Turbidity instrument

Abstract

In turbidity instrument 1, secondary optical chamber 5 receives a fluid sample (e.g. in vial 7) and is removably insertable into primary optical chamber 4 having removable lid (3, Fig. 1). Source channel 41 extends from the primary chamber; first transmissive optical component 410 provides a barrier to fluid passage towards light source 46. Detector channel (42, Fig. 3) extends from the primary chamber; second transmissive optical component (420, Fig. 3) provides a barrier to fluid passage towards optical detector 47. In use, first and second windows (560, 570, Fig. 4) over first and second apertures (56, 57, Fig. 4) of the secondary chamber are aligned with the source and detector channels respectively. The primary chamber is fluidly isolated from an exterior of the instrument. Optionally, in additional detector channel 43, third transmissive optical component 430 provides a barrier to fluid passage towards optical detector 48. In use, third window and third aperture (580, 58, Fig. 5) are aligned with the additional channel.

Description

Field of invention The present invention relates to instruments to measure turbidity in a liquid sample, and in particular to instruments to measure turbidity in a liquid sample by nephelometry. Background art Measurement of the turbidity of a liquid (such as water) is a key test to determine the quality and clarity of the liquid. High turbidity in water samples is indicative of high levels of suspended material (e.g. sediments, solid or oil-based pollutants, or biological material such as phytoplankton). High turbidity in drinking water supplies increases the risk of gastrointestinal disease due to the presence of viruses and bacteria on or in suspended material. High turbidity also reduces the penetration of light into bodies of water such as lakes, rivers and reservoirs, inhibiting the growth of submerged aquatic plants. This can impact species which depend on the aquatic plants, and thus an increase in turbidity can negatively impact the health of an ecosystem. Regular measurements of turbidity are often carried out to track changes in the quality of a body of water (such as a river, lake or reservoir) over time and thus observe and measure the impact of both human activities (such as nearby construction work, mining, quarrying, sewage discharge or agricultural activity) and natural events (such as a storm or heavy rainfall). The turbidity of drinking water supplies must be regularly measured to ensure the quality and clarity remains within acceptable ranges. Tracking changes in the quality of a body of water often involves measuring the turbidity of a large number of water samples taken from a number of different locations (for example at different positions along a river or on a lake) at different times. Sample transport and storage (and a delay between collection and measurement) can affect measurement accuracy. It is therefore practical to provide portable turbidity instruments which enable rapid and accurate measurement of a sample in situ (i.e. without having to transport samples to a laboratory for analysis). Turbidity is commonly measured by either by turbidimetry or (more commonly) by nephelometry. Both are optical techniques: turbidimetry measures attenuation in the intensity of a beam of light passing through a sample; whereas nephelometry detects and measures diffuse light (i.e. light scattered by suspended particles). Both turbidimeters and nephelometers thus have a light source and an optical detector: for turbidimeters the detector is positioned in or close to the same axis (i.e. at or around 180 °) as the light source and incident beam; whereas for nephelometers the detector is positioned in a different axis (typically at or around 90 ° relative to the incident beam). Increasingly, devices have detectors at or around both 180 ° and 90 °, thus enabling simultaneous measurement of both attenuated and diffuse light. The configuration, specification and arrangement of the light source, sample and detector(s) in turbidity instruments is governed by EPA180.1 (USA) and ISO 7027-1:2016 standards. The electrical systems (power supply, light source, detector(s), data processors etc) within turbidity instruments must be adequately protected against damage by water ingress (both from the water sample itself and the environment). Turbidity instruments, particularly portable instruments, are typically rated against ingress protection to a level of IP67 or greater (according to EN 60529). In existing instruments, the light source and detector(s) are usually separated from the sample chamber and exterior of the device by at least one optical component or lens. These lenses are therefore part of the primary ingress barrier, protecting the electrical components against water damage. Prolonged, repeated contact of the lenses by water samples (which may be acidic, basic or contain high levels of contaminants), sample vials, a user’s hands or other external contaminants results in a steady degradation in optical performance of the lens due to fouling, scratching and chemical attack. Turbidity measurement by nephelometry and / or turbidimetry is very susceptible to measurement error from stray light or changes in the performance of optical components. A contaminated or damaged lens may itself increase attenuation or light scattering, thus impacting the accuracy of measurements and increasing measurement noise and uncertainty. Measurement instruments therefore require regular recalibration as optical components age to ensure continued accuracy. Recalibration requires the use of a series of standard suspensions (usually of formazin) at differing concentrations. Consequently recalibration of instruments in the field is generally not possible. Where optical components become highly damaged, it may not be possible to correctly calibrate the instrument. The damaged optical components may need to be replaced before calibration, requiring servicing of the instrument. Instruments must therefore be returned to the manufacturer or a service centre for recalibration and service, resulting in the instrument being unavailable for a prolonged period. Summary of the Invention The present invention seeks to provide a turbidity instrument which has increased resilience against damage of optical components, and which can be serviced in the field. Viewed from a first aspect the present invention provides a turbidity instrument comprising: a primary optical chamber defined by one or more primary chamber walls and having an opening at a first end; a source channel extending from the primary optical chamber along a source axis to a light source, wherein the source channel contains a first transmissive optical component positioned between the primary optical chamber and the light source and providing a barrier to fluid passage along the source channel; a detector channel extending from the primary optical chamber along a detection axis to an optical detector, wherein the detector channel contains a second transmissive optical component positioned between the primary optical chamber and the optical detector and providing a barrier to fluid passage along the detector channel; a secondary optical chamber defined by one or more secondary chamber walls, a base and an opening at a first end opposite the base, wherein the secondary optical chamber includes first and second windows respectively positioned over first and second apertures within the secondary optical chamber; wherein the secondary optical chamber is removably insertable into the primary optical chamber, and wherein the secondary optical chamber is configured to receive a fluid sample; a removable lid positionable to cover at least a portion of the opening of the primary optical chamber; wherein when the secondary optical chamber is inserted into the primary optical chamber in an operational position, the first and second windows are respectively aligned with the source channel and the detector channel; and the primary optical chamber is fluidly isolated from an exterior of the turbidity instrument. By providing a secondary optical chamber which is removably insertable into the primary optical chamber, a secondary protective layer is provided between the sample and the first and second transmissive optical components. Thus these optical components are less likely to become damaged by physical or chemical processes through repeated or prolonged contact with sample fluids or physical touching / scratching by a user as samples are inserted or removed. Although the first and second windows of the secondary optical chamber may become damaged over time through exposure to samples and physical contact with repeated use, a user can simply remove and replace the entire secondary optical chamber to ensure continued machine effectiveness. This advantageously eliminates (or at least significantly reduces) the need for the instrument to be returned to the manufacturer or a service centre due to damaged optical windows. The primary optical chamber is fluidly isolated from the instrument exterior when the secondary optical chamber is in an operational position. In this context, “operational position” is intended to mean the position of the secondary optical chamber within the primary optical chamber required for the turbidity instrument to be operational. The fluid isolation may be provided by an annular seal around the secondary chamber walls at or towards the first end, which may be engageable with an annular seat provided either on an internal facing surface of the primary chamber walls at or towards the first end; or on an internal facing portion of an upper body part (e.g a casing) of the turbidity instrument; or on the removable lid. The first window is preferably configured to be arranged substantially perpendicular to the source axis; and the second window is preferably configured to be arranged substantially perpendicular to the detection axis, when the secondary optical chamber is inserted into the primary optical chamber and the first and second windows are respectively aligned with the source channel and detector channel. Preferably the secondary optical chamber is configured to fit snugly within the primary optical chamber, i.e. such that an average gap between an interior facing surface of the primary chamber walls and an exterior facing surface of the secondary chamber walls is minimal (e.g. 1 cm or less; preferably 0.5 cm or less; even more preferably 0.2 cm or less). The primary and secondary optical chambers may have a generally cylindrical shape. Where the primary and secondary optical chambers have a generally cylindrical shape, an external diameter of the secondary optical chamber is preferably at least 90% (e.g at least 95%) of an internal diameter of the primary optical chamber. An interior facing surface of the primary chamber walls may include one or more alignment features reversibly engageable with one or more corresponding features provided on an exterior facing surface of the secondary chamber walls, to ensure correct positioning and alignment of the secondary optical chamber within the primary optical chamber, and to prevent movement (e.g. rotation) of the secondary optical chamber within the primary optical chamber. For example, the interior facing surface of the primary chamber walls may include one or more ridges, grooves or channels extending generally perpendicularly to the opening; and the exterior facing surface of the secondary chamber walls may include one or more grooves or channels to receive the ridges of the primary chamber walls, or ridges receivable within the one or more grooves of the primary chamber walls. Where the chambers are generally cylindrical, the alignment features may be spaced apart unequally around a circumference of the primary and secondary chamber walls, thus permitting the secondary optical chamber to only be insertable into the primary optical chamber in a single orientation whereupon the first and second windows are respectively aligned with the source and detector channels. The first and second windows may both be positioned in the secondary chamber walls. Alternatively either the first window or the second window may be positioned in the base, with the other positioned in the secondary chamber walls. The source channel and detector channel are preferably arranged such that the source axis and detection axis are substantially perpendicular. In this context, “substantially perpendicular” means within ± 3° of perpendicular (i.e. 90° ± 3°). The source axis and detection axis are preferably substantially coplanar and together define a measurement plane. Thus in use, the optical detector is configured to detect diffuse light which has been dispersed / scattered by the sample. Preferably the secondary optical chamber is configured to receive a fluid sample within a sample vial. The sample vial may be composed of glass, or any plastic transparent in the visible or infrared region, such as polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), cyclic olefin copolymer (COC), clear polystyrene, or a polycarbonate (PC). The sample vial may be any sample vial used for turbidity measurement. Such sample vials are generally elongate (e.g. cylindrical) and have a longitudinal axis. The secondary optical chamber may be configured to receive a sample vial insertable along an insertion axis extending between the opening and the base. The secondary optical chamber may be configured to receive a sample vial such that the longitudinal axis of the sample vial and the insertion axis are substantially parallel. Alternatively, the secondary optical chamber may be configured to receive a sample vial such that the longitudinal axis and the insertion axis are angled with respect to each other. An angle between the longitudinal axis and the insertion axis is preferably 20° or less; or more preferably 10° or less. The secondary optical chamber may be configured such that when a sample vial is inserted, its longitudinal axis is substantially perpendicular to (i.e. 90° ± 3°) the measurement plane. Alternatively, the longitudinal axis may be angled relative to the measurement plane by between 3° and 45° from the perpendicular; preferably between 3° and 20° from the perpendicular; or more preferably between 5° and 15° from the perpendicular (e.g. about 10° from the perpendicular). Such angling of the sample axis relative to the measurement plane advantageously reduces the impact of light reflection / glare by the sample vial on measurement accuracy. Preferably the secondary optical chamber has internal dimensions (e.g. length and width; or diameter) at least 1.5 times the dimensions (e.g. length and width, or diameter) of the sample vial. Even more preferably the secondary optical chamber has internal dimensions of between at least 2 times the dimensions of the sample vial (e.g. between 2 and 3 times). Thus when a sample vial is positioned in the secondary optical chamber, there is a gap between the sample vial and the secondary chamber walls. Providing a gap between the vial surfaces and the first and second windows and the secondary chamber walls reduces the effects of stray light impacting the accuracy of turbidity measurement. However increasing the dimensions of the secondary optical chamber necessarily increases the overall size of the turbidity instrument. Thus, where the turbidity instrument is intended to be portable (e.g. for use in the field), the secondary optical chamber preferably has internal dimensions of less than or equal to 5 times the dimensions of the sample vial. The turbidity instrument may further comprise an additional detector channel extending from the primary optical chamber along an additional detection axis to an additional optical detector, wherein the additional detector channel contains a third transmissive optical component positioned between the primary optical chamber and the additional optical detector and providing a barrier to fluid passage along the additional detector channel; and the secondary optical chamber may further comprise a third window positioned over a third aperture in the secondary chamber walls configured to be alignable with the additional detector channel when the secondary optical chamber is inserted into the primary optical chamber. Where the detection axis is perpendicular to the source axis, the additional detection axis is preferably coaxial with the source axis. Thus the additional optical detector is configured to measure attenuation of light from the light source by the sample. The additional detector channel thus enables simultaneous measurement of both scattered / diffuse light and attenuated light. The secondary optical chamber may comprise one or more baffles protruding from the interior face of the secondary chamber walls and positioned circumferentially between the first, second and (if present) third windows. The baffles act to limit or prevent stray light from the light source reaching the optical detector(s) without passing through the sample. Where the secondary optical chamber is cylindrical, each baffle may protrude radially inwards from the interior face of the secondary chamber walls to a point spaced apart from the insertion axis (i.e. a central point of the secondary optical chamber) by a distance approximately equal to the sample vial radius. Thus the baffles additionally provide alignment means for correctly positioning the sample vial within the secondary optical chamber. The baffles may include a pair of blocking baffles, positioned either side of the second window and defining a passageway between a sample vial (when in position) and the second window. In use, the blocking baffles act to prevent any stray light entering the passageway other than through the sample vial. The blocking baffles may each have a generally triangular prism shape, with a proximal face in a plane parallel to and spaced apart from the detection axis to define the passageway; and a distal face. The distal face may be angled perpendicularly to the proximal face. The distal faces of the two blocking baffles may be coplanar. The baffles may include a third baffle, positioned circumferentially opposite the second window. The third baffle (together with the pair of blocking baffles) acts as alignment means for positioning the sample vial. Where the instrument has an additional detector channel coaxial with the source axis, the third baffle prevents stray light reaching the additional detector channel by reflecting off surfaces and passing around the sample vial. The third baffle may have a different profile / shape to the pair of blocking baffles. The third baffle may be generally wedge shaped and may taper to be thicker towards the base than towards the opening. An angle between the base and exposed surfaces of the third baffle may be between 90° and 100°; preferably between 90° and 95°; e.g. 92.5°. This facilitates removal of the secondary optical chamber from a mould tool during manufacture and ensures that any surface finish or coating can be more easily applied to the exposed baffle surfaces. The baffles and the secondary chamber walls (and optionally the base) may be formed as a single unitary component (i.e. the baffles may be integrally formed in the secondary chamber walls). The baffles may be formed as indentations in the secondary chamber walls such that an exterior surface of the secondary chamber walls has troughs corresponding to the internal baffles. The troughs may act as the alignment feature and may correspond to ridges in the primary chamber walls. Interior surfaces of the secondary optical chamber (optionally including exposed surfaces of the baffles) may be treated to minimise optical reflectivity. For example, the interior surfaces may have a high surface roughness, for example a VDI 3400 standard surface roughness grade of between 30 and 36 (preferably 32 to 34; e.g. 33). Alternatively (or additionally) an anti-reflective coating such as an anti-reflection paint or a powder finish paint; or an optical coating (such as the CERA-mat coating provided by Eschmann Textures International) may be applied to some or all interior surfaces. The interior surfaces of the secondary chamber walls may be fluted. Fluting increases (on average) the number of times a beam of light reflects off a surface when travelling between two points. Each reflection reduces the beam intensity; and thus fluting reduces the impact of reflected stray light on measurement accuracy. The removable lid is preferably moveable between an open position which enables insertion and removal of the secondary optical chamber; and a closed position where the removable lid prevents insertion or removal of the secondary optical chamber. The removable lid is composed of an optically opaque material. The removable lid may include a central opening into the secondary optical chamber (when installed) to enable a sample vial to be inserted. In use, when a sample vial is inserted, the removable lid (in a closed position) and a lid of the sample vial may together block light from outside the instrument entering the primary or secondary optical chambers. Nevertheless, the instrument may further comprise an optic cover removably positionable over the removable lid to provide a secondary optical barrier to external light. The removable lid and or optic cover may be hingedly attached to the primary optical chamber. The secondary chamber walls (and preferably base) are composed of a material that is of a thickness sufficient to be opaque to visible light and near-infrared (i.e. >99% of electromagnetic radiation in the wavelength range 380 nm to 1400 nm blocked). The secondary optical component may be composed of polycarbonate (PC), acrylonitrile butadiene styrene (ABS), polypropylene, or mixtures thereof. The first, second and (if present) third windows are composed of a material which is substantially transparent (i.e. >90% transmittance) to electromagnetic radiation in a required wavelength range for measurement. For example, the first, second and (if present) third windows may be composed of a material which is substantially transparent in a wavelength range of 380 to 1400 nm; or 400 to 900 nm. The first, second and (if present) third windows may be composed of polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), a polycarbonate (PC) or glass. The first, second and (if present) third windows may be coated on at least one face with one or more of an anti-glare coating, an anti-reflective coating or an anti-scratch coating. Preferably any coating is applied on an interior facing surface of each window. The first, second and (if present) third windows may be removably attachable over the respective apertures. The secondary optical chamber may include seals disposed between each of the first, second and (if present) third windows and an interior facing surface (or preferably exterior facing surface) of the secondary optical chamber. The seals may be (for example) an o-ring seal. The seals may be composed of any suitable material, e.g. PTFE, silicone, neoprene, nitrile rubber, EPDM rubber, a fluorocarbon rubber or natural rubber. The first, second and (if present) third windows may be adhered to the interior facing surface (or preferably exterior facing surface) of the secondary optical chamber with any suitable adhesive. The interior (or preferably exterior) facing surface of the secondary optical chamber may include slots configured to receive the windows and (optionally) the seals. The first, second and (if present) third transmissive optical component may be sealed within the respective channel with an o-ring seal composed of any suitable material as described above with respect to the seals. This ensures that the detector(s), light source and other optical / electronic components on the opposite side of the transmissive optical components to the primary optical chamber are protected from water ingress. One or more of the source channel and detector channel(s) may contain an aperture component positioned between the respective transmissive optical component and the light source or detector. Each aperture component may have a fixed aperture, and the sizes and shapes of each aperture may be different for each aperture component. Alternatively one or more of the apertures may be adjustable (e.g. manually or by an electronic control system) to adjust the amount of light passing through from the light source or reaching the detector. The primary optical chamber may include a pouch or cage within the primary optical chamber configured to receive and contain a desiccant. The desiccant acts to absorb any moisture which enters the primary optical chamber. The primary optical chamber may have a bottom wall opposite the opening at the first end which is configured to be removable to open a bottom opening at a second end. The secondary optical chamber may be insertable into the primary optical chamber through the bottom opening; and the removable lid may be fixedly attached to and cover at least a portion of the opening of the primary optical chamber. Viewed from a second aspect the present invention provides a secondary optical chamber for use in a turbidity instrument as hereinbefore described, wherein the secondary optical chamber is defined by one or more secondary chamber walls, a base and an opening at a first end opposite the base, and includes first and second windows respectively positioned over first and second apertures; wherein the secondary optical chamber is removably insertable into a primary optical chamber of a turbidity instrument as hereinbefore described, and wherein the secondary optical chamber is configured to receive a fluid sample. Other features of the secondary optical chamber may be as hereinbefore described. Viewed from a third aspect the present invention provides a kit of parts, comprising: a turbidity instrument comprising: a primary optical chamber defined by one or more primary chamber walls and having an opening at a first end; a source channel extending from the primary optical chamber along a source axis to a light source, wherein the source channel contains a first transmissive optical component positioned between the primary optical chamber and the light source and providing a barrier to fluid passage along the source channel; a detector channel extending from the primary optical chamber along a detection axis to an optical detector, wherein the detector channel contains a second transmissive optical component positioned between the primary optical chamber and the optical detector and providing a barrier to fluid passage along the detector channel; a removable lid positionable to cover at least a portion of the opening of the primary optical chamber; and one or more secondary optical chambers defined by one or more secondary chamber walls, a base and an opening at a first end opposite the base, wherein the secondary optical chamber includes first and second windows respectively positioned over first and second apertures within the secondary optical chamber; wherein the secondary optical chamber is removably insertable into the primary optical chamber of the turbidity instrument, and wherein the secondary optical chamber is configured to receive a fluid sample; wherein the secondary optical chamber is configured to be insertable into the primary optical chamber to be in an operational position where the first and second windows are respectively aligned with the source channel and the detector channel; and the primary optical chamber is fluidly isolated from an exterior of the turbidity instrument. Brief description of drawings A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure lisa perspective view of a turbidity instrument according to a first embodiment. Figure 2 is a simplified exploded view of the turbidity instrument of Figure 1. Figure 3 is a simplified exploded view of the primary optical chamber, source channel and detector channels of the turbidity instrument of Figure 1. Figure 4 is a perspective view of the secondary optical chamber of the turbidity instrument of Figure 1. Figure 5 is another perspective view of the secondary optical chamber of the turbidity instrument of Figure 1. Figure 6 is yet another perspective view of the secondary optical chamber of the turbidity instrument of Figure 1. Figure 7 is a top face view of the secondary optical chamber of Figures 4, 5 and 6. Figure 8 is a cross-sectional view of the turbidity instrument of Figure 1, taken along the source axis. Figure 9 is a perspective view of an alternative secondary optical chamber compatible with the turbidity instrument of Figure 1. Figure 10 is an exploded section view of another turbidity instrument according to another embodiment. Detailed description of preferred embodiment of the invention Referring initially to Figure 1, a turbidity instrument 1 is shown. The turbidity instrument 1 is intended to be portable and usable in the field, and has a body / casing 6 which encloses a power source (battery), as well as hardware for a control system. The casing 6 includes an upper body part 60 and a lower body part 65, and has an opening 61 in the upper body part 60. As shown in Figure 1, the opening 61 is partially covered by a removable lid 3 which is hingedly mounted to an outer surface of the upper body part 60 and is shown in Figure 1 in a closed position. An optical cover 2 is also hingedly mounted to the outer surface of the upper body part 60 and, although shown in an open configuration in Figure 1 is closeable to cover the removable lid 3 and close the opening 61. Referring now to Figure 2, a simplified exploded view of part of the internal structure of the turbidity instrument 1 is shown. As shown in Figure 2, the opening 61 opens into a secondary optical chamber 5 which itself is received within a primary optical chamber 4. The secondary optical chamber 5 is configured to be removably received within the primary optical chamber 4; i.e. the secondary optical chamber 5 can be removed upwards through the opening 61 when the lid 3 and the optical cover 2 are in an open position. Referring now to Figures 4 to 7, the secondary optical chamber 5 is defined by secondary chamber walls 50 and a base 51. The secondary optical chamber 5 is generally cylindrical and is open at an upper end opposite the base 51 to be generally cup-shaped with a rim 52 at the upper end. The secondary chamber walls 50 have an interior facing surface 54 and an exterior facing surface 55. The secondary chamber walls 50 have a first aperture 56, a second aperture 57 and a third aperture 58 disposed therein. In this particular embodiment, all three apertures 56, 57 and 58 are positioned in the secondary chamber walls 50, however other embodiments may have the first aperture 56 positioned in the base 51 (where the secondary optical chamber 5 has only two apertures 56, 57); or the second aperture 57 may be positioned in the base 51. Each aperture has a generally circular cross-section. As best illustrated in Figures 5 and 7, the first aperture 56 and third aperture 58 are positioned diametrically opposite each other and are coaxial. As best illustrated in Figure 6, the second aperture 57 is positioned further from the base 51 (i.e. more towards the rim 52) than the first and third apertures 56, 58. On an exterior side of each aperture 56, 57, 58, an optical window 560, 570, 580 is positioned (two of which are shown in Figure 4). Each optical window 560, 570, 580 is composed of polycarbonate and is substantially transparent to electromagnetic radiation within the visible and near-IR wavelength range. An interiorly facing surface of each optical window 560, 570, 580 is coated with an anti-reflective coating. The optical windows 560, 570, 580 are received within frames 562, 572, 582 provided on the exterior surface 55. Figure 4 shows the optical windows 560, 570 in position. In Figures 5 and 6 the windows 560, 570, 580 are removed. As shown in Figure 6, the third frame 582 also receives a window seal 583. The window seal 583 is generally ring-shaped, with an opening corresponding to the aperture 58. In this particular embodiment, double-sided tape cut to the required shape as shown in Figure 6 is used as the seal 583. In other embodiments, the window seal 583 may be composed of an elastomeric material (such as PTFE). Similar window seals are also provided in the first and second frames 562, 572 (not shown in Figure 6). Each window seal is received between a window seal seat (second and third window seal seats 571, 581 shown in Figure 5) and an interior face of the respective window. The window seals provide a fluid seal between the interior and exterior of the secondary chamber walls 50 when the windows 560, 570, 580 are installed. In this particular embodiment, each optical window 560, 570, 580 and window seal is received within its respective frame 562, 572, 582 in a tight friction fit, and is held in place by the adhesive of the double-sided adhesive tape which makes up the window seal. Thus each window 560, 570, 580 is removable (for example for cleaning or replacement) by pushing outwards through the respective aperture 56, 57, 58 to separate the window from its respective window seal seat. In other embodiments, an adhesive may be applied between the interior face (or side edges) of each window and the frame / exterior surface to permanently bond the window in place (i.e. such that removal of the window is not possible without considerable force, risking failure of the window). As best shown in Figure 4, the windows 560, 570, 580 are sized larger than the apertures 56, 57, 58. This increases contact area between the window seal and the interior face of the window (for example to allow adhesive to be applied) without obscuring the aperture. As best illustrated in Figures 5 and 7, three baffles 540, 541 and 542 are provided within the secondary optical chamber 5. The baffles 540, 541, 542 are elongate and are provided on the interior surface 54 of the secondary chamber walls 50. In this particular embodiment, the baffles 540, 541, 542 comprise two blocking baffles 540, 541 positioned either side of the second aperture 57; and a third baffle 542 positioned diametrically opposite the second aperture 57. As shown in Figures 4 and 5, the blocking baffles 540, 541 in this particular embodiment are formed integrally as part of the secondary chamber walls 50. Thus two alignment channels 550, 551 are defined on the external surface 55 either side of the second aperture 57, corresponding to the inverse of the two blocking baffles 540, 541. The blocking baffles 540, 541 have a generally triangular cross-section, and each have a baffle tab 543, 544 protruding radially inwards from an interior-most edge. The blocking baffles 540, 541 taper inwardly towards the rim 52. An angle of the taper in this particular embodiment (with respect to an axis perpendicular to the interior surface of the base 51) is 2.5°. The third baffle 542 has a slimmer profile than the two blocking baffles 540, 541. As shown in Figure 5, the third baffle 542 has a generally triangular cross-section but tapers inwardly towards the rim 52. An angle of the taper in this particular embodiment (with respect to an axis perpendicular to the interior surface of the base 51) is 2.5°. An interior edge of the third baffle 542 and the two baffle tabs 543, 544 together define a holder to receive a sample vial 7 within a central area of the secondary optical chamber 5. The tabs 543, 544 and the interior edge of the third baffle 542 are spaced apart around a circumference of the central area which is slightly larger (100-105%) the circumference of the sample vial 7. Thus the tabs 543, 544 and the third baffle 542 are configured to hold the sample vial within the central area. Referring again to Figure 4, adjacent the rim 52 is provided an annular seal 53 extending around the exterior surface 55. The annular seal 53 in this embodiment is composed of silicone, although other elastomeric materials commonly used for such seals may be equally applicable. With reference to Figures 2 and 8, the annular seal 53 is configured to engage a corresponding annular seat 63 on an annular protrusion 62 of the upper body part 60 to provide a fluid tight seal. Referring again to Figures 2 and 8, the removable lid 3 has an annular body 30 around a central bore 31. In an assembled configuration (as shown in Figures 1 and 8), when the removable lid 3 is closed, the annular body 30 contacts a rim 52 of the secondary optical chamber 5 and an upper edge of the annular protrusion 62. A primary seal 64 is provided between the upper body part 60 and a rim 401 of the primary chamber walls 40. Thus, although a primary fluid seal into the primary optical chamber 4 is provided by the secondary chamber 5, annular seal 53 and upper body part 60, the annular body 30 of the lid 3 can act as an additional seal to prevent fluid (and light) ingress into the primary optical chamber 4 when the secondary optical chamber 5 and removable lid 3 are in place. The optical cover 2 acts to prevent exterior light and contaminants (fluid or airborne solid particles) from entering the secondary optical chamber 5 when closed during measurement or between uses. Figure 3 shows an exploded view of the primary optical chamber 4 and the source channel 41 and two detector channels 42, 43. Figure 8 is a cross-sectional view of the turbidity instrument 1 taken along the axis of the source channel 41 and the detector channel 43. The primary optical chamber 4 has a generally cylindrical shape defined by primary chamber walls 40. A base (not visible in Figure 3) is provided at the bottom / lower end of the primary chamber walls 40 to close the lower end of the primary optical chamber 4. Referring both to Figure 3 and Figure 8, a source channel 41 extends through the primary chamber walls 40 along a source axis (not shown) to a light source 46. The light source 46 in this embodiment is an LED array provided on a PCB and operable to deliver light within the required wavelength for turbidity measurement (as defined by the relevant standards). In alternative embodiments an alternative light source may be used (e.g. an incandescent lamp; or an arc lamp), coupled with one or more optical filters to achieve the desired emission wavelengths. An LED light source is preferred, particularly in portable turbidity instruments such as the turbidity instrument 1, owing to its relatively low power usage and resistance to damage as a result of movement. Also within the source channel 41 is an aperture component 491. The aperture component 491 is controllable (in this embodiment by a computerised control system, not shown) to alter the amount of light from the light source 46 passing along the source channel 41. A first transmissive optical component 410 is received within the source channel 41 and is secured in place using an o-ring 411. The o-ring 411 is composed of silicone, although other elastomeric materials may be equally applicable. The o-ring 411 and the first transmissive optical component 410 together provide a fluid barrier within the source channel 41. The turbidity instrument 1 also includes a detector channel 42 (not visible in Figure 8) extending through the primary chamber walls 40 along a detection axis (not shown) to an optical detector 47. The detection axis is configured to be perpendicular (i.e. at 90°) to the source axis. In this particular embodiment, a measurement plane containing the source axis and the detection axis is tilted (i.e. not perpendicular) with respect to the primary axis of the cylindrical primary optical chamber 4; and the longitudinal axis of the sample vial 7 (which is coaxial with the primary axis of the primary optical chamber 4). The angle of tilt in this particular embodiment is 10° from the perpendicular. Angling the measurement plane with respect to the sample advantageously reduces the effect of glare / reflection of the light on the sample vial 7. As a result of the tilt, an opening of the detector channel 42 is positioned higher in the primary chamber wall 40 compared to the source channel 41. The detector channel 42 also includes an aperture component 492 operable to control the amount of light passing through the detector channel 42 to the optical detector 47. A second transmissive optical component 420 is received within the detector channel 42 and is secured in place using an o-ring 421. The o-ring 421 and the second transmissive optical component 420 together provide a fluid barrier within the detector channel 42. In this embodiment, the turbidity instrument 1 includes an additional detector channel 43 extending through the primary chamber wall 40 along an additional detection axis (not shown) to an additional optical detector 48. The additional detection axis is coaxial with the source axis, such that the additional detector channel 43 and the source channel 41 are diametrically opposed around the primary optical chamber 4, as best shown in Figure 8. The additional detector channel also includes an aperture component 493 operable to control the amount of light passing through the additional detector channel 43 to the additional optical detector 48. A third transmissive optical component 430 is received within the additional detector channel 43 and is secured in place using an o-ring 431. The o-ring 431 and the third transmissive optical component 430 together provide a fluid barrier within the additional detector channel 43. Thus the primary chamber walls 40, together with the three transmissive optical components 410, 420, 430 received within the three channels 41, 42, 43 provide a fluid-tight barrier between the interior of the primary optical chamber 4 and any electronic components within the turbidity instrument 1. The fluid tight barrier may be rated at IP67 or greater. Referring again to Figure 3, a pair of ridges 44 are provided on an interior facing surface 45 of the primary chamber walls 40. The ridges 44 are positioned either side of the detector channel 42 and are configured to be received in the corresponding alignment channels 550, 551 defined by the two blocking baffles 540, 541. Thus the secondary optical chamber 5 is only insertable into the primary optical chamber in a single orientation; with the two ridges 44 received within the alignment channels 550, 551, such that the source channel 41 aligns with the first aperture 56; the detector channel 42 aligns with the second aperture 57; and the additional detector channel 43 aligns with the third aperture 58. Thus when the second optical chamber 5 is inserted in the primary optical chamber 4, the arrangement of the channels 41, 42, 43, the light source 46, the detectors 47, 48, and the apertures 56, 57 and 58 enable the optical detector 47 to be operable to detect scattered / diffuse light from a sample in a sample vial 7 by nephelometry, and the additional optical detector 48 to detect attenuated light (i.e. the reduction in intensity of light passing through a sample in the sample vial 7). As best illustrated in Figures 2 and 8, when the optical cover 2 is in an open position, a sample vial 7 is insertable through the central bore 31 of the removable lid 3 and into the central area of the secondary optical chamber 5 (defined by the baffle tabs 543, 544 and the third baffle 542). An opaque lid 70 of the sample vial 7 includes a lip 71 which contacts a corresponding seat around the central bore 31 of the removable lid 3. Thus a transparent main body 72 of the sample vial 7, in which a fluid sample to be tested may be contained, is received within the central area of the secondary optical chamber 5. In the configuration shown in Figure 8, the opaque lid 70 and the removable lid 3 together prevent external light from entering the secondary optical chamber 5 (and the primary fl optical chamber 4). The optical cover 2 may be closed to provide additional protection against external light. Once a sample is in place as shown in Figure 8, the turbidity instrument 1 can be switched on and the light source 46 can be activated to direct light along the source axis through the source channel 41, the first window 560 and the first aperture 56 into the secondary optical chamber 5. The light then passes through the transparent walls of the sample vial 7 into the sample. Depending on the turbidity of the sample, light may then be scattered by particles within the sample. Light scattered along the detection axis (i.e. 90° to the source axis) then passes through the wall of the sample vial 7, through the second aperture 57 and the second window 570 and into the detector channel 42 where it is detected by the optical detector 47. Simultaneously, light continuing to pass through the sample along the source axis (i.e. light which is not scattered or absorbed by the sample) passes through the wall of the sample vial 7, through the third aperture 58 and the third window 580 and into the additional detector channel 43, where it is detected by the additional optical detector 48. Data from the two optical detectors 47, 48 and information on the settings (aperture sizes) of the aperture components 491, 492, 493 and the intensity of light produced by the light source 46 can then be used by a control system to provide a measurement of the turbidity of the sample. In this embodiment, the aperture components 491, 492, 493 each have a fixed aperture. The blocking baffles 540, 541 together block light which has not passed through the sample vial 7 (e.g. light which reflects off the interior surfaces of the secondary optical chamber 5) from entering the detector channel 42. Particularly for low turbidity samples, scattered light may be of a low intensity and thus the measurement of scattered light 47 at the optical detector 47 is vulnerable to noise as a result of external and reflected light. The third baffle 542 also acts to block light which has not passed through the sample vial 7 from entering the additional detector channel 43. In this embodiment, the secondary optical chamber 5 is composed of black coloured polypropylene. The interior facing surface 54 of the secondary optical chamber 5, including the exposed surfaces of the baffles 540, 541, 542, is treated to increase surface roughness and thus reduce reflectivity, to further reduce the impact of light reflections within the secondary optical chamber 5 on measurement accuracy. Over prolonged use of the turbidity instrument 1, particularly in the field, the windows 560, 570, 580 in the secondary optical chamber 5 may become scratched or otherwise damaged due to physical contact and chemical damage. When such damage becomes significant enough to affect the performance / accuracy of the turbidity instrument 1, or at a set interval, a user can simply remove the entire secondary optical chamber 5 by opening the optical cover 2 and the removable lid 3, and lifting the secondary optical chamber 5 out of the primary optical chamber 4 with the aid of tabs 59 disposed around the rim 52. A replacement secondary optical chamber 5 can then be inserted into the primary optical chamber 4 and the turbidity instrument 1 can continue to be used without requiring recalibration or servicing. The old secondary optical chamber 5 can then either be discarded, or the windows 560, 570, 580 can be removed and replaced or cleaned. Referring now to Figure 9, an alternative secondary optical chamber 8 is shown. The secondary optical chamber 8 is compatible with the turbidity instrument 1 and has a generally similar construction to the secondary optical chamber 5 with a generally cylindrical cup-like shape defined by secondary chamber walls 80 and a base 81 at a bottom end. A rim 82 extends around a top (open) end of the secondary optical chamber 8. Around the rim 82 on an exterior surface 85 is an annular seal 83 similar to the annular seal 53 and engageable with the annular seat 63 on the upper body part 60. First, second and third apertures 86, 87, 88 are provided in the secondary chamber walls 80, with corresponding frames 872, 882 on the exterior surface 85 to receive windows (not shown). In the secondary optical chamber 8, an interior facing surface 84 is provided with flutes 89 extending longitudinally from the base 81 towards the rim 82. The flutes preferably have internal angles of <90°; more preferably between 50° and 70°. In use, the flutes 89 act to increase the number of times any stray light reflects off a surface before (possibly) reaching a detector. The intensity of reflected light decreases by c. 95% with each reflection, so increasing the number of reflections acts to reduce the impact of any stray light on measurement accuracy. The secondary optical chamber 8 also has differently shaped baffles 840, 841, 842. Each baffle has a slimmer profile, and the two blocking baffles 840, 841 either side of the second aperture 87 extend partially inwards along a chord across the interior space of the cylindrical cavity, rather than extending radially inwards. In use, the two blocking baffles 840, 841 still function to prevent light which has not passed through the sample vial from entering the second aperture 87. Referring now to Figure 10, a simplified cross-sectional view of an alternative turbidity instrument 10 is shown. The turbidity instrument 10 is similar in construction to the turbidity instrument 1, but the primary optical chamber 140 includes a removable base 141 receivable in an opening of a bottom body plate 107. Thus the secondary optical chamber 150 (which may be similar to the secondary optical chambers 5, 8 already described) can be inserted into and removed from the primary optical chamber 140 by removing the base 141. An opening 102 in an upper body portion 106 of the turbidity instrument 10 allows a sample vial (not shown) to be inserted.

Claims

1. A turbidity instrument comprising:a primary optical chamber defined by one or more primary chamber walls and having an opening at a first end;a source channel extending from the primary optical chamber along a source axis to a light source, wherein the source channel contains a first transmissive optical component positioned between the primary optical chamber and the light source and providing a barrier to fluid passage along the source channel;a detector channel extending from the primary optical chamber along a detection axis to an optical detector, wherein the detector channel contains a second transmissive optical component positioned between the primary optical chamber and the optical detector and providing a barrier to fluid passage along the detector channel;a secondary optical chamber defined by one or more secondary chamber walls, a base and an opening at a first end opposite the base, wherein the secondary optical chamber includes first and second windows respectively positioned over first and second apertures within the secondary optical chamber; wherein the secondary optical chamber is removably insertable into the primary optical chamber, and wherein the secondary optical chamber is configured to receive a fluid sample;a removable lid positionable to cover at least a portion of the opening of the primary optical chamber;wherein when the secondary optical chamber is inserted into the primary optical chamber in an operational position, the first and second windows are respectively aligned with the source channel and the detector channel; and the primary optical chamber is fluidly isolated from an exterior of the turbidity instrument.

2. The turbidity instrument of any preceding claim, wherein the primary and secondary optical chambers each have a generally cylindrical shape.

3. The turbidity instrument of claim 2, wherein an external diameter of the secondary optical chamber is at least 90% of an internal diameter of the primary optical chamber.

4. The turbidity instrument of any preceding claim, wherein an interior facing surface of the primary chamber walls includes one or more ridges extending generally perpendicularly to the opening; and the exterior facing surface of the secondary chamber walls includes one or more grooves or channels to receive the ridges of the primary chamber walls.

5. The turbidity instrument of claim 4 when dependent on claim 2, where the ridges and grooves or channels are spaced apart unequally around a circumference of the primary and secondary chamber walls respectively.

6. The turbidity instrument of any preceding claim, wherein the first and second windows are both positioned in the secondary chamber walls.

7. The turbidity instrument of any preceding claim, wherein the source channel and detector channel are arranged such that the source axis and detection axis are substantially perpendicular.

8. The turbidity instrument of any preceding claim, wherein the secondary optical chamber is configured to receive a fluid sample in a sample vial.

9. The turbidity instrument of claim 8, wherein a longitudinal axis of the sample vial is angled relative to a measurement plane containing the source and detection axes by between 3° and 20° from perpendicular.

10. The turbidity instrument of claim 8 or 9, wherein the secondary optical chamber has internal dimensions of between 1.5 times and 5 times the dimensions of the sample vial.

11. The turbidity instrument of any preceding claim, further comprising:an additional detector channel extending from the primary optical chamber along an additional detection axis to an additional optical detector, wherein the additional detector channel contains a third transmissive optical component positioned between theprimary optical chamber and the additional optical detector and providing a barrier to fluid passage along the additional detector channel; andwherein the secondary optical chamber further comprises a third window positioned over a third aperture in the secondary chamber walls configured to be alignable with the additional detector channel when the secondary optical chamber is inserted into the primary optical chamber.

12. The turbidity instrument of claim 11, wherein the detection axis is perpendicular to the source axis and the additional detection axis is coaxial with the source axis.

13. The turbidity instrument of any preceding claim, wherein the secondary optical chamber comprises one or more baffles protruding from the interior face of the secondary chamber walls and positioned circumferentially between the first, second and (if present) third windows.

14. The turbidity instrument of claim 13, wherein the baffles and the secondary chamber walls are formed as a single unitary component, with the baffles formed as indentations in the secondary chamber walls such that the exterior facing surface of the secondary chamber walls has troughs corresponding to the baffles, wherein the troughs act as alignment features and correspond to ridges in the primary chamber walls.

15. The turbidity instrument of any preceding claim, wherein the interior facing surface of the secondary chamber walls has an anti-reflective coating applied.

16. The turbidity instrument of any preceding claim, wherein the interior facing surface of the secondary chamber walls is fluted.

17. A secondary optical chamber for use in a turbidity instrument according to claim 1, wherein the secondary optical chamber is defined by one or more secondary chamber walls, a base and an opening at a first end opposite the base, and includes first and second windows respectively positioned over first and second apertures; wherein the secondary optical chamber is removably insertable into the primary optical chamber of the turbidityinstrument, and wherein the secondary optical chamber is configured to receive a fluid sample.

18. A kit of parts, comprising:5 a turbidity instrument according to any of claims 1 to 16; andone or more secondary optical chambers according to claim 17.

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