Monitoring fluid at a processing plant

Abstract

Apparatus for monitoring fluid at a processing plant 20 comprises; first fluid sensor 21 for measuring conductivity and temperature of an inlet fluid to the processing plant and second fluid sensor 22

Description

BACKGROUND Clean-in-Place (CIP) generally describes the process of cleaning the internal surfaces of a processing plant without a need to disassemble the plant. For example, a beverage processing plant may stop production for a period of time while a CIP process is performed to clean the internal surfaces of the plant. During a CIP process, cleaning products are pumped through the plant. The final stage of a CIP process is a water flush with clean water. The water flush may use hot or cold water. There is a need to determine when it is safe to reuse the processing plant after cleaning. It is important to thoroughly cleanse the internal surfaces of cleaning products (e.g. nitric acid or sodium hydroxide) so as to avoid contamination of liquids subsequently passed through the plant. Removal of sodium hydroxide may be performed by flushing the internal surfaces of the plant with local town water or recirculated / recovered water. One way of determining when it is safe to reuse the processing plant after cleaning is to perform the water flush for a fixed period of time. This is wasteful of water and causes a long period of downtime to the processing plant. Another way of determining when it is safe to reuse the processing plant after cleaning is to monitor a fluid flowing out of the plant using a conductivity sensor. A conductivity of the fluid varies with the presence of cleaning products. However, the measurements obtained from a conductivity sensor may be unreliable, and the properties of town water or recirculated / recovered water, may also vary between different plant locations, and over time. This may also lead to an unnecessarily long period of downtime to the processing plant. There are other situations where there is a need to determine when a fluid flowing from a plant has reached a required state. SUMMARY OF THE INVENTION There is provided an apparatus for monitoring a fluid at a processing plant comprising: a first fluid sensor configured to measure conductivity and temperature of an inlet fluid to the processing plant; a second fluid sensor configured to measure conductivity and temperature of an outlet fluid from the processing plant; and a controller; wherein the controller is configured to: cause the first fluid sensor to determine a plurality of inlet fluid conductivity measurements and inlet fluid temperature measurements to obtain a set of reference conductivity data about the inlet fluid over a range of temperatures; and determine a relationship between conductivity and temperature for the inlet fluid from the set of reference conductivity data; and subsequently the controller is configured to: cause the first fluid sensor to determine a current inlet fluid conductivity measurement and a current inlet fluid temperature measurement of the inlet fluid and determine if the inlet fluid has changed; cause the second fluid sensor to determine an outlet fluid conductivity measurement and an outlet fluid temperature measurement of the outlet fluid; determine an expected outlet fluid conductivity value at a reference temperature; and, compare: (i) the outlet fluid conductivity measurement, or a temperature-adjusted outlet fluid conductivity measurement, at the reference temperature; and (ii) the expected outlet fluid conductivity value at the reference temperature, determine, based on the comparison, a concentration of an ionic contaminant or an unwanted fluid in the outlet fluid. An advantage is a more accurate determination of a concentration of ionic contaminants (e.g. cleaning products) or an unwanted fluid in the outlet fluid, and therefore a more accurate determination of when the concentration has reduced to an acceptable level. An advantage of the apparatus is that downtime of the plant may be minimised. For CIP and other flushing operations, there is an advantage of reducing the amount of water used to flush the plant and a saving in energy. Optionally, the controller is configured to determine the expected outlet fluid conductivity value at the reference temperature based on the current inlet fluid conductivity measurement and the current inlet fluid temperature measurement. An advantage of using the current inlet fluid conductivity measurement is that the expected outlet fluid conductivity value can adapt to changes in the composition of the inlet fluid and can therefore provide a better value for the comparison. Optionally, the controller is configured to determine the expected outlet fluid conductivity value at the reference temperature by adjusting the current inlet fluid conductivity measurement using the determined relationship between conductivity and temperature for the inlet fluid. Optionally, the controller is configured to determine the expected outlet fluid conductivity value at the reference temperature from the determined relationship between conductivity and temperature for the inlet fluid at the reference temperature. Optionally, the controller is configured to: determine an expected conductivity value at a temperature using the determined relationship between conductivity and temperature for the inlet fluid; compare the current inlet fluid conductivity measurement, or a temperature-adjusted current inlet fluid conductivity measurement, to the expected conductivity value at the same temperature; and determine, based on the comparison, if there has been a change in the inlet fluid. Optionally, the controller is configured to determine if a difference between: (i) the current inlet fluid conductivity measurement, or a temperature-adjusted current inlet fluid conductivity measurement; and (ii) the expected conductivity value at the same temperature is more than a threshold value. Optionally, the controller is configured to acquire a new set of reference data if there has been a change in the inlet fluid. Optionally, the controller is configured to acquire a new set of reference data after a period of time. Optionally, the controller is configured to adjust the outlet conductivity measurement to obtain a temperature-adjusted outlet conductivity measurement at the reference temperature using the determined relationship between conductivity and temperature for the inlet fluid. Optionally, the controller is configured to adjust the conductivity measurements to compensate for voids in the first fluid sensor or the second fluid sensor. Optionally, the first fluid sensor is configured to measure capacitance of the inlet fluid and the controller is configured to: cause the first fluid sensor to determine an inlet fluid capacitance measurement; adjust the inlet fluid conductivity measurement using the inlet fluid capacitance measurement, or the inlet fluid capacitance measurement after an adjustment for temperature. Optionally, the controller is configured to: store a relationship between capacitance and temperature for the inlet fluid; and adjust the inlet fluid capacitance measurement using the stored relationship between capacitance and temperature for the inlet fluid. Optionally, the second fluid sensor is configured to measure capacitance of the outlet fluid and the controller is configured to: cause the second fluid sensor to determine an outlet fluid capacitance measurement; adjust the outlet fluid conductivity measurement using the outlet fluid capacitance measurement, or the outlet fluid capacitance measurement after adjustment for temperature. Optionally, the controller is configured to: store data indicating a relationship between conductivity and a concentration of the ionic contaminant or unwanted fluid in the outlet fluid; and determine a difference between: (i) the outlet fluid conductivity measurement, or a temperature-adjusted outlet fluid conductivity measurement at the reference temperature; and (ii) the expected outlet fluid conductivity value; and use the stored data to determine whether the concentration of the ionic contaminant or unwanted fluid in the outlet fluid is less than a threshold amount. Optionally, the controller is configured to determine a time when, or a time until, the concentration of the ionic contaminant or unwanted fluid in the outlet fluid is less than the threshold amount. Optionally, the inlet fluid is a first fluid type and the outlet fluid is a mix of the first fluid type and an ionic contaminant or unwanted fluid. Optionally, the first fluid type is water and the ionic contaminant is a cleaning product. Optionally, the controller is configured to determine an end of a rinse stage of a clean in place (CIP) process on the plant. Optionally, the first fluid type is a beverage and the unwanted fluid is water. Another aspect provides a method of monitoring a fluid at a processing plant comprising: causing a first fluid sensor to determine a plurality of inlet fluid conductivity measurements and inlet fluid temperature measurements to obtain a set of reference conductivity data about the inlet fluid over a range of temperatures; and determining a relationship between conductivity and temperature for the inlet fluid from the set of reference conductivity data; and subsequently: causing the first fluid sensor to determine a current inlet fluid conductivity measurement and a current inlet fluid temperature measurement of the inlet fluid and determining if the inlet fluid has changed; causing a second fluid sensor to determine an outlet fluid conductivity measurement and an outlet fluid temperature measurement of the outlet fluid; determining an expected outlet fluid conductivity value at a reference temperature; and, comparing: (i) the outlet fluid conductivity measurement, or a temperature-adjusted outlet fluid conductivity measurement, at the reference temperature; and (ii) the expected outlet fluid conductivity value at the reference temperature, determining, based on the comparison, a concentration of an ionic contaminant or an unwanted fluid in the outlet fluid. Optionally, the method comprises determining the expected outlet fluid conductivity value at the reference temperature based on the current inlet fluid conductivity measurement and the current inlet fluid temperature measurement. Optionally, the method comprises determining the expected outlet fluid conductivity value at the reference temperature by adjusting the current inlet fluid conductivity measurement using the determined relationship between conductivity and temperature for the inlet fluid. Optionally, the method comprises adjusting the outlet conductivity measurement to obtain a temperature-adjusted outlet conductivity measurement at the reference temperature using the determined relationship between conductivity and temperature for the inlet fluid. Optionally, the method comprises adjusting the conductivity measurements to compensate for voids in the first fluid sensor or the second fluid sensor at the time of measurement. Another aspect provides a processing plant comprising: an inlet line; an outlet line; and the apparatus as described or claimed, wherein the first fluid sensor is configured to measure conductivity and temperature of an inlet fluid to the plant on the inlet line, and the second fluid sensor is configured to measure conductivity and temperature of an outlet fluid from the plant on the outlet line. Optionally, the processing plant comprises clean in place (CIP) apparatus, wherein the first fluid sensor is configured to measure conductivity and temperature of an inlet CIP fluid to the plant on the inlet line, and the second fluid sensor is configured to measure conductivity and temperature of an outlet CIP fluid from the plant on the outlet line. There is provided an apparatus for monitoring a fluid at a processing plant comprising: a first fluid sensor configured to measure conductivity and temperature of an inlet fluid to the processing plant; a second fluid sensor configured to measure conductivity and temperature of an outlet fluid from the processing plant; and a controller; wherein the controller is configured to: cause the first fluid sensor to determine a plurality of inlet fluid conductivity measurements and inlet fluid temperature measurements to obtain a set of reference conductivity data about the inlet fluid over a range of temperatures; and subsequently the controller is configured to: cause the first fluid sensor to determine a current inlet fluid conductivity measurement and a current inlet fluid temperature measurement of the inlet fluid and determine if the inlet fluid has changed; cause the second fluid sensor to determine an outlet fluid conductivity measurement and an outlet fluid temperature measurement of the outlet fluid; determine an expected outlet fluid conductivity value at a reference temperature; and, compare: (i) the outlet fluid conductivity measurement, or a temperature-adjusted outlet fluid conductivity measurement, at the reference temperature; and (ii) the expected outlet fluid conductivity value at the reference temperature, determine, based on the comparison, a property of the outlet fluid. There is also provided a method corresponding to the functionality performed by the controller as described or claimed. There is also provided computer-readable instructions that, when executed by a processor, cause the processor to perform the method as described or claimed. There is also provided a computer-readable storage medium carrying the computer-readable instructions. The functionality described in this document can be implemented in hardware, software executed by a processing apparatus, or by a combination of hardware and software. The processing apparatus can comprise a computer, a processor, a state machine, a logic array or any other suitable processing apparatus. The processing apparatus may be a general-purpose processor which executes software to cause the general-purpose processor to perform the required tasks, or a plurality of general-purpose processors which collectively execute software to cause the processors to perform the required tasks. Alternatively, the processing apparatus can be dedicated to perform the required functions. Another aspect of the invention provides machine-readable instructions (software) which, when executed by a processor, perform any of the described methods. The machine-readable instructions may be stored on an electronic memory device, hard disk, optical disk or other machine-readable storage medium. The machine-readable medium can be a non-transitory machine-readable medium. The term “non-transitory machine-readable medium” comprises all machine-readable media except for a transitory, propagating signal. The machine-readable instructions can be downloaded to the storage medium via a network connection. Within the scope of this application it is envisaged that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and / or in the following description and drawings, may be taken independently or in any combination. For example, features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures in which: FIGURE 1 shows an example of a processing plant with clean-in-place apparatus; FIGURE 2 shows an example timeline of operation of the apparatus shown in FIGURE 1; FIGURE 3 shows an example of conductivity measurements based on data acquired by the inlet fluid sensor and the outlet fluid sensor; FIGURE 4 shows an example method for acquiring a set of conductivity data of an inlet fluid; FIGURE 5 shows a relationship between relative dielectric permittivity of pure water and temperature; FIGURE 6 shows a relationship between capacitance and conductivity within a sensor cell when the volume occupancy of water is varied; FIGURE 7 shows a relationship between conductivity and temperature for use as a temperature compensation model (TCM); FIGURE 8 shows an example method for measuring conductivity of a fluid by the inlet fluid sensor and the outlet fluid sensor; FIGURE 9 shows adjustment of the measured conductivity; FIGURE 10 shows a parallel plate capacitor filled with a conductive fluid; FIGURE 11 shows an equivalent circuit of the parallel plate capacitor filled with a conductive fluid; FIGURES 12A and 12B show an example of a fluid sensor for use in the system; FIGURE 13 shows an apparatus for measuring properties of a fluid; FIGURE 14 shows part of the apparatus of FIGURE 11 in more detail; FIGURE 15 shows another example of a processing plant with fluid sensors; FIGURE 16 shows an example timeline of operation of the apparatus shown in FIGURE 15; FIGURE 17 shows an example of a computer system for implementing the controller. DETAILED DESCRIPTION FIGURE 1 schematically shows an example of a system 10 comprising a processing plant 20 and apparatus 12 to perform a clean-in-place (CIP) operation on the plant 20. The processing plant 20 may be an industrial plant for performing an industrial process, such as a plant which processes beverages, chemicals, pharmaceuticals, foodstuffs or any other product in fluid form. The processing plant 20 may perform one or more processes as part of manufacturing a product, such as heating or blending, or may package a product, such as bottling a beverage. Examples of processing plants include bottling factories, soft drinks manufacturers, breweries, distilleries, dairy producers and pharmaceutical plants. The processing plant 20 comprises a plant inlet line 15 and a plant outlet line 16. In normal operation of the plant, the plant 20 receives a fluid to be processed (“process fluid”, e.g. a beverage or a fluid ingredient of a beverage) via the plant inlet line 15. The plant inlet line 15 is connectable to a supply line 13 via a valve 14. Plant 20 processes the process fluid and outputs a processed fluid via the plant outlet line 16. The plant outlet line 16 is connectable to an outlet line 18 via a valve 17. CIP apparatus 12 comprises apparatus to perform a CIP process on the plant 20. CIP apparatus 12 is known in the art and typically comprises one or more storage vessels for storing a quantity of cleaning fluid. A cleaning fluid may comprise a solution of a cleaning product (e.g. sodium hydroxide or nitric acid) and water. CIP apparatus 12 comprises an inlet line 11 which is connectable to a water supply. The water supply is typically a supply of town water, or recovered / recirculated water. Town water is water which is provided by a local water authority. The CIP apparatus 12 may comprise one or more storage vessels for storing a quantity of water for use during the rinse stages of a CIP process. The CIP apparatus 12 also typically comprises one or more pumps for moving fluid through the plant 20. The CIP apparatus 12 is connectable to the plant 20 via a CIP feed line 23 and a CIP return line 24. The CIP apparatus 12 is connectable to a drain line 26 via a valve 25. A plant controller 30 controls operation of the plant 20 and the CIP apparatus 12. FIGURE 2 shows an example timeline of operation of the apparatus shown in FIGURE 1. The plant 20 operates for a period 41. At time T1 the plant 20 suspends operation and a CIP process 43 is performed. The CIP process 43 may be initiated in response to an elapsed time period since the last CIP process (e.g. one day or other time period) or in response to some other criterion, such as a quality of the processed fluid output by the plant 20. The CIP process 43 typically begins with a pre-rinse stage 44 followed by one or more cleaning stages 45 and one or more rinse / flush stages 46. The CIP process ends with a final rinse / flush stage 46. At time T2 the CIP process 43 ends and the plant 20 resumes operation after a period of downtime. Returning to FIGURE 1, when operation of the plant 20 is suspended for CIP, the plant controller 30 is configured to control valve 14 to block the flow of process fluid. The plant controller 30 is configured to control valve 14 to connect the CIP feed line 23 to the plant inlet line 15, and to control valve 17 to connect the plant outlet line 16 to the CIP return line 24. During a cleaning cycle of a CIP process, a quantity of cleaning fluid is passed through the plant 20. The cleaning fluid is pumped from the CIP apparatus 12 into the plant 20 along CIP feed line 23. The waste cleaning fluid is returned to the CIP apparatus 12 along the CIP return line 24 and may either be discharged to the drain outlet 26 via valve 25, stored within the CIP apparatus 12, or returned along CIP feed 23 for a further pass through the plant 20. During a pre-rinse or rinse stage of CIP, water is pumped from the CIP apparatus 12 into the plant 20 along the CIP feed line 23. The water during a pre-rinse or rinse stage may be discharged to the drain outlet 26 via valve 25, or returned along CIP feed 23 for a further pass through the plant 20. During the final rinse stage, the waste fluid comprises a mix of water and cleaning fluid. As the rinse stage progresses, the amount of cleaning product in the waste fluid will reduce. At some point, the amount of cleaning product in the waste fluid will reduce to an acceptable level. The acceptable level may be 1 part per million (PPM), or some other value. FIGURE 1 shows a system 10 with a CIP apparatus 12 and a single plant 20. The CIP apparatus 12 may be connectable to a plurality of plants 20. The CIP apparatus 12 may be used as a time-shared resource, and is connectable to one of the plurality of plants 20 at a time. FIGURE 1 shows fluid sensors 21, 22 used to monitor fluids in the system 10. A first fluid sensor 21 is positioned on an inlet side of the plant 20. A second fluid sensor 22 is positioned on an outlet side of the plant 20. Figure 1 shows two possible positions of the first fluid sensor 21. A first position of the fluid sensor 21 (shown in solid form) is in the CIP feed line 23. In this position, the fluid sensor 21 can: (i) monitor town water during a part of the CIP process when only water flows (e.g. during a rinse stage); and (ii) monitor cleaning fluid during a part of the CIP process when cleaning fluid flows (e.g. during a cleaning stage). A second position of the fluid sensor 21 (shown in dashed form) is in the water inlet line 11 to the CIP apparatus 12, or at any other point within the CIP apparatus 12 where only water flows. The second fluid sensor 22 is configured to monitor waste fluid flowing along the CIP return line 24. The type of waste fluid flowing along the CIP return line 24 will differ depending on the point in the CIP operation. During a cleaning stage, the second fluid sensor 22 monitors waste cleaning fluid. During a rinse stage, the second fluid sensor 22 monitors quality of the rinse water. In particular, measurements made by the second fluid sensor 22 can be used to determine an amount of cleaning fluid in the waste fluid. This allows an accurate determination of a safe end point of the final rinse stage. By determining an end point by accurate measurements, it is possible to reduce consumption of water and reduce downtime of the plant 20. A fluid sensor controller 32 connects to the fluid sensors 21, 22 and controls operation of the fluid sensors. The fluid sensor controller 32 may form part of the plant controller 30, or may be separate from the plant controller 30, and communicate with the plan controller 30. The fluid sensor controller 32 receives control signals from the plant controller 30 indicating, for example: a start of a CIP process; a current stage of a CIP process. The control signals may indicate the type of CIP process, e.g. hot clean, cold clean, nitric acid, caustic acid, sterilant etc. The fluid sensor controller 32 sends a control signal to the plant controller 30 indicating when it has detected an end of a rinse stage of the CIP process. Quantities measured by, and calculated by, the sensing system (i.e. the fluid sensors and fluid sensor controller) can also be sent to the plant controller 30 for monitoring purposes, if required. FIGURE 3 shows an example of conductivity measurements based on data acquired by the inlet fluid sensor 21 and the outlet fluid sensor 22. Town water is an ionic solution. The water comprises electrically charged ions which are responsible for observed conductivity. Some examples of ions found in town water are: calcium, magnesium, iron, sodium, hydrogen, chlorides, sulphates, nitrates, carbonates. The conductivity of the town water is dependent on factors such as: the type of ions present in the town water and concentration of ions in the town water. A mix of a cleaning product and town water is also an ionic solution, with a (much) higher conductivity. By accurately measuring conductivity of a fluid in the system 10, it is possible to determine an amount of cleaning product present in the waste fluid of the CIP return line 24. FIGURE 3 shows: conductivity S1 versus time for data acquired by the inlet fluid sensor 21 positioned in the CIP feed line 23; and conductivity S2 versus time for data acquired by the outlet fluid sensor 22 positioned in the CIP return line 24. At time T1 the CIP apparatus 12 supplies cleaning fluid to the plant 20. The conductivity of the inlet fluid sensor increases due to the higher conductivity of the acidic cleaning fluid. At a later time T2 (due to time taken to pass through the plant 20) the outlet fluid sensor 22 detects an increased conductivity due to the higher conductivity of the acidic cleaning fluid flowing along the CIP return line 24. At time T3 the CIP apparatus 12 stops supplying cleaning fluid to the plant 20 and begins a rinse stage with water. The conductivity of the inlet fluid sensor 21 falls due to the lower conductivity of the water. At a later time T4 (due to time taken to pass through the plant 20) the outlet fluid sensor 22 begins to detect a fall in conductivity in the rinse water. The rinse water carries an amount of cleaning product which has been washed from the internal surfaces of the plant 20. The conductivity measured by the outlet fluid sensor 22 continues to fall over a period of time as there is a diminishing concentration (amount) of cleaning product in the rinse water. At some point, the amount of cleaning product in the waste fluid will reduce to an acceptable level. The acceptable level may be 1 part per million (PPM), or some other value. The acceptable level of cleaning product corresponds to a difference between a conductivity measurement made by the outlet fluid sensor 22 and an expected conductivity value. The conductivity measurement and expected conductivity value are compared at the same temperature. When the controller determines that the conductivity difference is less than a threshold conductivity value representing an acceptable level of cleaning product, the controller determines that the final rinse stage has ended. In one example, the controller compares S2 and S1 after adjustment of one, or both, to the same temperature. The acceptable level of cleaning product corresponds to a difference in conductivity values (S2-S1). The town water is often sourced locally and thus varies widely in composition from site-to-site and sometimes even from year-to-year or week-to-week. This means that it is not possible to use a standard table of conductivity data at different geographical locations, as town water is likely to have a different composition at each location. It also means that it is not possible to use a standard table of conductivity data at a single geographical location over a period of time, as town water is likely to have a different composition over time. Issues with the performance of conductivity measurement are: • air entrainment in the flowing fluid causing significant process noise on measurements; • the conductivity of a solution of a given concentration is highly temperature dependent; and • conductivity of a given solution is highly temperature dependent. Method to acquire conductivity data using inlet fluid sensor FIGURE 4 shows an example method 50 for acquiring a set of conductivity data of an inlet fluid, such as town water. The method is performed for data acquired by the inlet fluid sensor 21. The method acquires a set of conductivity data over a range of temperatures. The acquired set of data forms a set of reference data about the current composition of the inlet fluid. The set of reference data may be used as a look-up table, or used to construct a model to define a relationship between conductivity and temperature. The set of reference data (or the model constructed using the set of reference data) has several uses. Firstly, the set of reference data / model can subsequently be used to determine if the composition of the inlet fluid has changed. Secondly, the set of reference data can be used to determine a quality of the outlet fluid in a CIP process and therefore determine an endpoint of a CIP process. The method may be performed by the fluid sensor controller 32 of FIGURE 1. At block 51 the controller measures the following quantities of a fluid using the inlet fluid sensor 21: conductivity; capacitance; and temperature. The purpose of this method is to acquire conductivity data about a particular type of fluid (e.g. town water). In the system shown in FIGURE 1, the inlet fluid sensor 21 is positioned in the CIP feed line 23. At certain times (e.g. pre-rinse and rinse stages of a CIP process) only water flows along the CIP feed line 23. At other times (e.g. clean stages of a CIP process) a mix of cleaning product and water flows along the CIP feed line 23. The controller 32 may receive control data from the plant controller 30 to indicate the current CIP stage. The controller 32 may use this control data to select times at which it acquires data about town water. Additionally, or alternatively, the controller 32 may determine if the conductivity and temperature fall within an expected range for town water. This is because cleaning fluid is highly conductive. Therefore, if the measured conductivity is below a threshold value, the controller can determine that only town water is flowing through the inlet sensor 21. At block 52 the controller compensates for voids in the measured fluid. The term “voids” means empty spaces in the fluid, such as air bubbles in the fluid. Presence of voids affects measurements made by the inlet fluid sensor 21. When voids are present in the fluid, the electrical path between electrodes of the fluid sensor comprises: (i) a path through the fluid and (ii) one or more paths through voids. Therefore, the measurement made by the fluid sensor will not be accurate. The capacitance measurement is used to adjust or compensate the conductivity measurement to compensate for voids. Temperature also affects measurements made by the inlet fluid sensor 21. At block 53 the controller 32 compensates the measured capacitance value for temperature. At block 54 the controller 32 compensates the measured conductivity with the temperature-compensated capacitance. Blocks 53 and 54 may be performed in either order. The dielectric constant of water varies with temperature. Polynomial equation [1] defines a relationship between relative dielectric permittivity and temperature t: er = 87.740 - 0.4008 t + 9.398 x 10“4 t2 - 1.410 x 10“6 t3 [1] This equation is described in: “Dielectric Constant of Water from 0° to 100°C”, Malmberg C.G., A.A. Maryott, Journal of Research of the National Bureau of Standards Vol 56, No 1. January 1956. FIGURE 5 shows this relationship plotted on a graph. The sensor cell has two high-resolution temperature sensors: one at the inflow and one on the outflow of the cell. The average measured temperature can be used to predict the expected capacitance of a fully filled cell (i.e. a sensor cell fully filled with water, without any voids). Using this relationship between dielectric permittivity and temperature it is possible to deduce the full cell capacitance for a given temperature. It has been found that the fractional amount of reduction of conductivity due to a void is proportionally linked to the reduction in capacitance. FIGURE 6 shows a graph of capacitance versus conductivity within a sensor cell when the volume occupancy of water is varied. Dividing the measured capacitance by the full cell capacitance gives a value which can be validly used as a scaling factor to correct the conductivity: Scaling factor = Measured Capacitance I Full cell capacitance (from [1]) [2] The corrected conductivity (compensated for voids and temperature) is: Corrected conductivity = conductivity I scaling factor [3] At block 55 the controller stores the compensated conductivity value [3] and the temperature at which the conductivity value was measured. The relationship between conductivity and capacitance varies depending on the type of fluid being measured. For example, the relationship is different for water and beer. Block 52 uses stored data for the type of fluid being measured. At block 56 the controller determines if it has acquired sufficient data to begin constructing a model of conductivity and temperature. Various criteria can be used. For example, the controller may wait until it has acquired conductivity data over a minimum range of temperatures either side of a nominal operating temperature (e.g. 20°C +1- 2°C), or any other required nominal operating temperature and range. The controller may wait until it has acquired a minimum number of data points. At block 57 the controller determines it has acquired sufficient data and begins to construct a model of conductivity versus temperature. One way of constructing the model is to determine a mathematical function which represents the relationship between the compensated conductivity values and measured temperatures. The mathematical function may be a polynomial function of any desired power. The mathematical function can be derived in various ways. One suitable method is polynomial regression. Another suitable method is curve fitting. The aim is to find a function which best approximates, or fits, the data set. In one example, the acquired data is fitted to a low-order (e.g. quadratic) polynomial curve which describes the expected conductivity of the water for a given temperature. FIGURE 7 shows an example of a set of data points (after void compensation) collected over a range on inlet temperatures, fitted to a quadratic curve. This curve is defined by an equation which gives conductivity as a function of temperature, e.g. Conductivity = a(T2) + b(T) + c [4] where: T is temperature and a, b and c are coefficients. An alternative to finding functions which approximate the data is to store the actual set of data values which relate conductivity and temperature and to use the set of data values as a look-up table. However, this is less desirable as it does not readily give a value of conductivity corresponding to a temperature which was not stored as part of the set of data values. It is advantageous that the model is derived from data points whose temperature is as close as possible to that expected of the outlet temperature at the end of a rinse / flush. This is due to the fact that the gradient of the model varies with temperature due to its non-linear nature. When employing a model which fits a polynomial to a dataset, care is taken when using the model when extrapolating to values outside of the domain of the training data. For example, if measurements are acquired between 20 and 30°C to train the model, it is likely that extrapolations to 40°C are erroneous. This phenomenon becomes more extreme with higher order polynomial fits. The above method constructs a model which relates conductivity to temperature for a particular composition of a fluid (e.g. town water) measured by the inlet fluid sensor 21. This will be called a temperature compensation model (TCM). At block 58 the controller can begin to use the model TCM of conductivity versus temperature. The functionality shown in FIGURE 4 may be performed by the fluid sensor controller 32, or may be distributed between a controller at the fluid sensor 21 and the fluid sensor controller 32. Method using inlet and outlet fluid sensors and stored data FIGURE 8 shows an example method for measuring conductivity of a fluid by the inlet fluid sensor 21 and the outlet fluid sensor 22. This can be used to determine an endpoint of a CIP process. The method may be performed by the fluid sensor controller 32. Blocks 61-67 are performed for the inlet fluid sensor 21. Blocks 68-72 are performed for the outlet fluid sensor 22. There is a possibility that the composition of the inlet fluid (e.g. water) has changed since the TCM was constructed. As described above, the composition of water changes over time and is affected by events such as recent rainfall or environmental factors. The controller checks, at blocks 61-66 whether the composition of the inlet fluid has changed. This check may be performed every time an outlet fluid measurement is made (block 68), or less frequently. At block 61 the controller measures the following quantities of the inlet fluid using the inlet fluid sensor: conductivity; capacitance; and temperature TJnlet. At blocks 62-64 the controller compensates for voids in the measured fluid and for the temperature TJnlet at which the measurement is made. The method is the same as described above for FIGURE 4. In summary: • determine the full cell capacitance for the measured temperature using equation [1]; • determine a scaling factor to correct the conductivity using equation [2]; • determine a corrected conductivity (compensated for voids and temperature) using equation [3], At block 65 the controller determines an expected conductivity value at the measurement temperature TJnlet, or at a reference temperature T_o, using the TCM. At block 66 the controller determines if the compensated conductivity measurement Condjnlet at temperature TJnlet (from block 62), or the temperature-compensated conductivity measurement at temperature T_o, is within a predetermined threshold value. If the conductivity measurement differs from the expected value by more than the threshold value, this indicates that the composition of the inlet fluid has changed. The TCM will need updating. The controller proceeds to block 67 and begins to acquire a new set of data using the method shown in FIGURE 4. The controller may continue to use the existing model TCM until a new model has been constructed. If the conductivity measurement differs from the expected value by less than the threshold value, this indicates that the composition of the inlet fluid is the same (or closely similar) to the composition of the inlet fluid at the time of acquiring the data for the TCM. Therefore, the TCM can be validly used for comparisons. Block 66 outputs an indication to block 74 that the model data is still usable. At block 68 the controller measures the following quantities of the outlet fluid using the outlet fluid sensor: conductivity; capacitance; and temperature T_outlet. At blocks 69-71 the controller compensates for voids in the measured fluid and for temperature at which the measurement is made. The method is the same as described above for blocks 62-64. At this point, the method has determined a compensated outlet conductivity measurement Cond_outlet at the outlet for a temperature T_outlet. It has been found that the measured conductivity is proportional to concentration of ions in a fluid. At the end of a rinse / flush stage of a CIP process the concentration of contaminant ions is low, such that the conductivity is dominated by the contribution of the naturally occurring ions in the water. Therefore, it is reasonable to apply a model based on uncontaminated water in the determination of the endpoint. At block 73 the controller determines an expected conductivity value at a required temperature, e.g. at the reference temperature T_o. At block 74 the controller compares two conductivity values: (i) the compensated outlet measurement from block 69 (optionally, with adjustment for temperature to the reference temperature T_o by block 72); and (ii) an expected value of the outlet conductivity from block 73. The two conductivity values indicate conductivity values at the same temperature e.g. the reference temperature T_o. This is a pair of measurements which can be compared Iike-for-like. There are several ways in which this part of the method may be implemented. Option A - use inlet fluid conductivity measurement Firstly, block 73 may determine an expected value of the outlet conductivity from the inlet fluid conductivity measurement. Block 69 outputs an inlet fluid conductivity measurement which has been compensated for voids. The inlet fluid conductivity measurement represents an expected conductivity of the outlet fluid, because the same fluid (e.g. water) first flows past the inlet sensor and then flows past the outlet sensor. An advantage of this approach is that the inlet fluid conductivity measurement is a recent measurement of the inlet fluid, and will represent any recent change in the composition of the inlet fluid. Option B - use conductivity value from model Secondly, block 73 may use an expected value of the outlet conductivity which is obtained from the model TCM. The model defines a relationship between conductivity and temperature for the inlet fluid. The model can be used to determine a conductivity corresponding to a required temperature. The two conductivity values (i) and (ii) may be compared at a reference temperature T_o. To accurately compare the conductivity values (i) and (ii), at least one of the conductivity values is adjusted such that both relate to the same temperature. The reference temperature T_o may be any desired temperature, advantageously within the range of data acquired by the TCM. T_o may be selected as a nearby convenient round number reference temperature e.g. 20°C, preferably within ~ 5°C of both TJnlet and T_outlet. This simplifies comparison of measurements made by the system over a longer period of time. For Option A above, where an outlet fluid conductivity measurement and an inlet fluid conductivity measurement are compared, the adjustment for temperature can be achieved by performing one of the following: (a) adjusting the inlet fluid conductivity measurement to the temperature T_outlet, i.e. determine an adjusted inlet conductivity measurement at the temperature T_outlet; (b) adjusting the outlet fluid conductivity measurement to the temperature TJnlet, i.e. determine an adjusted outlet conductivity measurement at the temperature TJnlet; (c) adjusting the inlet fluid conductivity measurement and the outlet fluid conductivity measurement to the same reference temperature T_o, i.e. determine an adjusted inlet conductivity measurement at the temperature T_o and determine an adjusted outlet conductivity measurement at the temperature T_o. Block 72 performs any required adjustment of the outlet conductivity measurement to a different temperature, such as the reference temperature T_o. For Option B above, where an outlet fluid conductivity measurement and an expected value from the model value are compared, the adjustment for temperature can be achieved by performing one of the following: (d) adjusting the outlet fluid conductivity measurement to the reference temperature T_o, i.e. determine an adjusted outlet conductivity measurement at the temperature T_o; (e) not performing any adjustment, and using the outlet temperature T_outlet for the comparison. At block 75 the controller determines if the difference in conductivity is less than a threshold value. The controller stores data which maps conductivity to acceptable levels of cleaning product (e.g. a conductivity value of 0.5 mS / m may correspond to 1 PPM of cleaning product in water.) The method of blocks 61-75 is repeated. For example, every 5 seconds. More generally, the interval between measurements may be any required value, such as a value in the range of 1 to 60 seconds. Block 75 determines if the difference between the conductivity measurements is less than the threshold value for a prescribed period of time (e.g. for X measurements, or for a period of Y seconds). This prevents against spurious isolated readings. If the difference between the conductivity measurements is less than the threshold value for the required period of time, the method proceeds to block 76. Controller 32 signals to the plant controller 30 that the rinse / flush stage has ended. The plant controller 30 can then restart the plant 20. If the difference between the conductivity measurements is above the threshold value, or if the difference between the conductivity measurements does not remain less than the threshold value for the required period of time, the method proceeds to block 77. Controller 32 signals to the plant controller 30 that the rinse / flush stage should continue. The plant controller 30 signals to the CIP apparatus to continue the rinse stage of the CIP process. The method returns to blocks 61 and 65 to acquire new measurements from the fluid sensors 21, 22. Optionally, if block 66 indicates that the model data is no longer current (i.e. that the inlet fluid has changed), block 74 may use a different criterion / criteria to make a decision, such as a larger threshold value. FIGURE 9 shows adjustment, or compensation, of a conductivity measurement in more detail. In this example, the outlet fluid sensor 22 has reported a conductivity measurement k at a measured temperature T. For simplicity it is assumed that k is a conductivity value which has already been adjusted to compensate for voids (FIGURE 8, block 69). The aim of the adjustment is to determine an adjusted conductivity value kr at the reference temperature To. The method determines a scaling factor which is then used to scale the conductivity measurement k at measured temperature T to an adjusted conductivity value kr at the reference temperature To. In this example, it is assumed that the relationship between conductivity and temperature is defined by a polynomial of order 2 of the form ax2 + bx + c. More generally, the TCM can be any function of temperature, i.e. f(T). The adjusted conductivity value kr can be calculated by: , __ i (a-^o2 + bT0 + c) T “ k- + bT + c) [5] The term + c) in the numerator is a constant. It evaluates to the approximate conductivity of the measured inlet water in the inlet fluid sensor at the reference temperature To. The term - in the denominator evaluates to the expected conductivity of inlet water at the current temperature T. Dividing one polynomial by another gives a scaling ratio for use as a multiplier to produce a compensated value. The technique shown here can be applied to any conductivity measurement which requires adjustment / compensation for temperature. It can also be used to adjust the inlet fluid conductivity measurement to a reference temperature (or to T_outlet). End point prediction In some systems, it is desirable to know when the CIP process will end. The method 60 may use the determined difference in conductivity values at block 73 to predict when the difference between conductivities will no longer be greater than threshold difference value. The method may do this, for example, by periodically storing values of the difference, and extrapolating the measured values to determine the time at which the difference will no longer be greater than the threshold value. Extrapolation may be performed by fitting a mathematical expression to the measured data values, such as an exponential decay function, a polynomial expression such as a second or third (or higher) order expression, or any other suitable mathematical function. Predictive functionality in terms of the time at which the plant is expected to be clean (‘end point prediction’) may be helpful in applications where industrial processes may be stopped or paused in order to facilitate cleaning of product outlet lines. Knowing the time remaining before cleaning will be complete can be helpful in enabling processes to be restarted before cleaning is complete so that product is ready to flow through the outlet lines when or soon after cleaning is complete. Other scenarios may benefit from advance knowledge of the time remaining before cleaning is complete. In some systems the controller 32 may output data to the plant controller 30 indicating the local time at which cleaning is expected to be complete, e.g. “Cleaning expected complete at 11:30am” or in terms of time remaining before cleaning is complete, with a timer countdown feature, e.g. “Time remaining before cleaning is complete: 0 h :23 mins”. Void compensation The presence of voids, or air bubbles, in the sensor cell affect the quality of conductivity measurements. Each of the sensors 21, 22 measure electrical impedance over the entire volume of the sensor cell. In addition to conductivity information, capacitance data is also available. The capacitance can be used to determine the error (reduction), if any, in measured conductivity arising from parts of the volume containing air, allowing corrections to be made. The following passage explains how capacitance can be used to determine the error in measured conductivity. For simplicity, the principles will be explained using a parallel plate capacitor. FIGURE 10 shows a parallel plate capacitor 80 with a pair of conductive plates 81, 82, each plate having an area A. An electrical lead 84 connects to each plate. The region 83 between the parallel plates 81, 82 is the sensing region, and contains a conductive aqueous fluid 85. Some voids (air bubbles) 86 are shown in the fluid 85. The sensor cell will conduct current due to the finite conductivity of the fluid and will store charge on the plates 81, 82 due to the finite capacitance of the structure. To the first order, the sensor cell can be considered as a parallel RC circuit shown in FIGURE 11, where: A C=^ d P is electrical resistivity and € is the dielectric permittivity of the fluid. Therefore, the relationship between total cell impedance Z, R and C can be written: Substituting for R and C we get Noting that Z 1 is defined as cell admittance Y and the fluid conductivity a is defined as the reciprocal of resistivity P, we can now write: r i Z-1 = Y = — cr + iuje a L J Observing the parameters above, the admittance is seen to be a product of two functions: A • d which describes the geometry of the cell; and • a + zwc which is a frequency dependent function of the fluid. [Note: cr + zwe can be regarded as a complex conductivity analogous to the use of complex dielectric permittivity in related literature where the complex part of the dielectric permittivity represents resistive loss.] The concept of combining both the conductive and dielectric properties of the fluid together into a single complex property of the fluid is important for the following discussion. If we undertake the above exercise for an idealised co-axial cylinder or spherical shell structure, the resulting expression for impedance still can be written as a product of two functions, one of which is determined by the geometry of the system and the other is a + iwe. In this way, the geometry should be chosen such that conductivity scales with dielectric constant. The relationship between conductivity and capacitance has been tested experimentally. A beaker full of tap water was allowed to equalise to an ambient of temperature of 22°C together with the sensor cell. The sensor cell was initially approximately 65% full. Repeated additions of 10ml of tap water (Birmingham UK) were made to the cell, slowly increasing the fraction of the volume occupied by the tap water. Note that at any point the exact volume of the water in the cell was not known or required for this study. After the final addition, the cell was seen to be full and it was ensured that there was no trapped air in the system. The relationship between the conductivity and capacitance is seen to be highly linear as would be expected from the conclusions of the above discussion. FIGURE 6 shows results of the above experiment, showing the linear relationship between capacitance and conductivity within a sensor cell when the volume occupancy of water is varied. The fluid sensors 21, 22 are axial cylindrical capacitors. From electrostatic considerations the electric field is symmetrical, but not uniform in strength throughout the sensing volume, it being lowest near the outer conductor and highest near the inner conductor. Therefore, although the quantities measured by the cell can be considered as an average value over the whole volume, different weighting is given to parts of the fluid in different positions within the sensor. For example, the presence of a void near the central conductor will cause a larger decrease in the conductivity and capacitance as that from an equal sized void nearer to the outer conductor. However, due to the inseparability of the two quantities in a manner shown above, the fractional amount of reduction of conductivity due to a void is seen to be proportionally linked to the reduction in capacitance. This means that no further consideration needs to be taken regarding the position or absolute volume of the voids within the system. All that is needed to perform a correction to the conductivity is the ratio of the capacitance value to the known full-volume capacitance value. Fluid sensors FIGURES 12A and 12B shows an example of a fluid sensor 610 for use as the inlet fluid sensor 21 and the outlet fluid sensor 22. The fluid sensor comprises a capacitive sensor cell with coaxial electrodes. FIGURE 12A shows the fluid sensor 610 in cross-section along a longitudinal axis (A-A’ in FIGURE 12B). FIGURE 12B shows the fluid sensor 610 in crosssection along line B-B’ of FIGURE 12A. The fluid sensor 610 is configured for measuring properties of a flowing fluid. The fluid sensor 610 is a form of capacitive sensor. The sensor has a first, outer, electrode 611 and a second, inner, electrode 612. The outer electrode 611 is tubular. The inner electrode 612 is a cylindrical rod. The electrodes 611, 612 are coaxial. A fluid flow channel 613 is defined in the region between the electrodes 611, 612. Fluid can flow along the fluid flow channel 613. This allows measurements to be made without a need to interrupt a process which uses the fluid. A feed through conductor 615 connects the inner electrode 612 to the drive signal generator 120 located outside the fluid sensor. The conductor 615 is insulated. A plurality of supports 614, shown here in the form of a crossshaped array, support the inner electrode 612 within the outer electrode. The supports 614 are formed of an insulating material. The flow channel 613 extends through apertures between the supports 614. One of the sets of supports 614 may incorporate the feed through conductor 615. The support 614 around the feed through conductor 615 provides a fluid-tight seal to prevent fluid loss from the sensor 610. The fluid sensor 610 can have any suitable length and diameter. The fluid sensor 610 comprises at least one temperature sensor. FIGURE 12A shows a first temperature sensor 616 is positioned across the fluid flow channel 613 near a first axial end of the fluid sensor 610. A second temperature sensor is positioned across the fluid flow channel 613 near the opposite axial end of the fluid sensor 610. Each of the temperature sensors is mounted on a support 617 across the fluid flow channel 613. Each of the temperature sensors measures a respective temperature of fluid flowing in the fluid flow channel 613. The fluid sensor controller 32 may use one of the measured temperatures, or may use a temperature value which is an average of the two temperatures measured by the temperature sensors 616. The average value may be calculated locally at a controller of the fluid sensor 610, or by the fluid sensor controller 32. FIGURE 13 shows apparatus 100 for measuring properties of a fluid. The apparatus 100 comprises a fluid sensor cell 110, such as the fluid sensor 610 shown in FIGURES 12A and 12B. The capacitive fluid sensor cell 110 has two main properties: (i) capacitance; (ii) conductance. These properties will vary according to the type of fluid between the electrodes 111, 112. Capacitance of the sensor is the ability of the sensor to store electric charge. Capacitance varies according to the permittivity of the dielectric material (i.e. fluid) between the electrodes 111, 112 of the capacitor. A dielectric material with a high dielectric constant (i.e. a good insulator) will increase the capacitance. Conductance is the flow of charge between the electrodes, through the dielectric material between the electrodes 111, 112. Conductance also depends on the properties of the dielectric material (i.e. fluid) between the electrodes 111, 112 of the capacitor. A high impedance fluid will cause a small conductance between the electrodes 111, 112. A low impedance fluid will give a higher conductance between the electrodes 111, 112. A drive signal generator 120 generates a drive signal. The drive signal is an alternating current electrical signal at a suitable frequency. The drive signal is applied to the fluid sensor cell 110. The drive signal may be applied to the inner electrode 112, with the outer electrode 111 connected to a reference ground. In an example, the alternating current electrical signal has a frequency which is in the low radio frequency (RF) range, of less than 10MHz, such as 5.05MHz. The drive signal generator 120 can be implemented by a Direct Digital Synthesis integrated circuit feeding a wideband operational amplifier. Direct Digital Synthesis is a technique which generates a sinusoidal analogue signal using a sequence of digital values representing amplitude of the signal at points in time. The digital values are converted into an analogue signal by a digital-to-analogue converter. The digital values required to generate the signal may be stored, and retrieved from memory, or calculated on-the-fly using an algorithm. A signal processing stage 130 is implemented, for example, by a microcontroller 200. The signal processing stage 130 receives an alternating electrical signal SENSE from the fluid sensor cell 110. The drive signal applied to the fluid sensor cell 110 will be modified by properties of the fluid in the fluid sensor cell 110. SENSE is indicative of the fluid. The signal processing stage 130 also receives the drive signal as a signal DRIVE or REF. It is possible to supply the drive signal by directly connecting an output of the drive signal generator 120 to the processing stage 130. Alternatively, the drive signal may be tapped from a different point, REF, in the system as described below. FIGURE 14 shows a schematic of the front-end of the system 100, showing analoguedomain components. The sensor cell 110 can be represented as an equivalent circuit network with a capacitance C in parallel with a resistance R. The value of C in this network is determined by the dielectric constant and R is a function of the conductivity of the fluid. Cblock is a DC blocking capacitor. The impedance of the sensor cell 110 equivalent circuit (R and C in parallel) can be expressed as: Z=[l+ywC] 1 where co is 2TTf, where f is the drive signal frequency. Rs and Z form a potential divider and the voltage across Z is the main sensor feedback signal SENSE. Z is a complex impedance. Ls is lead inductance from the connections to the sensor cell 110. The signal processing stage 130 is configured to determine a capacitance of a fluid and a conductivity of a fluid flowing through the fluid sensor cell. The signal processing stage 130 may be implemented in various ways. One way is described in International Patent Application WO 2019 / 097239 A1. Operation of the signal processing stage 130 will now be summarised. The signal processing stage 130 determines a first complex quantity Cs representing amplitude and phase of the SENSE signal and a second complex quantity Or representing amplitude and phase of the DRIVE / REF. signal. Cs and Cr are obtained over a plurality of cycles of the alternating drive signal. The signal processing stage 130 then determines a third complex quantity A representing amplitude and phase of a ratio of the first complex quantity Cs and the second complex quantity Cr. The quantity A has an amplitude equal to the ratio of the amplitudes of Cs and Cr, and a phase equal to a difference in phase of Cs and Cr. The quantity A represents a relationship between the SENSE and DRIVE / REF signals and is independent of supply voltage or ADC reference voltage variation. The signal processing stage 130 then determines a complex impedance of the fluid sensor cell 110 using an algorithmic model of the fluid sensor cell defined in terms of A and other known quantities. This algorithmic model may also compensate for parasitic components in the sensor, such as the lead inductance Ls. The complex impedance of the fluid sensor cell 110 determined above comprises: a real part representing resistance of the fluid sensor cell; and an imaginary part representing capacitance of the fluid sensor cell 110. The signal processing stage 130 then determines: capacitance of the fluid under test from the real part of the determined complex impedance and known properties of the fluid sensor cell (e.g. length, diameters of the electrodes); and conductivity of the fluid under test from the imaginary part of the determined complex impedance and known properties of the fluid sensor cell (e.g. length, diameters of the electrodes). The known properties of the fluid sensor cell 110 may be determined during a calibration operation (e.g. before installation) and stored at the fluid sensor for use by the signal processing stage 130. An advantage of the apparatus 100 is that it determines both capacitance and conductivity of a fluid from the complex impedance of a single fluid sensor cell 110, rather than separate capacitance and conductivity sensors. Other applications FIGURE 15 schematically shows another example of a system 10 comprising a plant 20. The system 10 is the same as the one shown in FIGURE 1, with some additional fluid sensors 27, 28. The additional fluid sensors 27, 28 allow monitoring of process fluid flowing into and out of the plant 20. The process fluid may be a beverage, chemical, pharmaceutical, foodstuff or any other product in fluid form. A fluid sensor 27 is positioned on an inlet side of the plant 20. The fluid sensor 27 is shown positioned in supply line 13, but other positions are possible, such as the plant inlet line 15. The fluid sensor 28 is positioned on an outlet side of the plant 20. The fluid sensor 28 is shown positioned in the plant outlet line 16. A fluid sensor controller 32 connects to the fluid sensors 27, 28 and controls operation of the fluid sensors. The fluid sensor 27 can monitor process fluid during a period when the process fluid flows into the plant 20. The fluid sensor 28 can monitor fluid flowing out of the plant 20. One application of the fluid sensors 27, 28 is to monitor outlet fluid flowing out of the plant 20 following the end of a ClP process. FIGURE 16 shows an example timeline of operation of the apparatus shown in FIGURE 15. The timeline is similar to FIGURE 2. The plant 20 operates for a period 41. During period 41 the controller 32 may acquire data about the process fluid flowing into the plant 20. The controller 32 may acquire a set of conductivity measurements for the inlet fluid (e.g. a type of beverage) and construct a TCM, similar to the one described above in FIGURE 4. At time T1 the plant 20 suspends operation and a CIP process 43 is performed. The CIP process 43 ends at time T2. At time T2 the plant 20 has been cleaned, and the waste fluid flowing from the plant 20 has a safe level of cleaning product. At this time, the plant 20 still contains water. When the plant 20 is restarted after T2, process fluid is pumped into the plant 20 via lines 13, 15. There is an initial period 47 during which the process fluid mixes with water. Fluid flowing from the plant outlet line 16 is a mix of process fluid diluted with water remaining from CIP. This period 47 may be called “push through”. After a period of push through, the concentration of water within the plant 20 has diminished to an acceptable level. During the period T2-T3 the controller 32 monitors the conductivity of outlet fluid flowing out of the plant 20. The controller 32 compares the conductivity of the outlet fluid with the stored data about the inlet process fluid. When the amount of water within the outlet fluid has diminished to an acceptable level, the controller 32 signals to the plant controller 30 that the push through stage has ended. By accurately determining conductivity of the outlet fluid, the push through period is minimised and wastage of the process fluid is reduced. The plant controller 30 begins to pump the process fluid into the plant 20. Another application is to monitor a product flushing operation at a processing plant. A product flushing operation is performed between processing a first fluid product (e.g. a first type of beverage) and processing a second fluid product (e.g. a second type of beverage). During a product flushing operation, the first fluid product is flushed from the processing plant by pumping water into the inlet line 15 of the plant 20. It is necessary to monitor when the product has been fully removed from the plant 20. The system is similar to the one shown in FIGURE 1, with the CIP apparatus 12 shown in FIGURE 1 performing a water flush / rinse rather than a cleaning process. The inlet fluid sensor 21 and outlet fluid sensor 22 may be positioned similarly to FIGURE 1. The fluid sensor controller 32 acquires measurements for water in a similar manner as described above. The data may be acquired during any period when water is flowing past the inlet fluid sensor 21. The controller 32 may acquire a set of conductivity measurements for water and construct a TCM, similar to the one described above in FIGURE 4. After a period of flushing, the concentration of the first fluid product within the plant 20 has diminished to an acceptable level. The controller 32 monitors the conductivity of outlet fluid flowing out of the plant 20. The controller 32 compares the conductivity of the outlet fluid with the stored data about the inlet fluid (water). When the amount of the first fluid product within the outlet fluid has diminished to an acceptable level, the controller 32 signals to the plant controller 30 that the product flushing operation has ended. By accurately determining conductivity of the outlet fluid, the duration of the product flushing operation is minimised and wastage of water is reduced. FIGURE 17 shows an example of a processing apparatus 300 which may implement at least part of the fluid sensor controller, such as the method of FIGURES 4 and / or 8. Processing apparatus 300 comprises one or more processors 301 which may be any type of processor for executing instructions to control the operation of the fluid sensing apparatus. The processor 301 is connected to other components of the device via one or more buses 306. Processor-executable instructions 303 may be provided using any data storage device or computer-readable media, such as memory 302. The processor-executable instructions 303 comprise instructions for implementing the functionality of the described methods. The memory 302 is of any suitable type such as non-volatile memory, a magnetic or optical storage device. The processing apparatus 300 comprises input / output (I / O) interfaces 307. The I / O interfaces 307 are configured to communicate with other apparatus, such as receiving raw sensor measurements from the fluid sensors 21, 22 and sending / receiving control signals to / from the plant controller 30. The I / O interfaces 307 may comprise a communications interface, such as an Ethernet interface for Local Area Network communications or an Internet Protocol (IP) interface. The processing apparatus 300 connects to a user interface 308. The user interface 308 may provide information to a user, such as status indications. Memory 302, or a separate memory, stores data used by the processor. This can include: a store of measurement data; a TCM; thresholds used by the method, e.g. threshold values corresponding to acceptable levels of cleaning product in a fluid. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Claims

1. Apparatus for monitoring a fluid at a processing plant comprising:a first fluid sensor configured to measure conductivity and temperature of an inlet fluid to the processing plant;a second fluid sensor configured to measure conductivity and temperature of an outlet fluid from the processing plant; anda controller;wherein the controller is configured to:cause the first fluid sensor to determine a plurality of inlet fluid conductivity measurements and inlet fluid temperature measurements to obtain a set of reference conductivity data about the inlet fluid over a range of temperatures; anddetermine a relationship between conductivity and temperature for the inlet fluid from the set of reference conductivity data;and subsequently the controller is configured to:cause the first fluid sensor to determine a current inlet fluid conductivity measurement and a current inlet fluid temperature measurement of the inlet fluid and determine if the inlet fluid has changed;cause the second fluid sensor to determine an outlet fluid conductivity measurement and an outlet fluid temperature measurement of the outlet fluid;determine an expected outlet fluid conductivity value at a reference temperature; and,compare: (i) the outlet fluid conductivity measurement, or a temperature-adjusted outlet fluid conductivity measurement, at the reference temperature; and (ii) the expected outlet fluid conductivity value at the reference temperature,determine, based on the comparison, a concentration of an ionic contaminant or an unwanted fluid in the outlet fluid.

2. The apparatus of claim 1 wherein the controller is configured to determine the expected outlet fluid conductivity value at the reference temperature based on the current inlet fluid conductivity measurement and the current inlet fluid temperature measurement.

3. The apparatus of claim 2 wherein the controller is configured to determine the expected outlet fluid conductivity value at the reference temperature by adjusting the current inlet fluid conductivity measurement using the determined relationship between conductivity and temperature for the inlet fluid.

4. The apparatus of claim 1 wherein the controller is configured to determine the expected outlet fluid conductivity value at the reference temperature from the determined relationship between conductivity and temperature for the inlet fluid at the reference temperature.

5. The apparatus of any one of the preceding claims wherein the controller is configured to:determine an expected conductivity value at a temperature using the determined relationship between conductivity and temperature for the inlet fluid;compare the current inlet fluid conductivity measurement, or a temperature-adjusted current inlet fluid conductivity measurement, to the expected conductivity value at the same temperature; anddetermine, based on the comparison, if there has been a change in the inlet fluid.

6. The apparatus of claim 5 wherein the controller is configured to determine if a difference between: (i) the current inlet fluid conductivity measurement, or a temperature-adjusted current inlet fluid conductivity measurement; and (ii) the expected conductivity value at the same temperature is more than a threshold value.

7. The apparatus of any one of the preceding claims wherein the controller is configured to acquire a new set of reference data if there has been a change in the inlet fluid.

8. The apparatus of any one of the preceding claims wherein the controller is configured to acquire a new set of reference data after a period of time.

9. The apparatus of any one of the preceding claims wherein the controller is configured to adjust the outlet conductivity measurement to obtain a temperature-adjusted outlet conductivity measurement at the reference temperature using the determined relationship between conductivity and temperature for the inlet fluid.

10. The apparatus of any one of the preceding claims wherein the controller is configured to adjust the conductivity measurements to compensate for voids in the first fluid sensor or the second fluid sensor.

11. The apparatus of claim 10 wherein the first fluid sensor is configured to measure capacitance of the inlet fluid and the controller is configured to:cause the first fluid sensor to determine an inlet fluid capacitance measurement;adjust the inlet fluid conductivity measurement using the inlet fluid capacitance measurement, or the inlet fluid capacitance measurement after an adjustment for temperature.

12. The apparatus of claim 11 wherein the controller is configured to:store a relationship between capacitance and temperature for the inlet fluid; andadjust the inlet fluid capacitance measurement using the stored relationship between capacitance and temperature for the inlet fluid.

13. The apparatus of any one of claims 10 to 12 wherein the second fluid sensor is configured to measure capacitance of the outlet fluid and the controller is configured to:cause the second fluid sensor to determine an outlet fluid capacitance measurement;adjust the outlet fluid conductivity measurement using the outlet fluid capacitance measurement, or the outlet fluid capacitance measurement after adjustment for temperature.

14. The apparatus of any one of the preceding claims wherein the controller is configured to:store data indicating a relationship between conductivity and a concentration of the ionic contaminant or unwanted fluid in the outlet fluid; anddetermine a difference between: (i) the outlet fluid conductivity measurement, or a temperature-adjusted outlet fluid conductivity measurement at the reference temperature; and (ii) the expected outlet fluid conductivity value; anduse the stored data to determine whether the concentration of the ionic contaminant or unwanted fluid in the outlet fluid is less than a threshold amount.

15. The apparatus of claim 14 wherein the controller is configured to determine a time when, or a time until, the concentration of the ionic contaminant or unwanted fluid in the outlet fluid is less than the threshold amount.

16. The apparatus of claim 14 or 15 wherein the inlet fluid is a first fluid type and the outlet fluid is a mix of the first fluid type and an ionic contaminant or unwanted fluid.

17. The apparatus of claim 16 wherein the first fluid type is water and the ionic contaminant is a cleaning product.

18. The apparatus of any one of the preceding claims wherein the controller is configured to determine an end of a rinse stage of a clean in place (CIP) process on the plant.

19. The apparatus of claim 16 wherein the first fluid type is a beverage and the unwanted fluid is water.

20. A method of monitoring a fluid at a processing plant comprising:causing a first fluid sensor to determine a plurality of inlet fluid conductivity measurements and inlet fluid temperature measurements to obtain a set of reference conductivity data about the inlet fluid over a range of temperatures; anddetermining a relationship between conductivity and temperature for the inlet fluid from the set of reference conductivity data;and subsequently:causing the first fluid sensor to determine a current inlet fluid conductivity measurement and a current inlet fluid temperature measurement of the inlet fluid and determining if the inlet fluid has changed;causing a second fluid sensor to determine an outlet fluid conductivity measurement and an outlet fluid temperature measurement of the outlet fluid;determining an expected outlet fluid conductivity value at a reference temperature; and,comparing: (i) the outlet fluid conductivity measurement, or a temperature-adjusted outlet fluid conductivity measurement, at the reference temperature; and (ii) the expected outlet fluid conductivity value at the reference temperature,determining, based on the comparison, a concentration of an ionic contaminant or an unwanted fluid in the outlet fluid.

21. The method of claim 20 comprising determining the expected outlet fluid conductivity value at the reference temperature based on the current inlet fluid conductivity measurement and the current inlet fluid temperature measurement.

22. The method of claim 20 comprising determining the expected outlet fluid conductivity value at the reference temperature by adjusting the current inlet fluid conductivitymeasurement using the determined relationship between conductivity and temperature for the inlet fluid.

23. The method of any one of claims 20 to 22 comprising adjusting the outlet conductivity measurement to obtain a temperature-adjusted outlet conductivity measurement at the reference temperature using the determined relationship between conductivity and temperature for the inlet fluid.

24. The method of any one of claims 20 to 23 comprising adjusting the conductivity measurements to compensate for voids in the first fluid sensor or the second fluid sensor at the time of measurement.

25. Computer-readable instructions that, when executed by a processor, cause the processor to perform the method of any one of claims 20 to 24.

26. A processing plant comprising:an inlet line;an outlet line; andthe apparatus of any one of claims 1 to 19, wherein the first fluid sensor is configured to measure conductivity and temperature of an inlet fluid to the plant on the inlet line, and the second fluid sensor is configured to measure conductivity and temperature of an outlet fluid from the plant on the outlet line.

27. The processing plant of claim 26 comprising clean in place (CIP) apparatus wherein the first fluid sensor is configured to measure conductivity and temperature of an inlet CIP fluid to the plant on the inlet line, and the second fluid sensor is configured to measure conductivity and temperature of an outlet CIP fluid from the plant on the outlet line.

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