Device for capacitive measurements in a multi-phase medium

Abstract

A device comprises at least one pair of excitation electrodes forming a capacitor; a floorplan (e.g., a ground plane); and an electronic circuit. The device comprises at least one control electrode arranged at a distance from the capacitor. A switching circuit, of the device, comprises a switch having an open state and a closed state. The switching circuit is designed to apply, to the control electrode, an electric potential common to the floorplan when the switch is in the closed state. The switching circuit is also designed to leave a floating electrical potential for the control electrode when the switch is in the open state. The electronic circuit is designed to measure the mutual capacitance between the pair of excitation electrodes when the switch is in the open state and when it is in the closed state.

Claims

1. A device for taking capacitive measurements in a multiphase medium, comprising: at least one pair of excitation electrodes, forming a capacitor; a ground plane; and an electronic circuit, arranged to electrically connect the pair of excitation electrodes to the ground plane, and configured to: apply an electrical potential (V.sub.+, V.sub.−) to each excitation electrode at a working frequency, and measure a transcapacitance between the pair of excitation electrodes; wherein the device also comprises: at least one control electrode, arranged at a distance from the capacitor, and intended to be inserted into the multiphase medium; and a switching circuit comprising a switch having an open state and a closed state, in which states the switch electrically disconnects and connects the control electrode from/to the ground plane, respectively, the switching circuit being configured to: apply, to the control electrode, an electrical potential common to the ground plane, when the switch is in the closed state, and leave the electrical potential of the control electrode floating, when the switch is in the open state; and wherein the electronic circuit is configured to measure the transcapacitance between the pair of excitation electrodes when the switch is in the open state and when the switch is in the closed state.

2. The device of claim 1, wherein: the multiphase medium comprises a phase containing species that are electrically conductive at the working frequency; the species possessing a cut-off frequency, below which the species equalize the electrical potential, left floating, in the multiphase medium, over the distance at which is arranged the control electrode of the capacitor; and the working frequency is chosen so as to be lower than or equal to the cut-off frequency.

3. The device of claim 2, wherein: the capacitor has a characteristic distance; and the distance at which is arranged the control electrode of the capacitor is chosen so that: $0<l<100\times d\frac{f_c}{f}\mathrm{and}\mathrm{preferably}\mathrm{so}\mathrm{that}$ $0<l<10\times d\frac{f_c}{f}$ where: l is the characteristic distance of the capacitor, d is the distance at which is arranged the control electrode of the capacitor, f.sub.c is the cut-off frequency, and f is the working frequency.

4. The device of claim 1, further comprising a set of control electrodes, of the at least one control electrode, the control electrodes of the set being arranged at various distances from the capacitor and being intended to be inserted into the multiphase medium, the switching circuit comprising one dedicated switch for each of the control electrodes.

5. The device of claim 1, wherein: the electronic circuit comprises a virtual ground connected to one excitation electrode of the pair of excitation electrodes, and wherein the electronic circuit is configured to measure the transcapacitance between the pair of excitation electrodes using a three- or four-wire method.

6. The device of claim 5, wherein the electronic circuit further comprises an operational amplifier used as an inverter, the operational amplifier comprising: a non-inverting input, connected to the ground plane; and an inverting input, connected to one excitation electrode of the pair of excitation electrodes.

7. The device of claim 1, further comprising: a dielectric layer, comprising a first surface and an opposite second surface, the pair of excitation electrodes extending to the first surface of the dielectric layer; and a counter-electrode, extending to the second surface of the dielectric layer, and forming the ground plane.

8. The device of claim 1, wherein the pair of excitation electrodes is covered with a dielectric film.

9. The device of claim 1, wherein the capacitor formed by the at least one pair of excitation electrodes is selected from a parallel-plate capacitor, a capacitor with interdigitated electrodes, and a coaxial-cylinder capacitor.

10. An installation, comprising: the device of claim 1; and a vessel containing the multiphase medium of claim 1, the control electrode of the device being inserted into the multiphase medium.

11. A system for taking capacitive measurements in the multiphase medium of claim 1, comprising: a floating device, intended to float in the multiphase medium; and at least one of the device of claim 1, securely fastened to the floating device.

12. The system of claim 11, wherein: the floating device comprises a separating wall forming a barrier to the multiphase medium, the wall possessing an internal surface, and the device of claim 1 is mounted inside the wall, against the internal surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages will become apparent from the detailed description of various embodiments, the description containing examples and references to the appended drawings.

(2) FIG. 1 is a schematic view illustrating the circuitry of a device according to the present disclosure.

(3) FIG. 2 is a schematic perspective view, illustrating one embodiment of a device according to the present disclosure.

(4) FIG. 3 is a schematic perspective view, illustrating one embodiment of a device according to the present disclosure.

(5) FIG. 4 is a schematic perspective view, illustrating one embodiment of a device according to the present disclosure.

(6) FIG. 5 is a schematic perspective view, illustrating one embodiment of a device according to the present disclosure.

(7) FIG. 6 is a schematic cross-sectional view, illustrating one embodiment of a device according to the present disclosure.

(8) FIG. 7 is a graph, the x-axis of which represents the observed oil level (in mm) and the y-axis of which represents the phase level (in mm) determined from the capacitive measurements of a device according to the present disclosure. “A” is an aqueous phase, “B” is an oil phase, and “C” is an air phase.

(9) FIG. 8 is a graph, the x-axis of which represents the observed water level (in % of the total height of the multiphase medium) and the y-axis of which represents the phase level (in % of the total height of the multiphase medium) determined from the capacitive measurements of a device according to the present disclosure. “A” is an aqueous phase, “B” is an oil phase, and “C” is an air phase.

(10) FIGS. 9a to 9e are schematic cross-sectional views, illustrating various embodiments of such a system according to the present disclosure.

DETAILED DESCRIPTION

(11) For the sake of simplicity, elements that are identical or that perform the same function have been designated with the same references in the various embodiments.

(12) One subject of the present disclosure is a device for taking capacitive measurements in a multiphase medium M, comprising: at least one pair of excitation electrodes 1, 2, forming a capacitor; a ground plane PM; and an electronic circuit 3, arranged to electrically connect the pair of excitation electrodes 1, 2 to the ground plane PM, and configured to: apply an electrical potential V+, V− to each excitation electrode 1, 2 at a working frequency, and measure a transcapacitance between the pair of excitation electrodes 1, 2;
the device being noteworthy in that it comprises: at least one control electrode 4, arranged at a distance l from the capacitor, and intended to be inserted into the multiphase medium M; and a switching circuit 5 comprising a switch 50 having an open state and a closed state, in which states the switch 50 electrically disconnects and connects the control electrode 4 from/to the ground plane PM, respectively, the switching circuit 5 being configured to: apply, to the control electrode 4, an electrical potential common to the ground plane PM, when the switch 50 is in the closed state, and leave the electrical potential of the control electrode 4 floating, when the switch 50 is in the open state; and
in that the electronic circuit 3 is configured to measure the transcapacitance between the pair of excitation electrodes 1, 2 when the switch 50 is in the open state and when the switch 50 is in the closed state.
Multiphase Medium

(13) The multiphase medium M contains species forming phases P.sub.1, P.sub.2, P.sub.3. The species possess a cut-off frequency, below which the species equalize the electrical potential, left floating, in the multiphase medium M, over the distance l at which is arranged the control electrode 4 of the capacitor. It is assumed that one of the species is electrically conductive at the working frequency. By way of example, the working frequency may be set to 1 kHz to detect the presence of an aqueous phase P.sub.1. The multiphase medium M sets the electrical potential (left floating by the switch 50) of the control electrode 4 when the switch 50 is in the open state.

(14) In the case illustrated in FIG. 6, the multiphase medium M contains three phases P.sub.1, P.sub.2, P.sub.3 that are stratified, that differ in their dielectric properties, and that have a height h.sub.1, h.sub.2, h.sub.3 in a vessel R of height H, respectively.

(15) Capacitor

(16) The capacitor formed by the pair of excitation electrodes 1, 2 is advantageously selected from a parallel-plate capacitor (as illustrated in FIG. 2), a capacitor with interdigitated electrodes (as illustrated in FIGS. 4 to 6), and a coaxial-cylinder capacitor (as illustrated in FIG. 3).

(17) The capacitor has a characteristic distance, denoted d. For example, for a parallel-plate capacitor, the characteristic distance d is the distance separating the two plates. For a capacitor with interdigitated electrodes, the characteristic distance d is equal to λ/4, where λ is the period of the interdigitated structure. Lastly, for a coaxial-cylinder capacitor, the characteristic distance d is the radial distance between the two cylinders.

(18) The excitation electrodes 1, 2 may have different forms, such as planar forms, cylindrical forms, interdigitated forms, etc.

(19) The pair of excitation electrodes 1, 2 is advantageously covered with a dielectric film 20. By way of non-limiting example, the dielectric film 20 may be made from a dielectric selected from a polyimide, a polytetrafluoroethylene, and a photosensitive resin.

(20) By way of non-limiting example, the excitation electrodes 1, 2 may be made from a metal preferably selected from Cu, Ag, Au, and Al. However, the excitation electrodes 1, 2 may be made from a plastic (e.g., a polyphthalamide) into which carbon fibers have been incorporated in order to make the excitation electrodes 1, 2 electrically conductive.

(21) Ground Plane

(22) The device advantageously comprises: a dielectric layer 10, comprising a first surface and an opposite second surface, the pair of excitation electrodes 1, 2 extending to the first surface of the dielectric layer 10; and a counter-electrode, extending to the second surface of the dielectric layer 10, and forming the ground plane PM.

(23) Such a dielectric layer 10 allows the excitation electrodes 1, 2 and the counter-electrode to be electrically insulated from each other so as to avoid short-circuiting them.

(24) By way of non-limiting example, the counter-electrode may be a plate made from a metal. The metal is preferably selected from Cu, Ag, Au, and Al. However, the counter-electrode may be made from a plastic (e.g., a polyphthalamide) into which carbon fibers have been incorporated in order to make the counter-electrode electrically conductive.

(25) By way of non-limiting example, the dielectric layer may be made from a dielectric selected from a polyimide and a polytetrafluoroethylene.

(26) Electronic Circuit

(27) The working frequency, at which the electronic circuit 3 applies an electrical potential V+, V− to each excitation electrode 1, 2, is chosen so as to be lower than or equal to the cut-off frequency of at least one of the phases P.sub.1, P.sub.2, P.sub.3. As mentioned above, the working frequency may be set to 1 kHz to detect the presence of an aqueous phase P.sub.1. As illustrated in FIG. 1, the electronic circuit 3 may comprise an excitation source (e.g., an AC voltage generator) for charging one excitation electrode 1 of the pair in order to apply thereto an electrical potential V+. The other excitation electrode 2 of the pair may be connected to the ground plane PM in order to apply thereto the electrical potential V.

(28) The electronic circuit 3 advantageously comprises a virtual ground 30 connected to one excitation electrode 1, 2. The electronic circuit 3 is advantageously configured to measure the transcapacitance between the pair of excitation electrodes 1, 2 using a three- or four-wire method.

(29) The electronic circuit 3 advantageously comprises an operational amplifier 31, which is used as an inverter and comprises: a non-inverting input, connected to the ground plane PM; and an inverting input, connected to an excitation electrode 2.
The operational amplifier 31 is employed in linear regime so as to place the excitation electrode 2 connected to the inverting input at ground potential virtually.
Control Electrode(s)

(30) The distance at which is arranged the control electrode 4 of the capacitor, which distance is denoted I, is chosen so that:

(31) $0<l<100\times d\frac{f_c}{f}\mathrm{and}\mathrm{preferably}\mathrm{so}\mathrm{that}$$0<l<10\times d\frac{f_c}{f}$
where: f.sub.c is the cut-off frequency, and f is the working frequency.

(32) As illustrated in FIG. 4, the device advantageously comprises a set of control electrodes 4 that are arranged at different distances I.sub.1, I.sub.2, I.sub.3 from the capacitor, and that are intended to be inserted into the multiphase medium M. Each of the different distances (denoted I.sub.1) at which are arranged the control electrodes 4 of the set advantageously respects:

(33) $0<l_i<100\times d\frac{f_c}{f}\mathrm{and}\mathrm{preferably}\mathrm{so}\mathrm{that}$$0<l_i<10\times d\frac{f_c}{f}$

(34) As illustrated in FIG. 4, the control electrodes 4 of the set are spaced apart horizontally, this allowing the multiphase medium M to be studied in this dimension.

(35) As illustrated in FIG. 5, the device may comprise a set of control electrodes 4, which electrodes are arranged at the same distance l from the one or more capacitors, and spaced apart vertically so as to be coplanar, this allowing the multiphase medium M to be studied in this dimension.

(36) The control electrode 4 or the control electrodes 4 may take the form of a grid, or the form of meanders. In the case of a control electrode 4 taking the form of a grid, one advantageous effect thereof is to promote contact with the conductive phase of the multiphase medium M, whether the phase be distributed surfacewise or volumewise, for example when the conductive phase is a foam or an emulsion. In the case of a control electrode 4 taking the form of meanders, the electrode may advantageously be planar, and parallel to the interdigitated excitation electrodes 1, 2. The pitch of the meanders will possible be chosen so as to detect all or some of the conductive phase of the multiphase medium M, when the conductive phase is distributed alternately (for example, when droplets that are separate from one another wet a surface of the meanders) in order to allow the fraction of the conductive phase covering the meanders to be detected.

(37) By way of non-limiting example, the control electrode 4 or the control electrodes 4 may be made from a metal, which is preferably selected from Cu, Ag, Au, and Al. However, the control electrode 4 or the control electrodes 4 may be made from a plastic (e.g., a polyphthalamide) into which carbon fibers have been incorporated in order to make the control electrodes 4 electrically conductive.

(38) Switching Circuit

(39) When the device comprises a set of control electrodes 4, the switching circuit 5 comprises one dedicated switch 50 for each control electrode 4.

(40) By way of non-limiting example, the switch 50 may be an on/off switch or an electrical relay.

(41) Application to Phase Detection

(42) The device, according to the present disclosure, may be a detector of presence of one of the phases P.sub.1, P.sub.2, P.sub.3 provided that there is a difference between the transcapacitance measured by the electronic circuit 3 between the pair of excitation electrodes 1, 2 when the switch 50 is in the open state (denoted C.sub.off), and when the switch 50 is in the closed state (denoted C.sub.on). The difference measured between C.sub.off and C.sub.on is indicative of the difference in dielectric response (in terms of electrical permittivity e) of the detected phase when the latter is excited by an electrical potential, whether the latter is an exterior potential or not.

(43) In practice, a detection threshold will be defined for the difference between C.sub.off and C.sub.on, above which threshold the presence of the phase is ensured.

(44) Application to Phase Quantification

(45) Let the following be considered: a multiphase medium M containing three stratified phase P.sub.1, P.sub.2, P.sub.3 that differ in their dielectric properties, and that have a height h.sub.1, h.sub.2, h.sub.3 in a vessel R of height H, respectively; and that the phase P.sub.1 has a differentiated dielectric response in the presence of an exterior electrical potential.

(46) Noting x.sub.1=h.sub.1/H; x.sub.2=h.sub.2/H; x.sub.3=h.sub.3/H

(47) it is possible to establish the following equations:
x.sub.1+x.sub.2+x.sub.3=1
C.sub.off=x.sub.1C.sub.1,off.sup.H+x.sub.2C.sub.2.sup.H+x.sub.3C.sub.3.sup.H
C.sub.on=x.sub.1C.sub.1,on.sup.H+x.sub.2C.sub.2.sup.H+x.sub.3C.sub.3.sup.H
where: C.sub.2.sup.H and C.sub.3.sup.H are the transcapacitances between the pair of excitation electrodes 1, 2 when the vessel R is filled with a phase P.sub.2 and filled with a phase P.sub.3, respectively, and C.sub.1,on.sup.H and C.sub.1,off.sup.H are the transcapacitances between the pair of excitation electrodes 1, 2 when the vessel R is filled with a phase P.sub.1, and when the switch 50 is in the closed state and in the open state, respectively.

(48) It is then possible to obtain the following relationships:

(49) $x_1=\frac{C_{on}-C_{off}}{C_{1,\mathrm{on}}^H-C_{1,\mathrm{off}}^H}$$x_2=\frac{\left(\left(C_{off}-x_1{C}_{1,off}^H\right)-C_3^H\left(1-x_1\right)\right)}{C_2^H-C_3^H}$$x_3=\frac{\left(\left(C_{on}-x_1{C}_{1,on}^H\right)-C_2^H\left(1-x_1\right)\right)}{C_3^H-C_2^H}$

(50) The value x.sub.1 (and therefore h.sub.1) is perfectly determined because: C.sub.on and C.sub.off are values measured during the acquisition, and C.sub.1,on.sup.H and C.sub.1,off.sup.H are values measured by prior calibration (see the following section).

(51) In the same way, it is possible to determine x.sub.2 (and therefore h.sub.2) and x.sub.3 (and therefore h.sub.3) on the basis of the acquisition measurements and of the calibration measurements.

(52) Calibration of the Device

(53) If the device is to be used for phase quantification, it is necessary to calibrate the device beforehand in order to determine the values of C.sub.1,on.sup.H, C.sub.1,off.sup.H, C.sub.2H, C.sub.3.sup.H.

(54) These calibrations may also be performed based on two other conditions of known levels, or by similarity with another medium, or even by numerical simulation.

(55) It is possible to calibrate the device in-situ, using additional capacitive sensors (compensating capacitors) and techniques known to those skilled in the art.

(56) Installation

(57) One subject of the present disclosure is an installation, comprising: a vessel R containing a multiphase medium M; and a device according to the present disclosure, the control electrode 4 being inserted into the multiphase medium M.

(58) The term “vessel” has a broad meaning and covers any means allowing the multiphase medium M to be contained.

(59) The vessel R is advantageously electrically insulated from the multiphase medium M in order not to apply electrical potential to the control electrode 4 (left floating by the switch 50 in the open state). The electrical potential thus remains set by the multiphase medium M when the switch 50 is in the open state. When the vessel R is not electrically insulated from the multiphase medium M (e.g., a vessel R with metal walls), then the capacitor and the control electrode 4 are arranged at a sufficiently large distance from the metal walls of the vessel R so that the vessel R does not influence the electrical potential of the multiphase medium M.

(60) The pair of excitation electrodes 1, 2 may be inserted into the multiphase medium M. By way of variant, the pair of excitation electrodes 1, 2 may be placed on the exterior side of a dielectric wall serving to contain the multiphase medium M.

Examples of Embodiments

(61) As illustrated in FIGS. 7 and 8, tests were performed on a multiphase medium M containing an aqueous phase A, an oil phase B and an air phase C in a vessel R consisting of a test-tube of a height H.

(62) The capacitor was formed by a pair of interdigitated excitation electrodes 1, 2. Each excitation electrode 1, 2 had a width of 250 μm. The inter-electrode distance was 250 μm. The capacitor had a height of 300 mm. The capacitor extended over one face of a dielectric layer 10 made of polyimide. The dielectric layer 10 had a thickness of 25 μm. The pair of excitation electrodes 1, 2 was covered with a dielectric film 20 made of polyimide. The dielectric film 20 had a thickness of 25 μm.

(63) The working frequency was 1 kHz. The transcapacitance between the pairs of excitation electrodes 1, 2 was measured using an LCR meter.

(64) The calibration to determine the values C.sub.A,on.sup.H, C.sub.A,off.sup.H was carried out by filling a test-tube with water and by submerging the pair of excitation electrodes 1, 2 and the control electrode 4 in the test-tube. The same protocol was observed for the oil.

(65) In the case illustrated in FIG. 7, oil was gradually poured into a test-tube that initially contained only water. In the case illustrated in FIG. 8, water was gradually poured into a test-tube that initially contained only oil. In both cases it was observed that the fluid-level values determined on the basis of the capacitive measurement of the device, according to the present disclosure, coincided with the fluid levels observed by eye.

(66) System for Taking Measurements with a Floating Device

(67) As illustrated in FIGS. 9a to 9e, one subject of the present disclosure is a system for taking capacitive measurements in a multiphase medium M, comprising: a floating device 6, intended to float in the multiphase medium M; and at least one device 7 according to the present disclosure, which device is securely fastened to the floating device 6.

(68) As illustrated in FIG. 9e, the floating device 6 may comprise a separating wall 60 forming a barrier to the multiphase medium M, the wall 60 possessing an internal surface, and the device 7 for taking capacitive measurements is mounted inside the wall 60, against the internal surface.

(69) The multiphase medium M may comprise three stratified phases comprising, in succession: a liquid first phase P.sub.1, which is electrically conductive at the working frequency; a liquid second phase P.sub.2, which is liable to contain one or more pollutants such as oils or hydrocarbons, and which is dielectric at the working frequency; and a gaseous third phase P.sub.3, which is dielectric at the working frequency.

(70) The floating device 6 is arranged to float on the surface of the liquid second phase P.sub.2. The floating device 6 may be a buoy. The floating device 6 may comprise a number of floats 6a, 6b. The floating device 6 may be of conical shape.

(71) Of course, the electronic circuit 3 and the switching circuit 5 of the device 7 for taking capacitive measurements are seal-tight with respect to the multiphase medium M. The device 7 for taking capacitive measurements advantageously comprises a module 70 configured to transmit the height of the liquid second phase P.sub.2, the transmission possibly being performed via a wireless communication. The device 7 for taking capacitive measurements is advantageously powered electrically: by a battery, or by a system for harvesting energy from the movement or variations in temperature of the floating device 6, or by a power source connected to a solar collector,
in order to make the operation of the device 7 for taking capacitive measurements autonomous.

(72) For an application for limiting the height of the liquid second phase P.sub.2, as illustrated in FIGS. 9a, 9c and 9d, the excitation electrodes 1, 2 and the one or more control electrodes 4 of the device 7 for taking capacitive measurements may be arranged to extend vertically, in contact with the liquid first and second phases P.sub.1, P.sub.2, and to a depth slightly larger than the height of the liquid second phase P.sub.2.

(73) For an application for detecting pollutants, as illustrated in FIG. 9b, the excitation electrodes 1, 2 and the one or more control electrodes 4 of the device 7 for taking capacitive measurements are advantageously arranged to lie entirely in the liquid second phase P.sub.2, preferably substantially parallel to the free surface of the liquid second phase P.sub.2.

(74) The invention is not limited to the described embodiments. Those skilled in the art will be able to consider technically workable combinations thereof, and to substitute equivalents therefor.