EP4649295A1

EP4649295A1 - Leak detection

Info

Publication number: EP4649295A1
Application number: EP24701255.2A
Authority: EP
Inventors: Stefan Dzisiewski-Smith
Original assignee: Laiier Ltd
Current assignee: Laiier Ltd
Priority date: 2023-01-09
Filing date: 2024-01-09
Publication date: 2025-11-19
Also published as: GB2626040A; CN120752504A; WO2024149982A1; GB2626040B

Abstract

There is described a method for detecting presence of a liquid, the method comprising: receiving a plurality of signals, each signal being indicative a corresponding impedance across a respective sensing circuit of a plurality of sensing circuits, each sensing circuit being configured such that the corresponding impedance across the sensing circuit decreases when the sensing circuit is exposed to a liquid; determining, for each of the plurality of sensing circuits, whether the impedance across the sensing circuit is below a threshold impedance; and, in response to determining that the impedances for each of a set of the plurality sensing circuits are below the threshold impedance, outputting an indication of presence of the liquid.

Description

Leak Detection

TECHNICAL FIELD

[0001] The present application relates to systems and methods for detecting a liquid, such as for leak detection.

BACKGROUND TO THE INVENTION

[0002] A variety of leak sensors operate by detecting a short circuit when any portion of a sensor element becomes wet. For example, conductive rope sensors comprise an elongate rope containing conductive elements. When a portion of the rope becomes wet, a short-circuit is created, completing an electric circuit until the rope dries out.

[0003] However, the sensitivity of such sensors is often unsuitable for a desired situation, for example because small, anticipated amounts of water trigger false positive events in the sensors, or because they are not sufficiently sensitive to alert to a leak event early enough in the process to mitigate damage.

SUMMARY OF THE INVENTION

[0004] According to an aspect of the invention, there is provided a method for detecting a liquid, the method comprising: receiving a plurality of signals, each signal being indicative of a corresponding impedance across a respective sensing circuit of a plurality of sensing circuits; each sensing circuit being configured such that the corresponding impedance across the sensing circuit changes when the sensing circuit is exposed to a liquid; determining, for each of the plurality of sensing circuits, whether the impedance across the sensing circuit is within a predefined range of impedances (e.g. an impedance range); and in response to determining that the impedances for a set of the plurality of sensing circuits are each within the predefined range of impedances, outputting an indication of the liquid.

[0005] In use, the impedance across each sensing circuit may depend upon an amount of water or other liquid to which the sensing circuit is exposed (e.g. that is on the sensing circuit or that is near to the sensing circuit). For example, exposing the sensing circuit to a greater amount of water or other fluid may decrease the impedance across that sensing circuit. Therefore, the impedance across a sensing circuit being below a threshold impedance may indicate that water or another liquid is on that sensing circuit or the impedance across a sensing circuit being above a threshold impedance may indicate that that sensing circuit is dry. The impedance of the sensing circuit may be affected by the liquid conducting electricity across (short-circuiting) parts, such as tracks, of the sensing circuit, or by decreasing the capacitance between such parts of the sensing circuits, for example when the sensing circuit is proximal to.

[0006] Alternatively, another type of liquid (e.g. a non-conductive liquid) may result in a decrease in impedance. In this case, the presence of this liquid may be detected by an increase in impedance, or the impedance across a sensing circuit being above a threshold impedance. Depending on the particular use case, a liquid may be detected based on the impedance crossing a threshold impedance. In one embodiment this involves detecting whether the impedance is above the threshold impedance. In an alternative embodiment, this involves detecting whether the impedance is below the threshold impedance. Whether the impedances across sensing circuits are above or below thresholds may therefore be used to determine whether water is on or near to those sensing circuits.

[0007] In some embodiments, the predefined range of impedances may be a range of impedances below a threshold impedance (e.g. a range from the threshold impedance to zero impedance). In alternative embodiments, the predefined range of impedances may be a range of impedances above a threshold impedance (e.g. a range from the threshold impedance to infinite impedance).

[0008] Existing water sensors that output indications of leaks when the impedance decreases across only a single sensing circuit, such as conductive rope sensors, are prone to oversensitivity and false-positive leak indications. Determining a set of a plurality of sensing circuits that each have impedances below the threshold impedance allows the accuracy of the leak detection to be improved.

[0009] In some embodiments, an indication of a liquid may be an indication of presence of the liquid output in response to determining that the set comprises at least a threshold number of sensing circuits. In such embodiments, the method may comprise, in response to the set comprising less than the threshold number of sensing circuits, outputting an indication of an absence of the liquid.

[0010] The higher the threshold number, the lower the sensitivity of the sensor and the less likely false positive detections are to be made (the more robust the system is to the effect of false positives). Avoiding such false positives in this manner may advantageously prevent time, money, equipment or consumable devices being used in unnecessarily responding to less serious leaks, and may avoid fatigue or a loss of confidence in a leak detector, thereby improving staff readiness and response time to actual serious leak events.

[0011] Each of the plurality of sensing circuits may comprise a pair of conductive tracks separated by a corresponding gap. The conductive tracks may be on a substrate. In use, the gap between the conductive tracks may be partially or entirely spanned by water or another conductive fluid, thereby reducing the impedance between the two tracks and/or across the circuit, or entirely short circuiting the two tracks and significantly decreasing the impedance across the circuit. Each of the tracks of a sensing circuit may comprise an electrode for connecting to a leak detector. The impedance across a sensing circuit comprising a pair of electrodes may be measured between the electrodes comprised by the pair of tracks.

[0012] In some embodiments, the method may be performed by a liquid sensor, such as a leak sensor, or by a processor thereof. The sensor or a system comprising the sensor may comprise the plurality of sensing circuits. Each of the sensing circuits may be connected to the same device or sensor. The method may be computer-implemented.

[0013] The two electrodes of each sensing circuit may be connected to different voltages, for example, by one or more of biasing circuitry, a generalised impedance, a resistor, a current source, a current sink, and a voltage divider, any of which may form part of the sensor. In some embodiments, one of the two conductive tracks of each sensing circuit may be connected to ground voltage either directly, or via one or more of biasing circuity, a generalised impedance, a resistor, a current source, a current sink, and a voltage divider. The other of the two conductive tracks may be connected to a voltage source or voltage line via one or more of biasing circuitry, a generalised impedance, a resistor, a current source, a current sink, and a voltage divider. In some embodiments, each of the electrodes may be connected to its corresponding voltage by an equal impedance, substantially equal impedance, or an impedance of the same magnitude (or said electrodes may be controlled to do so when the impedance between them and another electrode is being measured). The impedance of each electrode (and consequently of each track connected thereto) to its respective voltage being more similar when impedances are measured between the electrodes may result in the common-mode noise experienced by the two tracks being similar in magnitude and phase, reducing the impact in noise on the measurement.

[0014] In some embodiments, one or more conductive tracks and/or electrodes connected thereto may be shared between a plurality of the sensing circuits. For example, in some embodiments, each sensing circuit may comprise one respective track and/or electrode that only forms part of that sensing circuit, and a common conductive track and/or electrode that is shared between one or more of other sensing circuits, for example between all of the other sensing circuits. The common conductive track and/or electrode may be connected to ground either directly or via one or more of biasing circuitry, a generalised impedance, a resistor, a current source, a current sink, and a voltage divider as described above. A common conductive track and/or electrode may experience common-mode noise broadly similarly in phase and magnitude to the individual tracks and/or electrodes. Consequently measurements of impedances or voltage differences between the common track and/or electrode and the individual tracks and/or electrodes may reduce noise within the measurements.

[0015] The tracks forming part of each sensing circuit may be substantially parallel to each other, or may comprise a plurality of segments parallel to corresponding segments of the other track(s). The tracks of each sensing circuit may comprise a plurality of interdigitated segments. Such an arrangement may enable the sensing circuit to cover a comparatively large area. The separation between the two tracks of each sensing circuit may be substantially constant along the lengths of the tracks and/or to the separations between the tracks of the other sensing circuits.

[0016] The impedance between the two conductive tracks of a sensing circuit may be determined by measuring the voltages of each of the tracks.

[0017] The plurality of sensing circuits may include three or more sensing circuits, four or more sensing circuits, may include eight or more sensing circuits, or may include twelve or more sensing circuits.

[0018] In some embodiments, the plurality of circuits may be located at - and/or may extend over - different portions of the substrate. This may enable detection of where a leak has occurred in an area spanned by the substrate. In some embodiments, the substrate on which the conductive tracks of the plurality of sensing circuits are located may be an elongate strip. The plurality of sensing circuits may be located at different points - and/or may span different lengths - along the length of the elongate strip. This may allow a leak to be located along a length of the strip.

[0019] The substrate may be a substantially planar, allowing it to be placed on surfaces and/or within narrow gaps between structures or systems, and to reduce the volume of space occupied by the system. The substrate may be formed of a water impermeable material, such as a plastics material. For example, the substrate may be a polyethylene terephthalate (PET) substrate. The substrate may be formed of a flexible material, in order to enable it to confirm to surfaces in use.

[0020] In some embodiments, each of the conductive tracks is printed on the substrate, for example, using a conductive ink. The conductive ink may be an ink that is robust (e.g. chemically inert, physically stable, insoluble, etc.) in (e.g. in contact with) a specific target liquid or set of target liquids in order to aid detection of these particular targets. For instance, the target liquid may be water or an aqueous solution. For example, the conductive ink may be a carbon-based ink. Unlike metal-based inks, carbon-based inks do not form ions when in contact with water, which could otherwise cause the ink to react and possibly creep, potentially causing false positive or false negative results. Each sensing circuit may span a different portion of the substrate. The substrate may be an elongate strip and each sensing circuit may span a different portion of a length of the substrate.

[0021] In some embodiments, each of the conductive tracks is connected to a corresponding voltage by a circuit having a corresponding impedance, wherein the corresponding impedances are substantially equal to each other.

[0022] In some further embodiments, each of the conductive tracks is connected to a corresponding voltage by a variable impedance circuit having a variable impedance, and wherein the method further comprises for each conductive track determining a corresponding impedance for the conductive track and configuring the corresponding variable impedance circuit to have the determined impedance. The impedance for a given conductive track may be determined from configuration data stored in memory of the liquid sensor. The impedance for each conductive track may be determined by determining a type of each sensing circuit and, for each sensing circuit, determining an impedance for each conductive track based on configuration data storing an association between the impedance and the conductive tracks for each type of sensing circuit. The type of sensing circuit may be determined through sensing electrical characteristics of the sensing circuit.

[0023] In some embodiments, the substrate may comprise adhesive elements, such as an adhesive surface or backing, which may be used to position the substrate in a desired position. For example, a substrate may be adhered to a fluid-carrying pipe such that it extends along the length of the pipe, in order to detect any leaks from said pipe.

[0024] The substrate may be configured to releasably connect to a sensor performing the method, and/or electrodes comprised by such a sensor. This may allow the substrate and the sensing circuits to be replaced, for example if they become damaged, or to provide a different arrangement or number of sensing circuits to the sensor.

[0025] In some embodiments, the method may further comprise detecting when one or more of the sensing circuits have been compromised by damage, for example, as a consequence of a crack or tear in a substrate as described above. Such damage may be detected when the impedance across a sensing circuit exceeds a breakage threshold, the breakage threshold being higher than an impedance threshold as described above and may correspond to a significant increase in impedance when the circuit is physically broken. Alternatively, or additionally, damage may be detected when the impedance across a breakage circuit (which may comprise an uninterrupted conductive loop track extending across or around a substrate, such as around a perimeter thereof) decreases or falls below a breakage circuit threshold impedance. A substrate may comprise one or more of such breakage circuits. Upon detecting damage, an indication of such damage may be output.

[0026] The method comprises determining whether the impedance across each of the sensing circuits is within a predetermined range of impedances (such as below a threshold impedance) and in response to determining that the impedances for a set of the plurality sensing circuits are each within the predetermined range, outputting an indication of the liquid.

[0027] As described above, water or another liquid on or near a sensing circuit may decrease the impedance across that sensing circuit, therefore the impedance across a sensing circuit being below or above the threshold impedance may indicate that more or less than a threshold amount of water is on or near that sensing circuit respectively. A dryness or wetness of the sensing circuits may therefore be determined and the indication may be output based on such a determination.

[0028] The method may therefore be used to detect the undesired presence of liquids, for example, in order to detect leaks, when the impedances for a set of sensing circuits are below a threshold impedance. However, in some situations, if only relatively small amounts of water condense, fall or splash onto the sensing circuit - such as onto a sensing circuit defined by tracks on a substrate as described above - a leak detection may not be desired. Therefore, the indication may be an indication of the presence of the liquid which may only be output if a set of the sensing circuits across each of which the impedances are determined to be below the threshold impedance meets certain criteria, for example if the set comprises at least a threshold number of sensing circuits that have impedances across them below the impedance threshold (e.g. wherein the threshold number is two or more). In such embodiments, determining whether the impedance across each of the sensing circuits is within a predetermined range may comprise determining, for each of the sensing circuits, whether the impedance across the sensing circuit is below a threshold impedance.

[0029] Alternatively, or additionally, an indication or the absence of the liquid may be output, in response to a set of the sensing circuits across each of which the impedances are determined to be above a threshold impedance (which may be the same as, or different to, a threshold impedance used for an indication of the presence of a liquid as described above) meets certain criteria, for example if such a set comprises at least a threshold number of sensing circuits (which also may be the same as, or different to, a threshold number used for an indication of the presence of a liquid as described above). In such embodiments, these threshold impedances and/or numbers of sensing circuits may comprise any of the features outlined below with reference to the threshold impedances and/or numbers used for an indication of the presence of the liquid. [0030] The threshold number of sensing circuits and/or the threshold impedance may be varied and/or selected, for example, depending on the situation in which the method and/or sensor is employed. For example, a sensor positioned on a surface beneath a hot water heater, which would be expected to be dry with no condensation or other source of false positives may be configured with a lower threshold number of circuits than a sensor positioned near an exterior door, where condensation or splashing may be expected outside.

[0031] In some embodiments, the threshold number is adjustable based on user input.

[0032] In some embodiments, the threshold number of sensing circuits may be one, or may be selectable from any number between one and the number of sensing circuits connected to or comprised by the sensor. In other embodiments, the threshold number of sensing may only be, or may only be selectable as or variable between numbers two or greater.

[0033] In some embodiments, the threshold number may depend upon previous detections, previous output indications, and/or a state of the system. The threshold number may decrease after an indication of the presence of the liquid is output and/or after the set of sensing circuits is determined to comprise at least the threshold number of sensing circuits. After outputting such an indication and/or making such a determination, a “wet” state may be transitioned into, within which a lower threshold number may be used. This may ensure that a sufficient number of sensing circuits have dried before water is determined to no longer be present, and/or may prevent small fluctuations in the number of wet circuits, such as noise fluctuations, causing rapid transitions into and out of the wet state. Similarly, after an indication of the absence of the liquid is output and/or after the set of sensing circuits is determined to comprise less than the threshold number of sensing circuits, the threshold number may increase, for example, when entering a “dry” state. Alternatively, the threshold number may increase after an indication of the presence of the liquid is output, after the set of sensing circuits is determined to comprise at least the threshold number of sensing circuits, and/or within a wet state, and/or the threshold number may decrease within a dry state. That is, each time the number of sensing circuits in the set crosses a boundary between two ranges (e.g. the threshold number), the boundary may be moved in the opposite direction (e.g. increased when the number of sensing circuits drops below the threshold number or decreased when the number of sensing circuits becomes greater than or equal to the threshold number). This avoids the system repeatedly switching between ranges when the number of sensing circuits in the set is oscillating around the boundary.

[0034] The impedance threshold to be compared to the impedance across each of the sensing circuits may be the same. Alternatively, the impedance threshold to be compared to the impedance across each sensing circuit may depend on the identity of that sensing circuit. In such embodiments, the different impedance thresholds may be used for different sensing circuits.

[0035] In some embodiments, when determining whether the impedance across each of the plurality of sensing circuits is below or above a threshold impedance, the threshold impedance for all of the sensing circuits or for individual sensing circuits may be derived from current and/or historical values of the impedance across one, some or all of the sensing circuits, for example the impedance threshold of an individual sensing circuit may be derived from current and/or historical values of the impedance across that sensing circuit and/or across adjacent sensing circuits. Such an impedance value may be derived to be outside a range of fluctuations in the impedances across said sensing circuits, for example, as a result of noise. In some embodiments, for each of the plurality of sensing circuits, the predefined range of impedances against which the impedance across that sensing circuit is compared is a range of impedances below a corresponding threshold impedance, and the corresponding threshold impedance is increased after determining that the impedance across that sensing circuit is below the corresponding threshold impedance. In some embodiments, for each of the plurality of sensing circuits, the predefined range of impedances against which the impedance across that sensing circuit is compared is a range of impedances greater than or equal to a corresponding threshold impedance, and the corresponding threshold impedance is decreased after determining that the impedance across that sensing circuit is greater than or equal to the corresponding threshold impedance. That is, each time an impedance for a given sensing circuit crosses a boundary between two ranges of impedances, the boundary may be moved in the opposite direction (e.g. increased when the impedance drops below the threshold impedance or decreased when the impedance becomes greater than or equal to the threshold impedance). This avoids the system repeatedly switching between ranges when the impedance is oscillating around the boundary.

[0036] In some embodiments, the threshold impedance may depend upon previous detections, previous output indications, and/or a state of the system. The threshold impedance may decrease after an indication of the presence of the liquid is output and/or after the set of sensing circuits is determined to comprise at least the threshold number of sensing circuits. After outputting such an indication and/or making such a determination, a “wet” state may be transitioned into, within which a lower impedance threshold may be used. This may ensure that the sensing circuits have dried significantly before water is determined to no longer be present, and/or may prevent small fluctuations in impedances, such as noise fluctuations, causing rapid transitions into and out of the wet state. Similarly, after an indication of the absence of the liquid is output and/or after the set of sensing circuits is determined to comprise less than the threshold number of sensing circuits, the threshold impedance may increase, for example, when entering a “dry” state. Alternatively, the threshold impedance may increase after an indication of the presence of the liquid is output, after the set of sensing circuits is determined to comprise at least the threshold number of sensing circuits, and/or within a wet state, and/or the threshold impedance may decrease within a dry state.

[0037] In some embodiments, the indication of the liquid threshold is an indication of presence of the liquid output in response to determining that the set comprises at least a threshold number of adjacent sensing circuits for which the impedances are below the threshold impedance. In such embodiments, the method may comprise in response to the set comprising less than the threshold number of adjacent sensing circuits, outputting an indication of an absence of the liquid.

[0038] The threshold number of adjacent circuits may comprise any of the optional features of a threshold number of circuits described herein.

[0039] The method may comprise detecting the number of sensing circuits across which the impedance is below the threshold impedance (i.e. the number of sensing circuits within the set). In such embodiments, the indication of the liquid, and/or another indication which may be output, may indicate a severity of the leak and/or the number of sensing circuits across which the impedance is below the threshold impedance. Such an indication may specify the exact number of below-threshold-impedance sensing circuits and/or may correspond to a range of numbers of below-threshold-impedance sensing circuits. Such indications may advantageously allow the severity of a leak to be determined by a user or device receiving the indication. For example, understanding the severity of a leak may enable a facilities manager to determine whether a leak requires immediate attention or is not urgent, allowing them to allocate their limited resources in an efficient manner.

[0040] In some embodiments, whether the number of sensing circuits within the set is within a plurality of different ranges may be evaluated, and in response to determining that the number of sensing circuits within the set is within one of the ranges, an indication may be output that indicates which of the ranges the number of sensing circuits in the set is within. The indicated range may provide a user or system receiving the indication information on the amount of liquid to which the sensing circuits are exposed, and by extension the severity of a leak detected by the sensor. A lowest of these ranges, or the number being below any of the ranges, may indicate that the sensing circuit is not exposed to any liquid. The boundaries of these ranges may comprise any of the optional features of a threshold number of circuits described herein.

[0041] In some embodiments, the indication of the liquid is an indication of presence of the liquid output in response to determining that the number of sensing circuits in the set for which the impedances are below the threshold impedance is within one of a plurality of ranges, and the indication of presence of the liquid indicates the range that the number of sensing circuits in the set is within. In such embodiments, the method may comprise in response to the set comprising less than a lowest bound of a lowest of the plurality of ranges, outputting an indication of an absence of the liquid. The plurality of ranges may comprise three or more ranges.

[0042] In some embodiments, a plurality of states may be transitioned between, each corresponding to a different number of the sensing circuits across which the impedance is below the threshold impedance (i.e. to a different number of sensing circuits within the set). The number of states may be one greater than the number of sensing circuits, for example, such that each possible number of below-impedance-threshold sensing circuits has a corresponding state, including a state with no below-impedance-threshold sensing circuits. Alternatively, individual states may correspond to ranges of numbers of below-impedance- threshold sensing circuits. In some embodiments, an indication may be output when transitioning between states.

[0043] When within a given state, the number of sensing circuits across which the impedance is below the impedance threshold may be evaluated. After evaluating the number of below- impedance-threshold circuits, whether the number of below-impedance-threshold sensing circuits corresponds to a different state may be determined and if so, that state may be transitioned to. In other embodiments, after evaluating the number of below-impedance- threshold circuits, whether the number of below-impedance-threshold sensing circuits is above a value for transitioning to a sequentially more severe state (a state corresponding to a greater number of below-impedance-threshold sensing circuits) may be determined and/or whether the number of number of below-impedance-threshold sensing circuits is below a value for transitioning to a sequentially less severe state (a state corresponding to a smaller number of below-impedance-threshold sensing circuits) may be determined. In some such embodiments, the value for transitioning from a first less severe state to a second more severe state may be lower than the value for transitioning from the second more severe state to the first less severe state.

[0044] In some embodiments, as described above, the impedance threshold and/or the numbers of sensing circuits for transitioning between states may depend upon which state the system is currently in. In response to determining that that the number of sensing circuits in a set is within one of the plurality of ranges, that one of the plurality of ranges may be broadened, for example by decreasing the threshold number of sensing circuits at a boundary between that range and a range corresponding to a fewer number of sensing circuits. Alternatively, the threshold number may be increased. Alternatively or additionally, the threshold number of sensing circuits at a boundary between that range and a range corresponding to a greater number of sensing circuits may be increased or decreased.

[0045] In some embodiments, the impedance across each of the plurality of sensing circuits may be compared to the threshold impedance, and/or the number of below-impedance- threshold sensing circuits may be determined, periodically, for example once every minute. Evaluating the impedances and/or the number of below-impedance-threshold circuits less frequently may reduce the power consumption of the sensor. In some embodiments, the number of below-impedance-threshold sensing circuits may be re-evaluated immediately or shortly after transitioning from one state to another, and after a set time period after evaluating the number below-impedance-threshold sensing circuits and determining that no state transition is required.

[0046] In some embodiments, in addition to detecting the number of sensing circuits across which the impedance is below the threshold impedance (i.e. the number of sensing circuits within the set), the method may comprise detecting which of the plurality of sensing circuits have impedances across them exceeding the threshold impedance (i.e. the identities of the sensing circuits within the set). The identification of the liquid may indicate the identity or location of one or more sensing circuits in the set.

[0047] In some embodiments, the threshold number of sensing circuits, ranges of numbers of sensing circuits, and/or values for transitioning between states as described above, may be, or may be capable of being set as, a number of adjacent and/or continuous sensing circuits, such as three or more adjacent and/or continuous sensing circuits. This may prevent multiple sensing circuits at different points across a substrate experiencing splashing or condensation simultaneously from triggering an indication of the presence of liquid while allowing a leak on multiple adjacent sensing circuits to trigger such an indication.

[0048] In some embodiments, the method may comprise, for each of a plurality of pre-set combinations of sensing circuits, determining for each sensing circuit in the pre-set combination, whether the impedance across the sensing circuit is below the threshold impedance, wherein the indication of the presence of liquid, and/or another output indication indicates the identity and/or location of each pre-set combination for which the impedance across each sensing circuit in the pre-set combination is below the threshold impedance and/or whether specific combinations of sensing circuits have impedances below the threshold impedance.

[0049] Some embodiments may comprise evaluating whether all of the sensing circuits in one or more pre-set combinations have impedances across them below the impedance threshold. If the impedances across all the sensing circuits in a given pre-set combination are below the impedance threshold, an indication of the identity of that combination, such as index value, may be added to a field. After all the combinations have been evaluated, an indication including or derived from the field may be output. Some or all of the pre-set combinations may comprise a plurality of sensing circuits. Some or all of the pre-set combinations may comprise only a single sensing circuit.

[0050] In some embodiments, indications as described above may only be output if the impedances across the sensing circuits meet the requirement for doing so for more than a threshold period of time (and/or more than a threshold number of repeated measurements). For example, a sensor may only output the indication of the presence of liquid if at least the threshold number of sensing circuits have impedances across them below the impedance threshold for more than the threshold period of time. This may avoid indications being output based on only short-term changes in impedance.

[0051] Each output indication, such as the indication of the presence of the liquid, may be or may comprise an electronic signal, such as a wireless electronic signal. The wireless electronic signal may be transmitted according to a long range wide area network (LoRaWAN), Bluetooth, Wi-Fi, Global System for Mobile Communications (GSM), Long-Term Evolution (LTE), and/or Narrow Band Internet-of-Things (NB-loT) radio technology. The electronic signal indication may be sent to a separate system, such as a controller or central monitoring system, for example, a controller for a building in which the leak sensor is located. Alternatively, or additionally, the electronic signal indication may be transmitted to a cloud-computing system or platform.

[0052] Alternatively, or additionally, each indication may be or may comprise a signal to a user, such as a visual indication, which may be output by a display and/or one or more other light emitting components, such as LEDs which may be comprised by a leak sensor performing method. Alternatively, or additionally, an indication may comprise and/or be an audible signal, which may be output by a loudspeaker, which may be comprised by a leak sensor performing the method.

[0053] In some embodiments, an output indication, such as the indication of the presence of the liquid, may be output continuously or repeatedly until the conditions that triggered its output is are no longer satisfied, for a set period of time, until a situation triggering an update in the indication (such as an increase in a number of below-impedance-threshold sensing circuits that is specified in the indication), and/or until a cancellation or reset instruction is received.

[0054] For example, after an indication is triggered to be transmitted, a sensor may enter a triggered state in which the indication is output continuously or intermittently, until the sensor is reset. Continuing to output the indication, even if the conditions that triggered it are no longer satisfied may ensure that a user or controller is notified that a leak has occurred, even if the leak subsequently dries.

[0055] It will be appreciated that for each of the embodiments described above in which an indication of the presence of a liquid is output in response to some criteria being met by a set of the plurality of sensing circuits for which the impedances are below a threshold impedance, in another embodiment, an indication of the absence of a liquid may be output in response to the same criteria being met by a set of the plurality of sensing circuits for which the impedances are above a threshold impedance.

[0056] According to a second aspect of the invention there is provided a sensor configured to: receive a plurality of signals, each signal being indicative of a corresponding impedance across a respective sensing circuit of a plurality of sensing circuits, each sensing circuit being configured such that the corresponding impedance across the sensing circuit changes when the sensing circuit is exposed to a liquid; for each of the plurality of sensing circuits, comparing the impedance across the sensing circuit to a threshold impedance; and in response to determining that the impedances for a set of the plurality of sensing circuits are each below or each above the threshold impedance, output an indication of presence of the liquid.

[0057] The leak sensor may be configured to perform any of the optional feature of the method described above, and/or may comprise any of the optional features of a sensor described above. For example, the leak sensor may comprise the one or more sensing circuits.

[0058] The leak sensor may comprise a processor, which may be configured to perform the steps described herein; a memory, which may store computer instructions which when executed by the processor cause it to perform the steps described herein; a housing; one or more electrodes for connecting to sensing circuits or tracks thereof as described above; a power source such as a battery; a communication means, such as a wireless interface, for outputting indications as described herein; and/or other indicators for outputting an indication as described herein to a user, such as a display, a loudspeaker, one or more lights, or other suitable means.

[0059] According to a third aspect of the invention, there is provided one or more storage media storing computer executable instructions which when executed by a processor to cause a method according to the first aspect described above to be performed. The storage media may be non-transitory. [0060] Embodiments of the invention will be understood and appreciated more fully from the following detailed description, made by way of example only and taken in conjunction with the following figures.

BRIEF DESCRIPTION OF THE FIGURES

[0061] Fig. 1 is a diagram of a first example of a leak detection system;

[0062] Fig. 2 shows a second example of a leak detection system;

[0063] Fig. 3a shows the sensing circuit substrate of the leak detection system of Fig. 2;

[0064] Fig. 3b shows an alternative sensing circuit substrate;

[0065] Fig. 4 is a system diagram of a leak detection system;

[0066] Fig. 5 is a flowchart of a method of operating a processor of a leak detection system;

[0067] Fig. 6 is a flowchart of a categorisation algorithm in which the leak sensor is categorised into a state based on the number of its sensing circuits that have an impedance across them below the impedance threshold; and

[0068] Fig. 7 is a flowchart of a categorisation algorithm in which the leak sensor is categorised into a state based on the locations at which leaks occur on the sensing circuits.

DETAILED DESCRIPTION OF THE FIGURES

[0069] Referring to the figures generally, there are shown embodiments of leak detection systems comprising a plurality of sensing circuits and a processor configured to output an indication of the presence of a liquid based on impedances across the sensing circuits. Such leak detection systems may advantageously provide means for detecting leaks with adjustable sensitivity, may classify different types or severities of leaks, and/or may enable the location of a leak to be determined.

[0070] Fig. 1 shows a diagram of an example of a leak detection system 100 comprising a leak sensor 110 and a plurality of sensing circuits 120. The leak sensor 110 is configured to measure an impedance across each of the plurality of sensing circuits 120 and to output an indication through an output 150 in response to determining a set of the sensing circuits 120 have impedances across them below a threshold impedance.

[0071] The illustrated example system 100 comprises three sensing circuits 120a, 120b, 120c. However it will be appreciated that systems 100 may comprise other numbers of sensing circuits 120. The sensing circuits 120 are separated in space so as to detect the presence of water or other conductive fluids in different locations covered by the leak detection system 100.

[0072] Each of the sensing circuits 120 comprises two electrically-conductive tracks 130, 140 at different voltages separated by a gap which provides an impedance between them. In the example system 100, the two conductive tracks are a positive track 130 and a ground track 140.

[0073] Each of the ground tracks 140 is connected to a ground voltage source 142 provided by the leak sensor 110 via a respective switch 148, a respective impedance 144 (e.g. a circuit having a particular impedance, such as a resistor) and a respective ground electrode 146. The ground electrode 146 of each positive track electrically connects that track to the leak sensor 100. The positive tracks 130 are all connected to a positive voltage source 132 provided by the leak sensor 110 via a shared impedance 134 and a shared positive electrode 132. The shared ground electrode connects all of the positive tracks 130 to the leak sensor 100. The impedance across each sensing circuit 120 may be measured between the electrodes 136, 146 where its tracks 130, 140 are connected to the leak sensor 100, after its ground track 140 is connected to the ground voltage source 142 by its respective switch 148 and allowed to settle for a dwell period. While the measurement is being taken, the other ground tracks 140 are disconnected from the ground voltage source 142 by their respective switches 148. This ensures that the voltage measured is only over the selected electrodes.

[0074] As shown in Fig. 1 the two tracks 130, 140 of each sensing circuit may comprise a plurality of interdigitated fingers, allowing the sensing circuit to cover a wider area. The interdigitated fingers may have substantially equal separation to provide consistent sensitivity across the sensing circuit.

[0075] As a consequence of this arrangement, the positive tracks 130 and ground tracks 140 of each sensing circuit 120 are biased at different voltages, with an impedance between them provided by the gap. When water or another conductive fluid falls onto sensing circuit 120, it partially or entirely spans the gap, thereby reducing the impedance between the two tracks 130, 140. Therefore, when the impedance across that sensing circuit 120 falls below the impedance threshold, the leak sensor 110 may determine that water or another conductive material is present on that sensing circuit 120.

[0076] The impedance 144 between each ground track 140 and the ground voltage source 142 may be equal to, substantially equal to, or the same order of magnitude as, the impedance between the positive track 130 and the positive voltage source 130, or may be configured or controlled to be so when the impedance between the across that sensing circuit is being measured and/or when its corresponding switch 134, 144 is closed. The impedance of each track 130, 140 to their respective source 132, 142 being more similar during the measurement may result in the common-mode noise experienced by the two tracks 130, 140 being similar in magnitude and phase, reducing the impact in noise on the measurement.

[0077] In some examples, the impedances 134, 144 between the tracks 130, 140 and their corresponding voltage sources 132, 142 may be provided by resistors. These resistors may have fixed impedances, which may be sufficiently large to substantially dominate the impedances between 130, 140 and their corresponding voltage sources 132, 142 (with the impedances of other components between said tracks and sources being negligible in comparison). In alternative embodiments, the impedances 134, 144 may be variable, for example, electronically such as through a potentiometer (e.g. a digital potentiometer).

[0078] The impedances 134, 144 between the tracks 130, 140 and their corresponding voltage sources 132, 142 may be set by component selection during the design and/or construction of the sensor 110. Alternatively, these impedances 134, 144 may be set at boot time by the sensor 110. The sensor 110 may identify a sensing circuit 120 attached to it or a configuration thereof and may set the impedances in response thereto. For example, the sensor 110 may be configured to read an impedance configuration corresponding to the sensing circuit 120 from memory (e.g. a table stored in firmware) and to then configure a variable impedance circuit (e.g. a digital potentiometer) to implement that particular impedance between the tracks 130, 140 and their corresponding voltage sources 132, 142.

[0079] When specific numbers or combinations of the sensing circuits 120 have threshold impedances across them below the impedance threshold, the leak sensor 110 may determine that the impedance drops are not the result of minor and/or isolated splashing, condensation, or other environmental conditions, but is instead the consequence of a more severe leak. When such a detection occurs, the leak sensor 110 outputs an indication 150 of the presence of such liquid has been detected.

[0080] It will be appreciated that in alternative leak sensor systems 100, the two tracks of some or all of the sensing circuits may be connected to two different voltages rather than to a positive voltage and to ground; that the impedance(s) between one of the voltages (for example, the positive voltage 132) provided by the leak detector 110 and the respective electrode(s) may be omitted; and/or that both sets of tracks 130, 140 may be connected via separate respective impedances and/or electrodes, rather than a shared impedance 134 and electrode 136 as shown above; amongst other suitable modifications.

[0081] Fig. 2 shows a detailed view example leak sensor system 105 comprising all of the features of the system 100 described above with reference to Fig. 1 . The sensor system 105 comprises twelve sensing circuits 120 but only the first two 120a, 120b are shown in their entirety.

[0082] The leak sensor 110 comprises a housing 112 (e.g. a waterproof housing) that contains a power source (e.g. a battery), a processor, a memory storing computer instructions that when executed by the processor, cause it to perform steps as described herein, a LoRaWAN transceiver for outputting an indication 150; and the electrodes 136, 146 for connecting to the tracks 130, 140 of the sensing circuits 120.

[0083] The sensing circuits 120 are provided on an elongate flexible substrate 125 (e.g. a polyethylene terephthalate (PET) substrate) and are defined by tracks 130, 140, 145 printed thereon in conductive ink. The tracks 130, 140, 145 are printed in multiple layers to enable tracks to cross over each other without electrically connecting to each other. The electrodes 136, 146 at which each track 130, 140, 145 connects to the leak sensor are provided on a tab at an end of the substrate 125, which extends into the interior of the housing 112 of the leak sensor 110 in use.

[0084] Each of the ground tracks 140, 145 comprises a first surface portion 140 that defines a plurality of interdigitated fingers with a corresponding portion of the positive track 130 on a limited portion of the length of the substrate, and a second below-surface portion 145 that extends from the electrode 146 for that ground track to the first surface portion. The below- surface portion 145 of each track extends below the surface portion 140 of each conductive track covering an area closer to the electrodes than its own track.

[0085] The substrate 125 comprises a stack of layers consisting of a base layer, a below- surface conductive layer comprising the second below-surface portions 145 of the tracks on the base layer, one or more insulating dielectric layers on the first conductive layer to insulate the second below-surface portions 145 from the first surface portions 140 where they cross, and a final surface conductive carbon layer comprising the first surface portions 140 of the tracks. The substrate may comprise multiple insulating dielectric layers between the two conductive layers, in order to minimise the possibility of pinholes through the dielectric layers aligning and providing an unwanted electrical connection between the first and second portions 140, 145 of the tracks. Portions of the below-surface conductive layer where the first second below-surface portions 145 are to meet the first surface portions 140 of the surface conductive layer are not covered by the dielectric layers in order to allow the surface layer to be printed directly onto the below-surface portions 145 of the tracks. That is, insulated layers are provided to allow certain tracks to cross without short-circuiting the system. The sensing areas (the interdigitated fingers) are exposed so that no insulating layers may cover the tracks over the sensing areas. [0086] The portion of the substrate 125 shown in Fig. 2 shows the first and second portions 140a, 145a of a first ground track of a first sensing circuit 120a, the first and second portions 140b, 145b of a second ground track of a second sensing circuit 120b, as well as some of the second portions of third through twelfth ground tracks, including the second portion 145c of a third ground track for a third sensing circuit.

[0087] Fig. 3a shows an overall view of the substrate 125 and sensing circuits 120 as used in the system 105 of Fig. 2. The below-surface portions 145 of the tracks are shown more faintly than the surface portions 140.

[0088] Fig. 3b shows an overall view of an alternate arrangement of sensing circuits 120 for covering a broader rectangular area instead of an elongate strip. The alternative arrangement comprises a generally rectangular flexible substrate 125 and conductive tracks 130, 140 printed thereon. A single positive track 130 extends around a perimeter of the substrate and comprises a plurality of parallel finger portions extending inwards towards a central axis of the substrate 125 perpendicular to the plurality of parallel fingers. Eleven ground tracks 140 comprise portions extending along the central axis of the substrate 125 and portions comprising parallel finger portions interdigitated with those of the positive track 130.

[0089] Fig. 4 is a diagram of a leak detection system comprising a leak detector 110 and plurality of sensing circuits 120. The leak detector 110 comprises a processor 160 and a computer memory 170 storing computer executable instructions. The processor 160 is in communication with the memory 170 and is configured to receive signals from the sensing circuits 120 and to output an indication 150. The memory 170 stores computer instructions that when executed by the processor cause it to perform a method as described herein.

[0090] Fig. 5 is a flowchart of an example of a method of operating a processor 160 of a leak sensor 110, in which the leak sensor periodically performs a categorisation algorithm, such as an algorithm as shown in Fig. 6 or Fig. 7 in order to determine a state of the leak sensor 110 and the environment in which it is deployed. In the method, the leak sensor initially categorises the leak sensor as being in a “normal” state before waiting for a set period of time. After the set period of time has elapsed, the leak sensor performs an analogue to digital conversion (ADC) scan, measuring the impedance across each of the sensing circuits. In performing the scan, the leak sensor may measure the voltage difference between the two electrodes 136, 146 of the sensing circuit, from which the impedance between these electrodes 136, 146 may be derived. After measuring the impedances across each of the sensing circuits, the state of the leak sensor and its environment is categorised using a categorisation algorithm, such as a categorisation algorithm as described below with reference to Figs. 6 and 7. After categorising the state of the leak sensor and its environment the sensor sleeps for a period of time before performing a further ADC scan. The sensor may periodically perform a self-test sequence in which it may check whether its battery voltage is low, if its sensing circuits 120 are disconnected or damaged, if the state indicates a critically severe leak, and/or the state of its internal circuitry for internal failures. If any of these checks identify a fault, a message 150 may be output by the sensor indicating them.

[0091] Fig. 6 shows an example of a categorisation algorithm in which the leak sensor is categorised into a state based on the number of its sensing circuits that have an impedance across them below an impedance threshold. A plurality of states are defined, each comprising one or more possible numbers of below-impedance-threshold sensing circuits. The illustrated example categorisation algorithm uses four states, state A, state B, state C and state D, each corresponding to a corresponding number of below-impedance-threshold sensing circuits. The maximum number states is one plus the number of sensing circuits, so as to allow one state to exist for zero below-impedance-threshold sensing circuits.

[0092] Each state (with the exception of the final state) has a corresponding threshold number of below-impedance-threshold sensor circuits for moving to the next state with more below- impedance-threshold sensor circuits. The threshold number of below-impedance-threshold sensor circuits for moving from state A to state B is defined as T(A^B). Each state has a higher threshold than the state before it such that T(A^B) < T(B^C) < T(C^D).

[0093] In an initial step of the categorisation algorithm, the current state of the leak sensor is identified, from which the thresholds for moving to any adjacent states are determined. In a first evaluation, the leak sensor determines whether the number of below-impedance- threshold sensor circuits is greater than or equal to the threshold for moving the next state, and if so moves to that state and outputs a message indicating this change of state before ending the categorisation algorithm.

[0094] In the example shown in Fig. 6, the initial state is determined to be stage B, such that in the first evaluation, whether the number of below-impedance-threshold (“Num wet electrodes”) is greater than or equal to the threshold number for moving from state B to state C (T(B^C)) is determined,

[0095] If the number of below-impedance-threshold sensor circuits is not greater than or equal to the threshold for moving the next state, in a second evaluation, the leak sensor determines whether the number of below-impedance-threshold sensor circuits is less than the threshold for moving to the current state, and if so moves to the previous state and outputs a message indicating this change of state before ending the categorisation algorithm. [0096] In the example shown in Fig. 6, if the number of below-impedance-threshold is not equal to or greater than T(B^C), in the second evaluation, whether the number of below- impedance-threshold (“Num wet electrodes”) is less than the threshold number for moving from state A to state B (T(A^B)) is determined.

[0097] In alternative embodiments, in the second evaluation, the number of below- impedance-threshold sensor circuits may be compared to a specific threshold for moving to the previous state that differs from the threshold to moving from the previous state. For example, the number of below-impedance-threshold circuits may be compared to a threshold T (B^ A) instead of (T(A^B)) as shown in Fig. 5. This may include hysteresis into the system to bias the system against moving to less severe states, or may prevent rapid changes in the state of the system in response to a small flections in the number of sensing circuits with impedances close to the impedance threshold.

[0098] Alternatively, or additionally, the threshold impedance may depend upon the state of the system. For example, in states corresponding to more wet sensing circuits, the threshold impedance may be raised.

[0099] In other embodiments, after the moving to a new state and/or after sensing a message indicative of moving to the new state, the original step may be returned to and the evaluations may be repeated for the new state, instead of exiting the evaluation. This may allow the state to change by multiple steps in a single evaluation without waiting for a pre-set period to elapse, allowing the system to be responsive to rapid changes in the number of wet sensing circuits. Alternatively, or in addition, the system may be configured to move up or down more than one step at a time. That is, whilst FIG. 6 shows a method that determines whether the number of below-impedance-threshold is within one of two ranges (move up one state or move down one state), more ranges may be provided, each range relating to a certain state. Accordingly, the method may determine if the below-impedance-threshold falls within one of a predefined number of ranges, and set the state accordingly.

[0100] If neither of the first and second evaluations result in a change of state, the leak sensor determines whether the time since it sent a most recent message is greater than a pre-set interval (such as a regular message interval, RMI, as shown in the figure) and if so, output a message confirming the current state of the leak sensor. Whether or not such a message is output, the leak sensor then ends the categorisation algorithm.

[0101] If the current state of the leak sensor determined in the initial step of the categorisation algorithm, is the highest state (state D in the illustrated example), the first evaluation may be skipped, and if it is the lowest state (state A in the illustrated example), the penultimate evaluation may be skipped. [0102] The different states may correspond to different event types, such as the absence of a leak, and a plurality of different severities of leaks. Classifying leaks differently based on severity may enable resources to be more efficiently allocated when responding to messages indicating that leaks have occurred.

[0103] Fig. 7 shows an example of categorisation algorithm in which the leak sensor is categorised into a state based on the locations at which leaks occur on the sensing circuits.

[0104] A plurality of combinations of sensing circuits are defined, each comprising one or more of the sensing circuits. The illustrated example categorisation algorithm uses four combinations, combination A, combination B, combination C and combination D. The maximum number of combinations is 2ⁿ, where n is the number sensing circuits (including the combination with no sensing circuit).

[0105] Each combination of sensing circuits corresponds to a specific event, such as a leak covering a specific area spanned by the sensing circuits of that combination. For example, a combination consisting of a single sensing circuit in a specific area may correspond to the location of that sensing circuit on a substrate being wet. In the illustrated example, combination A corresponds to event A, combination B corresponds to event B, et cetera.

[0106] The algorithm comprises evaluating whether all of the sensing circuits in each combination have impedances across them below the impedance threshold. If the impedances across all the sensing circuits in given combination are below the impedance threshold the event corresponding to that combination, or an identifier thereof, is added to the current state. The combinations are evaluated in turn and after all the combinations have been evaluated, the leak sensor determines whether any events have been added to the current state, or whether the current state is empty. If any events, or identifiers thereof, have been added to the current state, a message identifying the events is output by the leak sensor before ending the categorisation algorithm.

[0107] If no events have been added to the current state and it is empty, the leak sensor determines whether the time since it sent a most recent message is greater than a pre-set interval and if so, output a message confirming the current state of the leak sensor. Such a message may allow a user of a control system to determine that the leak sensor is operating correctly and is not detecting any leaks. Whether or not such a message is output, the leak sensor then ends the categorisation algorithm.

[0108] After a categorisation algorithm is run as described above, the leak sensor may remain inactive for the set period of time, before re-measuring the impedances across each of the sensing circuit and repeating the categorisation algorithm. [0109] It will be appreciated that the conjunction and/or as used herein means one, some, or all, of the options it connects. For example, an subject comprising features A, B, and/or C may comprise feature A, feature B, feature C, features A and B, features A and C, features B and C, or features all A, B, and C.

[0110] While certain embodiments have been described, they have been presented by way of example only, and are not intended to limit the scope of protection. The inventive concepts described herein may be implemented in a variety of other forms. In addition, various omissions, substitutions and changes to the specific implementations described herein may be made without departing from the scope of protection defined in the following claims.

Claims

1 . A method for detecting a liquid, the method comprising: receiving a plurality of signals, each signal being indicative of a corresponding impedance across a respective sensing circuit of a plurality of sensing circuits, each sensing circuit being configured such that the corresponding impedance across the sensing circuit changes when the sensing circuit is exposed to a liquid; determining, for each of the plurality of sensing circuits, whether the impedance across the sensing circuit is within a predefined range of impedances; and in response to determining that the impedances for a set of the plurality sensing circuits are each within the predefined range of impedances, outputting an indication of the liquid.

2. A method according to claim 1 , wherein the predefined range is a range of impedances below a threshold impedance.

3. A method according to claim 1 or claim 2, wherein the indication of the liquid is an indication of presence of the liquid output in response to determining that the set comprises at least a threshold number of sensing circuits.

4. A method according to claim 3, comprising, in response to the set comprising less than the threshold number of sensing circuits, outputting an indication of an absence of the liquid.

5. A method according to claim 3 or claim 4, wherein the threshold number is adjustable based on a user input.

6. A method according to any preceding claim, wherein the indication of the liquid is an indication of presence of the liquid output in response to determining that the set comprises at least a threshold number of adjacent sensing circuits.

7. A method according to claim 6 comprising, in response to the set comprising less than the threshold number of adjacent sensing circuits, outputting an indication of an absence of the liquid.

8. A method according to any of claims 3 to 7 wherein the threshold number decreases in response to determining that the set comprises at least a threshold number of sensing circuits or of adjacent sensing circuits.

9. A method according to any of claims 3 to 8 when dependent upon claim 2 wherein the threshold impedance decreases in response to determining that the set comprises at least a threshold number of sensing circuits or of adjacent sensing circuits.

10. A method according to any preceding claim, wherein the indication of the liquid is an indication of presence of the liquid output in response to determining that the number of sensing circuits in the set is within one of a plurality of ranges, and wherein the indication of presence of the liquid indicates the range that the number of sensing circuits in the set is within.

11. A method according to claim 10, comprising, in response to the set comprising less than a lowest bound of a lowest of the plurality of ranges, outputting an indication of an absence of the liquid.

12. A method according to claim 10 or claim 11 , wherein the plurality of ranges comprises three or more ranges.

13. A method according to any of claims 10 to 12, wherein the one of the plurality of ranges is broadened in response to determining that the number of sensing circuits in the set is within the one of the plurality of ranges.

14. A method according to any preceding claim, wherein the indication of the liquid indicates the identity or location of one or more sensing circuits in the set.

15. A method according to any preceding claim, wherein the method comprises for each of a plurality of pre-set combinations of sensing circuits, determining, for each sensing circuit in the pre-set combination, whether the impedance across the sensing circuit is within the predefined range; and wherein the indication of the liquid indicates the identity or location of each pre-set combination for which the impedance across each sensing circuit in the pre-set combination is within the predefined range.

16. A method according to any preceding claim wherein, for each of the plurality of sensing circuits: the predefined range of impedances against which the impedance across that sensing circuit is compared is a range of impedances below a corresponding threshold impedance, and the corresponding threshold impedance is increased after determining that the impedance across that sensing circuit is below the corresponding threshold impedance; or the predefined range of impedances against which the impedance across that sensing circuit is compared is a range of impedances greater than or equal to a corresponding threshold impedance, and the corresponding threshold impedance is decreased after determining that the impedance across that sensing circuit is greater than or equal to the corresponding threshold impedance.

17. A method according to any preceding claim wherein each of the plurality of sensing circuits comprises a pair of conductive tracks separated by a corresponding gap.

18. A method according to claim 17 wherein each of the conductive tracks is printed on a substrate in conductive ink, and wherein each sensing circuit spans a different portion of the substrate.

19. A method according to claim 18 wherein the conductive ink is insoluble in the liquid.

20. A method according to claim 19 wherein the conductive ink is carbon based ink and the liquid is water or an aqueous solution.

21. A method according to any of claims 18-20 wherein the substrate is an elongate strip and wherein each sensing circuit spans a different portion of a length of the substrate.

22. A method according to any of claims 17 to 21 , wherein each of the conductive tracks is connected to a corresponding voltage by a circuit having a corresponding impedance, wherein the corresponding impedances are substantially equal to each other.

23. A method according to any of claims 17 to 22, wherein each of the conductive tracks is connected to a corresponding voltage by a variable impedance circuit having a variable impedance, and wherein the method further comprises for each conductive track determining a corresponding impedance for the conductive track and configuring the corresponding variable impedance circuit to have the determined impedance.

24. A method according to any of claims 17 to 23, wherein one of the conductive tracks is shared between multiple sensing circuits of the plurality of sensing circuits.

25. A method according to any preceding claim wherein whether the impedance across each of the plurality of sensing circuits is within the predefined range is determined periodically.

26. A liquid sensor comprising a processor configured to perform a method according to any preceding claim.

27. Storage media comprising computer instructions executable by one or more processors, the computer instructions when executed by one or more processors, causing the one or more processors to perform a method according to any one of claims 1 to 25.