the volume 102. The material mixture 110 leaves the volume 102 through the outlet 105. For example, the volume 102 may be at a higher pressure than the outlet 105, such that the pressure difference causes the material mixture 110 to flow out of the volume 102 via the outlet 105. Coupled to the mixing chamber 101, specifically the outlet 105 thereof, a measurement cell 114 is provided. That is to say, the measurement cell 114 is provided in fluid communication with the outlet 105 of the mixing chamber 101. The measurement cell 114 is a particular focus of the present application, and is advantageous for a number of reasons which are set out later in this document. For ease of reference, the measurement cell 114 is shown bound by a dashed line in Figure 1. It is within the measurement cell 114 that a parameter of the material mixture 110 is measured. That is, the system 100 determines a parameter of the material mixture in the measurement cell 114. The purpose of the measurement cell 114 is to provide a controlled environment in which to measure the parameter (e.g. homogeneity) of the material mixture 110 passing therethrough. For example, the mixing chamber 101 may have an irregular geometry, which may prevent comparable measurements being made using acoustic transducers sited at different positions. As another example, the dimensions of the mixing chamber 101 may not allow two (for example) acoustic transducers to be positioned sufficiently far apart from one another to prevent interference between the acoustic waves they emit. By way of further examples still, dissimilar materials within, and used to manufacture, the mixing chamber 101 may be detrimental to an accurate determination of the parameter of the material mixture measured in the mixing chamber 101. Flow properties of the material mixture within the mixing chamber 101 may also preventan accurate determination of the parameter of the material mixture (e.g. turbulence level, stagnant regions etc.). The measurement cell 114 is coupled to the outlet 105 of the mixing chamber 101, such that the parameter of the material mixture 110 is measured after it leaves the mixing chamber 101 (at least in the illustrated embodiment). As shown in Figure 1, the measurement cell 114 is positioned between the mixing chamber 101 and a mould 116. Hence, the parameter of the material mixture 110 within the mould 116 can be inferred from the parameter measured in the measurement cell 114. In some embodiments the material mixture 110, or fluid more generally, may not pass, or flow, directly into a mould. It may also be an open pour process, for example where a resin mixture foams around a component for insulation, or is used as an adhesive to bond two components together. The measurement cell 114 comprises an inlet 118, an inspection zone 120 and an outlet 122. The inspection zone 120 may be referred to as a constricted region. The inlet 118 of the measurement cell 114 is coupled to the outlet 105 of the mixing chamber 101. The inspection zone 120 is disposed between the inlet 118 and the outlet 122 of the measurement cell 114. The outlet 122 of the measurement cell 114 is coupled to the mould 116 (at least in the illustrated embodiment). As will be described in detail in connection with later Figures, a cross-section of a flow channel (e.g. 124) defined through the measurement cell 114 (e.g. from the inlet 118 to the outlet 122), varies along its extent due to the inspection zone 120. That is to say, the inspection zone 120 has a different cross-section geometry to other regions of the measurement cell 114. In particular, the inspection zone 120 ‘flattens’ the flow through the measurement cell (e.g. introduces an elongate cross-section) to improve the ease, and accuracy, of measuring a parameter of the material mixture passing through the measurement cell 114. By flattening out the flow through the inspection zone, more of the fluid can be inspected / measured. Furthermore, the flattened flow, owing to the elongate crosssection of the inspection zone 120, means that more of the fluid can be inspected whilst using fewer sensing apparatuses (e.g. acoustic transducers). This is in contrast to, for example, a circular pipe, which would require many sensing apparatuses be mounted circumferentially around the pipe to provide a reliable parameter of the bulk flow. The inspection zone 120 is (partly) defined by a first surface 126 and a second surface 128. In the illustrated embodiment the first and second surfaces 126, 128 are parallel with one another. However, in other embodiments these surfaces may not be parallel. The first and second surfaces 126,128 are defined by first and second portions 125,127 of the measurement cell 114 respectively. The first and second portions may be casing portions, in the style of a clamshell, in some embodiments, or may be a housing and insert (e.g. a sacrificial insert) receivable in the housing in other embodiments. These specific arrangements will be described in connection with later Figures. The first and second portions 125, 127 may be said to define an enclosure. Put another way, the first and second portions 125, 127 may be described as defining a chamber (e.g. through which a flow channel extends). Further detail regarding the geometry, and associated features, of embodiments of the measurement cell 114 will be provided in connection with later Figures. An acoustic (e.g. ultrasonic) transducer 130 is coupled to the measurement cell 114 (specifically to an external surface of the first portion 125 of the measurement cell 114 in the illustrated embodiment). The (first) acoustic transducer 130 is disposed along (e.g. aligned with) the inspection zone 120. Although only one (i.e. a single) acoustic transducer 130 is shown in the illustrated embodiment, the system 100 may comprise a plurality of acoustic transducers. As will be described in connection with later Figures, where the measurement cell 114 comprises multiple flow channels, each having an inspection zone, an acoustic transducer may be disposed along (or adjacent) each inspection zone (e.g. at least one acoustic transducer is disposed along each inspection zone). In any arrangement, the illustrated single pulse-echo acoustic transducer 130 may be exchanged for a plurality of acoustic transducers which operate in a pitch-catch mode (e.g. one acoustic transducer emitting acoustic waves, and another acoustic transducer receiving acoustic waves). The purpose of the acoustic transducer(s) 130 is to allow the parameter of the material mixture 110 to be determined, as will described in more detail below. The inspection zone 120 may have a substantially constant cross-section geometry, as indicated in Figure 1, or alternatively may have a cross-section geometry which varies along an extent of the inspection zone 120. The inspection zone 120 preferably has a square or rectangular cross-section when viewed along the direction in which the material mixture 110 flows through the measurement cell 114 (e.g. normal to arrow 124, indicating a first direction along which the first flow channel extends). The flow through the inspection zone 120 is consistent owing to the surrounding geometry. As previously mentioned, by determining a parameter of the material mixture 110 passing through the measurement cell 114, the parameter can be more readily, and accurately, determined in comparison to, for example, determining the same parameter whilst the material mixture 110 is within the mixing chamber 101. Difficulties arising from: flow conditions, dissimilar materials, uncertain geometries etc. can therefore be mitigated by ‘inspecting’ the material mixture 110 as it passes through the measurement cell 114. Maintaining a substantially constant cross-sectional area of at least the inspection zone 120 is also desirable for at least the reason that generation of an excessive backpressure can be avoided. The acoustic transducer 130 is positioned outside the measurement cell 114 in the illustrated embodiment. More specifically, the acoustic transducer 130 is positioned on an external surface of the measurement cell 114 (e.g. the first portion 125 thereof). The acoustic transducer 130 emits acoustic waves into, and receives acoustic waves from, a corresponding adjacent region of the inspection zone 120. It is not essential for the acoustic transducer(s) 130 to be positioned outside the measurement cell 130, and the acoustic transducer(s) could be positioned within the measurement cell 114 such that they are in contact with the material mixture 110. That said, it is preferable for there to be at least some material between the transducer(s) and the fluid as there is less reliance on the electronic hardware for measurement accuracy. The acoustic transducer 130 is oriented such that, in use, it emits an acoustic wave into the inspection zone 120 (e.g. a volume defined by the inspection zone 120) and into the material mixture 110 therein, and towards the second surface 128 (although it will be appreciated the orientation of the acoustic transducer 130 could be reversed). The acoustic transducer 130 then receives an acoustic wave from the inspection zone 120 and from the material mixture 110 (e.g. see Figure 2). As shown in Figure 2, the received acoustic waves (e.g. 150) are the result of the emitted acoustic waves (e.g. 148) being reflected by the second surface 128 of the measurement cell 114. The measurement setup shown in Figures 1 and 2 can thus be said to operate in pulse-echo mode (e.g. in which an acoustic transducer emits, and subsequently receives a reflected, acoustic wave). The system 100 further comprises a controller 140. The controller 140 comprises a processor 142, a memory 144, an analogue-to-digital converter (ADC) and a digital-to- analogue converter (DAC). The ADC and DAC are collectively represented by block 146. The processor 142 can be any suitable type of data processing device, such as a microprocessor, microcontroller, digital signal processor or application specific integrated circuit (ASIC). The processor 142 is communicatively coupled to the memory 144. The memory 144 can include a volatile memory, a non-volatile memory, or both volatile and non-volatile memories. The memory 144 stores a control program (not shown in Figure 1). The control program includes processor-executable instructions that, when executed by the processor 142, cause the system 100 to perform the method described below with reference to Figure 3. Returning to Figure 1, the ADC and DAC 146 are communicatively coupled to the acoustic transducer 130 (although, where a plurality of acoustic transducers are instead used, the ADC and DAC are communicatively coupled to each of the acoustic transducers). The ADC and DAC are typically coupled to the acoustic transducer(s) 130 by an electrical connection (e.g. using one or more electrically-conducting cables). The DAC converts a digital output from the processor 142 into an analogue electrical signal (e.g. a voltage signal), which is supplied to the acoustic transducer(s) 130. The analogue electrical signal produced by the DAC energises the acoustic transducer(s) 130, causing them to emit acoustic waves into the material mixture 110. Where a plurality of acoustic transducers are used, the DAC may supply the same signal to each acoustic transducer, or may supply a different signal to each acoustic transducer. The acoustic transducer(s) 130 generate analogue electrical signals when they receive acoustic waves transmitted through the material mixture 110. The ADC receives an analogue electrical signal from the acoustic transducer 130 and produces a digital representation of (each) analogue signal. More specifically, the ADC outputs a series of digital samples, which are supplied to the processor 142 whereupon they are processed to determine the parameter of the material mixture 110 in the volume 102. Turning to Figure 2, a magnified view of the measurement cell 114 is provided, along with the acoustic transducer 130 and controller 140. Figure 2 shows emitted and received acoustic waves 148, 150. Propagation of the emitted and received acoustic waves 148, 150 through the inspection zone 120 will be affected by the physical properties of the material mixture 110 passing therethrough. For example, the proportion of the first component 111 relative to the second component 112 (with reference to Figure 1) in the material mixture 110 may influence the speed at which the acoustic waves 148, 150 propagate through the inspection zone 120. As another example, the proportion of the first component 111 relative to the second component 112 in the material mixture 110 may influence the amount of attenuation experienced by the acoustic waves 148, 150 as they propagate through the inspection zone 120. As a further example, the proportion of the first component 111 relative to the second component 112 in the material mixture 110 may influence the magnitude of the reflection of the emitted wave 148 from the interface between the first portion 125 and the material mixture 110; this in turn will affect the physical properties (e.g., the amplitudes) of the received acoustic wave 150. A parameter of the material mixture 110 (e.g. homogeneity) may be determined by: comparing how the acoustic waves are influenced as they are transmitted through the inspection zone 120 to a computational model and / or by comparing how acoustic waves emitted and received by multiple acoustic transducers, at different spatial positions, are influenced as they are transmitted through the inspection zone 120. The exact way in which the parameter is determined is not of particular relevance to the present application. Rather, the focus is the incorporation of the measurement cell 114 to allow the parameter to be more readily, and accurately, determined. Briefly, taking homogeneity as an example, the homogeneity of the material mixture 110 can be determined by comparing how the acoustic waves are influenced as they are transmitted through different spatial regions of the inspection zone 120. For example, if the material mixture 110 is homogeneous, acoustic waves will have substantially the same speed and / or experience substantially the same attenuation when they propagate through different regions of the inspection zone 120. Conversely, if the material mixture 110 is heterogeneous (i.e., inhomogeneous), acoustic waves will have significantly different speeds and / or experience significantly different attenuation when they propagate through different regions of the inspection zone 120. It will be appreciated that the homogeneity comparison described above is particularly suited to a system comprising a plurality of acoustic transducers provided at different spatial positions (e.g. adjacent different flow channels within the measurement cell). As shown in Figures 1 and 2, the acoustic transducer 130 is configured to operate in pulse-echo mode. However, the measurement cell 114 could instead be provided with two or more acoustic transducers (e.g. in a pair) configured to operate in pitch-catch mode. Where acoustic transducers are configured to operate in pitch-catch mode, pairs of acoustic transducers (e.g. with one acting as an emitter, and one as a receiver) are preferably located opposite one another. For example, with reference to Figure 2, a second acoustic transducer 152 is preferably introduced opposite the first transducer 130, with one of the transducers 130,152 being configured to emit acoustic waves, whilst the other is configured to receive acoustic waves. That said, the use of acoustic transducers in pulse-echo mode has been found to be particularly advantageous for reasons of reducing the number of acoustic transducers which are needed. As will be described in connection with Figures 3 to 10, although the system 100 determines homogeneity (for example) by inspecting a material mixture 110 at one spatial position of a volume, additional acoustic transducers (and / or pairs thereof, when used in pitch-catch mode), can be introduced so as to allow the material mixture 110 to be sampled at more than one spatial position. By increasing the number of spatial positions at which acoustic transducers are located, a more complete understanding of homogeneity (for example) throughout the whole of the measurement cell can be achieved. Similarly, by introducing additional flow channels, a high flowrate of fluid (e.g. material mixture) through the measurement cell can be maintained whilst inspecting the fluid. Although the system 114 in Figure 2 shows controller 140 local to the measurement cell 114 (e.g. the cell 114 comprises a local processor), in other embodiments the system may process the received signals remotely. For example, the ADC and DAC may not be physically co-located with a controller. Instead, the ADC and DAC exchange data with the controller via a network. The network may comprise a local area network (LAN), a wide area network (WAN) or a combination thereof. The ADC and DAC and the controller may each be communicatively coupled to the network 147 by a respective network interface. The network interfaces may be wired or wireless interfaces, and may use any suitable communication protocol(s). When the alternative ‘remote’ system is used to perform the method 160 (shown in Figure 3), at operation 162 the controller causes the first acoustic transducer to emit acoustic waves by transmitting digital representations of excitation signals to the DAC via the network. The DAC converts the digital representations of the excitation signals to analogue voltages, which are applied to the first acoustic transducer to cause them to emit a first acoustic wave into the injection zone. At operation 164, the controller receives a first signal from the first acoustic transducer via the ADC and the network. Where multiple acoustic transducers are used (e.g. for a multiple flow channel measurement cell), at operation 166 the controller causes the second acoustic transducer to emit acoustic waves in a corresponding manner to the first acoustic transducer. The DAC converts the digital representations of the excitation signals to analogue voltages, which are applied to the second acoustic transducer to cause it to emit a second acoustic wave into a second injection zone. At operation 166, the controller receives a second signal from the second acoustic transducer via the ADC and the network. Of note, the second acoustic transducer described here is different than the second acoustic transducer 152 shown in Figure 2 (which is a receiving transducer of a pitch-catch pair). The described second acoustic transducer is preferably provided along a second flow channel, and aligned with a corresponding second injection zone (e.g. see second injection zone 235 in Figure 4). Figure 3 is a flow diagram of a method 160 of measuring a parameter of a fluid. The method may be a method of determining the homogeneity of the material mixture 110 in the inspection zone 120. The method 160 begins at operation 162, in which the processor 142 causes the first acoustic transducer 130 to emit a first acoustic wave 148 into the inspection zone 120. The processor 142 may cause the first acoustic transducer 130 to emit the first acoustic wave 148 by outputting digital values to the DAC. The digital values represent an excitation waveform which, after digital-to-analogue conversion by the DAC, is received by the first acoustic transducer 130. The excitation waveform causes the first acoustic transducer 130 to emit the first acoustic wave 148 into the inspection zone 120, whereupon it propagates through the material mixture 110. The excitation waveform may be any of a continuous wave, swept frequency wave, or a toneburst excitation. In the system shown in Figure 2, the first acoustic wave 148 propagates through the material mixture 110 until it reaches the second portion 127. The first acoustic wave 148 is reflected by the second portion 127, such that it propagates back towards the first portion 125 through the material mixture 110. A portion of the reflection of the first acoustic wave is received by the first acoustic transducer 130, and will be referred to as the first received acoustic wave 150. The first received acoustic wave 150 causes the first acoustic transducer 130 to produce an analogue electrical signal. At operation 164, the processor 142 receives a first signal that is indicative of the first received acoustic wave 150. For example, the processor 142 may receive a digital signal from the ADC, wherein the digital signal is a digital representation of the analogue electrical signal produced by the first acoustic transducer 130 in response to the first received acoustic wave 150. The processor 142 may store the first signal in the memory 144, such that the stored signal can be processed later to determine the parameter of the material mixture 110 (e.g. the homogeneity of the material mixture 110) as described in more detail in connection with operation 172. Operations 166 and 168 are substantially the same as operations 162 and 164, respectively, but are performed with respect to a second acoustic transducer. Operations 162 and 164 are optional steps of the method 160, hence they are shown with a dashed line. As mentioned above, where first and second acoustic transducers are used, the transducers are preferably located at different spatial positions and aligned with respective inspection zones (e.g. a first acoustic transducer is preferably disposed along a first inspection zone of a first flow channel, and a second acoustic transducer is preferably disposed along a second inspection zone of a second flow channel). That said, in some embodiments multiple transducers may be located at different spatial positions along the same flow channel (e.g. sequentially). In more detail, at operation 166, the processor 142 causes the second acoustic transducer (the second acoustic transducer not being shown in Figures 1 and 2) to emit a second acoustic wave into a corresponding second inspection zone. The processor 142 may cause the second acoustic transducer to emit the second acoustic wave by outputting digital values to the DAC. The digital values representan excitation waveform which, after digital-to-analogue conversion by the DAC, is received by the second acoustic transducer. The excitation waveform causes the second acoustic transducer to emit the second acoustic wave into the (second) inspection zone, whereupon it propagates through the material mixture 110 (in the second inspection zone). The excitation waveform may be any of a continuous wave, swept frequency wave, or a toneburst excitation. The second acoustic wave propagates through the material mixture 110 in a similar manner to that described in connection with the first acoustic transducer 130. The second acoustic wave propagates through the material mixture until it reaches the second portion 127. The second acoustic wave is reflected by the second portion 127, such that it propagates back towards the first portion 125 through the material mixture 110. A portion of the reflection of the second acoustic wave is received by the second acoustic transducer, and will be referred to as the second received acoustic wave. The second received acoustic wave causes the second acoustic transducer to produce an analogue electrical signal. At operation 168, the processor 142 receives a second signal that is indicative of the second received acoustic wave. For example, the processor 142 may receive a digital signal from the ADC, wherein the digital signal is a digital representation of the analogue electrical signal produced by the second acoustic transducer in response to the second received acoustic wave. The processor 142 may store the second signal in the memory 144, such that it can be processed later to determine the homogeneity of the material mixture 110 (for example). At operation 170, the processor 142 processes the first signal to determine the parameter of the fluid (e.g. material mixture 110) in the inspection zone. More specifically, one or more properties of the first signal is compared to determine the parameter. The determination may comprise comparing the determined one or more properties of the first signal to a computation model, calibrated data set, or signal from in-line calibration measurement cell(s), prior to mixing, to determine whether the property is as expected. The determination may comprise comparing the first signal to the second signal. When the comparison reveals that the one or more properties of the first and second signals are substantially the same, the material mixture 110 is homogeneous. However, when the comparison reveals that the one or more properties of the first and second signals are substantially different, the material mixture 110 is heterogeneous. In one implementation, times of flight associated with the first and second signals are compared. The time of flight of the acoustic waves associated with the first signal can be estimated by calculating the time elapsed between the first acoustic wave being emitted at operation 162 and the first signal being received at operation 164. Similarly, the time of flight of the acoustic waves associated with the second signal can be estimated by calculating the time elapsed between the second acoustic wave being emitted at operation 166 and the second signal being received at operation 168. When the time of flight associated with the first signal differs from the time of flight associated with the second signal, it can be inferred that the material mixture 110 is heterogeneous. In other words, the difference in the times of flight indicates that the composition of the material mixture 110 (and, hence, the speed of sound in the material mixture 110) differs between the first inspection zone 120 through which acoustic waves 148, 150 propagate and a second inspection zone through which acoustic waves associated with the second acoustic transducer propagate. This difference in the composition means that the material mixture 110 is, by definition, heterogeneous. The amount by which the time of flight associated with the first signal differs from the time of flight associated with the second signal is indicative of the extent to which the material mixture 110 is heterogeneous. In another implementation, the amplitudes of the first and second signals are compared. In this implementation, the amplitude of the first emitted acoustic wave 148 is the same as that of a second emitted acoustic wave. When the amplitude of the first signal differs from the amplitude of the second signal, it can be inferred that the material mixture 110 is heterogeneous. In other words, the difference in the amplitudes indicates that the composition of the material mixture 110 (and, hence, the extent to which the material mixture 110 attenuates acoustic waves) differs between the first inspection zone 120 through which acoustic waves 148,150 propagate and a second inspection zone through which acoustic waves associated with the second acoustic transducer propagate. This difference in the composition means that the material mixture 110 is, by definition, heterogeneous. The amount by which the amplitude of the first signal differs from the amplitude of the second signal is indicative of the extent to which the material mixture 110 is heterogeneous. In yet another implementation, times of flight associated with the first and second signals and the amplitudes of the first and second signals are compared to determine the homogeneity of the material mixture 110. The operations of the method 160 need not be performed in exactly the order shown in Figure 3, but can be performed in a different order that achieves substantially the same result. For example, operations 162 and 166 can be performed at substantially the same time (that is, the first and second acoustic transducers 120 can emit acoustic waves into the respective inspection zones at substantially the same time). Similarly, operations 164 and 168 can be performed at substantially the same time (that is, signals can be received from the first and second acoustic transducers 120 at substantially the same time). As another example, operation 166 can be performed before operation 164. Operations 162 and 166 are optional, in the sense that neither operation forms an essential feature of the subject-matter for which protection is sought. Turning to Figure 4, a perspective view of a measurement cell 200 according to an embodiment of the invention is provided. The cell 200 is an example of the measurement cell 114 schematically illustrated and described in connection with Figures 1 and 2 and referred to in the method 160 shown in Figure 3. The measurement cell 200 comprises an inlet 202 and an outlet 204. The measurement cell 200 is for inspection of a fluid (such as a material mixture) in a controlled and reliable manner, as previously described. The inlet 202 and outlet 204 are in fluid communication with one another via an inspection zone which is not visible in Figure 4 but is shown and will be described in connection with Figure 5. The inlet 202 is disposed proximate a first side 206 of the measurement cell 200. The outlet 204 is disposed proximate a second side of the measurement cell 200. In the illustrated embodiment each of the inlet and outlet 202, 204 is defined by a respective fluid connector 210, 212. Specifically, the fluid connectors 210, 212 take the form of nipples. However, it will be appreciated that various other fluid connectors could otherwise be used, and indeed the inlet and / or outlet may not take the form of a fluid connector whatsoever (e.g. they may be integral with the measurement cell itself). In a similar manner to that described in connection with the earlier figures, in preferred embodiments the inlet 202 is connectable to a volume, such as a mixing chamber, for receiving and mixing components of a material mixture. Put another way, a mixing volume is preferably disposed upstream of the inlet 202. The outlet 204 is connectable to a mould, the mould being for forming a moulded product. Put another way, a mould is disposed downstream of the outlet 204. In other embodiments, particularly for open pour processes, the mould may be omitted. In the illustrated embodiment the measurement cell 200 comprises first and second portions 214, 216. The first and second portions 214, 216 may be described as first and second casing portions respectively. The first and second portions 214, 216 are separable about a split line 218. This is advantageous for reasons of being able to access a chamber within the measurement cell 200, for reasons such as cleaning. For completeness, Figure 5 shows the measurement cell 200 in the absence of various fasteners and with the second portion 216 omitted. Returning to Figure 4, the first and second portions 214, 216 are secured in engagement with one another by a plurality of fasteners, two of which are labelled 218, 220 respectively (first and second fasteners). However, it will be appreciated that a total of eight fasteners are used in the illustrated embodiment (but a different number of fasteners could otherwise be used). The first and second portions 214, 216 are preferably manufactured from different materials, such as a polymer and a stainless steel. One material is preferably a polymer, acoustically matched to the fluid. The other material is preferably stainless steel (acoustically mismatched with the fluid). In some embodiments both portions may comprise the same material (e.g. steel). This advantageously increases the stiffness of the cell, particularly for smaller form factors and / or use with higher fluid pressures. The portion proximate the transducer (e.g. the second portion 216 in this embodiment) is preferably well acoustically matched to the fluid in order to transmit a large proportion of the acoustic signal into the fluid. The other portion (e.g. portion further away from the transducer, the first portion 214 in the illustrated embodiment) is preferably acoustically mismatched with the fluid so that a large proportion of the acoustic signal is reflected back towards the transducer. Both of these advantageously contribute towards a high signal-to-noise ratio (or signal quality) in a pulse-echo configuration. In a transmit-receive configuration (e.g. pitch-catch), both portions may be a polymer, and so well acoustically matched, which is advantageous for the same signal to noise ratio reasons above. Turning to Figure 5, a perspective view of the measurement cell 200 is provided with various fasteners (e.g. 218, 220) and the second portion 216 omitted. Figure 5 thus shows an interior of the measurement cell 200, and specifically shows part of a chamber 222 which is defined between the first and second portions 214, 216. The chamber 222 defined by the first and second portions 214, 216 (the second portion 216 being omitted in Figure 5) may be considered to extend from a first boss 224, proximate the first side 206 of the measurement cell 200, to a second boss 226 proximate the second side 208. At least one flow channel, two as shown in the illustrated embodiment, extends from the inlet 202 to the outlet 204. For the measurement cell 200, a first flow channel 228, and a second channel 230, each extend from the inlet 202 to the outlet 204. That is to say, the flow is split, by a wall 232, which may be referred to as a divider, between the inlet 202 and the outlet 204. The first and second flow channels 228,230 are also shown and labelled in connection with Figure 6, which shows a plan view. Returning to Figure 5, a particular focus of the present application is the inclusion of an inspection zone 234, which may be referred to as a constricted zone, between the inlet 202 and the outlet 204. In the inspection zone 234 the flow of fluid (through the first flow channel) is effectively flattened, by virtue of a cross-section of the flow channel through the inspection zone, and this aids the determination of parameters of the fluid via a sensing apparatus, such as an acoustic transducer. For the avoidance of doubt, a second inspection zone 235 is defined along the second flow channel 230 in the illustrated embodiment (although other embodiments may only incorporate one flow channel, and a corresponding one inspection zone). In Figure 5 first and second flows are schematically indicated 236, 238, through the first and second inspection zones 234, 235. The marker 234 schematically indicates an extent of the first inspection zone with respect to the measurement cell 200. Similarly, the marker 235 indicates an extent of the second inspection zone. The inspection zones 234, 235 are defined by two parallel surfaces in the illustrated embodiment. A first surface 240 defined by the first portion 214, and a corresponding second surface defined by the second portion 216 (not shown in Figure 5). The wall 232 effectively divides the first surface 240 into two portions: a first portion 242 and a second portion 244. Each of the first and second portions 242, 244 corresponds to the first and second flow channels 228, 230 respectively. In accordance with the invention, the inspection zones 234, 235 have an elongate crosssection in a plane normal to the corresponding first or second directions 236, 238 that the flow channels 228, 230 extend in. Taking the second inspection zone 235 as an example, a rectangular cross-section of the second inspection zone 235 in a plane normal to the second direction 238 is defined by a width dimension 246 and a height dimension 248 (see also Figure 10). Given that the width dimension 246 is larger than the height dimension 248 (e.g. preferably at least twice as large), an elongate (and rectangular) cross-section results. As previously mentioned, the second flow of material 238 is therefore flattened in at least the height dimension 248 as it passes through the second inspection zone 235. By virtue of the material choice, known dimensions and geometry of the surfaces which define the second inspection zone 235 (and a first inspection zone 234), the flow of fluid (e.g. material mixture) can be more accurately and readily sensed by sensing apparatuses, such as an acoustic transducer, to determine a parameter thereof. For example, undesirable additional reflections from other surfaces, which risk interference of an acoustic wave (for example) used to determine a parameter of the fluid, can be avoided. Further advantageously, the cross-sectional area of the flow channels 228, 230 is constant through the first and second inspection zones 234, 235 respectively. This avoids any flow behaviours of the fluid which could otherwise affect the determination of the parameter of the same. Briefly, Figure 5 also shows a gasket 250 which is seated in a corresponding recess and is disposed between the first portion 214 and the second portion 216 (not shown in Figure 5) when the portions are secured with respect to one another. Turning to Figure 6, a plan view of the first portion 214 of the measurement cell 200, as shown in Figure 5, is provided. Many of the features have already been described in connection with Figure 5, which will not be repeated here for brevity. Various features of the first portion 214 of the measurement cell 200 are shown in Figure 6. For example, both first and second inspection zones 234, 235 are schematically indicated extending along first and second portions 242, 244 of the first parallel surface 240. Dividing wall 232 is also labelled. Figure 6 schematically indicates the first and second flow channels 228, 230 extending in respective first and second directions between the inlet 202 and the outlet 204. Figure 6 also shows first and second ramped portions 252, 254 which interpose the inlet 202, inspection zones 234, 235 and the outlet 204 respectively. The first and second ramped portions 252, 254 may otherwise be described as tapering surfaces. As will be appreciated from Figure 7, the narrowing of the cross-section of the flow channels from the inlet 202, and along the ramped portion 252 reduces the risk of the fluid flow becoming turbulent, for example due to significant changes in direction, and provides a laminar flow of fluid through the inspection zones 234, 235. This is desirable for at least the reason that when a parameter of the fluid is determined in the inspection zones 234, 235, the respective material mixture is flowing smoothly and the parameter can therefore be determined more accurately and more readily. As has previously been described, by determining the parameter of the material mixture in the inspection zone or inspection zones, difficulties with determining the parameters in other areas (e.g. a mixing volume or in the mould itself) can be overcome. As will be appreciated from Figure 6, the first and second inspection zones 234, 235 are substantially identical to one another, being separated by the wall 232 and extending parallel to each other. In other embodiments, as will be described in connection with Figure 11 onwards, the measurement cell may only comprise a single flow channel therethrough and a corresponding single inspection zone. An advantage with incorporating a plurality of flow channels, and a corresponding plurality of inspection zones, is that more of the fluid (e.g. material mixture) flowing through the measurement cell can effectively be sampled, so the parameter of the material mixture be determined at a great number of data points whilst the flowrate remains comparatively high. It also reduces the likelihood of overlapping transducer beams (acoustic signal paths) which may reduce the accuracy of parameter (e.g. homogeneity) measurements in a single channel (overlapping measurement zones). In some embodiments a single flow channel may comprise a plurality of inspection zones, and a corresponding plurality of acoustic transducers disposed therealong. Turning to Figure 7, a cross-section side view of the measurement cell 200 through the first flow channel 228 is provided. Specifically, the Figure 7 cross-section view is taken through a centre of the first inspection zone 234. Figure 7 is included to provide a further view showing the interior (e.g. chamber 222 or part thereof) of the measurement cell 200 when the first and second portions 214, 216 are secured in engagement with other another (i.e. to form an enclosed space). As for the previous Figures, the inlet 202 and outlet 204 are shown, although Figure 7 shows the respective connectors 210, 212 as being threadably engaged to the first portion 214. Figure 7 shows the cross-sectional area of the first flow channel 230 reducing, in at least the height direction of the measurement cell 200, as it transitions from the inlet 202 to the converging ramped portion 252 and then to the first inspection zone 234. As previously described, through the inspection zone 234 the cross-sectional area of the first flow channel 230 is constant. However, in some embodiments the cross-sectional area may reduce slightly in order to provide a slight backpressure to ensure the inspection zone 234 is full of the fluid. The cross-sectional area of the flow channel then increases in at least the height direction of the measurement cell 200, at an outlet of the inspection zone 234 moving through the second ramped portion. Turning to Figures 8 to 10, cross-section views taken about a plane through an axial extent of the measurement cell 200 are provided. Figures 8 to 10 show the change in cross-section of the flow channel, or flow channels, through the measurement cell 200. Beginning with Figure 8, a cross-section view of the measurement cell 200 is provided about a plane adjacent an end of the first and second portions 214, 216 proximate the inlet 202. As such, Figure 8 shows a part cutaway view of the fluid connector 210 which defines the inlet 202. A corresponding cross-section of the flow channel through the inlet 202 is labelled 256. Cross-section 256 is circular, and there is only a single flow channel. That is to say, in the illustrated embodiment, where there are a plurality of flow channels through at least part of the measurement cell 200, at the inlet 202 and the outlet 204 there is only a single flow channel. However, it is envisaged that in other embodiments a plurality of inlets and a corresponding plurality of outlets could otherwise be used. Turning to Figure 9, a perspective cross-section view of the measurement cell 200 is provided just before the first ramped portion 252. As such, a cross-section 258 of the flow channel just upstream of the first ramped portion 252 is fully visible in Figure 9. Also visible in Figure 9 is a second inspection zone 235 after the single flow is divided into the two corresponding flow channels. As will be appreciated by comparing Figures 8 and 9, the geometry of the channel through which the fluid flows transitions from a circular cross-section 256 to a rectangular geometry 258 just upstream of the first ramped portion 252. Furthermore, and as will be appreciated from Figure 9, as the flow moves toward the inspection zones, along the first ramped portion 252, the cross-section area will continue to decrease towards the inspection zones. Described another way, the flow channel, or more particularly flow channels, converge or constrict. Turning to Figure 10, a cross-section end view of the measurement cell 200 is taken through the first and second inspection zones 234, 235. The cross-sections of the first and second inspection zones 234, 235 are labelled 260, 262 respectively. The crosssections 260, 262 are both rectangular. Furthermore, both cross-sections are the same as one another. As briefly described in connection with Figure 5, with initial reference to the second inspection zone 235, the width dimension 246 of the cross-section 262 is greater than the height dimension 248. In particular, the width dimension is approximately five times as large as the height dimension 248. Because the width dimension is greater than the height dimension, the cross-section may be described as being elongate. The cross-section may be described as being wider than it is tall. In preferred embodiments the height dimension 248 is at least around 0.4 mm, but could be lower depending on factors such as transducers, fluid viscosity etc.. Regardless, the height dimension 248 is a known quantity, facilitating the determination of a parameter of the fluid using, for example, an acoustic transducer. For the cross-section 260 of the first inspection zone 234, corresponding width and height dimensions 264, 266 are annotated. Again, like the second inspection zone 235, for the first inspection zone 234 the width dimension 264 is around five times as large as the height dimension 266. The cross-section 260 is therefore elongate. The crosssection 260 is also rectangular. A width 233 of the wall 232 is also indicated. The width 233 indicates a minimum separation between the adjacent inspection zones 234, 235. By providing a predetermined separation, when a respective acoustic transducer (for example) is mounted adjacent each inspection zone, interference between the acoustic transducers can be avoided. Similarly, side wall reflections can be avoided. For completeness, where a plurality of inspection zones (e.g. 234, 235) are incorporated, a corresponding plurality of sensing apparatuses (e.g. acoustic transducers) are preferably also incorporated ‘side-by-side’. That is to say, the plurality of sensing apparatuses are preferably located at the same distance along the measurement cell 200 (e.g. between the inlet and the outlet), but adjacent respective first and second inspection zones 234, 235. For example, as shown in Figure 10, a respective sensing apparatus is preferably disposed adjacent (e.g. aligned with) each of the inspection zones 234, 235 as indicated by boxes 269, 271. A minimum separation 273 is provided between the sensing apparatuses 269, 271 to reduce the risk of acoustic waves, emitted by the sensing apparatuses 269, 271, overlapping with one another (e.g. interference). In preferred embodiments at least one sensing apparatus is provided for each inspection zone. The sensing apparatus is preferably centrally aligned with a corresponding inspection zone 234, 235 in a widthways direction, as shown in Figure 10 (e.g. normal to a direction of fluid flow through the inspection zone). For example, the first sensing apparatus 269 is centrally aligned with the first inspection zone 234 along the width dimension 264. Similarly, the second sensing apparatus 271 is centrally aligned with the second inspection zone 235 along the width dimension 246. The sensing apparatus(es) are preferably bonded to a corresponding mount (e.g. a transducer mount, such as a pad). The mount may comprise a recess to reduce the risk of damage occurring to the sensing apparatus. The beam widths of the sensing apparatuses 269, 271 at the interface between the respective inspection zone 234, 235 and second portion 216 preferably do not overlap. Beam widths may be affected by one or more of: transducer frequency, dimensions (width / length / radius), electrode pattern, distance between transducer and fluid channel, material between transducer and measurement channel, transducer coupling. There are various advantages of providing the sensing apparatuses 269, 271 side-by-side, as shown in Figure 10. For example, as well as reducing interference (as mentioned above), the flow properties of the fluid through the inspection zones 234, 235 (e.g. pressure) should be consistent between the channels. This reduces the risk that pressure variation along the inspection zones detrimentally affect the measurement. A distance 267 is also labelled in Figure 10. Distance 267 is the distance that an acoustic wave, emitted by an acoustic transducer (for example), passes through to reach the inspection zone 234. The distance 267 is predetermined (e.g. known) to facilitate determination of a parameter of the fluid passing through the inspection zone 234. Selection of the distance 267 also influences whether the acoustic transducer (for example) operates in ‘far-field’ or ‘near-field’ mode. This distance 267 is preferably greater than the ‘near field distance’ of the sensing apparatus. The distance 267 should also be considered alongside the speed of sound of material of the second portion 216, the heights 266, 248 of the inspection zones 234, 235, and the speed of sound of the fluid to ensure repeat signals from interfaces between the second portion 216 and inspection zones 234, 235 do not interfere with reflections from interfaces between the first portion 214 and the inspection zones 234, 235. With brief reference to Figure 7, although not indicated in Figure 7 the sensing apparatuses are preferably centrally aligned with the corresponding inspection zone in the lengthways direction (e.g. along the direction of fluid flow through the inspection zone). In some embodiments the sensing apparatuses may be aligned slightly upstream (e.g. towards the inlet 202) of a lengthways centre of the inspection zone (whilst still being positioned along the inspection zone). An upstream of central alignment is desirable for stability of channel filling (where the fluid is resin). In all cases, it is desirable that the full beam width of the sensing apparatus falls within the corresponding inspection zone. The measurement cell 200 is particularly suited to low pressure operations, such as those under around 100 bar. This is owing to the fact that the first and second portions 214, 216 of the measurement cell 200 can be separated from one another to allow the chamber 222 to be cleaned between uses. Turning to Figure 11, a perspective view of a measurement cell 300 according to another embodiment is provided. The measurement cell 300 shares a number of features in common with the measurement cell 200. The measurement cell 300 is another example of the measurement cell 114 schematically illustrated and described in connection with Figures 1 and 2 and referred to in the method 160 shown in Figure 3. Like the measurement cells previously described, the purpose of the measurement cell 300 is also to provide a controlled environment to sample a fluid, such as a material mixture, in order to determine an associated parameter. Unlike the measurement cell 200, the measurement cell 300 is particularly suited to high pressure operation (e.g. greater than 100 bar). The measurement cell 300 comprises an inlet 302 (largely obscured from view in Figure 11 but shown in Figure 12) and an outlet 304. The inlet 302 and outlet 304 are in fluid communication with one another. The inlet 302 is disposed proximate a first side 306, and the outlet 304 is disposed proximate a second side 308. The first side 306 opposes the second side 308. The measurement cell 300 comprises first and second portions 310, 312. The first and second portions 310, 312 are coupled in engagement with one another. The first portion 310 comprises a housing 310. The second portion 312 comprises an insert 312. The insert 312 is received in the housing 310 as will be appreciated by Figures 13 and 20 (which will be described later in this document). Returning to Figure 11, the housing 310 comprises a boss 314 and a cylindrical portion 316. The inlet 302 is defined by the boss 314, whilst the outlet 304 is defined, at least in part, by the cylindrical portion 316. The boss 314 and cylindrical portion 316 are integrally formed with one another in the illustrated embodiment. The measurement cell 300 further comprises a transducer mount 318. The transducer mount 314 is disposed adjacent an inspection zone which will be described in connection with later Figures. The transducer mount 318 provides a surface on which a sensing apparatus, such as an acoustic transducer, can be mounted. The transducer, or sensing apparatus more broadly, can then be used to determine a parameter of the fluid as it passes through the measurement cell 300. A recess 320 is also provided to facilitate routing of cables to the transducer when mounted in-situ. As previously described, the insert 312 is received by the housing 314. The insert 312 is secured to the housing 314 by one or more of: geometric tolerances, fluid pressure and pins 326, 328, 330 abutting a mould. In other embodiments, a dowel pin may extend through the insert 312 and the housing 314 to align both components with respect to one another. Unlike the previous measurement cell 200, in which the outlet is defined by a fluid connector, as previously described the outlet 304 is defined by the combination of the housing 310 and the insert 312. A further distinction of the measurement cell 300 in contrast to earlier embodiments is that the insert 312, which forms part of the measurement cell 300, is directly coupled to a mould (although the mould is not shown in Figure 11). That is to say, no other components interpose the insert 312 and the mould. As a result, in practice the insert 312 is directly coupled to the mould. When a moulding process is finished, the insert 312 may remain connected to the mould. As such, the insert 312 may be described as a sacrificial insert insofar as it is decoupled from the measurement cell 300 and then destructibly detached from the mould. A replacement insert may then be inserted into the housing 310 for a subsequent moulding process. The sacrificial insert is advantageous where the fluid (such as resin) cures in the measurement cell to avoid disassembly. The insert 312 further comprises a plurality of location features 326, 328, 330. In the illustrated embodiment the location features take the form of projections, but it will be appreciated that, in other embodiments, other location features could otherwise be used (e.g. recesses, among other features). The location features 326, 328, 330 advantageously facilitate alignment of the insert 312, and so the measurement cell 300 more generally, with a mould. The housing 310 and insert 312 are preferably manufactured from different materials. For example, the housing 310 is preferably manufactured from a polymer. The insert 312 is preferably manufactured from stainless steel. The housing 310 material is preferably acoustically matched to the fluid (e.g. a polymer). The insert 312 is preferably acoustically mismatched with the fluid (e.g. steel). Whilst Figure 11 generally shows the measure cell 300 from the perspective of the second, outlet side 308, Figure 12 shows the measurement cell 300 from the perspective of the first, inlet side 306. Various features already described in connection with Figure 11 are shown in Figure 12, and will not be repeated here for brevity. Figure 11 also shows two fasteners 322, 324 retained in the insert 312. The fasteners 322, 324 are used to obscure, or ‘blank’, threaded bores in use. The threaded bores are configured to receive correspondingly threaded pull hammers. The pull hammers are used to withdraw the insert 312 from the housing 310. Figure 12 shows an inlet recess 332 defined at the first side 306 of the measurement cell 300. The inlet recess 332 is defined entirely in the housing 314. In some embodiments the inlet recess 332 maybe configured to receive a fluid connector, such as the fluid connectors 210, 212 shown in connection with the measurement cell 200. At the end of the inlet recess 332, towards the second side 308 of the measure cell 300, an aperture 334 is defined. The aperture 334 defines part of a flow channel which extends from the inlet 302 and through to the outlet 308 (not shown in Figure 12). Two planes are schematically indicated on Figure 12: a first plane 336 and a second plane 338. The first plane 336 is a vertical plane extending through a width wise mid-point of the measurement cell 300. The second plane 338 is a horizontal plane extending through a vertical mid-point of the measurement cell 300. With reference to Figures 13 and 14, which show cross-section views through the measurement cell 300, Figure 13 is taken through the plane 336, and Figure 14 is taken through the plane 338. For the avoidance of doubt, the perspective cross-section view shown in Figure 15 also is taken through the second plane 338 but is shown in a perspective view to illustrate the geometry surrounding the outlet 304. Turning to Figure 13, as previously mentioned Figure 13 is a cross-section side view through the measurement cell 300. Figure 13 thus shows the insert 312 received by the housing 310. Figure 13 also shows tapered surfaces 340,342 of the insert 312 in contact with corresponding angle surfaces 344,346 of the housing 310. The interaction between the surfaces 340, 342, 344, 346 facilitates the removal of the insert 312 from the housing 310 following a moulding operation, as well as alignment of the insert 312 with respect to the housing 310. It will be recalled that the insert 312 is a sacrificial insert, which may only be used for a single moulding process, whilst the housing 310 may be used for multiple moulding processes. A flow channel 354 extends from the inlet 302, through the aperture 334, through an inspection zone 348 to the outlet 304. Like the previous embodiment, a first ramped portion 350 is provided between the inlet 302 and the inspection zone 348. A second ramped portion 352 is provided between the inspection zone 348 and the outlet 304. An axial extent of the inspection zone 348 is labelled using a dashed arrow in Figure 13. The flow channel which the inspection zone 348 forms part of is labelled 354. As was the case in the previous embodiment, the inspection zone 348 flattens the flow as it passes therethrough and provides a region where more accurate and reliable measurements, for determination of a parameter of the flow, may be acquired. Also like in the previous embodiment, the inspection zone 348 is defined by two parallel surfaces: a first parallel surface 356 and a second parallel surface 358. Both of the first and second parallel surfaces 356, 358 are flat. In the illustrated embodiment the first parallel surface 356 is defined by the housing 310. The second parallel surface 358 is defined by the insert 312. The combination of the first and second surfaces 356, 358 thus at least partly defines the inspection zone 348. The combination of the first and second surfaces 356, 358 specifically defines at least part of an elongate geometry in a plane normal to the flow direction through the inspection zone 348. Figure 13 also shows the transducer mount 318 of the measurement cell 300 being disposed adjacent the inspection zone 348. Put another way, the transducer mount 318 is axially aligned with, and overlaps, the inspection zone 348. An acoustic transducer, for example, coupled to the transducer mount 318 can therefore be used to determine a parameter of the fluid as it flows through the flow channel 354, and specifically through the inspection zone 348, in a controlled and reliable manner. The transducer mount 318 is preferably disposed along a central region of the inspection zone 348 (e.g. along a central region of the dashed arrow 348) in an axial direction as shown in Figure 13. A sensing apparatus mounted to the transducer mount 318 preferably overlaps the central region of the inspection zone 348 (e.g. such that fluid is ‘inspected’ in the central region of the inspection zone 348). As described above, in some embodiments the sensing apparatus may overlap a region just upstream of a central region of the inspection zone 348 (e.g. towards inlet 302). Turning to Figure 14, a plan view of the measurement cell 300 is shown through the second plane 338 indicated in Figure 12. Figure 14 shows many of the same features shown in Figures 12 and 13, which will not be described again for reasons of brevity. However, additionally annotated in Figure 14 is a first direction 360 in which fluid flows through the first flow channel 354. Like Figure 13, an axial extent of the inspection zone is labelled 348. The first ramped portion 350 is shown between the aperture 334 and the inspection zone 348. A second ramped portion 352 is shown between the inspection zone 348 and the outlet 304. First, second and third widths 362, 364, 366 of the inspection zone 348 are also schematically indicated. These widths increase insofar as the second width 364 is greater than the first width 362. Similarly, the third width 366 is greater than both the first and second widths 362, 364. These widths are schematically indicated, in combination with the fixed height of the inspection zone 348 shown in Figure 13, to indicate that a cross-sectional area of the inspection zone 348, in a plane normal to the first direction 360, increases moving from a first, upstream end 368 towards a second, downstream end 370. The tapering visible in Figures 13 and 14 allows for separation of the insert 312, and any cured resin (for example), from the housing 310 without using an adhesive. The cross-sectional area in a central region of the inspection zone 348, normal to the first direction 360, is preferably similar to a cross-sectional area of a nozzle of a mix head upstream of the inlet 302. The taper at the end of the channel (e.g. 352) creates a slight back pressure for channel filling. Figure 14 also shows a funnel geometry of the second ramped portion 352. Specifically, the funnel geometry is defined by first and second angled sides 372, 374. These sides 372, 374 funnel the flow through the first flow channel 354 toward the outlet 304 (see also Figure 15). Although not shown in Figure 14, the transducer mount (e.g. 318 in Figure 13) is preferably centrally aligned with the inspection zone 348 in a widthways direction (e.g. normal to the first direction 360 through the inspection zone). For example, the transducer mount preferably overlies a centre of the widths 362, 364, 366 as shown in Figure 14. A sensing apparatus mounted to the transducer mount is thus also preferably centrally aligned with the inspection zone 348 in the widthways direction. Turning to Figure 15, a perspective view of the cross-section shown in Figure 14, i.e. corresponding to the plane labelled 338 in Figure 12, is provided. Figure 15 shows the aperture 334 opening into the first ramped portion 350, and the flow channel 354 that these features form part of. The first direction 360 is also labelled. Figure 15 shows, in more detail, the angled sides 372, 374 which define the funnelled geometry surrounding the second ramped portion 352. That is to say, the converging geometry of the flow channel 354, along the second ramp portion 354 and leading to the outlet 304, is visible. An extent of the inspection zone 348 is also labelled. Turning to Figures 16 to 19, various cross-section views through the measurement cell 300 are provided at different positions along the flow direction 360. These different cross-sections show the varying cross-sectional geometry, and cross-sectional area, of the first channel 354 through the measurement cell 300. Beginning with Figure 16, the cross-section is taken just before the aperture 334. At this position, a cross-section of the first flow channel is labelled 376 and is circular in accordance with the inlet recess 332. It will be appreciated that just downstream of this position the cross-sectional area of the flow channel is defined by the aperture 334. The first ramped portion 350 is also just visible in Figure 15. Turning to Figure 17, a cross-section view of the measurement cell 300 is provided normal to the first direction and towards an upstream end 368 of the inspection zone 348. For example, the cross-section may be taken at the first width 362 as indicated on Figure 14. In Figure 17 a cross-sectional geometry of the flow channel at this position is labelled 378. The cross-section 378 is generally rectangular insofar as it is defined by two parallel sides: a first side 358 defined by the insert 312 and a second side 356 defined by the housing 310. Although the housing 310 incorporates a rounded geometry in the upper corners of the cross-section shown in Figure 17, a major width of the crosssection 378 is labelled 380, with a corresponding height being labelled 382. Owing to the width dimension 380 being larger (significantly larger in this instance) than the height dimension 382, the cross-section 378 is elongate. As described in connection with the previous embodiment, this elongate geometry advantageously flattens the fluid flow through the inspection zone and provides for a more accurate and reliable reading to determine a parameter of the fluid. Turning to Figure 18, a further cross-section view of the measurement cell 300 is provided at a position further towards the second side along the flow direction. The position of the cross-section shown in Figure 18 corresponds to the second width measurement 364 shown in Figure 14. A cross-section of the flow channel at the position shown in Figure 18 is labelled 384. Like the previous Figure, the cross-section 384 is defined by a major width dimension 386 and a major height dimension 388. Again, owing to the width dimension 386 being larger than the height dimension 388, the cross-section geometry 384 is elongate. Similarly, the cross-sectional geometry is generally rectangular. Of note, the height dimension 388 is equal to the height dimension 382 shown in Figure 17. The major width dimension 386 is larger than the major width dimension 380 in Figure 17. As such, despite both cross-sections 378, 384 being provided in the inspection zone (e.g. having two sides defined by first and second parallel surfaces 386, 388) the cross-sectional area of the cross-section 384 is larger than the cross-sectional area of the cross-section 378. Of note, also shown in Figure 18 is a transducer amount 318. The transducer mount 318 is defined by a flat surface. The transducer mount 318 in the illustrated embodiment is parallel to the first and second flat surfaces 386, 388. As such, where an acoustic transducer is mounted to the transducer mount 318, acoustic waves travel through at least the distance 390 in Figure 18 to reach the fluid passing through the flow channel. The same description as set out above in connection with Figure 10 (e.g. relating to separations, dimensions etc.) applies equally to Figure 18 (and corresponding distance 390). Turning to Figure 19, a further cross-section through the measurement cell 300 is provided at a position closer to the outlet, further along the flow channel. Figure 19 is taken at a position that generally corresponds to the third width 366 marked on Figure 14. The cross-section of the flow channel at the position shown in Figure 19 is labelled 392 and is defined by major width and height dimensions 394, 396 respectively. Again, owing to the width dimension 394 being larger than the height dimension 396, the geometry is elongate (and is generally rectangular). Of note, the width dimension 394 does not extend to extreme ends of the void defined by the housing 310 in this position. Instead, just beyond the arrow heads of 394 the insert 312 obscures part of what would otherwise be the cross-sectional geometry through which the fluid flows. As was described in connection with Figure 18, the major height dimension 396 is the same as that of 388 in Figure 18 and 382 in Figure 17. In contrast, the major width dimension 394 is larger than the width dimension 386 in Figure 18. The cross-sectional area of the cross-section 392 is therefore greater than each of the cross-sections 384 and 378 shown in Figures 18 and 17 respectively. Turning to Figure 20, a perspective view of the housing 310 in isolation is provided. The perspective shown in Figure 20 generally corresponds to that shown in Figure 11 (i.e. from the second side 308) but again with the insert 312 (and fasteners 322, 324) omitted. Figure 20 is included to illustrate a cavity 398 which is defined by the housing 310 and in which the insert is received. The aperture 334 is also visible through the cavity 398 from the perspective of Figure 20. Although not entirely visible, the first flat surface 356 defined by the housing 310 is indicated. In a similar manner to that shown in Figure 14 in connection with the first to third widths 362, 364, 366, a width of the cavity 398 (e.g. in the direction labelled 400) generally decreases moving from the second end 308 towards the first end 306. As previously mentioned, this effective draft angle is to facilitate the removal of the insert from the cavity 398 after a moulding process is complete. Turning to Figure 21, a perspective view of the insert 312 in isolation is provided. Various features of the insert 312 already described are visible in Figure 21. For example, the flat surface 358, previously referred to as a second flat surface) is visible. Just visible is the first ramped portion 350, along with the second ramped portion 352. First and second angled sides 372, 374, which bound the second ramped portion 352, are also shown. The funnelled geometry of the second ramped portion 352 and surrounding sides 372, 374 is also visible in Figure 21. The location features 326, 328, 330 are also shown extending from the insert 312. Recesses 402, 404, configured to receive fasteners for securing the insert 312 to the housing, are also visible. Of note, an arcuate opening 406, which forms the outlet 304 when the insert 312 is inserted into the housing, is also labelled. As will be appreciated from Figure 21, the surface 358 extends along a majority of an extent of the insert 312. The insert 312 also comprises various alignment features for alignment of the insert with the housing. For example, the tapered surface 342, proximate the first ramped portion 350, is an example of one alignment feature as shown in Figure 13. Returning to Figure 21, sides of the insert 312, such as first and second sides 406, 408, also facilitate the alignment of the insert 312 within the cavity 398 of the housing. As will be appreciated from Figure 21, the insert 312 advantageously includes a flat surface 358 to aid the determination of a parameter of the fluid (e.g. to aid measurement) and a number of angled / draft surfaces to facilitate insertion and removal of the insert 312 from the housing. Turning to Figure 22, an end view of the insert 312 is provided. All of the features visible in Figure 22 have already been described, and will therefore not be described again here for brevity. Figure 22 is taken from the perspective of a second side of the insert 312 (e.g. when installed a side proximate the outlet of the measurement cell 300). Turning to Figure 23, a view of the insert 312 in isolation from a side closest to the inlet, when installed, is provided. Put another way, the Figure 23 view is taken from the opposite side to that of Figure 22. Features already described in connection with earlier Figures are visible, and will not be described again for brevity. However, briefly, the first ramped portion 350, and corresponding angled surface 340 generally underneath, are visible, as are the angled sides 372, 374 and the first flat surface 358. Ends of the arcuate aperture 406 are also visible. Stepped regions 410, 412, provided adjacent the first and second sides 406, 408 respectively, are also shown. The stepped regions 410, 412 are examples of alignment features used to align the insert 312 within the cavity defined by the housing. The stepped regions 410, 412 engage with a corresponding geometry of the housing 310 as shown in Figure 14 between the second and third width dimensions 364, 366. Figure 23 also illustrates the tapering nature of the insert 312. Specifically, at an outer tip of the ramped portion 350, i.e. an end of the insert 312, first, and narrowest, width geometry 414 is indicated. A second width dimension 416, corresponding to a narrowest width of the inspection zone in-situ is larger than the (first) width dimension 414. Just before the stepped portions 410, 412, the insert 312 has a (third) width 418. Beyond the stepped portions 410, 412 the insert 312 has a (fourth) width indicated 420. At a widest point of the insert 312, corresponding to the end shown in Figure 22, a (fifth) width 422 is labelled. The various widths of the insert 312 increase in the order: 414, 416, 418, 420, 422. As previously described, the tapering nature of the insert 312, insofar as the narrowest width 414 is provided at a first end of the insert 312 and a greatest width 422 is provided at an opposing second end of the insert 312, facilitates insertion and removal of the insert 312 from the cavity defined by the housing. The measurement cells described herein preferably comprise surfaces (e.g. defining the inspection zone(s)) that are accessible, such that a cleaning process (e.g. rubber pellet shot blasting) can be carried out. The measurement cells described herein are preferably manufactured from materials appropriate for operating temperatures in exemplary manufacturing processes. The measurement cells described herein are preferably manufactured from materials appropriate for operating pressures in exemplary manufacturing processes. The measurement cells described herein are preferably manufactured from materials that are solvent resistant, to aid cleaning. The measurement cells described herein are preferably manufactured from materials that are corrosion resistant, in at least the presence of resin materials. Suitable materials include stainless steel (e.g. 316L). The system 100, measurement cells 114, 200, 300 and the method 160 can be used for a variety of applications. As generally described, the primary use is to determine a parameter of a fluid flow, such as an aeration level, density or homogeneity. However, a preferred use is to determine a parameter of a material mixture, such as homogeneity. In addition to resin transfer moulding processes, the systems and methods disclosed herein can be applied to other manufacturing processes in which it is desirable to determine a parameter of a fluid. Regarding homogeneity specifically, the systems and methods herein can be applied to any situation where it is desirable to ensure component materials are homogeneously mixed. For example, the systems and methods disclosed herein can be used in extrusion, injection moulding and casting processes. In the case of extrusion, a material to be extruded is mixed within the mixing chamber 101 and, when it is determined to be sufficiently homogeneous, the material mixture 110 is dispensed via the measurement cell and forced through a die to form an extruded product. In the case of injection moulding and casting, a material to be moulded or cast is mixed within the mixing chamber 101 and, when it is determined to be sufficiently homogeneous by virtue of a parameter being determined as the mixture passes through the measurement cell, the material mixture 110 can be dispensed into a mould or die. A test shot of material may be extruded, or ejected, initially (e.g. into a different container), to avoid adding a poor homogeneity material into the mould. The systems and methods disclosed herein can be also used in coating processes. In this case, a coating material (such as a paint, adhesive or protective coating) is mixed within the mixing chamber 101 and, when it is determined to be sufficiently homogeneous by virtue of a parameter being determined as the mixture passes through the measurement cell, dispensed and applied to a surface using a sprayer. The systems and methods disclosed herein can be also used in mixing or blending processes. In this case, two or more materials are mixed or blended within the mixing chamber 101 and, when the material mixture 110 is determined to be sufficiently homogeneous by virtue of a parameter being determined as the mixture passes through the measurement cell, it is dispensed and optionally used as a raw material for another manufacturing process. The systems and methods described herein can also be used in connection with biological fluids. The method 160 can be implemented by instructions stored on a processor-readable 5 medium. The processor-readable medium may be: a read-only memory (including a PROM, EPROM or EEPROM); a random access memory; a flash memory; an electrical, electromagnetic or optical signal; a magnetic, optical or magneto-optical storage medium; one or more registers of a processor; or any other type of processor-readable medium. The method 160 and / or the functionality of the processor 142 can be 10 implemented by hardware, firmware, software or any combination thereof. Such hardware may include one or more application-specific integrated circuits or appropriately connected discrete logic gates. A hardware description language can be used to implement the method 160 and / or the functionality of the processor 142 with dedicated hardware. 15 It will be understood that the invention has been described above purely by way of example, and that modifications of detail can be made within the scope of the claims.

Claims

1. A measurement cell for inspection of a fluid, comprising:an inlet disposed proximate a first side of the measurement cell;an outlet disposed proximate a second side of the measurement cell; andat least one flow channel that extends in a first direction from the inlet to the outlet, the at least one flow channel having a cross-section, in a plane normal to the first direction, that varies along the flow channel to define an inspection zone;wherein the at least one flow channel comprises the inspection zone, the inspection zone having an elongate cross-section in a plane normal to the first direction.

2. The measurement cell according to claim 1, wherein the inspection zone is defined by two parallel surfaces.

3. The measurement cell according to claims 1 or 2, wherein the cross-sectional area of the at least one flow channel is substantially constant, or reduces slightly, in the inspection zone.

4. The measurement cell according to any preceding claim, wherein the at least one flow channel comprises first and second flow channels, wherein the first flow channel comprises a first inspection zone, and the second flow channel comprises a second inspection zone.

5. The measurement cell according to any preceding claim, further comprising a transducer mount disposed along the inspection zone.

6. The measurement cell according to claims 2 and 5, wherein the transducer mount is disposed parallel with one of the two parallel surfaces.

7. The measurement cell according to claims 4 to 6, wherein a first transducer mount is disposed, along the first flow channel, parallel with one of the two parallel surfaces that define the first inspection zone; andwherein a second transducer mount is disposed, along the second flow channel, parallel with one of two parallel surfaces that define the second inspection zone.

8. The measurement cell according to any preceding claim, wherein the measurement cell comprises first and second portions, the first and second portions being separable from one another.

9. The measurement cell according to claim 8, wherein the first and second portions are different materials.

10. The measurement cell according to claims 8 or 9, wherein first and second major sides of the elongate cross-section of the inspection zone are defined by the first and second portions respectively.

11. The measurement cell according to any one of claims 8 to 10, wherein the first and second portions comprise casing portions, the casing portions being separable about a split line.

12. The measurement cell according to any preceding claim, wherein the inlet and / or outlet are defined by a fluid connector.

13. The measurement cell according to any one of claims 8 to 10, wherein the first portion comprises a housing, and the second portion comprises an insert, wherein the insert is at least partially received in the housing.

14. The measurement cell according to claim 13, wherein the insert is a sacrificial insert for connection to a mould.is replaced with each moulding process.

15. The measurement cell according to claims 13 or 14, wherein the insert comprises one or more location features for alignment of the measurement cell with respect to a mould.

16. A system for measuring a parameter of a fluid, comprising:a measurement cell, comprising:an inlet disposed proximate a first side of the measurement cell;an outlet disposed proximate a second side of the measurement cell andat least one flow channel that extends in a first direction from the inlet to the outlet, the at least one flow channel having a cross-section, in a plane normal to the first direction, that varies along the flow channel;wherein the at least one flow channel comprises an inspection zone, the inspection zone having an elongate cross-section in a plane normal to the first direction;a first acoustic transducer disposed along the inspection zone of the at least one flow channel, the first acoustic transducer being configured to produce a first signal indicative of an acoustic wave transmitted through the fluid in the inspection zone and received at the first acoustic transducer; anda processor configured to:receive the first signal from the first acoustic transducer; andprocess the first signal to determine the parameter of the fluid within the at least one flow channel.

17. The system according to claim 16, wherein:the measurement cell comprises first and second flow channels, wherein the first flow channel comprises a first inspection zone, and the second flow channel comprises a second inspection zone;the first acoustic transducer is disposed along the first inspection zone, the first acoustic transducer being configured to produce a first signal indicative of an acoustic wave transmitted through the fluid in the first inspection zone and received at the first acoustic transducer; anda second acoustic transducer is disposed along the second inspection zone, the second acoustic transducer being configured to produce a second signal indicative of an acoustic wave transmitted through the fluid in the second inspection zone and received at the second acoustic transducer; andthe processor is configured to:receive the first signal from the first acoustic transducer and the second signal from the second acoustic transducer; andprocess the first signal and the second signal to determine the parameter of the fluid within the first and second flow channels.

18. A system according to claims 16 or 17, wherein one or more of the acoustic transducers is configured to:produce a signal indicative of an acoustic wave transmitted through the fluid at the corresponding inspection zone; and / oremit an acoustic wave into the fluid.

19. The system according to any one of claims 16 to 18, wherein the system is for measuring a parameter of a material mixture, further comprising:a volume for receiving, and mixing, components of the material mixture coupled to the inlet of the measurement cell; anda mould for forming a moulded product coupled to the outlet of the measurement cell.

20. A system in accordance with any of claims 16 to 19, wherein the processor is located remotely from the first acoustic transducer and the measurement cell.

21. A kit of parts for measuring a parameter of a fluid, the kit of parts comprising:a measurement cell, comprising:an inlet disposed proximate a first side of the measurement cell;an outlet disposed proximate a second side of the measurement cell; and at least one flow channel that extends in a first direction from the inlet to the outlet, the at least one flow channel having a cross-section, in a plane normal to the first direction, that varies along the flow channel;wherein the at least one flow channel comprises an inspection zone, the inspection zone having an elongate cross-section in a plane normal to the first direction;a first acoustic transducer disposed along the inspection zone of the at least one flow channel, the first acoustic transducer being configured to produce a first signal indicative of an acoustic wave transmitted through the fluid in the inspection zone and received at the first acoustic transducer; anda processor connectable to the first acoustic transducer, the processor being configured to:receive the first signal from the first acoustic transducer; andprocess the first signal to determine the parameter of the fluid within the at least one flow channel.

22. A method of measuring a parameter of a fluid, comprising:causing a first acoustic transducer, disposed along the first inspection zone of the first flow channel of the system according to any one of claims 16 to 20, to produce a first signal indicative of an acoustic wave transmitted through the fluid, in the first inspection zone, and received at the first acoustic transducer; and5 processing the first signal to determine the parameter of the fluid.

23. The method according to claim 22, further comprising:causing a second acoustic transducer, disposed along the second inspection zone of the second flow channel of the system according to claim 17, to produce a 10 second signal indicative of an acoustic wave transmitted through the fluid, in the second inspection zone, and received at the second acoustic transducer; andprocessing the second signal to determine the parameter of the fluid.

24. An insert for a measurement cell according to any one of claims 13 to 15.1525. An insert for a measurement cell, comprising:a flat surface extending at least partway along an extent of the insert;an alignment feature for alignment of the insert with a housing; andone or more location features for alignment of the insert with respect to a mould.

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