Liquid sensors and methods of making and using the same

Abstract

Liquid sensors and methods of making liquid sensors are disclosed herein. In preferred embodiments, the liquid sensors comprise a first electrode, a second electrode, an active shield and a temperature sensor. In preferred embodiments, a time division multiplexer multiplexes the first electrode, the second electrode, the active shield, and the temperature sensor with an electronic circuit to monitor a condition of a liquid using impedance spectroscopy, calculate a level of the liquid using time domain reflectometry and calculate a temperature of the liquid with the temperature sensor.

Description

PATENT APPLICATIONLIQUID SENSORS AND METHODS OF MAKING AND USING THE SAMEFIELD

[0001] The present patent document relates to new sensing probes and methods of making and using the same. In particular, the present patent document relates to liquid sensors and / or probes that combine a plurality of technologies to advance the art.BACKGROUND

[0002] Numerous technologies exist for liquid condition monitoring in tanks and vessels. Significant efforts have been devoted to the best ways to determine dielectric properties of fluids such as permittivity, as an indicator of contamination or ageing. Common methods for dielectric measurement consist of placing capacitive sensing electrodes into liquid, with an electronic circuit measuring response of the fluid under excitation from an alternating current (AC). For accurate measurements of capacitive, the electrodes must be fully immersed into the liquid, or the permittivity measurement will contain errors. Assuming variations of liquid level within the vessel, the sensing electrodes need to be sufficiently small, and placed at the bottom of the vessel. US Patent No. 7,043,402 depicts sensing electrodes as a miniature substrate with interdigitated structure. US Patent Publication No. 2022 / 0163505 shows a miniature disk with electrode structure. US Patent Publication No. 2018 / 0051793 explains electrodes as a set of short coaxial isolated tubes.

[0003] Measurements of permittivity can be performed with many different methods. US Patent No. 9,538,657 describes a resonant sensor: miniature sensing electrodes represent a small capacitor; that operates within a resonant circuit at a very high frequency. US Patent No. 6,922,064 shows permittivity determinations at three different frequencies of AC excitation. US Patent Publication No. 2005 / 0104607, Patents US 10,539,524, US 7,109,729,US 7,106,075, US 7,078,910, US 7,043,402, and many others describe the use of impedance spectroscopy with an AC excitation signal across a range of frequencies. Whereas single frequency AC excitation is limited, the use of impedance spectroscopy allows computation of complex permittivity, with the calculation of real and imaginary parts along with the excitation signal frequency sweep. Resulting functions are similar to spectra and characterize the condition of the fluid. Numerous studies show a strong correlation between fluid contamination and changes in complex permittivity determined with impedance spectroscopy.

[0004] Fully submerged small electrodes cannot determine the liquid level, obviously. Additional sensors must be used if the liquid level and temperature measurements are needed. As for a method for liquid level determination, a suitable liquid level sensor may be achieved using a capacitive method: liquid level tends to be proportional to measured capacitance between the sensing electrodes. This is the conventional method for liquid level determination. However, capacitive methods need an additional fully submerged sensor for calibration to account for unknown permittivity. Also, capacitance suffers drawbacks when trying to measure a liquid level: it is susceptible to contamination and build-up deposits. In contrast, Time Domain Reflectometry (TDR) methods offer a number of advantages. In particular, TDR delivers readings for liquid level only. In contrast, the capacitive method is sensitive to other physical parameters such as temperature, density and composition of the fluid.

[0005] A TDR sensor represents a radio frequency transmission line, consisting of sensing and ground electrodes, similar to capacitive electrodes. The method is based on measuring time delay between emitted and reflected radio frequency pulses, similar to guided wave radar, as known to those experienced with the arts. US Patent No. 9,453,755 and USPublication Nos. 2004 / 0027137, US 2009 / 0235737, US 2010 / 0153029, US 2011 / 0246100,and many others depict embodiments with TDR methods implemented with capacitive style electrodes. The current state of the art liquid level sensors are based on time domain reflectometer (TDR), capacitive, magnetostrictive, ultrasonic or other methods.

[0006] Packaging liquid level sensors, liquid condition monitoring probes and other sensors such as temperature sensors separately is not efficient from a weight, complexity and packaging standpoint, especially for modern compact engines. To this end, it would be advantageous to have a design wherein a single probe / sensor could sense liquid levels, provide liquid contamination monitoring and sense other parameters like temperature.Moreover, if such a sensor were constructed, the liquid level, liquid contamination and temperature data collected could all be cross-referenced to further improve the accuracy of each measurement.

[0007] Accordingly, there is a need in the art for a single simple sensor that can detect liquid levels, monitor the liquid condition and determine the liquid temperature. It’s an object of the current invention to provide a single sensor capable of monitoring liquid conditions, sensing the liquid level and determining the liquid temperature, thus avoiding any additional configurations or instruments. Products combining liquid condition monitoring, liquid level determination, and temperature methods in one miniature probe are not known.SUMMARY OF THE EMBODIMENTS

[0008] A novel liquid sensor is described and provided herein. In preferred embodiments, the liquid sensor comprises a first electrode, a second electrode, an active shield, a temperature sensor, an electronic circuit and a time division multiplexer. The time division multiplexer is in electrical communication with the first electrode, the second electrode, the active shield, and the temperature sensor.

[0009] The electronic circuit is configured to be in electric communication with the first electrode, the second electrode, and the active shield via the time division multiplexer andmonitor a condition of a liquid using impedance spectroscopy. The electronic circuit is also further configured to access the first electrode, the second electrode, and the active shield via the time division multiplexer and calculate a level of the liquid using time domain reflectometry. In addition, the electronic circuit is further configured to be in electrical communication with the temperature sensor via the time division multiplexer and calculate a temperature of the liquid.

[0010] In preferred embodiments, the liquid sensor has exactly two electrodes. In preferred configurations of the liquid sensor the first electrode is internal to and coaxial with the second electrode and the active shield.

[0011] In preferred embodiments the electronic circuit if configured to use the liquid level that is calculated to correct the condition of the liquid. The electronic circuit is configured to use the temperature of the liquid that is calculated to correct the condition of the liquid.

[0012] In some embodiments, the first electrode, the second electrode, and the active shield are separated for a portion of their longitudinal length by spacers. In embodiments with spacers, the spacers are made from high temperature chemically resistant plastic material. In embodiments with two spacers and an active shield, the active shield is preferably between the first spacer and second spacer.

[0013] In preferred embodiments, the active shield is electrically charged to the same voltage as the first inner electrode.

[0014] In some embodiments, the first electrode is a cylindrical tube and the second electrode is a cylinder that is coaxial with the first electrode. In other embodiments, all electrodes are implemented as flat electrodes. In yet other embodiments, the electrodes are flexible.

[0015] Methods for determining a liquid condition are also disclosed herein. In preferred embodiments, the methods comprise selectively placing a first portion of an electronic circuit in electrical communication with a first electrode and a second electrode via a multiplexer and monitoring a condition of a liquid using impedance spectrometry; selectively placing a second portion of an electronic circuit in electrical communication with the first electrode and the second electrode via the multiplexer and calculating a level of the liquid using time domain reflectometry; selectively placing a third portion of an electronic circuit in electrical communication with temperature sensor in thermal communication with the liquid and calculating a temperature of the liquid; and correcting the calculation of the condition of the fluid with the calculation of the level of the fluid and the temperature of the fluid.

[0016] In preferred methods, exactly two electrodes are used. In some methods, the first electrode is internal to and coaxial with the second electrode. In yet other embodiments, all the electrodes are implemented as flat electrodes. In some embodiments, the electrodes are flexible.

[0017] In some methods, the first electrode and second electrode are separated for a portion of their longitudinal length by spacers. In embodiments with spacers, the spacers are made from high temperature chemically resistive plastic. In yet even more preferred methods, the first electrode and second electrode are separated for a portion of their longitudinal length by a first spacer and a second spacer with a shield in between the first spacer and the second spacer.

[0018] In preferred embodiments of the methods, the shield is an active shield.BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Fig. la illustrates a liquid sensor that can detect both the liquid level and monitor the liquid conditions, and determine the temperature of liquid;

[0020] Fig. lb illustrates the liquid sensor of claim la with an electronic circuit downstream on an electrical cable instead of on the distal end of the sensor;

[0021] Fig. 2a illustrates a cross section of a time domain reflectometer (TDR) probe for determining the level of a liquid;

[0022] Fig. 2b illustrates the TDR probe of Fig. 2a in operation;

[0023] Fig. 2c is a plot of the magnitude of a TDR signal for the probe of Fig. 2a in operation;

[0024] Fig. 3a illustrates a cutaway view showing the internal design for one embodiment of a liquid sensor that can detect liquid level, monitor liquid conditions, and determine the temperature of liquid;

[0025] Fig. 3b shows cross-section 3b-3b of the liquid sensor of Fig. 3a;

[0026] Fig. 3c shows cross-section 3c-3c of the liquid sensor of Fig. 3a;

[0027] Fig. 3d shows cross-section 3d-3d of the liquid sensor of Fig. 3a;

[0028] Fig. 3e illustrates an exploded view of the liquid sensor of Fig. 3a;

[0029] Fig. 3f illustrates a break-away view of a temperature sensor coupled to a liquid sensor;

[0030] Fig 4 illustrates variations of the magnitude of Impedance Spectroscopy signal as a function of liquid level;

[0031] Fig. 5 shows the behaviour of Time Domain Reflectometry signal as a function of liquid level;

[0032] Fig. 6 illustrates the effect of temperature on the Impedance Spectroscopy signal;

[0033] Fig. 7 illustrates a schematic block diagram of an electronic circuit for use in an electronic sensor that can determine the liquid level, monitor the liquid conditions, and determine temperature of liquid.DETAILED DESCRIPTION OF THE DRAWINGS

[0034] This patent document describes liquid sensors and / or probes that are capable of performing multiple functions in a single sensor and / or probe and methods for using the same. In preferred embodiments, the liquid sensor can determine the liquid level of a vessel and also monitor the condition of the liquid. In an even more preferred embodiment, the liquid sensor can also monitor the temperature of the liquid.

[0035] Although the liquid sensors described herein may be used for any application, the primary applications are contemplated to be aircraft engines, turbines, pumps and machinery. In these applications, real-time determination of critical parameters of liquids such as lubricating oils, hydraulic fluids and fuels is critical. Sensing contamination, liquid level, stratification, and temperature of liquids is vital for the operation of machinery.

[0036] Fig. la illustrates a liquid sensor 1 that can detect both the liquid level and the monitor the liquid conditions. The liquid sensor 1 is placed inside a vessel 6 that contains a liquid 8. In preferred embodiments, the liquid sensor 1 comprises a main body 10 with an electronic circuit 2 at one distal end. As will be explained in more detail below, the main body 10 of the liquid sensor 1 is generally elongated to extend down into a liquid and contains a plurality of components.

[0037] In the preferred embodiments described herein, the liquid sensor 1 includes a temperature sensor 4 attached at the bottom end of the liquid sensor 1, i.e. the opposite distal end from the electronic circuit 2.

[0038] Fig. lb illustrates the liquid sensor 1 of claim la with the electronic circuit 2 downstream on an electrical cable instead of on the distal end of the sensor 1. Due to the high temperatures the sensor may be subjected to, for example in the use with engine oils, embodiments without the electronic circuit connected directly to the body of the sensor 1 may be desirable. As may be seen in Fig. lb, the electrical connector is connected with ahigh temperature cable 3. The high temperature cable 3 preferably has internal shielding for the cancellation of parasitic capacitance between the electrodes.

[0039] Impedance spectroscopy (IS) is the preferred method for liquid condition monitoring of the current invention. In addition, time domain reflectometry (TDR) is the preferred method of determining the liquid level in the current invention. Fig. 2a illustrates a cross section of a TDR probe for determining the level of a liquid. As may be seen in Fig. 2a, the cross-section is comprised by an inner electrode 20 surrounded by an outer electrode 10. In preferred embodiments, the inner electrode 20 and outer electrode 10 are coaxial. In even more preferred embodiments, the electrodes are tubular and coaxial however, other shapes such as planar, printed, or flexible electrodes could be used. In preferred embodiments, the inner electrode 20 is implemented as a hollow tube for the purpose of containing an additional wire. The wire will not interfere with operation of TDR or IS because those signals will not see the cavity inside the electrode 20 nor what’s arranged inside it.

[0040] Fig. 2b illustrates the TDR probe of Fig. 2a in operation. In operation, a TDR probe includes an electronic circuit that emits a short microwave pulse that propagates from the top of the probe along the waveguide, then reflects back from the liquid surface and returns back to the circuit. The circuit measures elapsed time between emitted and returned pulses. The operation of the TDR probe is similar to radar or sonar — but in a waveguide using an electrical signal. Fluids inside the probe create boundaries that reflect a portion of the incident signal. Unlike sonar and radar, the microwave pulse is contained within the coaxial waveguide, eliminating erroneous “echoes.” TDR can even determine liquid level boundaries for inner layers in a fluid, for example in the case of stratification between water and oil, as illustrated with Fig. 2b and clarified with Fig. 2c: each boundary is detected by a “step” in the magnitude of reflection caused by change of dielectric properties of the fluid at the boundary. Dielectric properties (permittivity) are a function of temperature for mostliquids. Accordingly, in preferred embodiments, a temperature sensor is preferred for correction and quantitative estimate of reflection magnitude. The magnitude also allows assessment of liquid properties such as concentration or composition, but the primary purpose is the determination of liquid level. Liquid properties monitoring is the goal of Impedance Spectroscopy.

[0041] Fig. 3a illustrates a cutaway view showing the internal design arrangement of one embodiment of a sensor that can detect a liquid level, monitor the liquid conditions, and determine the temperature of the liquid. When assembled, the body 10, which also serves as the airframe connected reference electrode or outer electrode 10, is coaxial with an inner electrode 20, which serves as an impedance spectroscopy sensing electrode 20. A couple spacers 24 and 28 and an active shield 26 are also coaxial with the electrodes 20 and 10. The spacers 24 and 28 help maintain the proper separation and spacing between the inner and outer electrodes. As may be appreciated, the spacers and active shield do not extend all the way to the distal tip of the electrodes 10, 20 but rather are located at the top and extend only a short portion of the way down the length the electrodes 10, 20.

[0042] As may be appreciated from Fig. 3a-3d, the body 10 of the sensor is a tubular coaxial design allowing liquid to enter the space between the airframe connected reference electrode 10 and the impedance spectroscopy sensing electrode 20 similar to that of the TDR sensor from Figs. 2a and 2b. In other embodiments, other configurations may be used. For example, a planar design for the electrodes may be used.

[0043] One major difference between the embodiments disclosed herein and the TDR sensor of Figs. 2a and 2b is that in the embodiments herein the inner electrode 20 also serves as an impedance spectroscopy sensing electrode 20. As will be explained in more detail below, it would not be obvious to one skilled in the art to use an impedance spectroscopy electrode 20 in this configuration because the impedance spectroscopy electrode 20 needs tobe completely submerged to work correctly. In the embodiments herein, the impedance spectroscopy electrode 20 will not be completely submerged. However, the present patent documents teach that correction may be done for such errors.

[0044] Referring now to Fig. 3a, the impedance spectroscopy sensing electrode 20 forms a capacitor in combination with the outer electrode 10. That portion of the probe is exposed to the liquid under test: The impedance spectroscopy signal related to that portion of the probe will determine dielectric properties of the liquid for characterizing the liquids composition such as contamination. Also, the top portion of the probe with electrical connector 22 and also electrical cable 3 of Fig. lb will contribute to the same impedance spectroscopy signal. Such unwanted contribution will affect the combined signal and result in errors. For the purpose of cancellation of that effect, the sensing electrode 20 is surrounded with a shield 26 driven to the same electric potential as the sensing electrode 20 by the circuit 2 of Fig. 1. Such an arrangement prevents the formation of any capacitance between the sensing electrode 20 and the outer electrode 10 where the shield 26 is present. Moreover, electrical connector 22 and connecting cable 3 would develop the same parasitic capacitance without shielding. The preferred embodiment employs triax connector 22 and double braided (shielded) cable 3, so the shield 26 electrically connecting to the inner shield of the triax connector 22 and then connecting to the inner shield of the cable 3 all form one continuous electrical shield around coaxial sensing electrode 20 and its connection all the way to electronic circuit 2. That continuous shield is driven to the same voltage as the sensing electrode 20 by electronic circuit 2 thus representing an active shield. Nevertheless, the outer braid (shield) of the cable 3 and the outer shield of the triax connector 22 are all connected to the outer electrode 10, so the resulting capacitor needed for impedance spectroscopy isn’t interrupted.

[0045] In the embodiment shown in Fig. 3, an electrical connector 22 is attached to one end of the sensor 1, assuming electronic circuit 2 is located off the body 10 of the sensor 1, connected with the cable as shown with Fig. lb. In preferred embodiments, this connector is a triax connector. However, in other embodiments, other connectors may be used. In still yet other embodiments, the electrical connector 22 is replaced with electronic circuit 2 attached to the top end of the body 10 of the sensor 1, as shown with Fig. la. For some applications, such an arrangement allows to save on installation space, but is only viable if the operating temperature range for the Probe does not exceed ratings for the electronic components.

[0046] Fig. 3b illustrates a cross-section 3b-3b of the top portion of the liquid sensor 1 shown in Fig. 3a. As may be appreciated from Figs. 3a-3d, the airframe reference electrode 10 and the impedance spectroscopy electrode 20 are coaxial. In addition, the first spacer 28, shield 26 and second spacer 24 are all coaxial and between the two electrodes.

[0047] As may be appreciated by returning to Fig. 3e, in some embodiments, the outer housing 10, which is also the outer electrode 10, may have holes to allow the liquid to more easily enter and exit the space between the electrodes 10 and 20.

[0048] Returning to Fig. 3a, 3b, and 3e, in preferred embodiments, the spacers are made from high temperature chemically resistant plastics such as Polyetheretherketone (PEEK). However, in other embodiments, other materials may be used.

[0049] Not all embodiments are required to have a shield and embodiments without a shield may be contemplated. However, in preferred embodiments, the shield 26 may be implemented in the form of an “active shield” with an additional (internal) braided jacket driven by the circuit to the same voltage potential as the active sensing electrode. With an equal voltage potential at both sensing electrodes and an active shield, no parasitic capacitance can develop.

[0050] Fig. 3a, 3d, and 3f illustrates breakaway and exploded views of the liquid sensor of Figs, lb showing the connection of a temperature sensor 4. In preferred embodiments, a temperature sensor 4 is in thermal communication with one or more of the electrodes or a housing. In general, the temperature sensor 4 just needs to be in thermal communication with the environment of the sensor. In preferred embodiments, the temperature sensor 4 is a resistance temperature detector (RTD) temperature sensor. In ever more preferred embodiments, the temperature sensor 4 is a platinum resistance temperature (Pt) RTD.

[0051] In the embodiment shown in Fig. 3a, 3d, and 3f, the temperature sensor 4 is located at the bottom end of the probe, such that it is subjected to the temperature of the liquid directly, even if liquid level is low covering just the distant end. The temperature sensor 4 is mounted with an insert 33 that seals the temperature sensor 4 against the liquid environment. In preferred embodiments, the insert 33 is implemented as a bead of plastic fixed in place with the help of swaging grooves 34. In even more preferred embodiments, high temperature chemically resistant epoxy is used for the purpose. The insert is small in size, so the temperature sensor 4 follows the temperature of the liquid environment with almost no lag. The temperature sensor 4 is connected to the electronic circuit 2 through the cable 3, inner shield of connector 22, and active shield electrode 26 by means of high temperature isolated wire 30. The wire 30 is connected to the shield 26 at the point of attachment 31. A small opening through the sensing electrode 20 is provided for the wire 30: Fig. 3f depicts the point of attachment 31 for the wire 30. The diameter of the wire 30 is small enough so to pass easily through the entire length of the sensing electrode 20, which is actually a thin tube. Presence of the wire 30 inside electrode 20 doesn’t affect functionality of the Probe: TDR and IS signals are developed around the sensing electrode 20, rather than inside it. The other end of the temperature sensor4 is attached to the outer electrode 10 at point of attachment 32 shown with Fig. 3a. Attachments 31 and 32 must be completed with alow Ohmic resistance connection such as soldering, brazing, or welding. As a result, the temperature sensor 4 becomes connected to electronic circuit 2 so it could read temperature sensor 4 resistance and convert it to the temperature of the liquid.

[0052] The sensing electrodes for liquid level monitoring have to stretch from the bottom of the tank with liquid, to the top of the tank, so the liquid may be determined at any level. In contrast, impedance spectroscopy sensors must always be completely submerged. This creates a seemingly impossible design criteria to rectify with a single set of electrodes. However, the impedance spectroscopy results may be corrected if one knows the liquid level. To this end, the liquid level sensor and liquid condition monitoring sensor are incorporated in a design with a single set of electrodes as explained herein.

[0053] Referring now to Fig. 4, the magnitude of impedance spectroscopy signal picked by electronic circuit 2 appears to be a function of liquid level. The impedance spectroscopy method itself relies on the characteristic response of the signal under a sweeping frequency of the excitation signal. The example shown in Fig. 4 depicts a typical response of an IS signal for a lubricating oil specimen, for a 10 Hz to 12 kHz scan frequency of excitation. Different chemical compositions of the liquid will result in different responses: that allows liquid condition monitoring. It could be appreciated from Fig. 4 that although the IS signal magnitude is roughly proportional to the volume of the liquid physically present in the space between the sensing and outer electrodes (i.e. proportional to liquid level), the resulting shape of IS response curve doesn’t change with variations of liquid level: it’s the same liquid substance obviously. That allows correction of the IS signal if the liquid level is known.

[0054] Referring now to Fig. 5, TDR liquid level determination operates by measuring time delay between an emitted and received microwave pulse. The pulse is reflected from the meniscus of the liquid as a result of the stepwise change in permittivity at the air / liquid boundary. It may be appreciated from Fig. 5 that time delay is directly reciprocal to liquidlevel. The time delay is measured on the base of i step height of the magnitude of each reflection: reflections are the sharpest at that point. Also, it may be appreciated that in the case of liquids with different permittivity, or if permittivity changes due to temperature variation, the resulting magnitude of reflections will change. Correspondingly, location of that point will follow half of the magnitude in the vertical direction at the chart of Fig. 5, but importantly its horizontal position will stay the same. That allows the TDR method to measure liquid level with no reference to the absolute value of dielectric permittivity. Now with the liquid level determined, the magnitude of the IS signal reflections may be corrected by means of calibration.

[0055] Both methods for IS condition monitoring and TDR liquid level determination depend on temperature of the fluid for accuracy. Accordingly, as explained above, a temperature sensor 4 for computation of correction factors needed for the two methods is included. Referring now to Fig. 6, the IS signal exhibits a strong function of temperature due to change in permittivity: that’s a natural characteristic for most liquids and substances. The temperature sensor 4 allows cancellation of that temperature function by means of calibration. Briefly, all three methods complement each other through correction algorithms and make each sensor more accurate.

[0056] Fig. 7 illustrates a schematic block diagram of an electronic circuit for use in an electronic sensor that can determine both the liquid level and monitor the liquid conditions. The electronic circuit 2 is in electrical communication to the electrodes 10, 20, and 30 shown in Fig. la. Alternatively, the electronic circuit 2 is in electrical communication with the electrodes 10, 20, and 30 via the cable 3, especially for the case of high temperature applications, as shown in Fig. lb. The cable 3 allows the electronic circuit 2 to be situated within a lower temperature zone.

[0057] Returning to Fig. 7, the electronic circuit 2 consists of a number of blocks for implementation of the three sensing methods: block 5 for impedance spectroscopy, block 6 for TDR, and block 7 for temperature. Since the Liquid Sensor Probe 1 employs a single set of the sensing electrodes, operation of the three methods is interleaved in time, with multiplexer 8, allowing access to the electrodes for each block in sequence. In some embodiments, the time multiplexing may not be even amongst the sensing methods and one method may be sampled more or less than others.

[0058] A circuit block Driver 9 allows transfer of the signals from the multiplexer 8 over the cable 3 or connector 22. As such, the sensing electrode 20 is driven and connected either to IS block 5 or TDR block 6 sequentially by means of MUX 8, whereas active shield electrode 26 is commutated either to temperature sensing block 7 during the temperature measurement phase, or to IS block 5 when the impedance spectroscopy phase is active, or to outer electrode 10 when the TDR phase acts. The outer electrode 10 is connected to the airframe (same as signal ground) permanently. Accordingly, the three methods operate the same sensing electrodes sequentially. It’s an object of the current invention to employ time interleaving for operation of the methods. The methods are implemented physically with the electronic circuit.

[0059] When the impedance spectroscopy block 5 is in electrical communication with the electrodes via the multiplexer, the electronic circuit creates an AC current and communicates it to the electrodes. The AC current is then swept through a range of frequencies and the impedance spectroscopy block 5 measures the change in impedance between the two electrodes. The temperature sensor 4 does not interfere with the IS signal, because it’s connected to the active shield electrode 26 driven by IS block 5 with low output impedance. In the preferred embodiments, a Platinum RTD with a standard resistance within 1-10 kOhm range is used for temperature sensor 4. At the same time, the IS block 5 output impedance isat the level of 3 orders of magnitude less, hence sensor 4 does not represent any load to the circuit, and is “invisible”.

[0060] When the TDR block 6 is in electrical communication with the electrodes via the multiplexer, the electronic circuit creates a microwave pulse and communicates it to the electrodes via the multiplexer. The TDR block 6 measures time delay between the emitted pulse and received reflections. In preferred embodiments, signal sampling over a semi- continuous time base operating principle is used, although other methods could be employed as well, as known to those familiar with the arts. The temperature sensor 4 does not interfere with TDR signal because it’s out of the circuit.

[0061] When the temperature block 7 is in electrical communication with the temperature sensor via the multiplexer, the temperature block 7 measures the voltage across the temperature leads from the temperature sensor.

[0062] In preferred embodiments, microcontroller block 12 controls timing operations for blocks 5-9, performs signal generation, data acquisition, signal conditioning and correction based on calibration, and does data exchange with connected equipment over interface block 11.

Claims

CLAIMSWhat is claimed is:

1. A liquid sensor comprising: a first electrode; a second electrode; an active shield; a temperature sensor; a time division multiplexer in electrical communication with the first electrode, the second electrode, the active shield, and the temperature sensor; and an electronic circuit configured to be in electric communication with the first electrode, the second electrode, and the active shield via the time division multiplexer and monitor a condition of a liquid using impedance spectroscopy, and further configured to access the first electrode, the second electrode, and the active shield via the time division multiplexer and calculate a level of the liquid using time domain reflectometry, and further configured to be in electrical communication with the temperature sensor via the time division multiplexer and calculate a temperature of the liquid.

2. The liquid sensor of claim 1, wherein the liquid sensor has exactly two electrodes.

3. The liquid sensor of claim 1, wherein the first electrode is internal to and coaxial with the second electrode and active shield.

4. The liquid sensor of claim 1, wherein the electronic circuit uses the liquid level that is calculated to correct the condition of the liquid.

5. The liquid sensor of claim 1, wherein the electronic circuit uses the temperature of the liquid that is calculated to correct the condition of the liquid.

6. The liquid sensor of claim 3, wherein the first electrode, the second electrode, and the active shield are separated for a portion of their longitudinal length by spacers.

7. The liquid sensor of claim 6, wherein the spacers are made from high temperature chemically resistant plastic material.

8. The liquid sensor of claim 3, wherein the first electrode and second electrode are separated for a portion of their longitudinal length by a first spacer and a second spacer with the active shield in between the first spacer and second spacer.

9. The liquid sensor of claim 8, wherein the active shield is electrically charged to the same voltage as the first electrode.

10. The liquid sensor of claim 1, wherein the first electrode is a cylindrical tube and the second electrode is a cylinder that is coaxial with the first electrode.

11. The liquid sensor of claim 1, wherein all electrodes are implemented as flat electrodes.

12. The liquid sensor of claim 11, wherein the electrodes are flexible.

13. A method for determining a liquid condition comprising the steps of: selectively placing a first portion of an electronic circuit in electrical communication with a first electrode and a second electrode via a multiplexer and monitoring a condition of a liquid using impedance spectrometry; selectively placing a second portion of an electronic circuit in electrical communication with the first electrode and the second electrode via the multiplexer and calculating a level of the liquid using time domain reflectometry;selectively placing a third portion of an electronic circuit in electrical communication with temperature sensor in thermal communication with the liquid and calculating a temperature of the liquid; and correcting the calculation of the condition of the fluid with the calculation of the level of the fluid and the temperature of the fluid.

14. The method of claim 13, wherein exactly two electrodes are used.

15. The method of claim 14, wherein the first electrode is internal to and coaxial with the second electrode.

16. The method of claim 14, wherein the first electrode and second electrode are separated for a portion of their longitudinal length by spacers.

17. The method of claim 15, wherein the spacers is are made from high temperature chemically resistive plastic.

18. The method of claim 14, wherein the first electrode and second electrode are separated for a portion of their longitudinal length by a first spacer and a second spacer with a shield in between the first spacer and the second spacer.

19. The method of claim 18, wherein the shield is an active shield.

20. The method of claim 14, wherein all electrodes are implemented as flat electrodes.

21. The method of claim 20, wherein the electrodes are flexible.

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