WO2010015025A1

WO2010015025A1 - A sensor using a bifilar coil

Info

Publication number: WO2010015025A1
Application number: PCT/AU2009/000992
Authority: WO
Inventors: Sliva Evgueni; Michael Tonkin
Original assignee: University Of South Australia
Priority date: 2008-08-04
Filing date: 2009-08-04
Publication date: 2010-02-11
Also published as: AU2009279368A1

Abstract

The invention relates to a simple and reliable sensor for measuring the value of an electrical property of a medium such as the complex impedance, conductivity, di-electric constant, or ion concentration of a species of interest. The sensing element comprises a bifilar coil in which only one end of each wire is connected to a measurement circuit and the opposite ends are free or unterminated. When the circuit is excited or driven at a particular frequency, this arrangement is lossy and the output signal or response is sensitive to the di-electric properties of the medium or fluid surrounding the sensor. The sensor thus has broad application in situations where it is desirable to detect the amount of, and/or changes in, the ion concentration level of a medium. Applications comprise measuring the salinity and water level in evaporative coolers, and the concentration of sodium metabisulphite in tanks holding seafood.

Description

A SENSOR USING A BIFILAR COIL PRIORITY DOCUMENTS

The present application claims priority from:

Australian Provisional Patent Application No. 2008903972 entitled "A SENSING ARRANGEMENT USING A BIFILAR COIL" and filed on 4 August 2008. The entire content of this application is hereby incorporated by reference.

INCORPORATION BY REFERENCE

The following publication is referred to in the present application: US Patent No. US 5,374, 380 entitled "Salinity control of sump water using conductivity probes" granted on 20 December 1993 to James and assigned to F F Seely Nominees Pty Ltd. The entire content of this patent is hereby incorporated by reference.

FIELD OF THE INVENTION The present invention relates to sensors for measuring the electrical properties of a medium.

BACKGROUND OF THE INVENTION

There exist many environments and applications where knowledge of, or change in, the electrical properties of a medium are of interest. For example the electrical properties of a medium, such as the conductivity and di-electric constant may be affected by the concentration of a particular ion species in the medium, as well as other factors such as temperature. Thus measurements of the electrical properties of a medium, or the change in the electrical properties of the medium can be used to infer information regarding the concentration, or changes in concentration of various ion species which may (or may not) be present, and appropriate action can then be taken

For example, increased soil salinity stresses many plants, affecting their ability to grow, and thus knowledge of soil salinity can be used in determining whether salt tolerant plant varieties should be planted. In yet another example the preservative sodium metabisulphite is added to tanks containing prawns and seawater to prevent discolouration of the prawns. In this case taking measurements of sodium metabisulphite concentration can be used to determine if the tank requires dosing, or alternatively re-dosing if the concentration has dropped below a desired or acceptable level.

In another example, consider the case of monitoring of salinity levels in the sump tank of evaporative coolers. Evaporative coolers are very attractive for regions with dry climates because they are energy efficient and environmentally friendly. Conventional evaporative coolers rely on the principle that evaporation of water by a passing air stream lowers the temperature of the air stream. Conventional coolers thus consist of a fan which directs air flow through evaporative pads, which are supplied with water from a sump tank through the use of a pump, and directing this cooled air into an area to be cooled. Whilst passage of the air through the pads evaporates some of the water, any salt present is not evaporated. This salt in solution either flows back down to the sump tank, or if the salinity is above some threshold, it may deposit on the evaporative pads which may lead to permanent damage and thus reduced cooling efficiency. As operation of the cooler continues, the water level in the sump tank will lower, and the salinity level will increase.

Even if the water level is maintained by the addition of water, the salt remains and continues to build up in concentration. The presence of high salt levels has a corrosive effect and is generally detrimental to the efficiency and life of the cooler. To counter the effects of high salt levels, water in the sump tank is typically periodically bled off or the entire sump tank is dumped and replaced with fresh water.

Thus in the above example monitoring and control of the salinity level of water in the sump tank is a crucial consideration and many evaporative coolers have conductivity sensors to monitor the salinity of the sump tank water to determine the appropriate point at which to refresh the water in the sump tank. Further, evaporative coolers are cost critical devices, and thus most manufacturers of evaporative coolers aim to reduce the cost of manufacture whilst also reducing running and maintenance costs. Accordingly typical evaporative coolers utilise cheap contact conductivity sensors to make such measurements, such as those described in US Patent 5,374,380, the contents of which are hereby incorporated by reference. However these contact conductivity sensors are prone to salt deposition.

Other sensors have also been developed for measuring ion concentration. For example optical based sensors have been developed which utilise measurements of the refractive index. However a disadvantage of such sensors is that they typically require complex hardware and significant computational resources to process such measurements. Other electrically based sensors such as non contact inductive sensors may be used to measure the ionic or electrical properties of the medium. However existing systems typically require sophisticated signal processing algorithms, typically based on Fourier transforms, which tends to make such devices expensive and thus they are generally not suitable for use in cost critical applications such as evaporative coolers due to increased cost and complexity of the sensor circuit.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a sensor, for determining a value of an electrical property of a medium comprising: a sensing element comprising a bifilar coil; and an excitation circuit connected to the sensing element for exciting the sensing element at a particular frequency; a measurement circuit for measuring the response of the sensing element; wherein the bifilar coil comprises a first conductor with ends Pl and P3 and a second conductor with ends P2 and P4, and ends Pl and P4 are connected to the excitation circuit and ends P2 and P3 are unterminated, and the response is representative of the value of an electrical property of the medium.

According to a second aspect of the present invention, there is provided a method for determining the value of an electrical property of a medium comprising: exciting a bifilar coil placed in or near to the medium at a particular frequency using an excitation circuit, wherein the bifilar coil comprises a first conductor with ends Pl and P3 and a second conductor with ends P2 and P4, and ends Pl and P4 are connected to the excitation circuit and ends P2 and P3 are unterminated; measuring a response of a circuit comprising the bifilar coil; determining a value of an electrical property of the medium based at least in part on the measured response.

According to a third aspect of the present invention, there is provided a method for calibrating a sensor for determining a value of an electrical property of the medium, the sensor comprising an excitation circuit, a measurement circuit and a sensing element wherein the sensing element comprises a bifilar coil placed in or near to the medium and excited a particular frequency using the excitation circuit, and the bifilar coil comprises a first conductor with ends Pl and P3 and a second conductor with ends P2 and P4, and ends Pl and P4 are connected to the excitation circuit and ends P2 and P3 are unterminated; the method comprising exciting the bifilar coil placed in or near a medium having a known value of an electrical property of the medium to be measured by the sensor at a particular frequency; measuring a response; repeating the step of exciting the bifilar coil at the particular frequency and measuring a response for the medium having a different known value of the property of the medium to be measured by the sensor. determining a relationship between the response and the value of the property of the medium to be measured by the sensor at the particular frequency.

According to a fourth aspect of the present invention, there is provided a method for determining the level of a fluid comprising: exciting a bifilar coil at a particular frequency using an excitation circuit, wherein the bifilar coil comprises a first conductor with ends Pl and P3 and a second conductor with ends P2 and P4, and ends Pl and P4 are connected to the excitation circuit and ends P2 and P3 are unterminated; measuring a response of a circuit comprising the bifilar coil; determining a value of the level of the fluid based at least in part on the measured response.

According to a fifth aspect of the present invention, there is provided a sensing element, for use in a sensor for measuring the value of an electrical property of a medium, comprising: a bifilar coil comprising a first conductor and a second conductor, wherein one end of each conductor is for connection to an excitation and measurement circuit, and the other end of each conductor is unterminated.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be discussed with reference to the accompanying drawings wherein:

FIGURE 1 is circuit diagram of a sensor according to an embodiment of the present invention;

FIGURE 2 is an equivalent circuit of the sensing coil with distributed parameters according to an embodiment of the present invention;

FIGURE 3 is an equivalent circuit of the sensing coil with lumped parameters according to an embodiment of the present invention;

FIGURES 4A and 4B are equivalent circuits showing the sensor coil in the H-bridge representing alternate switched configurations according to an embodiment of the present invention;

FIGURE 5 A is a circuit diagram of sensor according to an embodiment of the present invention;

FIGURE 5B is a simplified circuit diagram of the circuit diagram of Figure 5A according to an embodiment of the present invention;

FIGURE 6A depicts a simplified cross section of an evaporative cooler;

FIGURE 6B depicts a simplified cross section of an evaporative cooler utilising a combined water level and salinity sensor according to an embodiment of the present invention;

FIGURE 7 is a plot of the sensor voltage and current taken using the circuit of Figure 5 A; FIGURE 8 is a plot of the consumption current versus frequency response of the sensor for a range of salinity levels using the circuit of Figure 5 A;

FIGURE 9A is a representation of the bifilar coil according to one embodiment of the invention;

FIGURE 9B is a representation of the bifilar coil according to another embodiment of the invention;

FIGURE 9C is a representation of the bifilar coil according to another embodiment of the invention; FIGURE 9D is a representation of the bifilar coil according to another embodiment of the invention; FIGURE 10 is a plot of the load current versus frequency for 4 sodium metabisulphite concentrations over a frequency range from DC to 400MHz;

FIGURE 11 is a plot of the load current versus frequency for 4 sodium metabisulphite concentrations over a frequency range from 150MHz to 250MHz; FIGURE 12 is a flowchart of a method for determining the value of an electrical property of a medium according to an embodiment of the invention; and

FIGURE 13 is a flowchart of a calibration method for calibrating a sensor according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention describe a sensor utilising a bifilar coil arrangement for determining or estimating the value of an electrical or electrically related property (or properties) of the medium surrounding the coil or sensing probe. The electrical properties of a medium, such as the complex impedance which is a function of both the resistance and di-electric properties of the medium, are affected by the ion concentration of one or more species of interest in the medium. Thus measurements of the electrical response or suitable electrical parameters can be used to infer properties of the medium such as ion concentration or salinity. Such measurements can also be used to determine the fluid level of a medium in which the sensor is placed.

Figure 1 is circuit diagram of a sensor 100 for measuring the value of an electrical property of a medium according to an embodiment of the present invention. The circuit comprises a sensing element 110 which uses a bifilar coil arrangement in which only one end of each of the wires is connected to the rest of the circuit and the other ends are left free or unterminated. A bifilar coil is an inductor having two conductors, typically wires, which contain two closely spaced and substantially parallel or aligned windings. Provided the two conductors maintain this alignment, a bifilar coil need not be formed in a regular ring or "coil" shape. The bifilar coil in Figure 1 has a first wire or conductor 112 with ends Pl and P3 and a second wire or conductor 114 with ends P2 and P4, with both wires arranged in an inductive arrangement. In Figure 1, end Pl and P2 are near to one end of the coil and ends P3 and P4 are near to the opposite end of the coil, although as will be discussed below other arrangements are possible. The opposite ends of the two conductors or wires, Pl and P4, are connected to an excitation circuit 130 which uses an alternating potential to excite or drive the sensing element at a particular frequency, such as one close to the resonant frequency of the circuit which may be known from calibration data. A measurement circuit 140 measures the response or changes in electrical parameters of the sensor. In figure 1 load 120, such as a resistor, is placed in series with the bifilar coil sensing element and the measurement circuit measures the response as changes in electrical parameters across or in this load (voltage, current, power, current consumed etc). The alternating electric field close to the gap between the wires (conductors) in the bifϊlar coil arrangement is susceptible to changes due to the dielectic and ionic properties of the material adjacent the coils. The wires in the bifilar coil are effectively inductors connected in a similar fashion to a capacitor. Through leaving one end of each wire free, or unterminated, the mutual inductance between the two wires of the bifilar coil arrangement has greater losses and is more sensitive to the electrical properties of the medium surrounding the coil as compared to a conventional bifilar coil in which both ends are connected to circuit or ground.

Several characteristics may be observed. Firstly dielectric losses can be measured directly as the change of current in the load in leading quadrature to the excitation. In some embodiments the dielectric losses can also be separated into those dependent on the solvent and its permanent dissolved constituents and the additional components of the solution that are of interest. Secondly ionic conduction, as eddy currents induced in the near field by transformer action leads to direct ohmic losses, and may be measured as variation in the load current in phase with the excitation. Thirdly the frequency dependence of both of the above loss mechanisms exhibit specific variations related to ion resonances, and can be used to characterise particular solutes, combinations of solutes, and variations of concentration in both cases. Typically all of the above effects are also dependent upon temperature. However through taking appropriate sets of measurements and/or utilising calibration curves, lookup tables and other measurements such as temperature, the concentration of constituent solutes can be estimated.

An alternative view for understanding the circuit is that the bifilar coil arrangement acts as an antenna, injecting a first radio frequency wave into the medium surrounding the coil. The medium absorbs some of this energy and reflects it, based on the electrical properties of the medium such as the complex impedance. The measurement circuit connected to the coil measures the difference between the transmitted and reflected wave and the phase shift which can be used to determine the electrical properties of the medium.

To further understand the operation of the sensing element and the sensor, a simplified distributed circuit 200 of the sensing element 110 is shown in Figure 2. The bifilar coil can be represented as a π (pi) equivalent network with inductance dL 210 240, capacitance dC 220 and resistance dR 230 per unit length dl 250. The values of the resistance dR 230 and capacitance dC 220 are functions of the conductivity and di-electric constant of the surrounding medium in which the coil is placed. The sensing coil is electrically connected to a circuit by the opposite ends Pl and P4 whilst P2 and P3 are left open ended. This configuration is somewhat analogous to a capacitor configuration. The total impedance, Z(jω), of the sensor depends on the sum of the total inductance of one

coil L , and the integral of the length admittance combined of dC and dR:

Z(jω) =jLω + 1/Y(jω) where / is the length of the wire, L is the inductance, ω is operating frequency,./ = V- I , and Y is the admittance.

The coil can also be drawn as an equivalent lumped RLC circuit 300 as shown in Figure 3 where the inductance L 310 is effectively constant and will be largely determined by the properties of the exact coil configuration (metallurgical composition of the wire, number of turns, diameter of the coil etc) and the resistance R 330 and capacitance C 320 will depend upon the conductivity and the di-electric constant of the medium within or immediately surrounding the coil. The conductivity and di-electric constant depend upon the electric properties of the medium, such as the concentration of ions or the salinity, and the temperature of the medium. If an alternating current of a particular frequency is directed (driven) through the sensor, it will experience current losses which will depend upon the values of R, L and C. Hence by utilising the sensor in conjunction with a suitable circuit one can measure the response of the coil or a suitable output parameter such as current or voltage which will depend upon the electric properties and possibly the temperature of the medium. One can also utilise comparisons of the output measurement to the equivalent input measurement and thus measure current losses, power consumption or another suitable parameter. Calibration curves, lookup tables or related methods can then be used to estimate or determine the value of an electric or electrically related property, such as ion concentration or salinity, of the medium based on the output measurement. Such determinations or estimates may be improved by also measuring temperature, or otherwise taking temperature into account when performing the determination step.

It will also be appreciated by the person skilled in the art that such a series RLC circuit will have a natural resonant frequency given by a>o=l/sqrt(LC). At this resonant frequency, peak current flows in the circuit, and power losses due to conductivity of the medium will be highest. Hence if the circuit is driven at the resonant frequency (or indeed very close to the resonant frequency) then the maximum possible current based on the circuit parameters may be larger than the capacity of the supply source. In such cases one will lose sensitivity to detect changes in electric properties such as ion concentration. Alternatively, if the circuit is driven at frequencies substantially greater, or substantially less than the resonant frequency, then the power losses due to the conductivity of the medium will be less and an extremely sensitive measurement circuit will be required to detect changes in electric properties with sufficient resolution. Thus in order to be sensitive to the change in power losses (or an equivalent response or output parameter) due to changes in electric the circuit may be driven at a particular frequency, or frequencies, near to the resonant frequency. It will be appreciated by the person skilled in the art that such frequencies are those that correspond to those in the exponential decay region (between the peak and plateau zones) on a resonance curve.

The sensing element is driven by an alternating potential. A microcontroller, may be used to control pairs of switches at a desired frequency in order to excite the sensing element. Figures 4A and 4B show equivalent circuits showing the sensor coil in the H-bridge represents alternate switched configurations 400 402. Diagonally opposite pairs of switches are switched at a desired frequency to allow current to flow through the sensor in one direction, and then the opposite direction. For example in the first configuration 400, diagonally opposite switches 410 and 440 are closed, and the alternate diagonally opposite switches 430 and 420 are open. Then in the next configuration diagonally opposite switches 410 and 440 are open, and the alternate diagonally opposite switches 430 and 420 are closed.

A circuit diagram 500 according to an embodiment of the invention that was used to test the sensor suitable for use in measuring salinity levels in an evaporative cooler is shown schematically in Figure 5 A. The circuit used a Texas Instruments MSP430F149 microcontroller (not shown) which was used to control a first set of inverters 532 and a second set of inverters 534 which was used to excite the sensing element 510. A Motorola MC74HC04N hex inverter CMOS IC was used. An alternative would be a Texas Instruments SN74LVC04A-EP hex inverter. The inverters were connected in parallel (to ends Pl and P4 of the sensing element 510) to reduce the resistance of the switches. Pins 1,3,5,9,11 and 13 of the inverter chip were connected to timer output of the microcontroller 530. Pins 2, 4, 6 were connected to Pl and pins 8, 10, 12 were connected to P4.

The output terminals 7 and 14 of the inverter (ground and Vcc respectively) were connected to the ends Pl and P4 of the sensing element 510.

A constant voltage source V_cc (3.6V) 520 supplies current through inductor Ll (1OmH) 522 to the sensor via pin 14 of inverter chip (MC74HC04N, manufactured by Motorola), with pin 7 of the inverter chip connected to ground. A filter capacitor 521 is also shown (lmicroF). The switching of the H-Bridge by the microcontroller thus excites the sensor and the voltage commutated is stored across capacitor C2 (lμF) 524. The inductor Ll 522 averages the current flowing (consumed) in the circuit, and an output measurement is obtained by measuring the current flowing through shunt resistor Rl (27 Ω) 526 by measurement circuit 540. This measurement circuit comprises a non- inverting operational amplifier, a buffer amplifier (voltage follower) and an analogue to digital converter ADC 542, as shown in the bottom right hand quadrant of Figure 5 A. A simplified circuit diagram 550 is shown in Figure 5B, in which the sensing element has been represented by the equivalent H bridge circuit in Figure 4B.

This digital level is provided to the microcontroller, which converts the current measured into a salinity estimate, upon which decisions can be made. The power consumed by the inverter IC (MC74HC04N) is small (~20uA) and as will be seen below, is not significant in comparison to the sensor losses which are in the tens of milliamp range at frequencies near the sensors resonance frequency. Thus the resultant losses are those mainly due to the salinity of the water in the conduit to be measured.

A circuit according to Figure 5A was constructed to measure the power consumed by the sensor and to determine if such a circuit was capable of measuring salinity changes at sufficient resolution to enable decision making with regard to dumping and refilling the tank. To test the circuit and measure the current supplied into the sensor an additional shunt resistor of 67 Ohms was connected in series with the sensor at P4.

The sensing coil was formed from 100 turns of 0.315mm enamelled wire (polyesterimedem PEI-TS2). The dimensions of the coil, in terms of length, inner and outer diameters where 70, 14 and 19mm respectively. Bulk electrical parameters of the sensing coil were measured with an LRC meter (PEAC LRC40) tested for the coil filled with air, demineralised and saline water, and tests were performed at 22degrees Celsius. Inductance was measured for each of the single coils at a frequency of 200 kHz and the capacitance of the coils was measured at a frequency of 15 kHz. Measurements of inductance (microH), resistance (Ohm) and Capacitance (nanoFarads) were, respectively, 30, 1.6 and 1.4 in air, 30.2, 1.4 and 1.98 in demineralised water, and 30.8, 1.4 and 2.04 in water with a salinity of 2500ppm NaCl. In this embodiment the open ends of the coil extended approximately 10 cm from the coil and were left freely hanging in space although these could have been truncated.

A simplified cross section of a conventional evaporative cooler 600 is presented in Figure 6A. Evaporative pads 601 are placed in casing 602, the bottom part of which serves as water tank or a sump. The air to be cooled is forced by fan 604 from the outside through the wet evaporative pads. When the water evaporates from the pad surfaces, the temperature of the incoming air is reduced and it is supplied further to the rooms to be cooled. Water from the tank is delivered to the pads via a water supply conduit 610, by the pump 602. The level of water in the tank is controlled by float valve 605, which is connected to the water mains 611 through an electrically controlled valve 606. Water discharge rate is controlled by adjustable valve 608, which is preset during installation. Valve 607 opens the drain line 612 to dump the contents of the tank when the cooler is switched off for a long time. Some evaporative coolers are equipped with a salinity sensor, which is not shown in the diagram (for an example see Figure 1 of US 5,374,380). When the salinity (ion concentration) reaches a preset level the controller dumps the water from the tank and fills the tank again.

Figure 6B represents a simplified cross section of an evaporative cooler 620 incorporating a salinity and water level sensor according to an embodiment of the invention. The numbering of the parts is equivalent to those in figure 6A with the adjustable bleed off valve 608 and float valve 605 made obsolete by introduction of the sensing element 629. The coil of the sensing element is electrically isolated and placed around the conduit used to supply water to the evaporative pads. Placement of the coil around the conduit, and isolation of the coil from direct contact with the saline water prevents degradation due to a build up of salt deposition. Thus this configuration provides high reliability over existing sensors, and the capability of the sensor to additionally measure water level eliminates the need for the mechanical float sensor, thus further reducing the manufacturing cost, and eliminating any running or maintenance costs associated with the float sensor.

The placement of the coil around the conduit, and the axial dimensions of the coil are such that it is longer in its axial direction than the difference between the heights of the desired minimum and maximum water level in the tank. Sensor 629 is capable of measuring the salinity of the water flow through it and the level of water in it when pump 602 is switched off.

A typical operation cycle begins with the controller opening mains valve 606 until the level in the tank reaches a preset upper limit. The time the presetting is performed is typically, but not limited to, during manufacture, installation or subsequent maintenance. The pump 602 is then switched on to supply water to the evaporative pad 601. The controller periodically switches off the pump and measures the level of water in the pipe. If the level of water decreases beyond a preset lower limit, the controller opens the mains valve 606 to refill the tank.

The salinity of the water is measured when pump 602 is operating and thus water fills the conduit and the interior of the coil of the sensing element. When the salinity reaches a preset threshold, the controller opens valve 607 to dump the water from the tank, and then the fill and use cycle begins again.

Figure 7 shows oscillograms 700 of the voltages on either side of the shunt resistor 710, 720, as well as an oscillogram showing the difference between the two traces 730(trace 730 = trace 710 minus trace 720). At a frequency around 1.26MHz, the first trace 710 is approximately sinusoidal with amplitude of approximately 1.85 V corresponding to a supply current of around 3OmA.

To obtain calibration data for the salinity sensor, the frequency was varied from 100 kHz to 2MHz and for each frequency selected in this range, the current consumed in the circuit was measured for the case where there was no water within the coil. This process was then repeated for the case of saline water fully filling the cavity in the coil (as would be the case with the pump on) and for salinities ranging from 1000 to 4000 parts per million (ppm).

Figure 8 shows the consumed current (y axis) as function of frequency (x axis) for a range of salinities at a constant temperature of 22°C 800. Traces correspond to empty (no water) 820, tap water 830, water with salt at 2000ppm 840 and 4000ppm 850.From Figureδ we can observe that the resonance frequency in this case is approximately 700 kHz, and that at this frequency the circuit is no longer sensitive to changes due to salinity differences. Figure 8 shows that at frequencies either side of resonance, the circuit is sensitive to changes in salinity (at a fixed temperature).

Thus it is possible to calibrate the sensor in order to determine a relationship between the response or output parameter (eg current consumed) and the value of the electrical property of the medium (eg salinity) to be measured by the sensor at a particular frequency. For example one can take a series of current measurements at a particular frequency for a range of known values of salinity and then perform linear regression on the current readings to obtain a calibration or mapping relationship between the observed sensor current and estimated or determined salinity. Alternative approaches are also possible such as interpolating between measurements or fitting other (non linear) curves to the relationship. In this way a functional relationship or a look up table can be obtained. This procedure can be repeated for a range of temperatures, and a separate relationship or lookup table provided for each temperature, or a relationship which takes inputs of temperature and measured value and estimates the value of the electrical property of interest. The microcontroller can then be provided with calibration data or converter functionality.

Calibration data were obtained for the three right most frequency measurements shown in Figure 7 corresponding to frequencies of IMHz, 1.35MHz and 2MHz. At each frequency, linear interpolation was performed over the data points in Figure 7 (eg no water, tap water, and for salinities of 2000ppm and 4000ppm) to produce a relationship between current consumed and salinity. This interpolation was then tested using a water sample of the same temperature, but with a known salinity of 2500ppm. At each frequency a measurement of the current consumed was performed and the measured value of the current consumed was converted to a salinity estimate using the linear interpolation data for that frequency. This approach gave salinity estimates of 2542, 2681, and 2448ppm for the respective frequencies of IMHz, 1.35MHz and 2MHz (residuals of +42, +181 and -12ppm). These salinity estimates were then averaged to produce a final salinity determination of 2557ppm, an error of 57ppm. Using this sensor configuration at a temperature of 22°C the resolution of the sensor was approximately 250ppm which is comparable to existing salinity measurements and is sufficient to allow decisions regarding dumping or replenishment of tank water in an evaporative cooler.

It is apparent from figure 7 that the sensitivity to differences in salinity is frequency dependent (ie the range between the empty point and the 4000ppm point changes with frequency). In one embodiment estimates are performed at a single frequency, for example IMHz or 1.33 MHz for salinity estimates. In alternative embodiments, measurements are performed over a range of frequencies and estimates are combined so as to provide a more robust estimate. Hence one could apply various statistical techniques, regression analysis, fuzzy logic, or neural net techniques to combine measurements made at several frequencies to improve the determination of the salinity over a measurement made at a single frequency.

Further the microcontroller may be programmed to produce a digital signal based on the determined value of the electrical property of the medium. For example if the salinity rises above a predetermined threshold, then a digital signal could be produced which then leads to draining of the tank.

In addition to salinity measurements, the sensor and circuit may also be used to measure the level of water in the tank. The level of the water in the coil will affect the total resistance R and capacitance C of the sensing element. This effect can be seen in figure 7, where the power losses are reduced when the coil is empty, when compared to measurements made with the coil full of either tap water, or water with salinities of 2000ppm or 4000ppm. In an evaporative cooler, estimation of the level of water in the tank would be performed when pump 602 is switched off, and the level of the water in the water supply conduit is in equilibrium with the level of the water in the tank. The sensor is then operated at a suitable frequency and an output measurement is obtained. Calibration data can then be used to determine the water level based on the frequency and a previously obtained salinity measurement. The determined height level can then be compared against preset minimum and maximum levels to enable decisions regarding dumping or replenishment of water in the tank. Using the circuit described above, a frequency of 200 IcHz was used to determine if water was present within the coil. The different in the current for the sensing coil filled with water 860 (any salinity), and the empty sensor 862(no water) was about 5mA. As can be seen from Figure 7, the differences between different salinities were considerably smaller than the difference between water being present, or not being present. If the evaporative cooler had not been operated for some time one may wish to check if there is water present or not, or alternatively if the water level is greater than a minimum level. In such cases the last salinity measurement may have been made some considerable time previously and may no longer be accurate due to possible evaporation of water in the tank (thus increasing salinity). In such a case one could operate the sensor at a suitable frequency and compare the output value compared to a threshold value obtained with either an empty coil, or with the coil filled with tap water to the desired minimum level. If the measurement indicated a low water level, additional water could be added and then new measurements performed. Once water is known to be above a minimum level, a salinity estimate could be made, followed by an improved water level estimate.

Whilst the above techniques were all performed at a temperature of 22°C, they are equally applicable at other operating temperatures of evaporative coolers. Thus full calibration requires one to repeat the above calibration process at a range of temperatures over the operating range of the evaporative cooler. The method of determining the salinity thus consists of measuring the temperature, then measuring the circuit output (eg consumed current), choosing the conversion relationship appropriate for the measured temperature, and then using this relationship to determine the salinity from the circuit output measurement. One could make a new temperature measurement each time a salinity measurement is desired, or one could utilise a temperature measurement made within some preset time of making the circuit output measurement (with the microcontroller storing the values of the measurements to allow a determination of the salinity to be made).

Temperature measurements can be made by relatively minor additions to the sensor configuration. The resistance in a wire is dependent upon the temperature of the wire, hence temperature measurements can be obtained by measuring the resistance in one of the wires of the bifϊlar coil of the sensor (eg resistance from Pl to P3). This can be obtained by connecting a direct current (DC) current source to Pl and connecting P3 (the other end of the half coil) to ground and measuring the voltage drop over the (half) coil. Such measurements could be performed whilst the pump is on to ensure water is flowing through the coil. This sensor arrangement can also be used for level sensing when the pump is switched off. In such cases the resistance of the coil will be affected by the height of the water in the coil, and thus with calibration data one could estimate the height based on the measured resistance of the wire at the given temperature.

In the above configuration, a single coil is utilised to measure salinity and water level, with dimensions and placement of the coil such that it spans the range of acceptable water levels required for operation of the cooler. In an alternate configuration the single coil could be replaced with multiple coils of smaller axial lengths. These could be placed at the heights of the desired maximum and minimum water levels, along with any other heights at which measurements are desired. Another option would be to use multiple coils wherein each coil measures one or more of water level, temperature, or salinity. The choice of the best configuration will depend upon the exact application.

In one embodiment the sensing element is a single layer bifilar coil of close wound insulated wire with opposite ends connected to the sensor circuitry. Figures 9A and 9B illustrate two embodiments 900 930 in which a closely wound coil is located on the outside (900) and inside (930). In each case the first wire has ends Pl and P3 and the second wire has ends P2 and P4 and the bifilar coil has the shape of an open cylinder with ends Pl and P2 are near to one axial end of the cylinder and ends P3 and P4 are near to the opposite axial end of the cylinder. The sensing element may be in direct contact with medium 920, as in Figure 9A, or close proximity to the medium, or the medium will flow through, or reside within the coil of the sensor. In the case of an evaporative cooler placing the coil within the water supply pipeline will ensure that the coil is subject to constant washing during cooler operation, thereby minimising the build up of salt deposition.

In Figure 9A the coil is wrapped around a pipe or tube 910 and in Figure 9B the coil is wrapped around inside of a pipe or tube 91O.The wire core 918 is insulated 916. In Figure 9A close up section 912 shows the coil is attached to the pipe via a resin 914 and in Figure 9B close up section 942 illustrates a filling material 948 which fills the gap between the insulation and the pipe 940.

In the above configurations the sensing element is comprised of a bifilar cylindrical coil with a uniform circular cross section. Such coils are well known and easily manufactured. However it is considered that the effective operation of the sensor is not critically sensitive to the use of cylindrical coil with a uniform circular cross section. That is variations such as arrangements with non circular cross sections, or cross sections of varying diameters are considered acceptable alternative configurations, as provided the bifilar requirement is met, in which there is close spacing between the two conductors over their common length, then these configurations still allow the sensing element to operate based on inductive principles. That is current flow in one wire induces current flow in the second wire, and that this is influenced by the surrounding medium.

Figure 9C is a representation of a bifilar sensing coil made from a wire ribbon according to another embodiment of the invention. Various other bifilar coil arrangements are also possible. Figure 9D is a representation of a bifilar sensing coil on a printed circuit board (PCB) according to one embodiment of the invention. In Figure 9C a ribbon arrangement 950 is shown in a top view 960 and side view 970. Pairs of individual wires 916 are connected at the ends 962 to form a bifilar arrangement which is wrapped around a pipe 910 to form a bifilar coil as illustrated in top view 980 and side views 990. The connection at the ends of the ribbon is shown at 982. In Figure 9D an alternative embodiment 990 using a PCB 992 onto which the wires 918 are etched is shown. The spacing of the coils is deliberately enlarged to better illustrate the embodiment. The PCB board is coated with a resin or similar protective substance and may be placed directed in the fluid. Other circuit elements may also be placed on the board.

In the above example, the sensor was configured to measure the salinity of the water in the sump of the evaporative cooler. Salinity typically refers to the concentration of dissolved salts such as halide of chloride salts (sodium chloride being an exemplary example). Thus in other applications, ion concentration, rather than salinity may be measured. Another example relates to the measurement of sodium metabisulphite concentrations which has use in the aquaculture industry. Sodium metabisulphite is a complicated compound that assumes a multiplicity of properties when in aqueous solution. The main product of dissolution is sulphur dioxide which is the main constituent of the process for the control of prawn discolouration. Thus when prawns are caught they are placed in salt water and sodium metabisulphite may be added to preserve the prawns. Over time the concentration of sodium metabisulphite (or the dissolved sulphur dioxide) may change, and re-dosing may be required.

As sensing arrangement according to an embodiment of this invention was manufactured to detect the presence and concentration of sodium metabisulphite in salt water. In this case the sensor coil was similar to that shown in Figure 9D and comprised of a pair of parallel insulated conductors on a rigid substrate made by printed circuit techniques (a flexible substrate could also have been used). The test PCB board consists of a piece of standard double sided 1-6 mm thick teflon loaded glass fibre reinforced epoxy resin copper clad board etched in a pattern of parallel conductors that zig zag uniformly across the narrowest part of this board. The board is shaped to fit inside a standard test tube: ie, about 25 mm by 150 mm with an end chamfered to fit closely to the bottom of the tube. The top part of the board protrudes from the open end of the tube and is etched with tracks suited to carry the components needed to provide the alternating potential for the parallel conductors and also detect variations in current drawn by this device. The tracks are closely spaced and may zigzag or spiral around the board. The surface of the conductors may be protected by an insulating layer of epoxy resin.

Testing of the sensing arrangement was performed by exciting the sensor by a sinewave of voltage across its input terminals from an Agilent controllable signal generator under the control of a National Instruments Data Acquisition System. The results were observed on an Agilent digital spectrum analyser and collected directly by the Data Acquisition System. Figures 10 and 11 present screen shots showing measured signals for four different concentrations of sodium metabisulphite dissolved in seawater. In both figure 11 and 12 the y axis shows load current, measured as a voltage changes across a fixed load in series with the sensor, from a constant voltage source. The x axis covers a suitable range of excitation frequencies.

Figure 10 is a 'ranging shot' 1000 to determine where the effects are most sensitive to change. The initial frequency range is DC to 400 MHz and shows the load current for sensor in a medium with 5 g/£ (parts per thousand of sodium metabisulphite) 1010, 8 git 1020, 10 g/£ 1030 and >10 git 1040 (assumed to saturate). The voltage change was measured for a base line of 177-6 mV - 35 dB. Separation of the curves corresponding to different concentrations of sodium metabisulphite occurred most strongly over a frequency range of 150 MHz to 250 MHz indicated by line 1050.

Figure 11 corresponds to a second set of measurements taken over a frequency range of 150 MHz to 250 MHz, again with sodium metabisulphite concentrations of 5 g/€ 1110, 8 git 1120, 10 g/£ 1130 and >10 git 1240. The voltage change was measured from a base line of 375-4 mV - 16 dB.

From figures 10 and 11 it may be observed that the value of load voltage for the several dips in the trace for a particular solution do not show consistent behaviour over the frequency range with respect to another sample concentration. That is a constant separation between curves is not necessarily maintained over the frequency range. Similar to the case of the salinity sensor for the evaporative cooler (or indeed other applications), a calibration or characterisation process may be performed over the operating frequencies and expected range of concentration. Using such data estimates of other concentrations may be made using an interpolation algorithm or similar technique. For the data shown in figure 11, three points for each of three sufficiently different concentrations should be sufficient to determine the direction of change and extremes of concentration for proper operation.

Such a sensor may use a microcontroller which may be programmed to produce a digital signal based on the determined value of the concentration. For example if the concentration of sodium metabisulphite drops below a predetermined threshold, then a digital signal, or other output (eg LED, warning sound etc) could be produced which may alert a user that re-dosing is required. Such a sensor could be made portable for use on fishing boats or other field locations and utilise a simple green/red LED output (green indicating sodium metabisulphite acceptable, and red indicating dosing, or re- dosing is required. Alternatively the sensor could produce an analogue or digital estimate of the concentration. Such a sensor could use a PCB coil arrangement adapted to fit inside a test tube or other container. The container could be used to protect the PCB coil during storage, and then when in use it could be removable detached, and a sample of seawater from a holding tank obtained. The PCB coil could then be placed in the container and a determination performed.

Figure 12 presents a flowchart of a method for determining the value of an electrical property of a medium 1200 according to an embodiment of the invention. The bifilar coil sensing element is excited at a particular frequency 1210 and the response measured 1210. A decision can then be made whether to take a further measurement 1230. If yes, a new frequency is selected 1240 and the excitation process 1210 and measurement of response 1230 is repeated. Once sufficient measurements have been performed the estimation of an electrical property of the medium can be performed 1260 based upon these measurements. A lookup table or other calibration or mapping relationship may be used to perform the estimation of an electrical property of the medium. This estimation process may optionally utilise a temperature measurement 1250.

Figure 13 is a flowchart of a calibration method for calibrating a sensor 1300 according to an embodiment of the invention. The bifilar coil sensing element is placed in a medium with a known value of an electrical property 1310. The bifilar coil is excited at a particular frequency 1320 and the response measured 1340. A determination is made wither to take another measurement at a different frequency 1350. If yes 1352 then the previous steps 1320 to 1350 are performed 1352. Otherwise a determination is then made as to whether to adjust the value of the electrical property and/or temperature of the medium 1360. If yes 1362 the previous steps 1310 to 1360 are repeated. Once sufficient calibration measurements have been performed, a determination of the relationship between the value of the electrical property of the medium, and the measured responses as a function of frequency and/or temperature is performed. This information can then be loaded into a microprocessor used to control operation of a sensor according to an embodiment of the invention.

It will be appreciated by the person skilled in the art that embodiments of the bifilar coil configuration specified above in combination with a suitable circuit for providing a suitable input or excitation frequency to the sensor, and an circuit for measuring the response or output parameter, may be used to detect or measure changes in the electrical properties, such as conductivity, di-electric constant, ion concentration etc. Examples include measuring the salinity and fluid level of sump water in an evaporative cooler and the concentration of sodium metabisulphite in sea water. However it will be understood that these are only examples of the use of the sensor, and it will be apparent to those skilled in the art that there are many other applications to which the sensor can be put to use in.

Further the sensor, comprising the coil configuration and circuit of this description may be modified in regard to its physical and electronic configuration and still perform the same function particularly to the task of ion concentration and fluid level measurement in a range of medium such as soils or other environments in which it is desirable to measure the electric properties or dielectric properties of the medium. For example the bifilar coil arrangement may take various forms such as a cylindrical coil which could be placed in a fluid supply conduit of an evaporative cooler, a in the form of pairs of closely spaced tracks laid down on a PCB board, and of a size suitable for placing in a test tube containing a sample of a medium of interest

The sensing arrangement using a bifilar coil sensor is both simple to construct, and inexpensive to manufacture and is thus suitable for mass production. The sensor can comprise a microcontroller which has a converter for converting measured responses to an estimate of a value of an electrical property of the medium. Such a converter may be pre-programmed or preloaded with conversion functions, relationships or table for immediate use, or enable in situ calibration (or re-calibration).

The examples illustrate herein show that the circuit is capable of making measurements at sufficient accuracy to be useful. Further the circuit is simple, robust, and inexpensive to manufacture thereby allowing the sensor to be used in a range of applications and environments which prohibit the use of more expensive sensors. For example in the case of sodium metabisulphite, a relatively inexpensive and portable sensor could be built which could be carried and used on board fishing vessels. Similarly the existing float and contact conductivity sensors in evaporative coolers could be replaced with a single salinity and water level sensor according to embodiments of the invention.

Other potential applications in which it is of interest to measure ion concentration, salinity or conductivity include soil monitoring for agricultural purposes and water quality monitoring, such a for drinking water, or in desalination operations. Other applications are also possible as would be apparent to the person skilled in the art.

Throughout the specification and the claims that follow, unless the context requires otherwise, the words "comprise" and "include" and variations such as "comprising" and "including" will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge. It will be appreciated by those skilled in the art that the invention is not restricted in its use to the particular application described. Neither is the present invention restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the invention is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention as set forth and defined by the following claims.

Claims

THE CLAIMS

1. A sensor, for determining a value of an electrical property of a medium comprising, a sensing element comprising a bifilar coil; and an excitation circuit connected to the sensing element for exciting the sensing element at a particular frequency; a measurement circuit for measuring the response of the sensing element; wherein the bifilar coil comprises a first conductor with ends Pl and P3 and a second conductor with ends P2 and P4, and ends Pl and P4 are connected to the excitation circuit and ends P2 and P3 are unterminated, and the response is representative of the value of the electrical property of the medium.

2. A sensor according to claim 1, wherein the electrical property is related to the complex impedance of the medium.

3. A sensor according to claim 1 or 2, wherein the electrical property of the medium is the concentration of an ion species in the medium.

4. A sensor according to claim 1 , 2 or 3 wherein the particular frequency is a frequency close to a predetermined resonant frequency of the sensing element in the medium.

5. A sensing element as claimed in any one of claims 1 to 4, wherein the bifilar coil is formed from a first wire with ends Pl and P3 and a second wire with ends P2 and P4, and the bifilar coil has the shape of an open cylinder with ends Pl and P2 near to one axial end of the cylinder and ends P3 and P4 near to the opposite axial end of the cylinder.

6. A sensing element as claimed in any one of claims 1 to 4, wherein the bifilar coil is formed on a printed circuit board (PCB) such that each conductor forms a track on the PCB wherein the spacing between the two conductors is substantially constant.

7. A sensor according to claim 6, wherein the PCB board is constructed to fit within a test tube.

8. A sensor as claimed in any one of claims 1 to 7, wherein the measurement circuit comprises a load in series with the bifilar coil and the response is a measurement of the change in current in the load.

9. A sensor as claimed in any one of claims 1 to 7, wherein the measurement circuit comprises a load in series with the bifilar coil and the response is a measurement of the current consumed by the sensor.

10. A sensor as claimed in any one of claims 1 to 7, wherein the measurement circuit comprises a load in series with the bifilar coil and the response is a measurement of the voltage change in the load.

11. A sensor as claimed in any one of claims 1 to 10, further comprising a microcontroller, wherein the microcontroller receives the response, and uses the response to determine the value of the electrical property of the medium.

12. A sensor as claimed in claim 11, wherein the excitation circuit further comprises a plurality of inverters, and the microcontroller controls the switching of the inverters to excite the sensing element at a particular frequency.

13. A sensor as claimed in claim 12, wherein the microcontroller performs a series of measurements at a plurality of frequencies, and obtains a plurality of responses, each associated with a particular frequency, and combines the plurality of responses to determine the value of the electrical property of the medium.

14. A sensor as claimed in any one of claims 11 to 13, wherein the microcontroller further comprises a converter for determining the value of the electrical property of the medium from a measured response.

15. A sensor as claimed in claim 14, further comprising a temperature measurement circuit, wherein a measurement of the temperature is provided to the microcontroller and the converter also uses the temperature for determining the value of the electrical property of the medium.

16. A sensor as claimed in claim 14 or 15, wherein the converter comprises a lookup table.

17. A sensor as claimed in any one of claims 14, 15 or 16, wherein the converter comprises a function for mapping a response to a value of the electrical property of the medium.

18. A sensor as claimed in any one of claims 11 to 17, wherein the microcontroller produces a digital signal based on the determined value of the electrical property of the medium.

19. A sensor as claimed in any one of claims 1 to 18 wherein the sensor is further adapted to measure the level of fluid within the bifilar coil.

20. A sensor as claimed in claim 19 wherein the bifilar coil is located in the sump tank of an evaporative cooler to measure the salinity and the water level of water in the said sump tank.

21. A sensor as claimed in claim 19 wherein the bifilar coil is placed around or within a fluid supply conduit supplying water to evaporator pads of the evaporative cooler.

23. A sensor as claimed in any one of claims 1 to 18, wherein the medium is saline water and the electrical property is the concentration of sodium metabisulphite in saline water

24. A sensor as claimed in claim 23, wherein the bifilar coil arranged on a printed circuit board (PCB) such that each conductor forms a track on the PCB wherein the spacing between the two conductors is substantially constant.

25. A sensor according to claim 24, wherein the PCB board is constructed to fit within a test tube.

26. A method for determining the value of an electrical property of a medium comprising: exciting a bifilar coil placed in or near to the medium at a particular frequency using an excitation circuit, wherein the bifilar coil comprises a first conductor with ends Pl and P3 and a second conductor with ends P2 and P4, and ends Pl and P4 are connected to the excitation circuit and ends P2 and P3 are unterminated; measuring a response of a circuit comprising the bifilar coil; determining a value of an electrical property of the medium based at least in part on the measured response.

27. A method as claimed in claim 26 wherein the step of exciting the bifilar coil is performed at a plurality of frequencies, and a response is measured and associated with each of the plurality of frequencies, and the step of determining the value of the electrical property of the medium combines the plurality of responses to determine the one or more electrical properties of the medium.

28. A method as claimed in claim 26 or 27 further comprising receiving an estimate of the temperature of the medium and the step of determining uses the temperature estimate.

29. A method as claimed in claim 25, 26 or 27 wherein the value of the electrical property of the medium is the concentration of one or more ion species in the medium.

30. A method as claimed in any one of claims 26 to 27 wherein the plurality of frequencies are frequencies close to the resonant frequency of the bifilar coil in the medium.

31. A method as claimed in any one of claims 25 to 29 where in the response is the current consumed by the sensor.

32. A method for calibrating a sensor for determining a value of an electrical property of a medium, the sensor comprising an excitation circuit, a measurement circuit and a sensing element wherein the sensing element comprises a bifilar coil placed in or near to the medium and excited at a particular frequency using the excitation circuit, and the bifilar coil comprises a first conductor with ends Pl and P3 and a second conductor with ends P2 and P4, and ends Pl and P4 are connected to the excitation circuit and ends P2 and P3 are unterminated; the method comprising exciting the bifilar coil placed in or near the medium having a known value of an electrical property of the medium to be measured by the sensor at a particular frequency; measuring a response; repeating the step of exciting the bifilar coil at the particular frequency and measuring a response for the medium having a different known value of the electrical property of the medium to be measured by the sensor; and deteπnining a relationship between the response and the value of the electrical property of the medium to be measured by the sensor at the particular frequency.

33. A method as claimed in claim 32, wherein the method is repeated for a plurality of frequencies to determining a relationship between the response and the value of the electrical property of the medium to be measured by the sensor at a range of frequencies.

34. A method as claimed in claim 32 or 33, further comprising repeating the measurements at a plurality of temperatures to determine a relationship between the response and the value of the electrical property of the medium to be measured by the sensor at a range of frequencies and temperatures.

35. A method as claimed in any one of claims 32 to 34 wherein determining a relationship comprises performing one or more linear regressions over measurements taken at the particular frequency.

36. A method as claimed in any one of claims 32 to 35 wherein determining a relationship uses interpolation to create a lookup table for converting output measurements at a particular frequency and temperature to an estimate of a value of an electrical property of a medium.

37. A method as claimed in any one of claims 32 to 36, wherein the sensor comprises a microcontroller and the relationship is stored by the microcontroller.

38. A method for determining the level of a fluid comprising: exciting a bifϊlar coil at a particular frequency using an excitation circuit, wherein the bifϊlar coil comprises a first conductor with ends Pl and P3 and a second conductor with ends P2 and P4, and ends Pl and P4 are connected to the excitation circuit and ends P2 and P3 are unterminated; measuring a response of a circuit comprising the bifϊlar coil; determining a value of the level of the fluid based at least in part on the measured response.

39. A sensing element, for use in a sensor for measuring the value of an electrical property of a medium, comprising: a bifϊlar coil comprising a first conductor and a second conductor, wherein one end of each conductor is for connection to an excitation and measurement circuit, and the other end of each conductor is unterminated.

40. A sensing element as claimed in claim 39, wherein the opposite ends of each of the conductors is for connection to an excitation and measurement circuit.

41. A sensing element as claimed in claim 39 or 40, wherein the bifϊlar coil is formed from a first wire with ends Pl and P3 and a second wire with ends P2 and P4, and the coil has the shape of an open cylinder with ends Pl and P2 near to one axial end of the cylinder and ends P3 and P4 near to the opposite axial end of the cylinder.

42. A sensing element as claimed in claim 39 or 40, wherein the bifϊlar coil is arranged on a printed circuit board (PCB) such that each conductor forms a track on the PCB wherein the spacing between the two conductors is substantially constant.

43. A sensor according to claim 42, wherein the PCB board is constructed to fit within a test tube.