GB2586212A

GB2586212A - Liquid measurement device

Info

Publication number: GB2586212A
Application number: GB1910856.2A
Authority: GB
Inventors: Brett Steve
Original assignee: Hydracs Global Ltd; John Hilbrig; Shaun Taylor
Current assignee: Hydracs Global Ltd; John Hilbrig; Shaun Taylor
Priority date: 2019-07-30
Filing date: 2019-07-30
Publication date: 2021-02-17
Also published as: GB201910856D0; IE20200167A2

Abstract

A device 400 for measuring the total dissolved solids (TDS) of a liquid 410 includes first 420a and second 420b electrodes for contacting the liquid 410, and a test circuit. The test circuit applies a voltage to the electrodes 420a,b, measures an electrical signal from the electrodes 420a,b; varies the voltage applied based on a property of the measured signal; and determines the output indicative of the TDS of the liquid 410 when the property of the measured electrical signal satisfies a threshold condition. The threshold may be that the signal is stable and/or strong enough to determine a TDS value. There may be buffers 480a-f for controlling the applied voltage. The TDS value may be indicative of the health status or remaining lifetime of a water filter, and may indicate the hardness of water or presence of contaminants such as limescale.

Description

Liquid Measurement Device

Technical Field of the Invention

The invention relates to a device for determining an output indicative of the total dissolved solids of a liquid and/or an electrical property of the liquid, and a method for determining such measurements.

Background to the Invention

The quality of liquids (and in particular water) is important in many fields, including where they are consumed by humans or used in industrial processes. For instance, water may contain contaminants such as dirt, minerals, salts and various chemicals. These contaminants may be present as particulates, as dissolved solids or in other forms. Certain contaminants may be harmless when consumed by users, but some may be hazardous. In addition to such health risks, the quality of water can impact heavily on the taste of the water, which may be undesirable in the food and beverage industries.

In addition to the potential impacts when water is consumed by humans, the quality of water can affect the performance of machines that use it. For example, water that is too soft can be corrosive and water that that is too hard can create lime scale, especially within hot water systems. Therefore, both water types can result in expensive removal and repair of machinery from service. For instance, dishwashers, boilers, coffee machines, chemical syntheses, and various industrial and agricultural processes may operate more efficiently when the quality of water supplied is appropriate. Furthermore, the lifetimes of machines used in such processes may be improved by ensuring that unsuitable chemicals do not pass through the machines.

In light of the above difficulties, it is known to use filters to remove contaminants from water that is used by machines in order to attempt to remove such contaminants. It is furthermore known that many different types of water filter can be used to control the quality of water. For instance, known water filtration techniques include the use of physical barriers, chemical processes, and biological processes. Furthermore, different types of filters can be used alone or in combination to control the quality of water to different extents, depending on the intended use of the water.

However, water quality is neglected in many industries and water filters are often only replaced when the water quality has deteriorated to such an extent that a machine noticeably fails to function correctly. This negatively impacts users who consume such water or the machines that use the water. Furthermore, known devices for monitoring water quality are not always accurate over wide ranges of water hardness and can be difficult to install due to space constraints. Moreover, readings obtained from such known devices can be inaccurate and the devices can use relatively large amounts of energy.

Accordingly, there is required a device and method that overcomes these problems.

Summary of the Invention

Against this background and in accordance with a first aspect, the invention provides a device, as defined in claim 1. A method is also provided, as defined in claim 29.

The invention relates to a device that may be used to determine measurements related to the total dissolved solids (TDS) of a liquid such as water. The device includes electrodes that can be used to apply an electrical stimulus to the liquid, and the response of the liquid can be used to infer the contents of the liquid. For example, the stimulus may be a voltage applied to a measurement circuit that includes the electrodes, so that a voltage is applied across two electrodes in contact with the liquid. In this way, the electrical property that is measured following application of the stimulus may be any response of the liquid, such as the current passing through the liquid and/or the voltage across the liquid. The response may alternatively be a measure of a response of a different part of the device (e.g. a different electrical component in the device) following application of the stimulus.

The device may further comprise test circuitry arranged to vary the stimulus based on the measured response so that a measurement of an acceptable quality is obtained. For example, the device may recognise than an initial stimulus was not powerful enough to obtain a high quality (e.g. reliable) measurement of the liquid's response. Accordingly, the device may automatically increase the stimulus (for instance by causing a larger voltage to be developed across the measurement circuit) until a measurement or multiple measurements of an acceptable quality are obtained. Alternatively, the device may obtain a measurement of the liquid's response with the polarity of the electrodes reversed if the device detects that doing so would be likely to lead to an improvement in the quality of the measurement.

These steps may be iterated starting from a very small stimulus, to reduce power consumption relative to conventional devices that do not gradually increase the stimulus.

Therefore, the device may provide accurate measurements in a way that reduces battery consumption whilst being automatically capable of adapting to the different properties of different liquids. This may be particularly advantageous in situations where the properties of the liquid (and hence the magnitude of the response to a given stimulus) might be expected to vary dramatically over time. In such cases battery consumption may be reduced by starting measurement at a relatively low voltage and gradually increasing the voltage until a sufficiently accurate measurement of the response can be taken. This may be particularly advantageous in situations where battery powered devices are provided.

Alternatively, the device may apply an initial stimulus and it may be detected that the response is too large for an accurate measurement to be obtained. For example, a sensing circuit may have an error profile that leads to decreased accuracy above a certain signal level, for instance where the input to the sensing circuit is saturated. In this case, the device may automatically detect that the accuracy of the measured response would be improved by decreasing the magnitude of the stimulus until the magnitude of the measured response is reduced to an appropriate level. This may be implemented by determining whether the sensitivity of the device is sufficiently accurate for a given known stimulus and adjusting the stimulus so as to improve the sensitivity. Therefore, accuracy of measurement may be improved whilst simultaneously reducing power consumption.

The data provided by the devices of the invention may be analysed at an external system, such as a remote server. This may allow continuous monitoring of the quality of the liquid so that when the liquid quality drops to an unacceptable level, a person responsible for maintenance of the systems that use the monitored liquid is alerted. For example, the device may be installed in a region (e.g. under a sink) with limited access to wired network ports (e.g. Ethernet ports). Therefore, the device may comprise a wireless communication interface so that the data can be transmitted to an external system, such as a server in the vicinity of the device or a cloud server. This may allow processing and scheduling of maintenance to be performed remotely from the device, meaning that the maintenance of a potentially large number of machines that use liquid may be improved. 4 -

Furthermore, by providing a cloud-based solution, the water quality at a large number of geographical sites can be monitored and location-based trends can be identified. In this way, the present invention can use such trends to adjust its analysis. For example, it may be known that water in a particular locality is much harder than other localities and so the assessment of filter health can be dependent on the specific location in which it can be performed. Moreover, a cloud-based solution can allow seasonal or time-based trends to be identified and accounted for, in contrast with simplistic monitoring systems that do not include such information. For instance, if it is known that the water flowing through a particular filter is likely to become softer or harder (i.e. its TDS is likely to change) in the near future, then the prediction of the filter's remaining lifetime can be adjusted accordingly.

In summary, the device of the invention may be battery powered and allows the quantity of TDS in a liquid (e.g. water) supply to be inferred. Electrodes in the stream allow a processor to determine the conductivity of the water and from this measurement, the TDS value may be calculated. Accuracy of the result may be improved by monitoring the temperature of the water being measured.

The conductivity is preferably measured using a microprocessor based technique that prevents polarisation of the electrodes whilst minimising the number of electronic components used. A ratiometric measurement technique may be used to ensure accuracy of the readings under a wide range of battery conditions. Each measurement result may be the mathematical combination of several readings taken from the electrodes at different times. A further technique may allow the unit to operate over a very wide range of water conductivity values. An algorithm in the microprocessor may optimise or improve the electrode stimulus to provide the improved test conditions for the conductivity value being measured. Starting low, the stimulus may be increased in several steps until a reliable reading can be obtained.

In general terms, the present invention provides a device for determining an output indicative of the total dissolved solids, TDS, of a liquid, the device comprising: a measurement circuit comprising first and second electrodes for contacting the liquid; and a test circuit configured to: apply a voltage to the measurement circuit; measure an electrical signal from the first and second electrodes; vary the voltage applied to the measurement circuit based on a property of the measured electrical signal; and determine the output

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indicative of the TDS of the liquid when the property of the measured electrical signal satisfies a threshold condition.

The output may be a value of the TDS of the liquid or a value related to the TDS of the liquid. The output may also be an alert indicating that the TDS of the liquid is not at an acceptable level. The output may be in the form of the device being configured to take an action, such as shutting down a system, based on the TDS value.

The electrodes may be formed from any material suitable for measuring electrical properties of liquids and are part of the measurement circuitry that measures the properties of the liquid. Suitable materials for the electrode include various existing metals and alloys.

The test circuit is the portion of the device that causes a voltage across the electrodes. In some aspects, the measurement circuit includes: a controller that outputs a signal (which may be a digital signal); one or more impedances that connect the signal output to one of the electrodes; and the connection between the other electrode and the controller (which may also include an impedance). The effect of the impedances is to adjust the voltage across the electrodes even when the signal output by the controller is constant. In alternative implementations, the controller may be capable of providing a variable output to vary the voltage across the electrodes. The test circuit may be configured to provide any stimulus that causes a voltage difference to occur between the electrodes. For instance, the test circuit may be configured to input a current or voltage to one electrode (or to a component connected to the electrode).

Varying the voltage applied to the measurement circuit is preferably achieved by applying a digital signal to one electrode (optionally via an impedance, depending on the sensitivity required) and monitoring the other electrode with an analogue to digital converter. The use of one or more impedances in conjunction with a digital signal allows the voltage across the electrodes to be adjusted (even if the voltage generated by the controller is not changed), so as to improve the device sensitivity. In that case, varying the voltage may comprise activating one or more impedances to control the voltage across the electrodes. However, it will be appreciated that varying the voltage applied to the measurement circuit can additionally (or alternatively) be achieved by adjusting the voltage that the controller is configured to generate. 6 -

The test circuit is preferably configured to receive measurements from the other electrode (or at another location) in the form of an electrical signal. The electrical signal may be a voltage measurement, a current measurement or any other detected electrical property of a node or component of the device. Based on the electrical measurement, information about the liquid in contact with the electrodes may be inferred. For instance, an impedance, resistance, reactance, conductivity, voltage drop across the liquid, or any other measurement may be used to infer the electrical characteristics of the liquid, which may in turn be used to determine a TDS measurement (or a related quantity) for the liquid. Other types of electrical properties may be measured and other types of stimulus may be used.

The test circuit is the portion of the device that is arranged to apply and vary the magnitude and/or polarity of the applied voltage, current or stimulus. The test circuit may comprise switches, resistors (variable or fixed), buffers and various other components for controlling and/or varying the stimulus. The test circuit may further comprise logic to control the stimulus and to interpret the measurements received from the measurement circuit.

The measurement circuit is the portion of the device that contacts the liquid and generates the electrical signal that the test circuit measures. The measurement circuit includes the electrodes and the connections from the electrodes to the component of the device that receives such electrical signals (e.g. a controller or microprocessor). For example, the measurement circuit may be connected to an analogue-to-digital convertor in a microprocessor. The microprocessor may be arranged to receive the generated digital measurements, process the measurements and control the stimulus accordingly.

Advantageously, varying the voltage (or any other electrical stimulus) applied to the measurement circuit based on a property of the measured electrical signal may allow automated adjustment of the device. The property may be any value that allows the test circuit to determine whether the obtained measurement is reliable and/or accurate. Various statistical parameters may be used or the actual measured values may be used for the step of varying. Such feedback may provide a device that improves the quality of the readings taken without any user input being required. Therefore, a more effective measurement system may be provided. In this way, a plurality of measurements can be obtained in a way that provides good quality readings even when the properties of the liquid change significantly over time. 7 -

Preferably, the test circuit is configured to vary the voltage applied to the measurement circuit by increasing or decreasing the magnitude of the applied voltage. Advantageously, starting measurement with a small applied stimulus does not waste a significant amount of energy if the obtained reading is inaccurate. Thereafter, the magnitude of the applied stimulus may be increased so as to obtain a larger output measurement. Thus, the device provides an efficient way of obtaining accurate measurements whilst saving energy. The test circuit may also be configured to gradually reduce the magnitude of the applied stimulus, for instance when an initial measurement was unnecessarily high and decreasing the measurements would lead to increased accuracy of measurement. This may provide further energy savings and advantages.

Preferably, the test circuit is configured to vary the voltage applied to the measurement circuit by reversing the polarity of the applied voltage. Reversing the polarity of the applied voltage may be used to maintain the health of the electrodes by preventing accumulation of ions at the surfaces thereof. This may be used instead of or in conjunction with the step of varying the magnitude of the applied voltage. Therefore, the voltage V On Volts) between the electrodes may alternate between V=+K and V=-K (where K is a constant). Alternatively, the voltage between the electrodes may vary in amplitude and polarity. For instance, the voltage may be a function, V(t), of time where V(t) = tsin(t) (the voltage increasing in magnitude whilst changing in polarity) or V(t) = t/sin(t) (the voltage decreasing in magnitude whilst changing in polarity). It will be appreciated that these formulae are provided for illustrative purposes only and that various other forms for the voltage as a function of time may be used, including piecewise linear approximations and/or stepwise approximations to such functions.

Preferably, the test circuit is configured to measure the electrical property of the liquid iteratively at intervals so as to provide a group of measurements, and to iteratively vary the voltage applied across the electrodes at corresponding intervals until the property of the measured electrical signal satisfies the threshold condition. In this way, the electrical stimulus may be varied repeatedly and gradually until an acceptable value is reached (e.g. when a threshold condition is satisfied). Thereafter, repeated measurements at an appropriate level of stimulus may be taken and repeated as many times as is necessary for a desired degree of accuracy. 8 -

The test circuit may have two characteristic intervals (i.e. timescales), where the shorter characteristic interval is the time between successive measurements in a single group of measurements (which may be on the order of a fraction of a second, seconds, minutes or hours) and the longer timescale may be the time between groups of measurements (which may be on the order of days, weeks or months). Repeated measurements at the shorter interval may be used to identify a single, more accurate output for a specific point in time whereas the repeated measurements at longer intervals may be used to track long-term trends of the properties of the liquid (and optionally the long-term health of a filter).

Preferably, the test circuit is configured to adjust the interval between groups of measurements based on the property of the electrical signal. For example, certain types of water filter may degrade in a non-linear fashion, meaning that the rate of change of water properties may vary over time. In such cases, if the interval between groups of measurements was kept constant, then a water filter may degrade to an unacceptable condition unnoticed. By varying the interval between groups of measurements based on the property of the measured electrical signal (e.g. its value or its rate of change), the device may be able to detect and closely monitor liquid quality when it is approaching unacceptable levels of TDS. Conversely, if repeated measurements show little or no variation then it may be determined that such regular measurement is unnecessarily wasting energy. In such cases, the frequency of measurement may be decreased automatically thereby saving energy.

Preferably, the test circuit is configured to determine a sensitivity for the device and to vary the voltage applied to the measurement circuit so as to increase the sensitivity. For instance, the sensitivity may be a statistical parameter based on the reliability and/or accuracy of measurements. Therefore, the device may automatically increase its sensitivity, providing clear advantages in accuracy.

Preferably, the property of the measured electrical signal is one of more of: a value of the measured electrical signal; a value indicative of the TDS of the liquid; a variance of a plurality of measurements of the measured electrical signal; a signal-to-noise ratio of one or more measurements of the measured electrical signal; and an indication of whether the measured electrical signal satisfies a threshold condition. Such parameters may lead to a more accurate device, and various other parameters may be used.

Preferably, the test circuit is configured to obtain an average measurement indicative of the TDS of the liquid and/or an average value of the measured electrical signal. This may further increase accuracy and reliability of the device.

Preferably, the device further comprises a temperature sensor for measuring a temperature of the liquid, wherein the test circuit is configured to determine the measurement indicative of the TDS of the liquid based on the temperature of the liquid. Advantageously, using a temperature sensor may provide increased accuracy of the measurement of the TDS of the liquid.

Preferably, the device further comprises a flow-rate sensor for measuring a flow-rate of the liquid. Using flow-rate information in this way may provide further information about the health of a filter, since a blocked or damaged filter may fail to perform its filtering function adequately (affecting the TDS measurements) and may also lead to reduced or increased flow-rate. Therefore, providing a flow-rate sensor provides additional benefits that can be used to raise alerts (e.g. that a filter needs changing) more quickly than either TDS monitoring or flow-rate monitoring alone.

Preferably, the test circuit is configured to determine a health status for a water filter of the liquid based on the measurement indicative of the TDS of the liquid, preferably wherein the health status is a remaining lifetime. This may comprise performing a prediction of the remaining lifetime of any filter to which the device is attached. This may be performed by storing information about the filter and performing such predictions using the device, or by transmitting data from the device to an external system that makes such predictions. In either case, users of the device are informed of the health of the filter accordingly.

Preferably, the device further comprises a wireless communication interface configured to transmit the measurement indicative of the TDS of the liquid and/or the measured electrical signal to an external system. Advantageously, providing a wireless communication interface wireless technology makes the device simple to install. This may be particularly advantageous in scenarios where space is limited and/or wired connections to the internet are not available. For instance, kitchens, cafes, airing cupboards and underneath sinks may have plumbing for appliances that use water but may not have Ethernet ports in convenient locations nearby. Providing a wireless communication interface is therefore advantageous in such scenarios. The wireless communication interface may be a 20 chip -10 -which draws very little current and so is particularly advantageous for battery-powered implementations. Alternatively, a 4G chip may be provided for more sophisticated capabilities. In certain examples where smart devices (e.g. smart home appliances such as smart boilers or smart coffee machines) are used, the portal and/or the device may be configured to transmit a communication to the smart appliance causing the appliance to shut down when the water quality is not of an acceptable quality. This may pre-emptively prevent damage to such devices.

Preferably, the test circuit comprises a buffer connected to a node of the measurement circuit, wherein the test circuit is configured to vary the voltage applied to the measurement circuit by changing the state of the buffer. The buffer may be a tri-state buffer, which allows an output port thereof to assume an extremely high impedance state, preventing the output from affecting the rest of the circuit, in addition to providing high and low logic levels. The buffer may be directly connected to an electrode of the device or may be connected to the measurement circuit through one or more impedances for providing a drop in voltage. In any event, the buffer may be used for providing an electrical stimulus to the measurement circuit. Instead or in addition to a buffer, all embodiments of the device of the present invention may comprise any electrical component (digital or analogue), such as a variable impedance, that is suitable for controlling the electrical stimulus applied to the measurement circuit.

Preferably, the test circuit comprises a plurality of buffers connected to the node of the measurement circuit for providing coarse and fine control of the voltage applied to the measurement circuit. For instance, a plurality of different impedances may be provided between each buffer and the measurement circuit, so as to provide different voltage adjustments. This may provide a more flexible, accurate and efficient device.

Preferably, the test circuit comprises one more buffers connected to a second node of the measurement circuit and is configured to vary the voltage applied to the measurement circuit by changing the state of each buffer connected to the second node. For instance, when the test circuit is configured to reverse the polarity of the electrical stimulus, this may be achieved by turning buffers that are on and connected to one electrode off whilst simultaneously turning buffers that are off and connected to the other electrode on.

Preferably, the test circuit comprises a known impedance connected to one of the first and second electrodes, thereby forming a potential divider with the liquid. The known impedance may be fixed or variable and may be used to infer the electrical properties of the liquid in the circuit, For instance, if the only significant voltage drops in components in the circuit are in the liquid and the known impedance then the relative voltage drops and/or the current in the device can be used to infer the electrical properties of the liquid based on the applied stimulus and the known impedance. Preferably, the test circuit is configured to measure the electrical signal and/or determine the measurement indicative of the TDS of the liquid based on the known impedance. In other words, the known impedance may be used to directly calculate the TDS or it may be used to calculate an intermediate value, such as a conductivity value that may then be used to determine a TDS value by a subsequent unit conversion (which may be performed on-board the device or remotely to the device).

Preferably, the test circuit comprises a second known impedance connected to the other electrode, thereby forming a second potential divider with the liquid. This may be particularly advantageous in scenarios where the polarity of the stimulus is applied to preserve the electrodes, as measurements can be obtained with a stimulus of either polarity being applied. Preferably, the test circuit is configured to measure the electrical signal and/or determine the measurement indicative of the TDS of the liquid based on the second known impedance. This may be performed analogously to the first known impedance. Advantageously, the known impedances can be activated and deactivated (e.g. using buffers and/or switches) to provide control over the sensitivity of the device.

Preferably, the test circuit is configured to measure the response of one of the first and second electrodes to a known voltage applied to the measurement circuit. For instance, the application of a known voltage may have expected responses (in terms of current and/or voltage) from certain components in the device but these responses may be influenced by the presence of liquid in contact with the electrodes. From the known components of the device and the known voltage applied it may be possible to infer various characteristics of the liquid, as described above. For example, the test circuit is preferably configured to measure the current flow in a portion of the device and/or a voltage at a node of the device so as to measure the electrical signal and/or determine the measurement indicative of the TDS.

-12 -Preferably, the electrical signal is a measure of one or more of: the impedance of the liquid; the resistance of the liquid; the current through the liquid; the voltage across the liquid; and/or the conductivity of the liquid. It will be appreciated that various other measurements may be taken in accordance with the invention. Preferably, the measurement indicative of the TDS of the liquid is a conductivity of the liquid.

In preferred embodiments of the invention, the liquid is water. However, it will be appreciated that various other liquids are susceptible to analysis using the device of the present invention.

Preferably, the device comprises a test cell. For instance, the device may be a standalone component that is fitted in a liquid supply to monitor the supply and the device may comprise a test cell into which the liquid flows to be analysed. Therefore, the test cell may provide a region into which the electrodes extend, so that the electrodes can come into contact with the liquid.

Preferably, the device comprises a battery. This may be advantageous in kitchens and the like, where it may be impractical to install sockets adjacent plumbing (e.g. due to safety). Battery-powered devices benefit from the measurement method used by the device of the invention due to its energy-saving features such as dynamic variations in the magnitude of an electrical stimulus or the sensitivity of the device.

Preferably, the test circuit comprises a processor, a microprocessor, a controller, or an integrated circuit. This may be used to control the states of the various components that perform the measurements and apply the electrical stimuli.

The invention further provides a method comprising receiving, at an external system, data from the device according to any embodiment described herein, and determining a health status of a filter based on the received data, preferably wherein the health status is a remaining lifetime. This may reduce the computational burden of the actual device whilst allowing users of the device to monitor their water quality and/or filter health easily. The results may be provided back to the device from the external system, or the results could be processed remotely and presented to users via an online portal, which may be a website.

-13 -The invention further provides a method for determining an output indicative of the total dissolved solids, TDS, of a liquid, the method comprising: applying a voltage to a measurement circuit comprising first and second electrodes in contact with the liquid; measuring an electrical signal from the electrodes; varying the voltage applied to the measurement circuit based on a property of the measured electrical signal; and determining the output indicative of the TDS of the liquid when the property of the measured electrical signal satisfies a threshold condition. The method may further comprise providing any feature of the device described above, or performing any action that any component of the device performs.

For instance, the method may comprise varying the voltage applied to the measurement circuit by increasing or decreasing the magnitude of the applied voltage. Preferably, the method comprises varying the voltage applied to the measurement circuit by reversing the polarity of the applied voltage.

The method may comprise measuring the electrical property of the liquid iteratively at intervals to provide a group of measurements, and iteratively varying the voltage applied across the electrodes at corresponding intervals until the property of the measured electrical signal satisfies the threshold condition. The method may comprise adjusting the interval between groups of measurements based on the property of the electrical signal.

Preferably, the method includes determining a sensitivity and varying the voltage applied to the measurement circuit so as to increase the sensitivity. As noted previously, the property of the measured electrical signal may be one of more of: a value of the measured electrical signal; a value indicative of the TDS of the liquid; a variance of a plurality of measurements of the measured electrical signal; a signal-to-noise ratio of one or more measurements of the measured electrical signal; and an indication of whether the measured electrical signal satisfies a threshold condition. The method may further comprise obtaining an average measurement indicative of the TDS of the liquid and/or an average value of the measured electrical signal.

The method preferably comprises measuring a temperature of the liquid, and determining the measurement indicative of the TDS of the liquid based on the temperature of the liquid. Additionally or alternatively, the method may comprise measuring a flow-rate of the liquid.

-14 -The method preferably comprises determining a health status for a water filter of the liquid based on the measurement indicative of the TDS of the liquid, preferably wherein the health status is a remaining lifetime. Preferably, the method comprises wirelessly transmitting the measurement indicative of the TDS of the liquid and/or the measured electrical signal from a device to an external system.

The method may comprise varying the voltage applied to the measurement circuit by changing the state of one or more buffers connected to a first node. Preferably, the method comprises varying the voltage applied to the measurement circuit by changing the state of each buffer connected to a second node.

The method may comprise providing a known impedance connected to one of the first and second electrodes, thereby forming a potential divider with the liquid. The method may further comprise measuring the electrical signal and/or determining the measurement indicative of the TDS of the liquid based on the known impedance. Optionally, the method comprises providing a second known impedance connected to the other electrode, thereby forming a second potential divider with the liquid.

The method optionally comprises measuring the electrical signal and/or determining the measurement indicative of the TDS of the liquid based on the second known impedance.

The method may comprise measuring the response of one of the first and second electrodes to a known voltage applied to the measurement circuit.

The method may comprise measuring the current flow in a portion of a device and/or a voltage at a node of a device so as to measure the electrical signal and/or determine the measurement indicative of the TDS. The electrical signal may be a measure of one or more of: the impedance of the liquid; the resistance of the liquid; the current through the liquid; the voltage across the liquid; and/or the conductivity of the liquid. The measurement indicative of the TDS of the liquid may be a conductivity of the liquid. The liquid is preferably water and the methods are preferably applied to a device having a test cell and/or a battery. A processor, a microprocessor, a controller, or an integrated circuit may be used to cause any of the described methods to be performed.

It should be noted that any feature described above may be used with any particular aspect or embodiment of the invention.

-15 -

Brief Description of the Drawings

The invention may be put into practice in various ways, which will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 depicts a device according to a first embodiment; Figure 2 depicts a device according to a second embodiment; Figure 3 depicts a device according to a third embodiment; Figure 4 depicts a device according to a fourth embodiment; Figure 5 depicts a method according to a fifth embodiment; and Figure 6 depicts an online portal provided using devices and methods of the disclosure.

It should be noted that the figures are illustrated for simplicity and are not necessarily drawn to scale.

Detailed Description of Preferred Embodiments

In Figure 1, there is shown a device 100 suitable for determining an output indicative of the total dissolved solids TDS of a liquid 110. The device 100 comprises electrodes 120a and 120b in contact with the liquid 110. The electrodes 120a and 120b are connected respectively to nodes 100a and 100b of the device 100. The device further comprises impedance 190 having an impedance value of Zi. The impedance 190 is connected between nodes 100b and a further node 100c of the device 100. The device additionally comprises a controller 130 that is connected to each of nodes 100a, 100b and 100c.

During operation, the controller 130 applies an electrical stimulus to the liquid 110. The stimulus is a voltage V between nodes 100a and 100c. The controller 130 selects the value of the applied voltage V. Selecting the value of the applied voltage V can be achieved by varying the voltage that a voltage generator of the controller 130 outputs and/or by activating/deactivating one or more impedances (e.g. impedances that are internal to the controller 130 and not shown in Figure 1). In response to this applied voltage, current flows between electrodes 120a and 120b and the magnitude of the current that flows is affected by the properties of the liquid 110 and the value Li of the impedance 190.

-16 -The controller 130 is connected to a further node 100b of the device 100, and node 100b is between the second electrode 120b and the impedance 190. The voltage at node 100b is an input to the controller 130. The controller 130 may be capable of measuring the voltage between any two nodes to which it is connected. The controller 130 may also be capable of applying any stimulus (e.g. a voltage or a current) to any node to which it is connected. As the voltage at 100b is a function of the impedance Zi, the voltage V and the impedance of the liquid 110, the impedance of the liquid can be determined and then used to calculate the TDS, or the TDS can be determined directly. The TDS may then be determined at the controller 130 using the fact that TDS (measured in mg/L) is typically between 50-100% of the numerical value of the conductivity measured in Siemens (1S=1kg-1-m-2.s3,A2).

The device 100 of Figure 1 can be used to implement the present invention because, in the general terms described previously, the device 100 comprises a measurement circuit comprising first and second electrodes 120a and 120b for contacting the liquid 110 and a test circuit comprising controller 130 and impedance 190 for applying a voltage V to the measurement circuit. The controller 130 measures an electrical signal (the voltage at node 100b and/or the current following at nodes 100a/b/c) from the first 120a and second electrodes 120b. The controller 130 then causes a variation in the applied voltage V by changing the voltage V. Once the measured electrical signal (the voltage at node 100b and/or the current following at nodes 100ab/c) satisfies a threshold condition (e.g. the signal is stable and/or strong enough), then the controller 130 determines an output indicative of the TDS of the liquid. This allows automated, efficient and accurate analysis of the liquid.

In Figure 2 there is depicted a second embodiment of the invention. This embodiment is similar to the first embodiment in that the device 200 is suitable for determining an output indicative of the total dissolved solids TDS of liquid 210. Moreover, the device 200 comprises electrodes 220a and 220b in contact with the liquid 210. The electrodes 220a and 220b are connected respectively to nodes 200a and 200b of the device 200. The device further comprises impedance 290 having an impedance value of Z2b. The impedance 290 is connected between nodes 200b and 200c of the device 200. Similarly to the first embodiment, the device additionally comprises a controller 230 that is connected to each of nodes 200a, 200b and 200c.

-17 -The device of the second embodiment differs from that of the first embodiment in that an additional impedance 285 is provided between node 200a and a node 200d to which the controller 230 is connected. In the embodiment of Figure 1, the controller is connected to electrode 120a directly via node 100a, whereas the second embodiment provides the additional impedance Z2, between the node 200d of the controller 230 and the node 200a of the first electrode 220a.

The provision of further 285 impedance may be advantageous. For instance, impedance 290 may be a fixed resistance that can be used to provide a known voltage drop across it when a given amount of current flows through it. In this case, impedance 285 may be a variable resistance for providing variations in the total impedance. For example, this may be used to reduce the voltage between the first 220a and second 220b electrodes to fine tune the accuracy of any measurement taken. Therefore, the controller 230 may be able to both control the impedance value Z2 and to apply a known voltage V to the impedance 285. In this way, measurements of an electrical signal at the electrodes may be taken by the controller 230.

The device 200 of the second embodiment may measure an electrical signal from the electrodes 220a and 220b in a number of ways. Preferably, the controller 230 monitors a voltage across the electrodes 220a and 220b. However, the controller may determine the voltage at other nodes of the circuit and convert these into electrical signals related to the properties of the liquid. For example, for a given voltage between nodes 200c and 200d, the total current flow in the device will be determined by the sum of the impedances of the liquid and the two impedances 285 and 290.

The device 300 of Figure 3 is similar to the device 200 of Figure 2 and shows a preferred implementation. The device 300 comprises electrodes 320a and 320b in contact with the liquid 310. The electrodes 320a and 320b are connected respectively to nodes 300a and 300b of the device 300. The device further comprises impedance 390 having an impedance value of 73b. The impedance 390 is connected between nodes 300b and 300c of the device 300. The device also comprises impedance 385 having an impedance of Z3, that is connected between nodes 300a and 300d. The device additionally comprises a controller 330 that is connected to nodes 300c and 300d.

-18 -As in Figure 2, the controller 330 is connected to nodes 300a and 300b to sense the voltage across the electrodes. However, in contrast with Figure 2, the device 300 of Figure 3 further includes impedances 375 and 380, which have impedances of Z30and Z3d respectively. Impedance 375 is connected between the controller 330 and node 300b, whilst impedance 380 is connected between the controller 330 and node 300a.

The electrodes 320a and 320b alternate between driving a digital signal and receiving an analogue one. If 320a is driving, then controller 330 sets a logic 1 at this node and effectively removes Z3a, Zaa and Zaa from the circuit by setting the corresponding lines high impedance. The controller reads the voltage at node 300b. If the voltage is too high then the sensitivity can be reduced by deploying Z3 in parallel with Z3b. The action is accomplished by the controller driving its Z3 connection to zero volts. In this implementation, the combination of the controller 330 and the resistors 375, 380, 385 and 390 can be considered to be a test circuit that is capable of varying the voltage applied to the electrodes 320a and 320b.

In Figure 4, a fourth embodiment of the invention is depicted. Figure 4 is an extension of the design of Figure 3. Instead of a single paralleling resistor (Z2. in Figure 3above), two are provided, which allows the controller to select multiple sensitivities if required. In this embodiment, a liquid 410 to be measured flows in the direction 400L through a filter 460 (which does not form part of the device itself) and into a test cell 415. Within the test cell 415 are electrodes 420a and 420b, a temperature sensor 440 and a flow-rate sensor 450, each of which are connected to an analogue-to-digital convertor 405 of a microprocessor 430. The microprocessor 430 is connected to a wireless communication interface 470 for transmitting and/or receiving data wirelessly. The wireless communication interface 470 may be provided on the same circuit board as the microprocessor 330 or may be provided in a separate unit that is connected to the microprocessor 330. The device 400 is powered by battery 425. Due to the low power requirements of the device 400, battery 425 may be one of various known batteries that can provide an acceptable battery life for the device 400.

The microprocessor 430 further comprises a plurality of buffers 480a-f. The buffers 480a-f may be tri-state buffers and may be coupled to the electrodes 420a and 420b directly, as in the case of buffers 480c and 480f. Moreover, the buffers 480a, 480b, 480d and 480e are coupled to the electrodes 420a and 420b via impedances 485a, 485b, 485d and 485e -19 -respectively. The values of the impedances 485a, 485b, 485d and 485e may be selected so as to provide coarse and fine control over the voltage across the electrodes 420a and 420b.

Impedances 485a and 485b are connected to a node 400a of the device to which the buffer 480c is connected and to which the first electrode 420a is also connected. In this way, the voltage at the node 400a and hence the electrode 420a can be controlled by changing the state (e.g. high, low or impedance) of one or more of buffers 480a-c. Node 400a is furthermore connected to a ground of the device via a further impedance 490a.

Similarly, impedances 485d and 485e are connected to a node 400b of the device to which the buffer 480f is connected and to which the second electrode 420b is also connected. In this way, the voltage at the node 400b and the electrode 420b can be controlled by changing the state (e.g. high, low or impedance) of one or more of buffers 480d-f. Node 400b is also connected to a ground of the device via impedance 490b.

In use, the TDS of the liquid 410 may be measured by passing the liquid through the test cell 415. As the two electrodes 420a and 420b and temperature sensor 440 are connected directly to the microprocessor 430, firmware within the microprocessor 430 may cause the 420a and 420b electrodes to be controlled in the following manner so as to estimate the TDS value of the liquid 410.

In this embodiment, the TDS value of the liquid 410 is estimated by measuring its electrical conductivity (although other parameters may be determined). Referring to Figure 4, it can be seen that both electrodes 420a and 420b are controlled by similar (identical when corresponding components in the two branches have the same properties), but separate circuits. The electrodes 420a and 420b alternately operate as an output and as an input so that when electrode 420a is an input, 420b is an output and vice versa.

In an initial state, electrode 420a may be an output and 420b may be an input. The output is formed by setting buffer 480c to drive high with buffers 480a and 480b left in a high impedance state (so as to effectively remove them from the circuit). In this configuration, the test cell 415 forms a potential divider with impedance 490b. The voltage seen at electrode 420b is thus directly related (e.g. directly proportional) to the conductivity of the -20 -liquid 410. The analogue-to-digital converter 405 within the microprocessor 430 records this voltage and may store this value in memory.

Once a measurement has been taken, the roles of the electrodes 420a and 420b may be reversed in embodiments of the invention and the process repeated. In particular, electrode 420b is set to be an output and 420ba is set to be an input. The output is formed by setting buffer 480f to drive high with buffers 480d and 480e set to a high impedance state. In this configuration, the test cell 415 forms a potential divider with impedance 490a and the voltage at electrode 420a is again directly related to the conductivity of the liquid 410. The analogue-to-digital converter 405 within the microprocessor 430 again records this voltage.

The two readings obtained from these steps may be averaged to obtain an average conductivity value, so as to provide improved accuracy. Furthermore, the act of reversing the roles of the electrodes 420a and 420b reduces the risk of their polarisation.

Advantageously, the device 400 comprises impedances such as resistors 485a, 485b, 485d and 485e. These may be activated so as to adjust the sensitivity of the potential divider, so that liquids 410 of very low or very high conductivity may be examined and provide accurate results. If it is determined that they are required, then they are brought into action by setting the appropriate digital buffers (480a, 480b, 480d and/or 480e) to output mode rather than impedance.

The microprocessor 430 is capable of determining whether the sensitivity needs to be changed and acts accordingly so as to increase the sensitivity. The determination may be made based on one or more of: the impedance of the liquid; the resistance of the liquid; the current through the liquid; the voltage across the liquid; and/or the conductivity of the liquid. For instance, the determination may be based on a property of the measured electrical signal such as one or more of: a value of the measured electrical signal; a value indicative of the TDS of the liquid; a variance of a plurality of measurements of the measured electrical signal; a signal-to-noise ratio of one or more measurements of the measured electrical signal; and an indication of whether the measured electrical signal satisfies a threshold condition.

The above sequence may be repeated several times and an average conductivity value obtained. The accuracy of any such measurement may be increased further by combining -21 -the information with data received from the temperature sensor 440, so that a temperature-compensated value for the TDS of the liquid 410 may be determined. This may be a first group of measurements occurring at intervals of a first timescale (e.g. seconds, minutes or hours) that provides an average TDS measurement for a certain point in time. The above averaging process may then be repeated at a second, longer timescale (e.g. daily, weekly or monthly) so as to provide long-term trends on the filter 460 health.

The microprocessor 430 depicted in Figure 4 also receives information from a flow-rate sensor. The combination of the TDS value and the volume of liquid 410 passed through the filter 460 allows a prediction of the remaining lifetime of the filter 460 to be made. For instance, the microprocessor 430 may comprise models and algorithms that use existing charts for filter lifetimes in certain circumstances (which may be provided by the filter manufacturer) as well as creating a dynamic prediction based on the hardness of the water (e.g. using a TDS value) and the actual usage of the filter (using the total volume of liquid that has flowed through the filter). Such predictions provide advantages over existing TDS meters because existing systems lack the dynamic adjustment of measurements and predictive capabilities of the present invention.

The measurements taken on the liquid 410 described above may be processed by the microprocessor 430. For instance, the microprocessor may have filter 460 properties pre-loaded thereon so that it may make predictions of the remaining filter 460 life and/or raise an alert when the filter 460 needs to be changed. In such cases, the wireless communication interface 470 need not be provided and may be replaced with a visual output (e.g. a screen or a flashing light) so as to indicate that the filter 460 needs to be changed or to output the remaining lifetime of the filter 460. This may lead to a simple, lightweight device. However, the provision of a wireless communication interface 470 may be particularly advantageous for several reasons. For instance, the wireless communication interface 470 may receive filter data from a database stored in the cloud or on the internet. In this case, when a new filter 460 is fitted then the device 400 can continue to predict its lifetime even if the filter's properties were not pre-loaded on the device.

Alternatively, the raw sensor data (e.g. from temperature sensor 440, electrodes 420a and 420b, and flow-rate sensor 450) may be transmitted to the cloud for external analysis and monitoring.

-22 -The remaining filter 460 lifetime predictions may be performed in a number of ways. One way of assessing the health of a filter 460 may include identifying a consistent change in TDS level or by identifying that the manufacturer's recommended maximum volume of liquid has been filtered.

For example, the device 400 (or an external system) may be configured to monitor an average degradation rate for the filter 460 based on the rate of change of the TDS content of the liquid 410 over time. Then, this value may be used to extrapolate (e.g. using a linear or polynomial-based model) the remaining amount of time until the TDS falls below a threshold value (e.g. a threshold value of acceptability) based on the current rate of change. Similarly, an average value for the liquid 410 used per unit of time may be determined using data from the flow-rate sensor 450. This average flow-rate may then be used (again using a linear or polynomial-based model, for instance) to predict the remaining time until the total liquid 410 that has passed through filter 460 exceeds the filter manufacturer's recommendation for the total liquid that may pass through the filter.

Calculations of these two remaining lifetimes may be performed in parallel. Then, the lower of the two remaining lifetime prediction values may be used as the remaining lifetime of the filter 460, so as to provide a lower bound on its remaining lifetime. Alternatively, multivariate statistical analysis may be used to combine the TDS measurements and the flow-rate measurements into a single predicted lifetime for the filter 460.

Thus, the invention may improve upon existing techniques for managing filters, which typically rely on reacting to problems caused by degraded filters rather than pre-empting such degradation. The frequency of measurements of TDS values may be increased when it is determined that the filter 460 is nearing the end of its lifetime, so as to ensure that it is not replaced before it is too late to prevent damage from occurring.

The above-described techniques allow for a cost effective, low component count solution that provides automatic sensitivity adjustment in an efficient manner whilst also reducing the risk of electrode polarisation. Depending on the specific implementation, the circuitry may be potted in a resin to provide waterproofing.

It should be noted that various alterations may be made to the embodiments depicted in Figures 1, 2, 3 and 4. For instance, the microprocessor 430 and controllers 130 and 230 may be any type of controller, and the invention may be implemented in digital and/or -23 -analogue circuitry. Moreover, the buffers 480a-f may be replaced with any number of buffers (e.g. one or two buffers could be used rather than three) or could be replaced with any number of variable resistance or fixed resistances, such as those depicted in Figures 1 (190) and 2(285 and 290). The wireless communication interface may be a 2G, 30 or 40 chip. The preferred wireless communication interface is a GSM module. Furthermore, each unit may be serialised to allow identification on a web portal.

The devices 100, 200, 300 and 400 may be self-contained units that can be housed on a metal bracket to allow fitment to nearby surfaces, allowing easy installation. The compact nature of the devices 100, 200, 300 and 400 means that they can readily fit within the space provided for existing water filtration equipment. As the devices 100, 200, 300 and 400 may be battery powered and use 20 technology, they may be totally self-contained and long-lasting, requiring no physical interface with any other equipment.

The flow-rate sensor 450 and temperature sensor 440 may not be within the test cell 415 used for the TDS measurement. For instance, they may be installed as separate integers within a water supply and both connected to a communication interface. The device 400 then comprises distinct sensors 420a, 420b, 440 and 450 that are connected to a distinct controller 430 and communication interface 470.

In Figure 5, there is depicted a method 500 of implementing the invention. In particular, the method is a method 500 for determining an output indicative of the total dissolved solids, TDS, of a liquid, the method comprising: applying 510 a voltage to a measurement circuit comprising first and second electrodes in contact with the liquid; measuring 520 an electrical signal from the electrodes; varying 530 the voltage applied to the measurement circuit based on a property of the measured electrical signal; and determining 540 the output indicative of the TDS of the liquid when the property of the measured electrical signal satisfies a threshold condition. The method may be implemented using the embodiments of any of Figures 1 to 4. For instance, the method may comprise causing any action described above to be taken or may comprise providing any feature of the disclosed devices.

Data may be provided from the above-described devices and methods to any cloud-based or remote server. Such data may be processed and/or presented to users via a website, a smartphone app, or via any other appropriate means. In Figure 6, there is shown a web- -24 -based water telemetry portal 600 that a user can access to monitor the health of their water filters. The devices described above provide a daily (or any other frequency) capture of information that allows users to manage their water filters. In this specific portal shown, data regarding a combination of litres used and measured conductivity of water passing through individual hardware systems is transmitted through a network connection to a secure cloud based server.

The particular parameters that are displayed to a user can include: an identifier (11B009B) of the device; records of when the filter was last changed; the current filter installed; a measure of the water hardness; the precise bypass level that was set on installation; the calculated filter life in terms of volume; the water conductivity; the water temperature; the degree of hardness (dH); the filter renewal date; the volume of water filtered since the last renewal; an option to order a replacement filter; a forecast of the filter expiry date; a forecast of when a replacement filter should be ordered; a history of previous readings; an indication of the most recent communications received from the device. Advantageously, this allows remote monitoring and maintenance of a water filter without the need for a user to manually inspect the filter. In certain examples where smart devices (e.g. smart home appliances such as smart boilers or smart coffee machines) are used, the portal and/or the device may be configured to transmit a communication to the smart appliance causing the appliance to shut down when the water quality is not of an acceptable quality. This may pre-emptively prevent damage to such devices.

Although the disclosure has been described with reference to particular types of devices, data and applications, and whilst the disclosure provides particular advantages in such cases, as discussed herein the disclosure may be applied to other types of devices, data and applications. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

As used herein, including in the claims, unless the context indicates otherwise, singular forms of the terms herein are to be construed as including the plural form and vice versa. For instance, unless the context indicates otherwise, a singular reference herein including in the claims, such as "a" or "an" means "one or more". Throughout the description and claims of this disclosure, the words "comprise", "including", "having" and "contain" and -25 -variations of the words, for example "comprising" and "comprises" or similar, mean "including but not limited to", and are not intended to (and do not) exclude other components.

The use of any and all examples, or exemplary language ("for instance", "such as", "for example" and like language) provided herein, is intended merely to better illustrate the disclosure and does not indicate a limitation on the scope of the disclosure unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the disclosure.

Any steps described in this specification may be performed in any order or simultaneously unless stated or the context requires otherwise.

All of the aspects and/or features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the preferred features of the disclosure are applicable to all aspects of the disclosure and may be used in any combination. Likewise, features described in non-essential combinations may be used separately (not in combination).

Claims

-26 -CLAIMS: 1. A device for determining an output indicative of the total dissolved solids, TDS, of a liquid, the device comprising: a measurement circuit comprising first and second electrodes for contacting the liquid; and a test circuit configured to: apply a voltage to the measurement circuit; measure an electrical signal from the first and second electrodes; vary the voltage applied to the measurement circuit based on a property of the measured electrical signal: and determine the output indicative of the TDS of the liquid when the property of the measured electrical signal satisfies a threshold condition.
2. The device according to claim 1, wherein the test circuit is configured to vary the voltage applied to the measurement circuit by increasing or decreasing the magnitude of the applied voltage.
3. The device according to claim 1 or claim 2, wherein the test circuit is configured to vary the voltage applied to the measurement circuit by reversing the polarity of the applied voltage.
4. The device according to any preceding claim, wherein the test circuit is configured to measure the electrical property of the liquid iteratively at intervals so as to provide a group of measurements, and to iteratively vary the voltage applied across the electrodes at corresponding intervals until the property of the measured electrical signal satisfies the threshold condition.
5. The device according to claim 4, wherein the test circuit is configured to adjust the interval between groups of measurements based on the property of the electrical signal.
6. The device according to any preceding claim, wherein the test circuit is configured to determine a sensitivity for the device and to vary the voltage applied to the measurement circuit so as to increase the sensitivity.
-27 - 7. The device according to any preceding claim, wherein the property of the measured electrical signal is one of more of: a value of the measured electrical signal; a value indicative of the TDS of the liquid; a variance of a plurality of measurements of the measured electrical signal; a signal-to-noise ratio of one or more measurements of the measured electrical signal; and an indication of whether the measured electrical signal satisfies a threshold condition.
8. The device according to any preceding claim, wherein the test circuit is configured to obtain an average measurement indicative of the TDS of the liquid and/or an average value of the measured electrical signal.
9. The device according to any preceding claim, wherein the device further comprises a temperature sensor for measuring a temperature of the liquid, wherein the test circuit is configured to determine the measurement indicative of the TDS of the liquid based on the temperature of the liquid.
10. The device according to any preceding claim, wherein the device further comprises a flow-rate sensor for measuring a flow-rate of the liquid.
11. The device according to any preceding claim, wherein the test circuit is configured to determine a health status for a water filter of the liquid based on the measurement indicative of the TDS of the liquid, preferably wherein the health status is a remaining lifetime.
12. The device according to any preceding claim, wherein the device further comprises a wireless communication interface configured to transmit the measurement indicative of the TDS of the liquid and/or the measured electrical signal to an external system.
13. The device according to any preceding claim, wherein the test circuit comprises a buffer connected to a node of the measurement circuit, wherein the test circuit is configured to vary the voltage applied to the measurement circuit by changing the state of the buffer.
-28 - 14. The device according to claim 13, wherein the test circuit comprises a plurality of buffers connected to the node of the measurement circuit for providing coarse and fine control of the voltage applied to the measurement circuit.
15. The device according to claim 13 or 14, wherein the test circuit comprises one more buffers connected to a second node of the measurement circuit and is configured to vary the voltage applied to the measurement circuit by changing the state of each buffer connected to the second node.
16. The device according to any preceding claim, wherein the test circuit comprises a known impedance connected to one of the first and second electrodes, thereby forming a potential divider with the liquid.
17. The device according to claim 16, wherein the test circuit is configured to measure the electrical signal and/or determine the measurement indicative of the TDS of the liquid based on the known impedance.
18. The device according to claim 16 or claim 17, wherein the test circuit comprises a second known impedance connected to the other electrode, thereby forming a second potential divider with the liquid.
19. The device according to claim 18, wherein the test circuit is configured to measure the electrical signal and/or determine the measurement indicative of the TDS of the liquid based on the second known impedance.
20. The device according to any preceding claim, wherein the test circuit is configured to measure the response of one of the first and second electrodes to a known voltage applied to the measurement circuit.
21. The device according to any preceding claim, wherein the test circuit is configured to measure the current flow in a portion of the device and/or a voltage at a node of the device so as to measure the electrical signal and/or determine the measurement indicative of the TDS.
-29 - 22. The device according to any preceding claim, wherein the electrical signal is a measure of one or more of: the impedance of the liquid; the resistance of the liquid; the current through the liquid; the voltage across the liquid; and/or the conductivity of the liquid.
23. The device according to any preceding claim, wherein the measurement indicative of the TDS of the liquid is a conductivity of the liquid.
24. The device according to any preceding claim, wherein the liquid is water.
25. The device according to any preceding claim, wherein the device comprises a test cell.
26. The device according to any preceding claim, wherein the device comprises a battery.
27. The device according to any preceding claim, wherein the test circuit comprises a processor, a microprocessor, a controller, or an integrated circuit.
28. A method comprising receiving, at an external system, data from the device according to any preceding claim and determining a health status of a filter based on the received data, preferably wherein the health status is a remaining lifetime.
29. A method for determining an output indicative of the total dissolved solids, TDS, of a liquid, the method comprising: applying a voltage to a measurement circuit comprising first and second electrodes in contact with the liquid; measuring an electrical signal from the electrodes; varying the voltage applied to the measurement circuit based on a property of the measured electrical signal; and determining the output indicative of the TDS of the liquid when the property of the measured electrical signal satisfies a threshold condition.