FLUID SENSOR

Abstract

A fluid sensor (15) is disclosed. The sensor comprises a frame (21), first and second electrodes (271, 272) supported by the frame and separated by space occupiable by a fluid, and a third electrode (273) lying in a path (29) on the frame between the first and second electrodes.

Claims

1. A fluid sensor, comprising: a frame; first and second electrodes supported by the frame and separated by space occupiable by a fluid; and third electrode lying in a path on the frame between the first and second electrodes.

2. (canceled)

3. The fluid sensor of claim 1, wherein the frame comprises a slot to define a gap between first and second members, wherein the first and second electrodes are disposed on the first and second members respectively.

4. The fluid sensor of claim 1, wherein the frame comprises a main portion and first and second members extending from the main portion such that there is a gap between the first and second members, wherein the first and second electrodes are disposed on the first and second members respectively.

5. The fluid sensor of claim 3, wherein the third electrode is disposed at least in part of the first or second member.

6. (canceled)

7. The fluid sensor of claim 4, wherein the third electrode is disposed in the main portion.

8. The fluid sensor of claim 1, further comprising: a fourth electrode lying in the path.

9. The fluid sensor of claim 8, wherein the fourth electrode is disposed at least in part of the first or second member.

10-15. (canceled)

16. The fluid sensor of claim 1, wherein the frame is generally U-shaped.

17. The fluid sensor of claim 1, further comprising: a coating of dielectric material; wherein the frame and first and second electrodes and the at least third electrode are encapsulated by the coating.

18. (canceled)

19. (canceled)

20. A sensor system comprising: the fluid sensor of claim 1; and a measurement system configured to measure an impedance, Z, between the first and second electrodes, and to measure a voltage difference, V, between the third electrode and another electrode.

21. The sensor system of claim 20, wherein the other electrode is a fourth electrode which is different from the first and second electrodes.

22. The sensor system of claim 20, wherein the other electrode is one of the first and second electrodes.

23. (canceled)

24. The sensor system of claim 20, wherein the measurement system comprises: a signal source arranged to apply an excitation signal to at least one electrode; an impedance measurement circuit arranged to measure the impedance, Z, between the first and second electrodes; and a voltage difference measurement circuit arranged to measure the voltage difference, V, between the third electrode and the other electrode.

25. The sensor system of claim 24, wherein the impedance measurement circuit comprises a transimpedance circuit.

26. (canceled)

27. (canceled)

28. A pump control system comprising: at least one sensor system of claim 20; and a pump controller for controlling operation of a pump, wherein the at least one sensor system is arranged to provide signal(s) to the pump controller.

29. (canceled)

30. A float switch comprising the pump control system of claim 28.

31. A pump system comprising: at least one sensor system of claim 20; and a pump controller for controlling operation of a pump, wherein the at least one sensor system is arranged to provide signal(s) to the pump controller; and a pump arranged to be controlled by the pump controller.

32. (canceled)

33. A fluid analyser comprising: at least one sensor system of claim 20; and a user interface configured to output signals in dependence upon the at least one sensor system.

34. A method of signal processing, the method comprising: receiving a first signal indicative of electrical impedance between first and second electrodes; receiving a second signal indicative of voltage difference between a third electrode and another electrode; and determining, in dependence upon the first and second signals, whether a sensor including the first, second, and third electrodes and the other electrode is immersed in a fluid.

35-37. (canceled)

38. A method of determining presence of a fluid in a volume, the method comprising: measuring the electrical impedance of a medium in the volume; measuring a voltage difference between two electrodes; comparing the electrical impedance and voltage difference with defined thresholds; and determining presence of a fluid by using the results of said comparison.

39. (canceled)

40. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Certain embodiments of the present invention will now be described, by way of example, with reference to FIGS. 4 to 10 of the accompanying drawings, in which:

[0038] FIG. 1 illustrates immersing a simple capacitive sensor into a fluid;

[0039] FIG. 2 illustrates removing a capacitive sensor from a fluid;

[0040] FIG. 3 shows change in capacitance over time for a simple capacitive sensor after the sensor has removed from a solution of soap and water;

[0041] FIG. 4 is a schematic diagram of a space in which a fluid may gather or be stored, a fluid level sensor and instrumentation;

[0042] FIG. 5 is a schematic diagram of a first fluid sensor which includes a set of electrodes;

[0043] FIG. 6 is a schematic diagram of a second level sensor which includes a set of electrodes;

[0044] FIG. 7 is a schematic diagram of a third level sensor which includes a set of electrodes;

[0045] FIG. 8 is an equivalent circuit for electrodes shown in FIG. 5 and circuits used to provide excitation signal to the electrodes and process measurement signals;

[0046] FIG. 9 is an equivalent circuit for electrodes shown in FIGS. 7 and 8 and circuits used to provide excitation signal to the electrodes and process measurement signals;

[0047] FIG. 10 is an example of a first circuit for converting a current received from an electrode into a voltage;

[0048] FIG. 11 is an example of a second circuit for converting a current received from an electrode into a voltage;

[0049] FIG. 12 is an example of a circuit for amplifying a voltage difference between a pair of electrodes;

[0050] FIG. 13 is schematic diagram of a look-up table storing parameter threshold values;

[0051] FIG. 14 shows plots of real and imaginary components of current and real and imaginary components of voltage for a sensor when immersed and not immersed in water;

[0052] FIG. 15 shows plots of real and imaginary components of current and real and imaginary components of voltage for a sensor when immersed and not immersed in water and in soapy water;

[0053] FIG. 16 is a side elevation of a first sensor;

[0054] FIG. 17 is a schematic cross-section taken through a first prong of the first sensor shown in FIG. 16 along the line A-A;

[0055] FIG. 18 is a side elevation of a second sensor having an on-board controller;

[0056] FIG. 19 is schematic block diagram of a fluid level sensing system;

[0057] FIG. 20 is a process flow diagram of a method of sensing a fluid level;

[0058] FIG. 21 schematically illustrates float switch which includes a fluid sensor;

[0059] FIG. 22 schematically illustrates a water pump with an integrated fluid sensor;

[0060] FIG. 23 schematically illustrates a fluid analyser; and

[0061] FIG. 24 is an orthogonal view of a fluid sensor with annular electrodes.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

[0062] Referring to FIG. 1, a simple capacitive sensor 1 is able to detect the presence of a fluid 2 by measuring capacitance between a pair of electrodes 3. The electrodes 3 are positioned such that the electric field generated by a voltage difference between two electrodes 3 extends sufficiently into a space 4 between the electrodes 3 which is expected to be filled by the fluid 2.

[0063] When the sensor 1 (which is initially in air) is immersed in a fluid 2, the change in capacitance and/or resistance between the two electrodes 3 is detected as a change in an electrical signal, for example, current, frequency, and/or voltage.

[0064] Referring to FIG. 2, when the sensor 1 is removed from the fluid 2, it would normally be expected that the signal would quickly return to the value it had in air before the sensor 1 was immersed in the liquid. This might not happen, however, if the fluid 2 forms a layer 5 on the surface of the sensor 1 that persists after the sensor 1 is extracted from the fluid 2.

[0065] Referring to FIG. 3, a plot 6 of capacitance against time is shown.

[0066] FIG. 3 illustrates a change in capacitance over time after the simple capacitive sensor 1 is extracted from a fluid 2 in the form of a solution of soap and water. The sensor 1 is extracted from the fluid 2 at t=0. The plot 6 shows that the capacitive signal takes approximately 10 minutes to return to its original value in air, which is about 1 pF.

[0067] Examples of capacitive fluid sensors are herein described which can determine whether a capacitive signal is attributable to a layer of fluid on the surface of the sensor, rather than a volume of fluid. The sensors can be used to sense the presence of a fluid, without the need for moving parts, and can be used in a fluid pump. The sensors disclosed herein may take the form of a low-voltage (e.g., 12 V) sensor which is fully sealed against water ingress. The sensors herein described may be connected to a separate, remote pump control unit via a wired or wireless link. The sensors herein described can provide a more robust and reliable fluid presence sensor, and be one which is electrically safer.

[0068] Referring to FIG. 4, a space 11 in which a fluid 12 may be present, for instance, gather, be stored or pass through, is shown.

[0069] The space 11 may take the form of a part of a building such as a cellar or the bottom of a shaft (for instance, an elevator shaft), a storage structure such as a tank, cistern or pond, a collector such as a sump (which may be in a building or vehicle), a conveyance structure such as a pipe or channel, or a vehicle such as boat. The fluid 12 may take the form of water, and may include a contaminant, such as oil or waste material (such as human waste).

[0070] When fluid 12 is present in the space 11 (for example, due to flooding), if the surface 13 of the fluid 12 rises above a given level 14 it can be sensed by a sensor 15 and measurement system 16 (which may be integrated into a single unit). The measurement system 16 may provide data 17 for example, measurements or level information, via a wired or wireless link 18 to a control system 19 which can be used to control pump 20 arranged to pump fluid 12 from the space 11.

[0071] The sensor 15 may be able to distinguish between different types of fluid 12.

[0072] Referring also to FIG. 5, the sensor 15 comprises a generally inverted U-shaped frame 21, for example in the form of printed circuit board, or other substrate which may or may not be planar, comprising a main portion 22 (or body or trunk) and two or more members 23.sub.1, 23.sub.2 (herein also referred to as prongs, legs, or sides) extending away from the body 22 towards respective distal ends 24.sub.1, 24.sub.2. In this case, the members 23.sub.1, 23.sub.2 are elongated, in other words, elongate members. In other cases, the members 23.sub.1, 23.sub.2 may be defined by a slot, aperture, slit or cut-out. In this case, the prongs 23.sub.1, 23.sub.2 extend, in parallel, from one side 25 of the body, in the same direction. The prongs 23.sub.1, 23.sub.2 are arranged side-by-side (in other words, transversely spaced apart) so as to define a gap 26 therebetween. Typically, the sensor 15 is arranged to be oriented so that the prongs 23.sub.1, 23.sub.2 depend from the body 22 and, thus, point downwards and so detect fluid 12 rising upwards.

[0073] The sensor 15 includes at least three electrodes. For example, in a first example, the sensor 15, 15.sub.1 has four electrodes including first, second, third and fourth electrodes 27.sub.1, 27.sub.2, 27.sub.3, 27.sub.4 disposed on the prongs 23.sub.1, 23.sub.2. Each electrode 27.sub.1, 27.sub.2, 27.sub.3, 27.sub.4 generally takes the form of a rectangular pad which crosses the majority of the width of the prong 23.sub.1, 23.sub.2.

[0074] The first electrode 27.sub.1 is disposed proximate to the distal end 24.sub.1 of the first prong 23.sub.1, and the second electrode 27.sub.2 is disposed proximate to the distal end 24.sub.2 of the second prong 23.sub.2. The first and second electrodes 27.sub.1, 27.sub.2 lie on a first line 28.sub.1 running transversely across the first and second prongs 23.sub.1, 23.sub.2. Herein, the first and second electrodes 27.sub.1, 27.sub.2 are also referred to as the lower electrodes.

[0075] The third and fourth electrodes 27.sub.3, 27.sub.4 are disposed proximate to the body 22. Thus, the third electrode 27.sub.3 is interposed between the body 22 and the first electrode 27.sub.1, and the fourth electrode 27.sub.4 is interposed between the body 22 and the second electrode 27.sub.2. The third and fourth electrodes 27.sub.3, 27.sub.4 lie on a second line 28.sub.2 which is generally parallel to the first line 28.sub.1 and which is longitudinally offset (in this orientation upwardly) from the first straight line 28.sub.1. Herein, the third and fourth electrodes 27.sub.3, 27.sub.4 are also referred to as the upper electrodes.

[0076] The third and fourth electrodes 27.sub.3, 27.sub.4 lie on a path 29 running between the first and second electrodes 27.sub.1, 27.sub.2 through a bridge provided by the body 22. The path 29 is generally has an inverted U-shape and generally corresponds to the outline of the substrate 21 above the first and second electrodes 27.sub.31, 27.sub.2. A centre line 30 of the path 29 is shown. At least a portion of third and fourth electrodes 27.sub.3, 27.sub.4 are disposed in the first and second prongs respectively 23.sub.1, 23.sub.2. In some examples, the third and fourth electrodes 27.sub.3, 27.sub.4 lie entirely within the length of the prongs 23.sub.1, 23.sub.2. In other examples, the third and fourth electrodes 27.sub.3, 27.sub.4 may beyond the upper end of the prongs 23.sub.1, 23.sub.2 and extend into the body 22.

[0077] As will be explained in more detail later, if a film of fluid 12 covers the surface of the sensor 15 covering a sufficient area of the path 29 so as to provide a low-impedance path between the first and second electrodes 27.sub.1, 27.sub.2, then one or more electrodes disposed on the path 29, such as the third and fourth electrodes 27.sub.3, 27.sub.4, can be used to detect the presence of the film of fluid 12.

[0078] Each prong 23.sub.1, 23.sub.2 has a width W, a length L and a thickness T. The width W may be between 0.5 cm to 1 cm, 1 cm to 2 cm, 2 cm to 4 cm, or 4 cm to 10 cm. The length L may be between 2 cm to 5 cm, 5 cm to 10 cm, 10 cm to 20 cm, or 20 cm to 50 cm. The thickness T may be between 0.5 mm to 1 mm, 1 mm to 2 mm, 2 mm to 4 mm, or 4 mm to 10 mm. Each pair of prongs 23.sub.1, 23.sub.2 is separated by a distance S which may be between 0.5 cm to 1 cm, 1 cm to 2 cm, 2 cm to 4 cm, or 4 cm to 10 cm. The prongs 23.sub.1, 23.sub.2 may be rectangular in transverse cross section, or may have another shape, such as circular (in other words, the prongs may be cylindrical).

[0079] The electrodes 27.sub.1, 27.sub.2, 27.sub.3, 27.sub.4 are covered by a dielectric layer (or electrically-insulating layer), such as rubber, plastic, varnish, or glass, having a thickness, for example, between 0.02 mm and 5 mm.

[0080] Referring to FIGS. 6 and 7, the sensor 15 need not have four electrodes 27.sub.1, 27.sub.2, 27.sub.3, 27.sub.4. For example, the sensor 15 may have only three electrodes 27.sub.1, 27.sub.2, 27.sub.3. The third electrode 27.sub.3 is disposed in the path 29 running between the first and second electrodes 27.sub.1, 27.sub.2 and preferably covers the majority of the width of the path 29.

[0081] Referring in particular to FIG. 6, a second sensor 15, 15.sub.2 is substantially the same as the first sensor 15, 15.sub.1 differing mainly in that it lacks the fourth electrode 27.sub.3.

[0082] Referring in particular to FIG. 7, a third sensor 15, 15.sub.3 is substantially similar to the as the second sensor 15, 15.sub.3 but a third electrodes 27.sub.3 is not disposed in a prong 23.sub.1, 23.sub.2, but in the main portion 22.

[0083] Referring to FIGS. 8 and 9, a measurement system 16 is used to apply excitation signals to the sensor 15 and measure response signals from the sensor 15.

[0084] The measurement system 16 includes a signal source 31, an impedance measurement circuit 32 and a voltage difference measurement circuit 33.

[0085] Referring to FIGS. 5, 6, 7, 8 and 9, the lower electrodes 27.sub.1, 27 may be used to measure an impedance Z.

[0086] One of the two lower electrodes 27.sub.1, 27.sub.2, for example, the second electrode 27.sub.2, may be connected to the signal source 31 in the form of a voltage source. The other electrode, for example, the first electrode 27.sub.2, may be connected to ground. The voltage source 31 may be an ac source generating an excitation signal 34 (or drive signal) having an amplitude V.sub.0, for example, between 1 mV and 100 mV, between 100 mV and 1 V, between 1V and 10 V, or larger than 10 V, and a frequency f, for example, between 100 Hz and 1 kHz, between 1 kHz and 10 kHz, or between 10 kHz and 1000 kHz). The signal 34 may take the form of a sinusoidal wave, a triangular wave, a square wave, or another waveform pattern. The source 31 may generate excitation signal 34 continuously or for a length of time T in pulses (or bursts or wave trains). The excitation signal 32 may consist of pulses having an amplitude and a duration.

[0087] The other of the two lower electrodes 27.sub.1, 27.sub.2 may be connected to a virtual voltage reference.

[0088] FIG. 10 shows a first transimpedance amplifier circuit 35, 351 which can serve as an impedance-measuring circuit 32. The first transimpedance amplifier circuit 35, 351 has a node or terminal 36 for receiving the excitation signal 34 which convert an input current received from the first electrode 27.sub.1 into a proportional output voltage at node or terminal 37.

[0089] Referring also to FIG. 10, the virtual voltage reference may be the inverting input 38 of a transimpedance amplifier 40 in the form of an op-amp. The transimpedance amplifier 40 may be configured to convert an input current to an output voltage V.sub.OUT with a gain, for example, between 0.01 V/A and 0.1 V/A, between 0.1 V/A and 1 V/A, between 1 V/A and 10 V/A. The output voltage can be provided to an input of an ADC 41.

[0090] In this example, the transimpedance amplifier circuit 35, 351 includes first and second resistors 42, 43 and a capacitor 44. The first and second resistors 42, 43 are arranged in series between the excitation signal input node or terminal 36 and one of the first and second electrodes 27.sub.1, 27.sub.2, in this case, the first electrode 27.sub.1. The inverting input 38 of the amplifier 40 is connected to a first node 45 (or tap) between the first and second resistors 42, 43, and the capacitor 44 is connected between the first node 45 and ground. The non-inverting input 39 of the amplifier 40 is connected to the first electrode 27.sub.1.

[0091] The first and second resistors 42, 43 and capacitor 44 may have values, for example, of 1 k, 1 M and 22 nF respectively, although the components 42, 43, 44 may have other values.

[0092] FIG. 11 shows another transimpedance amplifier circuit 35, 35.sub.2 which can serve as an impedance-measuring circuit 32. The transimpedance amplifier circuit 35, 35.sub.2.

[0093] In this example, the transimpedance amplifier circuit 35, 35.sub.2 includes third, fourth, and fifth resistors 51, 52, 53, and second and third capacitors 54, 55. The third and fourth resistors 51, 52 are arranged in series between the second electrode 27.sub.2 and second node 56 which is connected to the inverting input 57 of a transimpedance amplifier 59 in the form of an op-amp. The transimpedance amplifier 59 may be configured to convert an input current to an output voltage V.sub.OUT with a gain, for example, between 0.01 V/A and 0.1 V/A, between 0.1 V/A and 1 V/A, between 1 V/A and 10 V/A. The output voltage can be provided to an input of an ADC 41.

[0094] The second capacitor 54 is connected to a third node 60 between the third and fourth resistors 51, 52 and to ground. The fifth resistor 53 and the third capacitor 55 are connected in parallel between the second node 56 (and, thus, the inverting input 57) and the output 37.

[0095] A voltage reference signal Vref is provided via node or terminal 60 to the non-inverting input 58.

[0096] The voltage reference signal Vref is provided by a voltage dividing circuit 61 provided by sixth and seventh resistors 62, 63 arranged in series between a supply rail at VCC and ground in a totem pole configuration. The voltage reference signal Vref is taken by a tap 64 between the sixth and seventh resistors 62, 63. A fourth capacitor 65 is arranged in parallel with the seventh resistor 63 between the tap 64 and ground.

[0097] The third, fourth, and fifth resistors 51, 52, 53, and second and third capacitors 54, 55 may have values, for example, of 1 k, 1 k, 1 M, 1 nF and 4.7 pF respectively, although the components 51, 52, 53, 54, 55 may have other values. The sixth and seventh resistors 62, 63 and the fourth capacitor 66 may have values of 10 k, 10 k and 100 nF, respectively, although the components 62, 63, 65 may have other values.

[0098] Other transimpedance amplifier circuit topologies may be used.

[0099] In yet another example, the second electrode 27.sub.2 is connected to a voltage reference. The excitation signal 34 is routed to the first electrode 27.sub.1 through a circuit able to measure the current, such as a shunt resistor connected to a difference amplifier, or an instrumentation amplifier, or a differential input of an ADC.

[0100] Referring again to FIGS. 5, 6, 7, 8 and 9, the impedance measuring circuits 32 hereinbefore described allow measurement of complex impedance Z between said electrodes 27.sub.1, 27.sub.2. The complex impedance Z is a function of the insulating material coating the electrodes and prongs, and of the medium between the two prongs 23.sub.1, 23.sub.2.

[0101] A measurement of impedance Z allows determination of whether the space 26 between the electrodes 27.sub.1, 27.sub.2 is filled with air or another fluid having a different conductivity and/or resistivity, such as water or oil.

[0102] As explained earlier, a sensor having just two electrodes might measure a value of impedance Z close to that of water even if only a thin layer of water, dirt or contamination covers the surface of the device, and the space 26 between the prongs 23.sub.1, 23.sub.2 is mainly filled with air. This is undesirable particularly in applications in which the measured impedance Z is used to decide whether or not to turn on a pump and thus result in dry pumping.

[0103] The third and fourth electrodes 27.sub.3, 27.sub.4 are connected to a circuit 33 for measuring the voltage difference V between the third and fourth electrodes 27.sub.3, 27.sub.4. The voltage-difference measuring circuit 33 may take the form of a difference amplifier, an instrumentation amplifier, a differential input of an ADC.

[0104] Referring to FIG. 12, an example of a suitable voltage-difference measuring circuit 33 is shown.

[0105] The circuit 33 includes a differential amplifier 70 having inverting and non-inverting inputs 71, 72 and an output 73 connected to a node or terminal 74. The third and fourth electrodes 27.sub.3, 27.sub.4 or, in the case that only three electrodes are used, the third and second electrodes 27.sub.3, 27.sub.4, are connected to the inverting and non-inverting inputs 71, 72 via eighth and ninth resistors 75, 76 respectively. The circuit 33 includes a tenth resistor 77 and a fifth capacitor 78 arranged in parallel between the inverting input 71 and the output 73. The circuit 33 includes an eleventh resistor 79 and a sixth capacitor 80 arranged in parallel between the non-inverting input 71 and a supply rail held at half the supply voltage, that is, 0.5 VCC. The circuit amplifies a voltage difference V between third and fourth electrodes 27.sub.3, 27.sub.4 and presents an amplified signal at the node or terminal 74.

[0106] The eighth, ninth, tenth and eleventh resistors 75, 76, 77, 79 have values of, for example, 10 k, 10 k, 1 M, and 1 M, respectively, and the fifth and sixth capacitors 78, 80 have a value of 2.2 pF, although the components 75, 76, 77, 78, 79, 80 may have other values. Furthermore, other voltage different measuring circuit topologies may be used.

[0107] Referring again to FIGS. 4, 5, 6, 7, 8 and 9, when the sensor 15 is immersed in water or other fluid 12, the measured voltage difference V will take a first value V.sub.1.

[0108] When a film of fluid 12 covers the surface of the sensor 15, for example, after the sensor has been removed from the bath of fluid, the measured voltage difference V takes a second value V.sub.2. The first value V.sub.1 is usually smaller than the second value V.sub.1 and typically is between 0 and 0.1, between 0.1 and 0.4, or between 0.4 and 0.8 the first value, in other words 0V.sub.10.1V.sub.2, 0.1 V.sub.2V.sub.10.4V.sub.2, or 0.4V.sub.2V.sub.10.8 V.sub.2. Preferably, a ratio, R, of V.sub.1/V.sub.2 (that is R=V.sub.1/V.sub.2) is between 0 and 0.8, that is, 0R0.8. Thus, V.sub.1 may take a value between being practically immeasurable to a value which is a similar, but less than V.sub.2.

[0109] The measurement of impedance Z and the measurement of the voltage difference V is can be used to determine if the sensor 15 is fully or partially immersed in a fluid, or if the sensor is in air, or if the sensor is in air but a layer of a fluid or of a contaminant is present on the surface of the sensor.

[0110] Referring to FIG. 13, one or more threshold values 81 of a first parameter and one or more threshold values 82 of a second parameter which can be used to identify the state 83 of the sensor 15, for example, whether the sensor 15 is immersed or not, and/or an identifier 84 for the fluid in which the sensor is immersed. The threshold values 81, 82 may be stored in a look-up table 85.

[0111] The first parameter may be impedance or representative of impedance. The second parameter may be voltage difference or representative of difference.

[0112] Referring to FIG. 14, first and second plots 91, 92 are shown. The current and voltage values correspond to inverse impedance (in other words, 1/Z, or admittance, Y) and V measurements.

[0113] The first plot 91 contains real and imaginary values of current obtained using the first and second electrodes 27.sub.1, 27.sub.2 when the sensor 15 is immersed in water (shown by the symbol ) and when the sensor 15 is not immersed in water (shown by the symbol o). The second plot 92 contains real and imaginary values of voltage obtained using the third and fourth electrodes 27.sub.3, 27.sub.4 when the sensor 15 is immersed in water (shown by the symbol ) and when the sensor 15 is not immersed in water (shown by the symbol o).

[0114] In the first plot 91, there is a threshold value 93 of imaginary current (of around 40) which can be used divide the plot into two regions 94, 95, namely a first region 94 when the sensor 15 is immersed in water and a second region 95 when the sensor is not immersed in water. In the second plot, there is a threshold value 96 of imaginary voltage (of around-50) which can be used divide the plot into two regions 97, 98, namely a first region 97 when the sensor 15 is immersed in water and a second region 97 when the sensor is not immersed in water.

[0115] Referring to FIG. 15, third and fourth plots 101, 102 are shown.

[0116] The first plot 101 contains real and imaginary values of current obtained using the first and second electrodes 27.sub.1, 27.sub.2 when the sensor 15 is immersed in a solution of soap and water (shown by the symbol +), when the sensor 15 is not immersed in the soapy water (shown by the symbol ). The second plot 102 contains real and imaginary values of voltage obtained using the third and fourth electrodes 27.sub.3, 27.sub.4 when the sensor 15 is immersed in soapy water (shown by the symbol +) and when the sensor 15 is not immersed in soapy water (shown by the symbol ).

[0117] In the third plot 101, there is a threshold value 103 of imaginary current (of around 50) which corresponds to a threshold value of imaginary impedance and which can be used divide the plot into two regions 104, 105, namely a first region 104 (or upper region) when the sensor 15 is immersed in soapy water and a second region 105 (or lower region) when the sensor 15 is not immersed in soapy water.

[0118] In the fourth plot, there is a first threshold value 106 of imaginary voltage (of around-50) which corresponds to a threshold value of imaginary impedance and which can be used divide the plot into two regions 107, 108, namely a first region 107 (or upper region) when the sensor 15 is immersed in soapy water and a second region 108 (or lower region) when the sensor is not immersed in soapy water.

[0119] In the fourth plot, there is a second threshold value 109 (of around 1000) which can be used divide the plot into two regions 110, 111, namely a third region 110 which corresponds to a low value of V (and, thus, to high resistance) when the sensor 15 is immersed or has been immersed in water, and a fourth region 111 which corresponds to a high value of (and, thus, a low resistance) when the sensor has a residual layer of soapy water.

[0120] Generally. the higher the capacitance, C, the lower the imaginary impedance, the higher the imaginary current, Im(I) and vice versa. The higher the resistance, the higher the real impedance, the lower the real current Re(I) and vice versa.

[0121] The imaginary current Im(I) reflects the capacitance attributable to the fluid, in addition to the double-layer capacitance that already exists between the electrode and medium between the electrodes. If it is assumed that water is entirely resistive and oil/air is entirely capacitive, then:

[00001]$C_{\mathrm{total}}=\frac{1}{\frac{1}{C_{\mathrm{electrode}}}+\frac{1}{C_{\mathrm{oil}}}+\frac{1}{C_{\mathrm{electrode}}}}$

[0122] When oil is between the electrodes, the total capacitance is the sum of the reciprocal of the three capacitors. Thus, the total capacitance is lower (and the imaginary impedance is higher) and, therefore, imaginary current Im(I) is lower.

[0123] When water is between the electrodes, the total capacitance is only the sum of the reciprocal of the two electrodes. Thus, when the sensor is immersed in water, the capacitance is higher (and the imaginary impedance is lower), and therefore, the imaginary current Im(I) is higher.

[0124] The real current Re(I) provides an indication of the resistance between the electrodes.

[0125] Water has lower resistance than oil and so the real current Re(I) is higher for water.

[0126] In relation to real voltage Re(V) and imaginary voltage Im(V), when a conductive layer is present, both real and imaginary values will have higher values as they lie entirely on the current path. When the sensor is dry or immersed, the electrodes are electrically floating.

[0127] Real and imaginary values of voltages and currents can be measured with or without a surface layer of a fluid of different compositions and for when the sensor is and is not immersed. This can be used to define regions in the plots of Re(V) and Im(V), and Re(I) and Im(I) to build a map having different regions corresponding to different states of the sensor (i.e., fluid type, not immersed and dry, not immersed with fluid layer and immersed). The regions are divided by threshold values (of Re(V), Im(V), Re(I) and Im(I)) which can be used to classify the state of the sensor.

[0128] The look up table 85 (FIG. 13) may store values of real and imaginary values of current and voltage of the thresholds and, thus, be used to determine whether of not the sensors is immersed or not, if not immersed, whether it has a surface layer of fluid and, optionally, to identify the liquid.

[0129] Referring to FIG. 16, a first example of a four-electrode sensor 15, 15.sub.1,1 is shown. The sensor 15.sub.1 is shown without encapsulation (that is, without a dielectric coating) and any housing. The dielectric coating (not shown) may cover the substrate 21, electrodes 27.sub.1, 27.sub.2, 27.sub.3, 27.sub.4 and any on-board electronics 121 (FIG. 19) so as to make the sensor 15 fluid-tight and prevent the fluid from coming into direct contact with the electrodes 27.sub.1, 27.sub.2, 27.sub.3, 27.sub.4 and on-board electronic components.

[0130] The sensor 15 comprises a generally inverted U-shaped frame 21 comprising a printed circuit board 22 and two or more prongs 23.sub.1, 23.sub.2. The sensor 15 includes at least four electrodes including first, second, third and fourth electrodes 27.sub.1, 27.sub.2, 27.sub.3, 27.sub.4 disposed on the prongs 23.sub.1, 23.sub.2. Each electrode 27.sub.1, 27.sub.2, 27.sub.3, 27.sub.4 generally takes the form of a rectangular pad which crosses the majority of the width of the prong 23.sub.1, 23.sub.2.

[0131] The printed circuit board 22 supports conductive tracks 111 (or conductive traces), for instance tracks formed of copper or other metal, and on-board components 112 (such as resistors and capacitors) and a connector 113 for a cable (not shown).

[0132] Referring also to FIG. 17, the printed circuit board 22 may take the form of a multi-layered board comprising three or more board layers 114.sub.1, 114.sub.2, 114.sub.3.

[0133] The printed circuit board 22 supports electrodes 27.sub.3 on opposite faces 115.sub.1, 115.sub.2. For example, the third electrode 27.sub.3 can be provided on opposite faces 115.sub.1, 115.sub.2. Conductive tracks 116 to the lower electrodes 27.sub.1, 27.sub.3 may be embedded in the multi-layered board 22 and conductive regions 117 may be provided on the faces 115.sub.1, 115.sub.2 of the board 22 running over the embedded traces to provide screening. The conductive regions 117 are electrically grounded, that is, connected to ground.

[0134] Referring to FIG. 18, a second example of a four-electrode sensor 15, 15.sub.1,2 is shown. The sensor 15.sub.1,2 is shown without encapsulation (that is, without a dielectric coating) and any housing. The dielectric coating may cover the substrate 21, electrodes 27.sub.1, 27.sub.2, 27.sub.3, 27.sub.4 and any on-board electronics 121 (FIG. 19).

[0135] The sensor 15.sub.1,2 is similar to the first example of the four-electrode sensor 15.sub.1,1 except that is arranged to support a microcontroller 122 (FIG. 19) on the body 22 in region 123. The microcontroller 120 is not shown in FIG. 18.

[0136] Sensors 15 with fewer electrodes (for example, three electrodes) or with more electrodes may support a microcontroller 122 (FIG. 19) on the body 22.

[0137] Referring to FIG. 19, a sensor system 120 is shown.

[0138] The sensor system 120 includes the sensor 15 and optionally on-board circuits 121, that is, supported on the body 22, which may include, for example, the impedance measurement circuit 32 and/or voltage difference measurement circuit 33. The controller 122 is preferably supported by the sensor 15, in other words, also is included in on-board circuits 121 (FIG. 19) so as to make the sensor 15 fluid-tight.

[0139] The system 120 includes a controller 122, for example, in the form of a microcontroller, which includes an ADC 41, a processor 123, memory 124, non-volatile memory 125 storing code 126 for processing measurement signals and configuration data 127 for example including look-up table 85 (FIG. 13).

[0140] The controller 122 may serve as the excitation source 31 (FIGS. 8 and 19). In some examples, the controller 122 may serve as the measurement circuit 32 and/or voltage difference measurement circuit 33. The sensor system 120 may be connected to a control system 19 for controlling the pump. In some examples the controller 122 provides the control system 19. The controller 122 can be used to determine if the sensor 15 is fully or partially immersed in a fluid 12, or if the sensor is in air, or if the sensor 15 is in air but a layer of a fluid or of a contaminant is present on the surface of the sensor. The controller 122 can be used to determine the type of contaminant, for example, soap, oil, waste matter and the like.

[0141] The controller 122 can be used to control a switch, which can be used to operate a motor or a pump.

[0142] At least two sensors can be used to detect the presence of a fluid in two different positions, for instance at two different levels. The sensor outputs may be used to switch on or off a pump.

[0143] The measured impedance Z and voltage difference V can be used to determine if contaminants, such as oil(s), are present in the fluid 12. The measured impedance Z and voltage difference V can be used to determine the type of contamination.

[0144] The sensor 15 can be incorporated into a variety of different products.

[0145] Referring to FIG. 21, a float switch 200 having a piggyback switch configuration is shown.

[0146] The float switch 200 includes a piggyback plug 201 (or switch plug) having male and female terminals 202, 203. The male terminals 202 are insertable into a power socket (not shown) and the female terminals 203 may receive a male plug (not shown) of an appliance, such as pump.

[0147] The piggyback plug 201 is connected via a cable 204 to a float switch unit 205 which includes a sensor 15, measurement system 16 and control unit 19 in the form of a switch. The float switch unit 205 may be placed, for example, on the floor or in the bottom of a sump, vessel or tank, and be used to detect the presence or absence of fluid.

[0148] Referring to FIG. 22, a pump unit 300 comprising a pump 20, a sensor 15 and measurement and control unit 17, 19 is shown.

[0149] The pump 20, sensor 15 and measurement and control unit 17, 19 are integrated into a single unit. The pump unit 300 can take the form of a drainage pump.

[0150] The sensor 15 may be provided in a deployable float switch unit (not shown) attached to a main unit (not shown) via a cable or wire-carrying tether (not shown).

[0151] Referring to FIG. 23, a fluid analyser 400 is shown which comprises a sensor 15, measurement and control unit 17, 19 and a display 401.

[0152] The fluid analyser 400 may be used to indicate the presence of contamination in a fluid, such as oil or waste matter in water. For example, the fluid analyser 400 based on values in a look-up table 85 (FIG. 5) may identify whether the fluid 12 (FIG. 4) is one of a given number of fluids. For example, the fluids may include water, water and oil, and/or oil.

[0153] As explained earlier, the electrodes need not be planar, that is pad-like, but can take a variety of different shapes and configurations.

[0154] Referring to FIG. 24, a third example of a four-electrode sensor 15, 15.sub.1,3 is shown.

[0155] The sensor comprises a main body 22 and cylindrical prongs 23.sub.1, 23.sub.2 extending from the main body 22. Annular first, second, third and fourth electrodes 27.sub.1, 27.sub.2, 27.sub.3, 27.sub.4 are disposed on the prongs 23.sub.1, 23.sub.2. The third or fourth electrode 27.sub.3, 27.sub.4 may be omitted.

Modifications

[0156] It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of fluid sensors and component parts thereof and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.

[0157] The frame need not be thin, but can be formed in a block. The first and second electrodes may be arranged to face each other (as opposed to face outward in the same direction). The first and second electrodes may be arranged closer together, for example, on inwardly projecting jaws, so as to improve sensitivity.

[0158] There may be more than four electrodes.

[0159] Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.