Measurement of optical properties of a fluid

Abstract

Apparatus (1) for measuring at least one optical property, for instance turbidity, of a fluid (2), such as drinking water, is disclosed. The apparatus comprises a measurement section (3) comprising a conduit (4) for the fluid, a light source (15), a photodetector (17), and a processing unit (22) for processing at least one signal received from the at least one photodetector for use in determining the at least one optical property of the fluid. The at least one light source, the at least one photodetector and the diffuse reflector are configured such that, when light (16) is emitted by the at least one light source and the fluid is in the conduit, the light passes through the fluid and at least some of the light is reflected from the diffuse reflector towards the at least one photodetector, the apparatus configured to restrict light at the second wavelength reaching the at least one photodetector.

Description

[0001] Measurement of optical properties of a fluid

[0002] Field

[0003] The present invention relates to measurement of optical properties of a fluid, in particular, drinking water.

[0004] Background

[0005] Drinking water is highly regulated, usually by a national regulator, such as, in the case of England and Wales, the Drinking Water Inspectorate. Standards and regulations specify requirements for water covering a range of parameters including microorganisms, chemicals, metals and the way water looks and tastes including optical properties, such as turbidity and discolouration.

[0006] Turbidity and discolouration are properties that are most noticeable to end users and, thus, are most likely to be reported to the supplier ( / .e., the utility company) by end users. It is desirable, therefore, for the utility company to be able to measure these optical properties, particularly at the point that it reaches the end user. This, however, may be impractical. Turbidity measuring instruments typically employ an arrangement in which a sample is illuminated with light at a specific wavelength from a given angle, and detecting the amount of light scattered at 90 degrees from the incident light direction. Such instruments are complex which places constraints on how and where they can be installed, which generally increases their cost, and ultimately places limits on the number of locations that turbidity can be monitored.

[0007] Reference is made to JP 2010 181150 A which describes measuring the concentration of a dissolved substance dissolved in a liquid. A light emitting body transmits light which passes through the solution to be measured, is reflected by a reflecting plate, passes through the solution again and is received by a light receiving body. The light emitting body and the light receiving body are arranged so that the optical axis of the light emitter is oriented at approximately 45 degrees with respect to the reflecting plate.

[0008] GB 2634585 A describes apparatus for measuring at least one optical property of a fluid, such as turbidity or discolouration. The apparatus comprises a measurement section comprising a conduit for the fluid, at least one light source, at least one photodetector, a diffuse reflecting surface, and a processing unit for processing at least one signal received from the photodetector for use in determining the optical property of the fluid. The at least one light source, photodetector and diffuse

[0009] 138393PCT1reflecting surface are configured such that, when light is emitted by the at least one light source and the fluid is in the conduit, the light passes through the fluid and is reflected from the diffuse reflecting surface towards the at least one photodetector.

[0010] Reference is also made to WO 2010 / 090391 A2, which discloses a turbidity measuring probe, JP 3842492 B2, which describes an algae concentration measurement system, WO 2023 / 153623 Al, which describes a turbidity monitoring device and WO 2023 / 172196 Al, which concerns an optical sensor for determining a real-time fluid property of particles in a turbid medium.Summary

[0011] According to a first aspect of the present invention there is provided apparatus for measuring at least one optical property of a fluid. The apparatus comprises a measurement section comprising a conduit for a fluid, a light source, a photodetector and a processing unit for processing at least one signal received from the photodetector to determine at least one optical property of the fluid. The light source and the photodetector are disposed on a first side of the conduit and configured such that, when light is emitted by the light source and a fluid containing particles is in the conduit, at least some of the light is scattered by the particles so as to be receivable by the photodetector. The apparatus is configured to restrict light reaching the photodetector other than by scattering from particles. This can help increase signal-to-noise ratio (SNR) of a measurement.

[0012] The at least optical property of the fluid may be turbidity. The light may lie in the infrared light spectrum.

[0013] The light source may be capable of emitting light across a first range of wavelengths which includes a first wavelength. The light source may be capable of emitting light across a second range of wavelengths which includes a second, different wavelength. The least one light source may be capable of transmitting light across a third range of wavelengths.

[0014] The light source may comprise a light-emitting diode. The light source may comprise a light-emitting laser diode. This can help to reduce stray light and, thus, improve signal-to-noise ratio. The light source may include a broadband light source (such as a white light source) and a filter to provide light at the first wavelength (or first range of wavelengths including the first wavelength).

[0015] The apparatus may be configured so as to direct the light in a predetermined direction and to absorb the light by a predetermined surface of the conduit and / or to block light reflected from a predetermined surface of the conduit.

[0016] The apparatus may further comprise a diffuse reflector disposed on a second side of the conduit, opposite the light source and the photodetector. This can help increase SNR since light scattered by the particle can be reflected back to the photodetector.

[0017] The light source may be capable of emitting infrared light. Thus, the at least one optical property may include turbidity. The fluid may be water or an aqueous solution.The apparatus may include a first lens for guiding light from the photodetector into the conduit. The apparatus may include a second lens for guiding light from the conduit to the photodetector.

[0018] The apparatus may include a shield interposed between the light source and detector so as to block light from the light source reflecting off an interface (such as a surface of a lens) towards the photodetector. The shield may extend between the first and second lenses.

[0019] The apparatus may comprise at least one light source including the light source, the at least one light source capable of emitting light at a first wavelength and at a second different wavelength and a diffuse reflector disposed on a second side of the conduit, opposite the light source and the photodetector. The apparatus may be configured such that, when light at the first wavelength is emitted by the at least one light source and the fluid is in the conduit, the light at the first wavelength passes through the fluid and is reflected from the diffuse reflector towards the photodetector. The apparatus may be configured such that, when light at the second wavelength is emitted by the at least one light source and the fluid is in the conduit, the light at the first wavelength is restricted from reaching the photodetector other than by scattering from particles.

[0020] According to a second aspect of the present invention there is provided apparatus for measuring at least one optical property of a fluid. The apparatus comprises a measurement section comprising a conduit for a fluid, at least one light source capable of transmitting light at a first wavelength and light at a second, different wavelength, at least one photodetector, a diffuse reflector and a processing unit for processing at least one signal received from the at least one photodetector for use in determining at least one optical property of a fluid.

[0021] The at least one light source, the at least one photodetector and the diffuse reflector may be configured such that, when light is emitted by the at least one light source and the fluid is in the conduit, the light passes through the fluid and at least some of the light is reflected from the diffuse reflector towards the at least one photodetector.

[0022] The apparatus may be configured to restrict and / or prevent light at the second wavelength reaching the at least one photodetector. The apparatus may be configured to allow light at the second wavelength that has been scattered by the fluid to be received by the photodetector, but to restrict and / or prevent light at the secondwavelength that has not been scattered by the said fluid to be received by the photodetector.

[0023] This can help increase signal-to-noise ratio (SNR) of a measurement based on light at the second wavelength.

[0024] The arrangement(s) may be configured to reduce the proportion of light at the second wavelength emitted by the at least one light source which is unscattered and which reaches the at least one photodetector at least 90% or by at least 99%.

[0025] The at least optical property of the fluid may comprise first and second optical properties. The first optical property may be colour of the fluid and the second optical property may be turbidity.

[0026] The second wavelength may be larger than the first wavelength. The first wavelength may lie in the visible light spectrum. The second wavelength may lie in the infrared light spectrum.

[0027] The apparatus may comprise at least one light source capable of emitting light at least three different wavelengths including the first and second wavelengths and a third wavelength. The third wavelength may lie in the visible light spectrum. For example, the first and third wavelengths may lie in green and red portions of visible light spectrum. Thus, the apparatus may determine if the liquid is green (for example, indicative of the presence of, for example, algae) and / or if the liquid is red or brown (for instance, indicative of the presence of rust, fecal matter and / or soil).

[0028] The at least one photodetector may consist of a photodetector (in other words, there may be only one photodetector).

[0029] The at least one light source may be capable of transmitting light across a first range of wavelengths which includes the first wavelength and / or across a second range of wavelengths which includes the second wavelength. The least one light source may be capable of transmitting light across a third range of wavelengths. The light across the first range of wavelengths is preferably visible light and the light across the second range of wavelengths is preferably infrared light. In other words, the least one light source is (are) preferably capable of transmitting visible light and infrared light.The at least one light source may include at least one light-emitting diode. The at least one light source may include at least one light-emitting laser diode. This can help to reduce stray light and, thus, improve signal-to-noise ratio. The at least one light source may include a broadband light source (such as a white light source) and at least one filter to provide light at the first wavelength (or first range of wavelengths including the first wavelength) and / or light at the second wavelength (or second range of wavelengths including the second wavelength).

[0030] The at least one light source may include a first light source capable of transmitting light at the first wavelength (or first range of wavelengths including the first wavelength) and a second light source capable of transmitting light at the second wavelength (or second range of wavelengths including the second wavelength). The first light source and the second light source are preferably spaced apart (for example by at least 1 mm, at least 2 mm, at least 5 mm or at least 10 mm).

[0031] The apparatus may be configured such that light at the first wavelength and light at the second wavelength emitted by the at least one light source start on different paths.

[0032] The apparatus may further comprise at least one optical component configured to restrict light at the second wavelength or the light across the second range of wavelengths reaching the at least one photodetector. The at least one optical component may be a shield arranged to block light from the at least one light source reflecting off an interface (such as, the surface of a lens) towards the photodetector. The shield may abut the interface (such as, the surface of a lens). The shield may abut a substrate which supports the at least one light source and the photodetector.

[0033] The at least one optical component may include a lens arrangement. The lens arrangement may consist of one lens. The lens arrangement may comprise first and second lenses. The second lens may be configured to receive only the light at the second wavelength or the light across the second range of wavelengths.

[0034] The at least one optical component may include an optical baffle.

[0035] The apparatus may further comprise a sensor unit including the at least one light source and the at least one photodetector, the sensor unit comprising a housing. The apparatus may further comprise a sealing member for sealing the sensor unit in theconduit. The sealing member may be an O-ring. The sealing member may also serve as an optical component, such as an optical baffle.

[0036] The apparatus may further comprise an optical filter. The optical filter may be configured to pass light in the first range of wavelengths and to restrict light in the second range of wavelengths. The optical filter may be disposed in front of or incorporated in the diffuse reflector.

[0037] The optical filter may be coating. The optical filter may be an additive, such a dye, in the diffuse reflector. The optical filter may restrict light by reflecting less than 95%, less than 80% or less than 50 % of incident light in the second range of wavelength. The optical filter may pass light by reflecting more than 50%, more than 75% or more than 90 % of incident light in the first range of wavelengths. The second range of wavelengths may fall within or at least overlap with 700 to 3,000 nm.

[0038] The optical filter may be configured to pass light of a first polarization and to restrict light of a second, different polarization. The optical filter may be disposed in front of or incorporated into a one of the at least one photodetector or the photodetector. The first polarization may be a first plane of linear polarization and the second polarization may be a second, orthogonal plane of linear polarization

[0039] The apparatus may further comprise at least one optical component configured to focus, reflect, and / or direct light from a first subset of the at least one light sources in a first direction, and to focus, reflect, and / or direct light from a second subset of the at least one light sources in a second, different direction.

[0040] The at least one optical component may comprise or be a lens arrangement. The lens arrangement may consist of a single lens. The lens arrangement may comprise at least two lenses.

[0041] The at least one optical component may be configured to focus, reflect, and / or direct light from a nthsubset of the at least one light sources in an nthdirection (where n is a positive non-zero integer). The first and second and optionally nthdirections are preferably mutually exclusive.

[0042] Light power of first and second subsets and optionally nthsubsets of the at least one light source preferably spatially overlap by less 60%, less than 40% or less than 20%.Light power of the first and second subsets and optionally nthsubsets of the at least one light source preferably do not spatially overlap.

[0043] The first and second directions are preferably predetermined.

[0044] The at least one optical component may cause light travelling in the second direction to fall outside of the diffuse reflector.

[0045] The diffuse reflector may be configured to reflect a lower proportion light at a first range of incident angles and to reflect a higher proportion of light at a second, different range of incident angles.

[0046] The apparatus may comprise at least one polarising filter interposed between the at least one light source and the at least one photodetector. The at least one polarising filter may be interposed between the at least one light source or the at least one photodetector and a lens assembly and / or conduit.

[0047] The at least one polarising filter may be interposed between the diffuse reflector and the conduit.

[0048] The at least one polarising filter may comprise first and second different polarising filters, wherein the first and second different polarising filters have orthogonal polarizations such that light passed through the first polarising filter is blocked from by the second polarising filter.

[0049] The diffuse reflector may be configured such that optical properties of the diffuse reflector are changeable. For example, the diffuse reflector may comprise a thermochromic material, an electrochromic material, an electro-optic material and / or a liquid crystal.

[0050] The conduit may include at least one optical component for restricting light. For example, the at least one optical component may be configured to block light. The at least one optical component may comprise an optical baffle or aperture stop. The at least one optical component may be configured to reflect light, to refract light and / or to deflect light.According to a third aspect of the preset invention there is provided a fluid meter comprising a unit housing apparatus for measuring fluid flow through the meter, and the apparatus of the first or second aspect.

[0051] The fluid may be water, such as drinking water, or include water, such as a beverage.

[0052] According to a fourth aspect of the present invention there is provided apparatus for measuring at least one optical property of a fluid. The apparatus comprises a measurement section comprising a conduit for a fluid, at least one light source, at least one photodetector, a diffuse reflecting surface, and a processing unit for processing at least one signal received from the at least one photodetector for use in determining at least one optical property of a fluid. The at least one light source, the at least one photodetector and the diffuse reflecting surface are configured such that, when light is emitted by the at least one light source and the fluid is in the conduit, the light passes through the fluid and is reflected from the diffuse reflecting surface towards the at least one photodetector.

[0053] In the case of water, the apparatus can be provided in, or together with, a water meter, and so help sensors capable of measuring turbidity and discoloration to be deployed more widely throughout the water delivery network, particularly at points of use.

[0054] The light emitted by the at least one light source may pass along an optical axis which is normal to the diffuse reflecting surface or which is within 10 degrees from the normal to the diffuse reflecting surface. The light emitted by the at least one light source may pass radially across a cylindrical conduit. The light emitted by the at least one light source may pass along an optical axis which is parallel to the normal to the diffuse reflecting surface. The light emitted by the at least one light source may pass along an optical axis which is perpendicular to the axis of a cylindrical conduit. The light emitted by the at least one light source may pass along an optical axis which is perpendicular to a plane containing the axis of the cylindrical conduit.

[0055] The at least one light source and the at least one photodetector may be disposed on one side of measurement section.

[0056] The conduit may be a tube. The conduit may me be circular, elliptical or rectangular in cross section.The at least one property may include colour of the fluid and / or turbidity of the fluid.

[0057] The diffuse reflecting surface may be curved such as an annular section of a cylinder of material or an arced section of a cylinder of material. The diffuse reflecting surface may be or may be substantially white. The diffuse reflecting surface may comprise a coating on an inner surface of the conduit.

[0058] The at least one light source or each light source may be or may be substantially monochromatic and the at least one photodetector is broadband (expressed differently, able to detect more than one different wavelength) or vice versa.

[0059] The at least one light source and the at least one photodetector may be provided in a sensor unit comprising a housing.

[0060] At least one of the at least one light source and / or at least one of the at least one photodetector may be operable in the infrared band of the electromagnetic spectrum.

[0061] The processing unit may be configured to calculate a given parameter indicative of the quality of the fluid in dependence upon the at least one signal received from the at least one photodetector.

[0062] The fluid may be a liquid. The liquid may be water, such as, drinking water, for example, for human consumption. The fluid may be a gas.

[0063] The at least one light source may be activated for a duration and / or at a repetition rate depending on flow rate of fluid and / or an accumulated volume of fluid. For example, the apparatus may take a measurement every N litres, where N may be positive (corresponding to flow in a forward direction) or negative (corresponding to flow in a reverse direction, i.e., back flow), non-zero number, for example, greater than or equal to 10. This can be used to help reduce power consumption.

[0064] According to a fifth aspect of the present invention there is provided a fluid meter comprising a unit housing apparatus for measuring fluid flow through the meter and the apparatus of the first, second and / or fourth aspect.

[0065] According to a sixth aspect of the present invention there is provided a system comprising a fluid meter and the apparatus of the first, second and / or fourth aspect.The fluid meter and the apparatus are discrete units arranged such that fluid is communicated through the meter and then through the apparatus or vice versa.

[0066] According to a seventh aspect of the present invention there is provided computer system comprising at least one processor, memory and a network interface. The at least one processor is configured, in response to receiving a set of messages, such as measurements, alarms and / or codes, from a set of remote water meters at known locations relating to at least one water property, such as oxidation reduction potential, discoloration and / or turbidity, to detect a change in water being delivered to a given water meter and / or to a given sub-set of meters, and / or to determine a cause of a change in water being delivered to a given water meter and / or to a given sub-set of meters, and / or to determine a statistical estimate of a point of origin in a water network resulting in a change in water being delivered to a given water meter and / or to a given sub-set of meters, and / or to determine a severity of change in water being delivered to a given water meter and / or to a given sub-set of meters.

[0067] According to an eighth aspect of the present invention there is provided a drinking water delivery system comprising a set of remote water meters provided with or including a respective apparatus of the first aspect and deployed through a water supply network; and the computer system of the fifth aspect. The computer system is arranged to communicate with the set of remote water meters so as to receive the set of messages, such as measurements, alarms or codes.

[0068] According to a ninth aspect of the present invention there is provided a method of using the apparatus of the first aspect, the method comprising causing activation of the at least one light source or at least two light sources sequentially and processing signals, such as a sequence of signals, received from the at least one photodetector to determine at least one optical parameter.

[0069] The at least one light source may be active at a given repetition rate or given pattern and the least one photodetector may be read out in synchronisation with activation of the at least one light source so as to increase read-out sensitivity.

[0070] According to a tenth aspect of the present invention there is provided an apparatus for measuring an optical parameter of a fluid comprising a measurement section that allows the measured fluid to flow through a round conduit, at least one light source, and at least one photodetector positioned substantially on one side of the measurement section, a curved diffuse reflecting surface positioned in a way to reflectoutgoing light, that has passed through the measured fluid from the light source, back to the photodetector and an electronic processing unit.

[0071] The apparatus may comprise a water meter. The apparatus may be adjacent and / or connected to a fiscal grade water meter. The apparatus may be located inside the main body of a fiscal grade water meter.

[0072] The fluid may be drinking water.

[0073] The measured optical parameter may be discoloration and / or turbidity.

[0074] The diffuse reflector may be white in colour. The diffuse reflector may comprise of a dye or coating applied to the measurement section.

[0075] The light source may be substantially monochromatic (either intrinsically or through filtering), and the photodetector is broadband or vice versa. Some or all the light sources and photodetectors may be contained in the same housing.

[0076] At least one light source or photodetector may be (are) substantially in the IR spectra.

[0077] The electronic processing unit may calculate water quality parameters, given the measurements taken by the photodetector(s).

[0078] According to an eleventh aspect of the present invention there is provided a method using the apparatus of the first, third and / or seventh aspect, wherein multiple light sources, preferably of different spectral content, are illuminated sequentially, in such a way as to allow the photodetector to measure the response from each light source individually.

[0079] According to an eleventh aspect of the present invention there is provided a drinking water delivery system comprising the apparatus of the first and / or seventh aspect, a collection of fiscal water meters installed in a water supply network wherein: at least some meters are capable of electronically communicating to a centralised storage and analysis unit; at least some meters have at least one additional sensor capable of measuring at least one property of the water; wherein the sensor is mounted within the water meter body and / or in line with the water meter in the main water stream, measuring at least one of the following ORP (oxidation reduction potential), discolouration, turbidity; and a centralised storage and analysis unit that uses storeddata to detect changes in the water being delivered to a water meter and / or a group of meters and / or inferring the cause of the change and / or providing a statistical estimate of the origin point in the network and / or the severity of changes in the water being delivered.

[0080] The centralised storage and analysis unit may combine the flow data with the ORP and / or discolouration and / or turbidity data. The centralised storage and analysis unit may combine the water pressure data with the ORP and / or discolouration and / or turbidity data. The communication between the water meter and the centralised storage and analysis unit may use wireless communication protocols. The meter may have the capability of controlling the water quality sensor and request the measurement to be performed. The water quality sensor may measure independently at regular intervals and reports the information to the water meter. The centralised storage system can request at least one of the water meters to capture and can report an additional measurement based on the values already stored in the system based on the values stored in the centralised storage system.Brief Description of the Drawings

[0081] Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:

[0082] Figure 1 is a schematic block diagram of apparatus for measuring optical properties of a fluid;

[0083] Figure 2 is a schematic perspective view of a conduit;

[0084] Figure 3 is a schematic side view of an example of a sensor unit;

[0085] Figure 4 is a schematic end view of an assembly of a sensor unit and a pipe;

[0086] Figure 5 is a schematic side view of an assembly of the sensor unit and a pipe shown in Figure 4 and a processing unit;

[0087] Figure 6 is a process flow diagram of a method of operating the sensor unit;

[0088] Figures 7A to 7C are side views of three reflectors for use in the measuring apparatus; Figure 8 is a schematic side view of a first arrangement for improving signal-to-noise ratio (SNR);

[0089] Figure 9 is a schematic side view of a second arrangement for improving SNR;

[0090] Figure 10 is a schematic side view of a third arrangement for improving SNR;

[0091] Figure 11 is a schematic side view of a fourth for improving SNR illustrating reflection of infrared light from a reflector;

[0092] Figure 12 is a schematic side view of a fourth arrangement for improving SNR illustrating reflection of infrared light from a suspended particle;

[0093] Figure 13 is a schematic side view of a fifth arrangement for improving SNR;

[0094] Figure 14 is a schematic side view illustrating a source of stray light;

[0095] Figure 15 is a perspective view of a sensor having a lens for visible light and a lens for infrared light;

[0096] Figure 16 is a cutaway perspective view of the sensor shown in Figure 15;

[0097] Figure 17 is a cross-sectional view of the sensor shown in Figure 15 and a pipe;

[0098] Figure 18 is schematic side view of the sensor shown in Figure 5;

[0099] Figure 19 is a plot of received infrared light intensity as a function of turbidity for the sensor shown in Figure 15;

[0100] Figure 20 is a plot of received infrared light intensity as a function of turbidity for a sensor similar to that shown in Figure 15 but having a single lens;

[0101] Figure 21 is a plot of received infrared light intensity as a function of turbidity for a sensor with and without a diffuse reflector;

[0102] Figure 22 is a schematic diagram of a water delivery network; and

[0103] Figure 23 is a schematic diagram of a part of a water delivery network.Detailed description of Certain Embodiments

[0104] Turbidity is caused by the presence of suspended solids in water, usually caused by contamination with clay, silt, organic matter and micro-organisms, such as plankton. Turbidity is measured as the amount of light that is scattered when passing through water, and is expressed in Nephelometric Turbidity Units (NTUs). Turbidity is visible from around 5 NTU. A high turbidity (typically considered to be greater than 5 NTUs) is an issue for water distribution utility companies because it can lead to customer complaints and also because it can be a contributing cause for an increase in bacterial contamination since bacteria find a suitable environment to replicate on the surface of the suspended particle. An acceptable level of turbidity for water being introduced into the water network is typically less than 1 NTU, and an acceptable level at the tap is usually less than 4 NTUs.

[0105] Discolouration takes place when soluble compounds are released into the water, with the compound absorbing at specific wavelengths. A change in colour is a direct indicator of the presence of dissolved molecules which can be caused by corrosion in the pipework or leaks in the pipe which lead to ingress of foreign material in the water stream. A change in colour is one of the main causes for customer complaints even in cases where the dissolved compound is safe for human consumption.

[0106] Utility companies are interested in monitoring these parameters since they can help to detect issues in the network which are responsible for the majority of customer complaints and it can facilitate manage network operation. Because deposits tend to form during period of constant flow, changing the flow conditions in the network (for example, by increasing flow in a specific section) can cause the deposits to be dissolved again. Without direct information, utility companies tend to change operating conditions very slowly to ensure water quality is not affected.

[0107] Apparatus is herein described which employs an optical sensor which can be mounted in-line in a water distribution system. The sensor can be installed in a dedicated port in a measurement section. The measurement section is connected to a water stream in an inline configuration so that all the water from the stream enters the inlet of the measurement section and is then returned from the outlet to the main stream. The sensor is able to measure the colour of the water and can report it as an RGB triplet of values, plus a reference dark current measurement (that is, the signal detected by the photodiode without any LEDs illuminated). The values can then be used to obtain an intensity of the discolouration and the potential cause. Additionally, the sensor canalso measure the turbidity level in the water using an infrared LED by detecting the light backscattered by the particles suspended in the water.

[0108] Turbidity can be used both as a standalone measurement and to correct the RGB values obtained via the discolouration measurement to compensate for the increased light levels detected on all three channels with increasing turbidity in the water.

[0109] Referring to Figure 1, apparatus 1 (or a "sensor") for measuring at least one optical property of a fluid 2 is shown. In this case, the fluid 2 is water, in particular, drinking water and so the fluid 2 is hereinafter referred to as water.

[0110] Referring also to Figure 2, water 2 flows, e.g., intermittently, through a measurement section 3 in the form of a section of pipe 4 (herein also referred to as a "conduit" or "flow tube"). The pipe 4 has a side wall 5, which defines a flow passage 6 (or "lumen"), and an inner surface 7. The pipe 4 includes an aperture 8 (or "port") in the side wall 5 on a first side 9 of the pipe 4. A typical size of the pipe 4 is between1 / 2 inch (13 mm) and 1 inch (25 mm) inner diameter, although the pipe 4 may have a smaller or bigger inner diameter.

[0111] The apparatus 1 includes an optical sensor 11 which can take the form of a single unit having a housing 12 and an optical window 13 which can be arranged to form a watertight enclosure and / or an optically-clear potting compound 14, such as a polyurethane-based resin, filling the housing 12. The optical window 13 may be formed of a suitable material, such as glass, Plexiglass (RTM) or quartz. The window 13 may be flat or curved. The optical sensor 11 includes at least one light source 15 (or "optical source"), for example, in the form of one or more light-emitting diodes (LEDs), for emitting light 16, and at least one optical detector 17 (or "light detector"), for example, in the form of one or more photodiodes, for measuring intensity of received light 18. The apparatus 1 also includes a diffuse reflecting surface 19 (or "diffuse reflector") which is disposed on the inside of the pipe 4 on a second side 10 of the pipe 4. As will be explained in more detail hereinafter, the diffuse reflector 19 can be provided by the inner surface 7 of the pipe 4. The optical sensor 11 is disposed in or over the aperture 8 and may be secured using a suitable sealant (not shown), for instance, epoxy, and / or using a suitable retainer (not shown), such as a clip or band.

[0112] The at least one optical source 15 and the diffuse reflector 19 are positioned and orientated so as to direct light 15 through the aperture 8 along an optical axis 21 into the pipe 4, towards the diffuse reflector 19 so that it passes through the flow passage6 and, thus, the water 2 when present in the pipe 4. The at least one optical detector 17 and the diffuse reflector 19 are positioned and orientated so that the diffuse reflector 19 reflects the light 16 back towards the at least one optical detector 16 substantially along the optical axis 21. The optical axis 21 is preferably normal to the diffuse reflector 19. Thus, the optical sensor 11 is arranged to be perpendicular to the longitudinal axis of the conduit 4.

[0113] A reflector may be considered to be a diffuse reflector if it reflects more than 90% of the incident light in a specific wavelength range, of which more than 50% is diffusely reflected according to Lambert's cosine law.

[0114] The apparatus 1 includes a processing unit 22 which is used to drive the at least one optical source 15 and to receive a signal from the at least one optical detector 17. The apparatus 1 may include a power source (not shown), for example in the form of a battery.

[0115] The apparatus 1 also includes a network interface 24 preferably in the form of a wireless communications network interface. The wireless communications network is preferably a low-power, wide-area (LPWA) network, such as LoRa (RTM), which may communicate directly with a central node 56 (Figure 8).

[0116] Referring to Figure 3, the optical sensor 11 may comprise a printed circuit board (PCB) 31 supporting the at least one light source 15 and at least one photodetector 17. The at least one light source 15 and at least one photodetector 17 may be housed in the same enclosure 12. Each light source 15 can take the form of a light-emitting diode (LED). An integrated circuit (not shown) may provide one, two, three, four or more light sources 15 and the light detector 17. The one of more light sources include first, second, third and fourth light sources 15 (that is, first, second, third and fourth LEDs) each light source providing light at a different wavelength. For example, there may be infrared, red, green, and blue light sources 15i, 15R, 15G, 15B. respectively. The blue light source 15B may be omitted. In some cases, the light sources may emit, for example, white light, and a set of colour filters (not shown) may be used to obtain light at different wavelengths. An optically-clear potting compound 14 may encapsulate the source(s) 15 and photodetector 17 and the optical window 13 may be omitted.

[0117] The measurement section 3 is configured such that the optical axis 21 (Figure 4) of the sensor 11 is arranged perpendicular to the direction of the fluid flow 2 in themeasurement section, thereby illuminating the cross section of the flow. The processing unit 22 and optical components 15, 17 can be housed in the same enclosure 12.

[0118] Referring also to Figure 4 and 6, the sensor 11 may be inserted into the port 8 in the measurement section 3.

[0119] Light 16 is sent through the optical window 13 and / or potting compound 14 into the inside 6 of the flow tube 4. A white diffuse reflector 19, for example, in the form of an annulus of suitable material, is disposed inside the pipe 4 to increase the signal-to-noise ratio.

[0120] Preferably, the at least one optical source 15 is orientated with respect to diffuse reflector 19 the so that light 16 is directed normally (that is, 90 degrees) at the diffuse reflector 19 so that it is reflected back on itself, back along optical axis 21. In other words, the light 16 is not directed at an oblique angle (for instance, 45 degrees) at the diffuse reflector 19.

[0121] Referring to Figure 6, the photodiode 17 measures the intensity of infrared light 18 reflected by particles from the infrared LED (step SI and S2). This is used to measure turbidity. The photodiode 17 measures the of light 18 reflected by the diffuse reflector from each of the LEDs 15 in sequence, one at a time (steps S3 to S8). In other words, a first light source 15 is activated for a given time (step S3) and, while it is active, the photodiode 17 measures the intensity of the reflected light (step S4). The first light source 15 is deactivated and then the next light source 15 is activated for a given time (step S5) and, while it is active, the photodiode 17 measures the intensity of the reflected light (step S6) and so on. The given time and a repetition rate (in other words, how often a measurement is taken) may depend on flow rate. The signal decreases if particles (not shown) in the water 2 absorb light at a specific colour ( / .e., a specific wavelength). The sensor 11 collects the values at different colours and computes a turbidity level and respective raw intensity level of each colour of light, e.g., intensity levels for red, green and, optionally, blue light. Other colours can be used, for example, yellow, depending on the colour space of interest. After correcting each value for the turbidity level, the sensor transmits a report 41 (or "message") of the four values, namely turbidity and RGB triplets or RG pairs (if only R and G channels are present) (step S9). The message 41 can include other information such as a code or alarm level.By using a simple optical sensor ( / .e., one in which the optical sources and detectors can be mounted in the same board and / or provided by a single IC), lower-cost, high-volume electronic circuits can be used. By using a backscattering geometry arranged perpendicular to the water flow, a simpler design can be used which reduces complexity of installation. By performing measurement in-line, there is no need for additional components for flow diversion and conditioning and waste management.

[0122] Referring in particular to Figure 1, the use of the diffuse reflector 19 can allow the optical components to be positioned on one side 9 of the measurement section 3 thereby reducing mechanical and manufacturing complexity. Additionally, the reflector 19 can be suitably shaped to facilitate its integration into arbitrarily shaped measurement sections.

[0123] Referring to Figure 7A, the diffuse reflector 19 can take the form of annulus or part of an annulus 19arc (or "arced cylindrical section") formed of suitable material, such as polycarbonate, PTFE or ODM98.

[0124] Referring to Figure 7B, the inside surface 7 of the pipe 4 itself may provide the diffuse reflector 19, especially if the pipe is formed from a suitable plastic material. In some cases, the surface 7 may be prepared, e.g., roughened, to provide sufficient diffusiveness.

[0125] Referring to Figure 7C, the diffusive reflector 19 may take the form of a coating of suitable material applied to inside surface 7 of the pipe 4. Suitable materials for the diffuse reflector include titanium dioxide nanoparticles, nanocellulose-based materials, PTFE-based materials and ODM98.

[0126] Alternative arrangements - Introduction

[0127] In the approaches herein described, measurement of turbidity relies on suspended particles scattering infrared light (also referred to as "infrared" or "IR") back to a detector. The infrared light is also reflected by the diffuse reflector.

[0128] The composition of the reflector (in other words, the material used to form the reflector) and / or configuration of the reflector may, however, result in the reflector reflecting a significant proportion of infrared light. In some cases, the reflector can approximate an integrating sphere, returning almost all the infrared light back to the detector regardless of it were scattered by suspended particles or not. As a result, theamount of infrared light received by the detector may barely change as turbidity changes.

[0129] It is desirable, therefore, to have an arrangement which restricts, even prevents, infrared light from reaching the detector from the reflector, but still allows visible light to reach the detector unhindered. In such an arrangement, the only infrared light that received by the detector is backscattered from suspended particles.

[0130] Arrangements will now be described which can help to ameliorate the problem of reflectors reflecting infrared light and, thus, increase signal-to-noise ratio (SNR) of a turbidity measurement.

[0131] At least one light source, at least one photodetector and a diffuse reflector may be configured such that, when light is emitted by the at least one light source and the fluid is in the conduit, the light passes through the fluid and at least some of the light is reflected from the diffuse reflector towards the at least one photodetector. Light at the second wavelength (in this case, infrared) is prevented from reaching the at least one photodetector. Expressed differently, the arrangements can restrict (e.g., attenuate) or prevent (e.g., block, refract and / or deflect) light at the second wavelength reaching the photodetector along or by a direct path (which can include reflection from the diffuse reflector). Preferably, the arrangement (or a combination of different arrangements) effectively limits or stops infrared light from being reflected a first time from the inside of the flow tube. The arrangement(s) may, however, additionally or alternatively restrict or prevent or limit or stop infrared light after a first reflection from the inside of the flow tube.

[0132] Thus, the arrangement(s) can allow scattered light at the second wavelength to be received by the photodetector but restrict and / or prevent unscattered light at the second wavelength from being received by the photodetector.

[0133] The arrangement(s) may be configured to reduce the proportion of emitted, unscattered light at the second wavelength reaching the photodetector by 90 % or more and, in some cases, by 99 % or more.

[0134] First arrangement for improving SNR

[0135] Referring to Figure 8, a first arrangement 30, 301 for improving SNR is shown.The first arrangement 30, 30i includes first and second light sources 15i, 152, which are spaced apart, photodetector(s) 17 and a diffuse reflector 19 generally set out in the same way to that hereinbefore described, for example, with light sources 15 and photodetector(s) 17 located on one side 9 of the pipe 4 and the diffuse reflector 19 located on the opposite side 10. The first light source 15i is a visible light source and the second light source is an infrared light source 152.

[0136] The first arrangement 30, 30i also includes an optical component 31 in the form of a lens disposed in front of the first and second light sources 15i, 152 and the photodetector 17 so that light 16i, I62 from the first and second light sources 15i, 152 passes through the lens 31 to enter the flow passage 6. The lens 31 is configured to cause the visible and infrared light I61, I62 to take different paths (and, thus, strike the reflector 19 at different points of incidence Pi, P2) and / or to have different angles (9i, 02).

[0137] The reflector 19 is configured to reflect light with different reflectance, R, according to position of incidence, P, and / or angle of incidence, 9.

[0138] For example, the reflector 19 may comprise at least two different parts 19i, 192, offset along the pipe 4, and having different optical properties. The first part 19i has a high degree of reflectance and the second part 192 has a low, or even zero, degree of reflectance. This may be achieved by coating the second part 192 of the reflector 19, for example, with a dye, coating or thin film, or placing an obstruction (not shown) in the path of the light.

[0139] The reflector 19 may have a component 32, such as an optical grating baffle, a microlouvers film or an aperture stop, disposed in front of the reflector 19 which only allows light to reach the reflector 19 or to leave the reflector 19 within a limited angular range.

[0140] Other optical components 31 can be used instead of the lens which employ diffraction, reflection, refraction, or combinations thereof. The light sources 15i, 152 can be colocated, for instance, by using a diode capable of emitting both visible and infrared light.

[0141] Second arrangement for improving SNR

[0142] Referring to Figure 9, a second arrangement 30, 302 for improving SNR is shown.The second arrangement 30, 302 is substantially the same as the first arrangement 30, 30i (Figure 8) differing mainly in that the optical component 31 is configured to cause infrared light I62 to fall outside of the reflector 19 and so avoid reflection of infrared light. The angle 82 of the infrared light I62 may be arranged to be sufficiently steep to avoid any return reflection.

[0143] Additionally, or alternatively, a baffle 33 may be used to block light that could return to the detector 17. In Figure 9, the visible light source is not shown for clarity, but may sit next to the infrared light source 152 for example, next to it in the x-y plane, for instance to the left or to the right, or in front of or behind the infrared light source 152 as viewed in Figure 9.

[0144] The baffle 33 may be provided by an O-ring used to seal the optical unit 11 in the pipe 4 or other ring which projects into the passage 6.

[0145] One, or both, of these arrangements can be used which means that the reflector 19 need not be modified or be provided with a filter, coating, paint or film.

[0146] Third arrangement for improving SNR

[0147] Referring to Figure 10, a third arrangement 30, 30s for improving SNR is shown.

[0148] The third arrangement 30, 30s is substantially the same as the first arrangement 30, 30i (Figure 8) differing mainly in that the reflector 19 is adapted or is provided with filter 34 to change with the reflective properties of the reflector 19 in the infrared spectra, specifically to increase the absorption (and decrease the reflectivity) in the infrared spectra only. Again, in Figure 10, the visible light source 15i is not shown for clarity but may sit next to the infrared light source 152.

[0149] The filter 34 can take the form of a paint or dye. If the reflector has a suitable composition, for example, it is made of plastic, then an infrared absorbing additive (such as an additive used in laser marking) can be included in the composition.

[0150] The filter 34 can, however, take the form of a separate element 34 which sits in front of the reflector 34 between the reflector and the passage 6.

[0151] Fourth arrangement for improving SNR

[0152] Referring to Figures 11 and 12, a fourth arrangement 30, 304 for improving SNR is shown.The fourth arrangement 30, 304 is substantially the same as the third arrangement 30, 303 (Figure 10) differing mainly in that the reflector 19 is adapted or is provided with a polarizing filter 35 and the infrared light source 152 and the detector 17 are provided with respective polarizing filters 36, 37. The lens 31 (Figure 10) may be omitted. Again, in Figures 11 and 12, the visible light source 15i is not shown for clarity but may sit next to the infrared light source 152 for example, next to it in the x-y plane.

[0153] The orientation of polarization of the reflector polarizing filter 35 and the infrared light source polarizing filter 36 are aligned. The orientation of the detector polarizing filter 37 is perpendicular to the other two filters 35, 36.

[0154] Outgoing light I62 is polarised and is reflected from the reflector 19, re-polarised by the reflector filter 35.

[0155] Referring in particular to Figure 11, in the case of no turbidity, light propagating towards the detector 17 is cross polarised with respect to the detector filter 37 and, thus, no light reaches the detector 17 or is at least significantly attenuated.

[0156] Referring in particular to Figure 12, in the case where there is turbidity, light reflecting from particles 38 will have random polarisation and, thus, some of the reflected light will be transmitted through the detector filter 37 and reach the detector 17.

[0157] Fifth arrangement for improving SNR

[0158] Referring to Figure 13, a fifth arrangement 30, 30s is shown.

[0159] In the fifth arrangement, the optical properties, such as reflective properties, of the reflector 19 can be altered electrically, for example, using electrodes 39 (which may be transparent), controlled by the processing unit 22. Thus, the processing unit 22 may decrease reflectiveness of the reflector 19 while a turbidity measurement using infrared light is performed and increase reflectiveness of the reflector 19 while a colour measurement using visible light is carried out.

[0160] The reflector 19 may include or be provided with thermochromic or electrochromic films, or a coating or additive which is responsive to an electric field. The reflector 19 may include or be provided with a controllable optical component, such as a liquid crystal device.Combination of arrangements

[0161] Two or more of the arrangements for improving SNR can be used.

[0162] Arrangement for reducing stray light

[0163] Additionally, or alternatively, an arrangement can be used to ameliorate the issue(s) of cross-coupling and / or the stray light. Stray light is light which does not pass through the intended optical path but instead reaches the detector by some other route. This effect reduces the signal to background ratio and makes the measurements more difficult and making it harder to resolve fluids of similar turbidity.

[0164] Referring to Figure 14, one source of stray light 40 is reflection from a shared lens 31. Infrared light I62 from the infrared light source 152 can travel as expected along an intended optical path, namely, from the light source 152, through the lens 31, into water 2 where it is scattered off a particle 38. The scattered light I62 returns to the photodetector 17. An alternative path exists resulting from light I62 being reflected by the surface 41 of the lens 31.

[0165] Reflections can occur at other interfaces, such as the interface 42 between water 2 and the lens 31.

[0166] To reduce or even prevent such reflection, a shield 43 (shown shaded for clarity) can be placed between the infrared light source 152 and the detector 17 which abuts (or extends through) the surface 41 of the lens 31 and a substrate (such as a PCB) on which the source and detector are mounted.

[0167] Accordingly, optical components are configured and elements, such as baffles, light shields (such as shield 43), elastomeric seals, lens coatings, and separate lenses for specific optical components, can be used to reduce and even eliminate these reflections.

[0168] One approach for reducing stray light is to use elastomeric seals around optical components.

[0169] Referring to Figures 15, 16, 17 and 18, a prototype sensor is shown.

[0170] The prototype sensor has use two lenses 31i, 312. A first lens 31i is a common lens used for the visible light source 15i and the detector 17. A second, separate lens 312 is used for the infrared source 152.A first shield 43 (shown lightly shaded) can be interposed between the lenses 31i, 312. As explained earlier, this can be used to block reflections. A second shield 44 having apertures 45 can be interposed between, on the one hand, the light sources 15i, 152 and detector 17 and, on the other, the lenses 31i, 312.

[0171] This arrangement can be combined with the arrangements for increasing signal-to-noise ratio. Thus, for example, the second lens 312 can be configured, for example, by having a chamfered face, to direct light outside the reflector 19, towards an absorber 46. In this way, only infrared light I62 that is scattered from a particle is returned to the detector 17.

[0172] Figure 19 is a graph of infrared light intensity received by the detector as a function of turbidity for the sensor shown in Figures 15, 16, 17 and 18.

[0173] Figure 20 is a graph of infrared light intensity received by the detector as a function of turbidity for a sensor similar to that shown in Figures 15, 16, 17 and 18 except that it uses a single lens.

[0174] Although the single-lens sensor exhibits better signal-to-noise, this is likely due to a smaller lens in the two-lens sensor and edge effects. Further optimisation is expected to increase the signal-to-noise ratio of the two-lens sensor shown in Figure 15.

[0175] Table 1

[0176] Signal to background (at 10 NTUs) Comparative example 0.002

[0177] A (1-lens; not shown) 0.183

[0178] B (2-lens sensor; Figure 15) 0.102

[0179]

[0180] Figure 21 illustrates the effect of the reflector (as opposed to using an absorbative surface) for the two-lens sensor shown in Figure 15.

[0181] Table 2

[0182] Signal to background (at 10 NTUs) Comparative example 0.002

[0183] Reflector (2-lens sensor; Figure 15) 0.102

[0184] No reflector (2-lens sensor; Figure 15) 0.055

[0185]

[0186] As can be seen from Table 2, the reflector approximately doubles the signal.

[0187] This suggests that the diffuse reflector serves more than one purpose. First, it serves as a means of measuring absorption properties of the liquid (colour) through transmission. Secondly, it serves as a means of increasing the signal of the turbidity measurement by virtue of using an "integrating sphere" like effect.

[0188] The device 1 can be made from low-cost, low-power consumer electronic components, such as those found in wearable electronic devices (such as, smart watches) which allow the system to be battery powered with long lifetime.

[0189] Referring to Figure 19, a drinking water distribution network 51 is shown which includes a source 52 of drinking water, a network of pipes 53, water measurement nodes 54, a wireless access point 55 and a central station 56.

[0190] Each water measurement node 54 includes a fiscal water meter 57, a water sensor 1 and a wireless network interface 59.

[0191] The central station 56 includes a first network interface 60 for interfacing with the water measurement nodes 54, a processing system 61, storage 62, a user interface 63 and an optional second network interface 64, e.g., for interfacing with the Internet.

[0192] The sensor 1 can communicate information back to a processing unit or gateway unit (not shown) which reports the information back to a centralised station 56 via wireless links 65 which can then remotely monitor the quality of the water.

[0193] The sensor 1 may be mounted inside or next to a smart water meter 57 and is directly connected to the meter electronics, which manages the measurement schedule and is responsible for reporting the measurement value to a centralised system 56.

[0194] The lower power consumption, lower cost and the possibility to install the sensor next to an existing or co-installed water meter while sampling the main water stream help to enable a large number of sensors to be deployed at the point of use in a drinking water distribution network 51 thereby providing valuable real-time data on the characteristics of the water being delivered to the end user.

[0195] The information allows for a faster response to issues which might cause customer complaints and cause issues with the quality of the delivered water and a bettermonitoring of network operation which can enable a more flexible operation of the network.

[0196] The information collected by the centralised system can be correlated with the geographical position of the water meter node 54 and the known topology of the network so that in case of an event which leads to changes in one of the measured properties the system can estimate the most probable location of the origin of the detected change.

[0197] Referring to Figure 20, a simple example of a pipe network is shown in which water flows through a first water measurement node 54i into a first pipe section 58i which splits into second and third pipe sections 582, 58s each having a respective node 542, 543. If the first and second water measurement nodes 54i, 542 measure low values of turbidity but the third water measurement node 54s measures a high value of turbidity, then the probable location is in the third pipe section 58s.

[0198] The central station 56 can use a model (not shown) to simulate ingress of contaminant. The results can be displayed via the user interface.

[0199] The most probable location of the origin of the detected change can be found, for example, by identifying the population of meters which measure a change or drift in the measured property and tracking the section of the network that feeds water to the meter population. Additionally, meters can be clustered based on the magnitude and time evolution of the measured change, to provide an estimate of the pipe section where the issue might be arising to mitigate the effect of false positive readings from single sensors.

[0200] The centralised system can also provide an estimate of the severity of the issue affecting the network by correlating the number of meters which detect the change in the measured property with the magnitude of the measured change. This may be used to determine whether urgent action is required.

[0201] For example, a slow drift of the measured parameter(s) can indicate a minor issue in the network, so a low priority alarm can be raised, linked to a time estimate for when the parameters will be out of tolerance and maintenance will be required. This can further be connected to the number of meters (hence the portion of the network) being affected by the issue. This way, smaller issues which however affect a large household population could still be considered as a severe issue and dealt with high priority.Furthermore, the centralised system can also infer the characteristics of the cause of the measured change by correlating the degree of discolouration with the colour of the dissolved material and / or the change in ORP, and / or the change in turbidity, for example distinguishing the ingress of soil through a damage pipe, such as clay or red sand, versus the presence of rust due to pipe corrosion. The information can be fed to the maintenance crews in order to provide them with more accurate instruction on the maintenance operation required, reducing the number of site visits.

[0202] As an example, a discolouration value indicating the presence of red / brown dissolved matter can be correlated with the change in ORP. The presence of rust has shown to induce a relatively small change in ORP potential, while the ingress of soil will quickly exhaust the chlorine disinfectant in the water supply causing a sharp decrease in ORP potential. So based on the change in ORP potential it is possible to discriminate between ingress of soil particles in a broken pipe against the presence of iron oxide due to corrosion in the pipe. The information can then be used to direct the maintenance effort based on the advanced knowledge on the type of issue being experienced in the network.

[0203] As an additional example, when an increase in turbidity is observed in a section of the network, this can be monitored against the measured ORP value. Suspended particles should not change ORP significantly, however they sustain bacterial growth. Bacterial proliferation will use up the disinfectant in the water supply, so that the issue can be considered a low or high priority based on the observed changed in ORP: low priority if no change in ORP is observed, high priority if a reduction in ORP potential is observed.

[0204] It will be appreciated that many modifications may be made to the embodiments hereinbefore described.

[0205] The sensor may include an arrangement which uses a combination of total internal reflection to allow larger deflection angles for the IR and one or more mechanical features to block or attenuate stray light. For example, an acute angle of incidence at the infrared lens-water interface can be used to refract a beam of light towards an absorbative part of the flow tube. A sufficiently acute angle may be needed due to the lower difference in refractive index between the lens material and water. The angle, however, is not larger the critical angle to avoid total internal reflection. The angle of deviation from the perpendicular may be between about 20 to30 degrees.As no absorbative material is perfect, the increased angle of incidence of the infrared relative to the absorbative material can be used to lessen any small specular reflection from said surface towards the photodiode.

[0206] It may be desirable to direct the beam in a way that places the middle of the beam opposite the photodiode. In this way, any light scattered off a particle would have a more direct path to the photodetector

[0207] The sensor may include an arrangement involving directing the beam substantially down the flow tube (i.e., along the longitudinal axis of the flow tube), along the axis of the fluid flow. This arrangement might eliminate the need for absorbative material since light will travel far along the flow tube and so any reflection will lead to little, if any light returning to the photodiode. This may be easier to achieve with a nonstraight flow tube (for example, one which is curved or bent), where the vertex meets at the point where the optical elements are located. This allows the infrared deflection angle to be substantially less than 80 to 90 degrees.

[0208] It may be further desirable to have the beam intersect the flow tube (which may have an area of absorbative material) substantially offset in the axis parallel to flow. This means that any light not absorbed by the absorbative material or any light reflecting off accumulated limescale or fouling does not make it back to the photodiode.

[0209] For arrangements where the beam path is much larger than 30 degrees, total internal reflection of the IR lens can be used.

[0210] Additional arrangements can take into account the effect of fouling on the key optical areas such as the infrared lens, the absorbative material, and the photodetector lens. For example, the flow tube can have baffles to block or attenuate light scattered from fouled surfaces.

Claims

Claims1. Apparatus for measuring at least one optical property of a fluid, the apparatus comprising:■ a measurement section (3) comprising:- a conduit (4) for a fluid;■ at least one light source (15) capable of transmitting light at a first wavelength and light at a second, different wavelength;■ at least one photodetector (17);■ a diffuse reflector (19); and■ a processing unit (22) for processing at least one signal received from the at least one photodetector for use in determining at least one optical property of a fluid;wherein the at least one light source, the at least one photodetector and the diffuse reflector are configured such that, when light (16) is emitted by the at least one light source and the fluid is in the conduit, the light passes through the fluid and at least some of the light is reflected from the diffuse reflector towards the at least one photodetector,the apparatus configured to restrict light at the second wavelength reaching the at least one photodetector.

2. The apparatus of claim 1, wherein the least one light source (15) is capable of transmitting light across a first range of wavelengths which includes the first wavelength and / or across a second range of wavelengths which includes the second wavelength.

3. The apparatus of claim 2, wherein the light across the first range of wavelengths is visible light and the light across the second range of wavelengths is infrared light.

4. The apparatus of claim 1 or claim 2, wherein the least one light source (15) includes at least one light-emitting diode.

5. The apparatus of claim 1, or any one of claims 2 to 4, wherein the least one light source (15) includes at least one light-emitting laser diode.

6. The apparatus of claim 1, or any one of claims 2 to 5, wherein the least one light source (15) includes a broadband light source and at least one filter to provide light at the first wavelength and / or second wavelength.

7. The apparatus of claim 1, or any one of claims 2 to 6, wherein the at least one light source (15) includes a first light source ( 15i) capable of transmitting light at the first wavelength and a second light source (152) capable of transmitting light at the second wavelength.

8. The apparatus of claim 7, wherein the first light source ( 15i) and the second light source (15z) are spaced apart.

9. The apparatus of claim 1, or any one of claims 2 to 8, wherein the apparatus is configured such that light at the first wavelength and light at the second wavelength emitted by the at least one light source start on different paths.

10. The apparatus of claim 1, or any one of claims 2 to 9, further comprising:■ at least one optical component (31, 32, 33, 34, 43) configured to restrict light at the second wavelength or the light across the second range of wavelengths reaching the at least one photodetector.

11. The apparatus of claim 10, wherein the at least one optical component (31, 32, 33, 34) includes a lens arrangement (31).

12. The apparatus of claim 10 or 11, wherein the at least one optical component (31, 32, 33, 34) includes an optical baffle (33).

13. The apparatus of claim 10, or claim 11 or claim 12, further comprising:■ a sensor unit (11) containing the at least one light source and the at least one photodetector and comprising a housing (12); and■ a sealing member for sealing the sensor unit in the conduit (4).

14. The apparatus of claim 13, wherein the sealing member provides an optical component.

15. The apparatus of claim 1 or any one of claims 2 to 14, further comprising:■ an optical filter.

16. The apparatus of claim 15, wherein the optical filter is configured to pass light in a least some of the first range of wavelengths and to restrict light in at least some of the second range of wavelengths.

17. The apparatus of claim 16, wherein the optical filter is disposed in front of or incorporated in the diffuse reflector.

18. The apparatus of claim 16 or 17, wherein the second range of wavelengths falls within or at least overlaps with 700 to 3,000 nm.

19. The apparatus of claim 15, wherein the optical filter is configured to pass light of a first polarization and to restrict light of a second, different polarization.

20. The apparatus of claim 1 or any one of claims 2 to 19, further comprising:■ at least one optical component configured to focus, reflect, and / or direct light from a first subset of the at least one light sources in a first direction, and to focus, reflect, and / or direct light from a second subset of the at least one light sources in a second, different direction.

21. The apparatus of claim 20, wherein the at least one optical component causes light travelling in the second direction to fall outside of the diffuse reflector.

22. The apparatus of claim 20 or 21, wherein the diffuse reflector is configured to reflect a lower proportion light at a first range of incident angles and to reflect a higher proportion of light at a second, different range of incident angles.

23. A fluid meter comprising:■ a unit housing:- apparatus for measuring fluid flow through the meter; and- the apparatus of claim 1 or any one of claims 2 to 23.

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