CA1271524A - Dielectric constant measuring apparatus - Google Patents
Dielectric constant measuring apparatusInfo
- Publication number
- CA1271524A CA1271524A CA000537062A CA537062A CA1271524A CA 1271524 A CA1271524 A CA 1271524A CA 000537062 A CA000537062 A CA 000537062A CA 537062 A CA537062 A CA 537062A CA 1271524 A CA1271524 A CA 1271524A
- Authority
- CA
- Canada
- Prior art keywords
- electrode
- sense
- sample
- electrodes
- chamber
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
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- Measurement Of Resistance Or Impedance (AREA)
- Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
Abstract
ABSTRACT
Disclosed is an apparatus for measuring the mean permittivity of a sample of material comprising a sense chamber for containing the sample, the sense chamber comprising a cylin-drical tube of low permittivity, low conductivity material and having an outer surface provided with at least three helical foil electrode structures extending along substantially the entire length of the tube. Each electrode structure comprises an inner sense electrode in contact with the tube and an outer shield electrode. The sense electrode and the shield electrode are separated by a thin barrier of insulating material. The apparatus further comprises a source for applying out of phase alternating voltage signals to the electrode structures to produce a rotating electrical field in the sense chamber and for deriving a voltage read-out signal proportional to the sum of the charging currents applied to the electrode structures, the read-out signal being directly related to the mean permittivity of said sample.
Disclosed is an apparatus for measuring the mean permittivity of a sample of material comprising a sense chamber for containing the sample, the sense chamber comprising a cylin-drical tube of low permittivity, low conductivity material and having an outer surface provided with at least three helical foil electrode structures extending along substantially the entire length of the tube. Each electrode structure comprises an inner sense electrode in contact with the tube and an outer shield electrode. The sense electrode and the shield electrode are separated by a thin barrier of insulating material. The apparatus further comprises a source for applying out of phase alternating voltage signals to the electrode structures to produce a rotating electrical field in the sense chamber and for deriving a voltage read-out signal proportional to the sum of the charging currents applied to the electrode structures, the read-out signal being directly related to the mean permittivity of said sample.
Description
127i~2A
This invention relates to apparatus for measuring the mean permittivity (dielectric constant) of a sample of material.
A measurement oE the mean permittivity of a sample of material in a chamber enables one to estimate the volume ratio of two materials of known properties making up the sample,at least one of the two materials being a fluid, i.e. gas or liquid. One can also measure the dielectric constant or permittivity of an unknown material of known volume when surrounded by a fluid of known properties. In all cases, the materials under test may be either static or in motion as for a contained flow situation.
Canadian patent No. 1,082,310 of Dechene, et al, issued July 22, 1980 discloses apparatus for measuriny relative fractions of liquid vapor in mixed phase fluid flow. Measurements are made in several current loops and the current loops are supplied by a rotating electrical field source by means of multi-phase voltage oscillations of 1-30 kHz. A reference sensor is used and the voltage across a load resistor represents conductivity.
Canadian patent No. 1,101,070 of Newton, et al, issued May 12, 1981, relates to measuring relative fractions of liquid (nonconductive) and vapor, or solids (nonconductive) and gases.
Sequential capacitance measurements are made across the cross section of a flow to be measured. Frequencies of 10-100 kHz are used and a rota-ting electric field is produced to average the dielectric constant of the entire cross section of the sensor.
U.S. patent 3,639,835 of Dammig, Jr., et al, issued February 1, 1972, discloses a capacitive tank gaging apparatus ~5~
based on the fact that the density of a dielectric liquid is directly related to its dielectric constant. The capacitance between electrodes separated by space containing liquid and gas of known characteristics is a measure of the quantity of liquid present. A measuring unit impresses different voltages on sets of electrodes in a manner that generates relatively uniform time-varying electric fields across the tank. The measuring unit senses the capacitive current -through the tank interior between the electrodes, which current is a function of the dielectric corlstan-ts of -the fluids in the -tank.
The present invention, which differs in a number of respects from the arrangements disclosed in -the above mentioned patents, enables simple and highly accurate estimation of the mean permittivity of the contents of a sense chamber. Briefly, the invention uses a sense chamber consisting of a cylindrical tube of relatively low permittivity, low conductivity material (e.g. poly-tetrafluoroethylene). This is wrapped with a plurality of helical foil sense electrodes, e.g. three, each covering about 30% of the tube circumference. A thin insulating barrier surrounds the sense electrodes. On top of the insulating barrier a set of foil shield electrodes is wrapped to overlap the sense electrodes, these extending slightly beyond the edges of the sense electrodes. One more insulating barrier surrounds the shield electrodes and a continuous grounded Faraday shield surrounds the entire sense chamber except for an opening at the top through which sample material may be poured.
Three sinusoidal signals of the same frequency, e.g. 10 kHz, and 120 of phase apart are applied to the sense and shield electrodes such that approximately the same voltage is applied to each side of a pair of electrodes. This results in a rotating electric field. A reading signal is developed which is a voltage proportional to the sum of the true RMS value of the charging currents applied to each of the three sense electrodes.
The ratio of the signal output span (difference between full and empty readings) with a known subs-tance and with a test sample i5 nearly proportional to the ratio of the permittivities of the two substances.
The arrangement according to the above mentioned Canadian patent No. 1,082,310 is similar in use of a three-phase rotating field but differs in that the fluid is in contact with the electrodes so that current can flow through the fluid. In the present invention there is no electrical contact with the sensed materials and measurements are not reliant on conductivity effects. In addition, the present invention requires only three electrodes whereas the system according to the patent requires six electrodes~ The system of the patent uses alternate grounded electrodes providing a more complex field pattern and it lacks the driven guard electrodes of the present invention which ensure that the electric field set up by the sense electrodes is virtually exclusively within the volume of the chamber. Should the system of the patent be used in a capacitance mode, a great deal of sensitivity would be lost due to fringed field effects and current ~i2~L
required to polarize the ma-terials behind and beside the electrodes.
Regarding Canadian patent No. 1,101,070, although it relates to a capacitance system it uses a distinctly different field geometry using six electrodes as opposed to three for the preferred embodiment of the present invention, and a complicated switching system is required to obtain rotation of the field. As with Canadian patent No. 1,082,310, the electrode plates are in intimate con-tact with the sample fluid and therefore the system would be quite sensitive to conductivity effects, i.e. readings would change based on ionic content of water, for instance. The system of the presen-t invention is nearly insensitive to dissolved salts in the water. The superior guard and shield arrangement of the present invention improves the sensitivity as there need be no energy input by sense electrodes to polarize materials outside or adjacent to these electrodes.
Regarding U.S. patent ~o. 3,639,835, the concept of the capacitance gauging system is correct but it does not use a rotating field and, due to the distinctly patchwork nature of the electrodes within the chamber, it is unlikely that the fields are uniform, nor would the time average of the fields be likely to be uniform throughout the volume of the chamber. Therefore, inaccuracies may occur for situations in which fluids within the tank had sloshed to one side or the other or towards the ends.
The patent requires a considerably more complicated network than the present invention in order to obtain readings. With respect to the flow through system of the patent, -there is no helical 7~497-1 electrode arrangement which would reduce sensitivity to non-homo-geneous fluid flows, nor is the selected electrode arrangement adequate to produce a rotating electrical field. In all cases, the patent is concerned with a static single phase AC field.
According to a broad aspect of the present invention there is provided apparatus for measuring the mean permittivity of a sample of material comprising a sense chamber for containing said sample, said sense chamber comprising a cylindrical tube of low permittivity, low conductivity material and having an outer surface provided with at least three helical foil electrode structures extending along substantially the entire length of said tube, each electrode structure comprising an inner sense electrode in contact with said tube and an outer shield electrode, said sense electrode and said shield electrode being separated by a thin barrier of insulating material, said apparatus further comprising means for applying out of phase alternating voltage signals to said electrode structures to produce a rotating electrical field in said sense chamber and for deriving a voltage read-out signal proportional to -the sum of the charging currents applied to the sense electrodes of said electrode structures, said read-out signal being directly related to the mean permittivity of said sample.
An embodiment of the invention will now be described in con~unction with the accompanying drawings, in which:
Figure 1 is a sketch of a sense chamber provided with three single-turn helical electrode structures, Figure 2 is a cross-sectional view of a sense chamber and associated circuitry with radial spacings exaggerated for clarity of illustration, and Figure 3 is a block diagram of drive and sense circuitry used in the apparatus according to the invention.
Referring to Figure 1, the sense chamber 10 comprises a cylindrical tube 12 of relatively low permittivity, low conduc-tivity material, e.g. polytetrafluoroethylene (sold under the trade mark TEFLON). The tube 12 is wrapped with three helical electrode structures 13, 14 and 15 which, as shown in Figure 2, each comprises a foil sense electrode 16, a thin insulating barrier 18 and a foil shield electrode 20. Each sense electrode 16 has a width sufficient to cover about 30~ of the tube circum-ference. The remaining 10~ of the circumference is taken up by gaps between electrodes, one of which is indica-ted at 21 in Figure 2. The insulating barrier 18 surrounds the sense electrodes and covers the gaps between them. The shield electrodes 20 overlap the sense electrodes 16 and are wider so as to extend beyond the edges of the sense electrodes by about 25~ of the gap width on each side. Another insulating barrier 24 surrounds the shield electrodes and a continuous grounded Faraday shield 26 surrounds the entire sense chamber except for an opening at the top through which sample may be poured.
Drive and sense circuitry 30, to be described in connec-tion with Figure 3, is connected to the shield and sense Eoils of each electrode structure via lines 32 and 3~. It is important to note that, while Figure 2 shows separate sets of lines 32, 34, for ease of illus-tration, the lines to the shie].d and sense electrodes are actually coaxial. Thus each sense electrode will be connected to circuitry 30 by the center conductor o~ a coaxial cable while the associated shield electrode will be connected by the shield conductor of the same coaxial cableO This ensures that the only capacitive effects are at the sense electrodes. Parallel lines or twisted pairs would have stray capacitances which might well exceed the small capacitance effects at the sense electrodes so that the apparatus would not function properly.
Turning now to Figures 3, a master oscillator 40 produces a sinusoidal signal of, for example, 10 kHz, which is fed to three phase shifters 42, 43 and 44. The phase shifters are adjusted to produce three signals of identical frequency but 120 of phase apart. The outputs of the phase shifters are connected to shield d~iving amplifiers 47, 48 and 49 having output lines 32a to 32c connected to the shield electrodes 20 (not shown in Figure 3). The outputs of the phase shi~ters 42 to 44 are also connected via matched resistors 50a to 50c and lines 51a to 51c (actually coaxial shields for lines 32a to 32c) to the sense electrodes 16 (Figure 2.) ~early the same voltage is applied to the sense and shield electrodes resulting in a rotating electric field. Charging curren-ts to the sense electrodes, dependent on the permittivity of the material in the sample chamber, develop reading voltages or sense voltages across resis-tors 50a to 50c which are amplified by amplifiers 54 to 56 and conver-ted to DC
signals by signal converters (rectifiers) 60-62. The DC signals æ~
from converters 60-62 are summed by summation stage 65 to produce an output Vout which is a voltage proportional to the -true RMS value oE the charging currents applied to each of the three sense electrodes.
The use of helical electrodes results in good linearity of measurements and reduces the sensitivity to orientation for nonhomogeneous flowing systems.
The sample chamber could be a section of tubing with fluid material flowing through it rather than a "static" sample chamber as described above.
The driven cable shields and guard electrodes maximize sensitivity by minimizing current in the sense lines and electrodes required to polarize materials other than those within the sense chamber.
Although Figure 1 shows single turn helical electrodes they could be, for example, double turn helices. More than three sets of electrodes can be used and the driving signals need not be sinusoidal.
This invention relates to apparatus for measuring the mean permittivity (dielectric constant) of a sample of material.
A measurement oE the mean permittivity of a sample of material in a chamber enables one to estimate the volume ratio of two materials of known properties making up the sample,at least one of the two materials being a fluid, i.e. gas or liquid. One can also measure the dielectric constant or permittivity of an unknown material of known volume when surrounded by a fluid of known properties. In all cases, the materials under test may be either static or in motion as for a contained flow situation.
Canadian patent No. 1,082,310 of Dechene, et al, issued July 22, 1980 discloses apparatus for measuriny relative fractions of liquid vapor in mixed phase fluid flow. Measurements are made in several current loops and the current loops are supplied by a rotating electrical field source by means of multi-phase voltage oscillations of 1-30 kHz. A reference sensor is used and the voltage across a load resistor represents conductivity.
Canadian patent No. 1,101,070 of Newton, et al, issued May 12, 1981, relates to measuring relative fractions of liquid (nonconductive) and vapor, or solids (nonconductive) and gases.
Sequential capacitance measurements are made across the cross section of a flow to be measured. Frequencies of 10-100 kHz are used and a rota-ting electric field is produced to average the dielectric constant of the entire cross section of the sensor.
U.S. patent 3,639,835 of Dammig, Jr., et al, issued February 1, 1972, discloses a capacitive tank gaging apparatus ~5~
based on the fact that the density of a dielectric liquid is directly related to its dielectric constant. The capacitance between electrodes separated by space containing liquid and gas of known characteristics is a measure of the quantity of liquid present. A measuring unit impresses different voltages on sets of electrodes in a manner that generates relatively uniform time-varying electric fields across the tank. The measuring unit senses the capacitive current -through the tank interior between the electrodes, which current is a function of the dielectric corlstan-ts of -the fluids in the -tank.
The present invention, which differs in a number of respects from the arrangements disclosed in -the above mentioned patents, enables simple and highly accurate estimation of the mean permittivity of the contents of a sense chamber. Briefly, the invention uses a sense chamber consisting of a cylindrical tube of relatively low permittivity, low conductivity material (e.g. poly-tetrafluoroethylene). This is wrapped with a plurality of helical foil sense electrodes, e.g. three, each covering about 30% of the tube circumference. A thin insulating barrier surrounds the sense electrodes. On top of the insulating barrier a set of foil shield electrodes is wrapped to overlap the sense electrodes, these extending slightly beyond the edges of the sense electrodes. One more insulating barrier surrounds the shield electrodes and a continuous grounded Faraday shield surrounds the entire sense chamber except for an opening at the top through which sample material may be poured.
Three sinusoidal signals of the same frequency, e.g. 10 kHz, and 120 of phase apart are applied to the sense and shield electrodes such that approximately the same voltage is applied to each side of a pair of electrodes. This results in a rotating electric field. A reading signal is developed which is a voltage proportional to the sum of the true RMS value of the charging currents applied to each of the three sense electrodes.
The ratio of the signal output span (difference between full and empty readings) with a known subs-tance and with a test sample i5 nearly proportional to the ratio of the permittivities of the two substances.
The arrangement according to the above mentioned Canadian patent No. 1,082,310 is similar in use of a three-phase rotating field but differs in that the fluid is in contact with the electrodes so that current can flow through the fluid. In the present invention there is no electrical contact with the sensed materials and measurements are not reliant on conductivity effects. In addition, the present invention requires only three electrodes whereas the system according to the patent requires six electrodes~ The system of the patent uses alternate grounded electrodes providing a more complex field pattern and it lacks the driven guard electrodes of the present invention which ensure that the electric field set up by the sense electrodes is virtually exclusively within the volume of the chamber. Should the system of the patent be used in a capacitance mode, a great deal of sensitivity would be lost due to fringed field effects and current ~i2~L
required to polarize the ma-terials behind and beside the electrodes.
Regarding Canadian patent No. 1,101,070, although it relates to a capacitance system it uses a distinctly different field geometry using six electrodes as opposed to three for the preferred embodiment of the present invention, and a complicated switching system is required to obtain rotation of the field. As with Canadian patent No. 1,082,310, the electrode plates are in intimate con-tact with the sample fluid and therefore the system would be quite sensitive to conductivity effects, i.e. readings would change based on ionic content of water, for instance. The system of the presen-t invention is nearly insensitive to dissolved salts in the water. The superior guard and shield arrangement of the present invention improves the sensitivity as there need be no energy input by sense electrodes to polarize materials outside or adjacent to these electrodes.
Regarding U.S. patent ~o. 3,639,835, the concept of the capacitance gauging system is correct but it does not use a rotating field and, due to the distinctly patchwork nature of the electrodes within the chamber, it is unlikely that the fields are uniform, nor would the time average of the fields be likely to be uniform throughout the volume of the chamber. Therefore, inaccuracies may occur for situations in which fluids within the tank had sloshed to one side or the other or towards the ends.
The patent requires a considerably more complicated network than the present invention in order to obtain readings. With respect to the flow through system of the patent, -there is no helical 7~497-1 electrode arrangement which would reduce sensitivity to non-homo-geneous fluid flows, nor is the selected electrode arrangement adequate to produce a rotating electrical field. In all cases, the patent is concerned with a static single phase AC field.
According to a broad aspect of the present invention there is provided apparatus for measuring the mean permittivity of a sample of material comprising a sense chamber for containing said sample, said sense chamber comprising a cylindrical tube of low permittivity, low conductivity material and having an outer surface provided with at least three helical foil electrode structures extending along substantially the entire length of said tube, each electrode structure comprising an inner sense electrode in contact with said tube and an outer shield electrode, said sense electrode and said shield electrode being separated by a thin barrier of insulating material, said apparatus further comprising means for applying out of phase alternating voltage signals to said electrode structures to produce a rotating electrical field in said sense chamber and for deriving a voltage read-out signal proportional to -the sum of the charging currents applied to the sense electrodes of said electrode structures, said read-out signal being directly related to the mean permittivity of said sample.
An embodiment of the invention will now be described in con~unction with the accompanying drawings, in which:
Figure 1 is a sketch of a sense chamber provided with three single-turn helical electrode structures, Figure 2 is a cross-sectional view of a sense chamber and associated circuitry with radial spacings exaggerated for clarity of illustration, and Figure 3 is a block diagram of drive and sense circuitry used in the apparatus according to the invention.
Referring to Figure 1, the sense chamber 10 comprises a cylindrical tube 12 of relatively low permittivity, low conduc-tivity material, e.g. polytetrafluoroethylene (sold under the trade mark TEFLON). The tube 12 is wrapped with three helical electrode structures 13, 14 and 15 which, as shown in Figure 2, each comprises a foil sense electrode 16, a thin insulating barrier 18 and a foil shield electrode 20. Each sense electrode 16 has a width sufficient to cover about 30~ of the tube circum-ference. The remaining 10~ of the circumference is taken up by gaps between electrodes, one of which is indica-ted at 21 in Figure 2. The insulating barrier 18 surrounds the sense electrodes and covers the gaps between them. The shield electrodes 20 overlap the sense electrodes 16 and are wider so as to extend beyond the edges of the sense electrodes by about 25~ of the gap width on each side. Another insulating barrier 24 surrounds the shield electrodes and a continuous grounded Faraday shield 26 surrounds the entire sense chamber except for an opening at the top through which sample may be poured.
Drive and sense circuitry 30, to be described in connec-tion with Figure 3, is connected to the shield and sense Eoils of each electrode structure via lines 32 and 3~. It is important to note that, while Figure 2 shows separate sets of lines 32, 34, for ease of illus-tration, the lines to the shie].d and sense electrodes are actually coaxial. Thus each sense electrode will be connected to circuitry 30 by the center conductor o~ a coaxial cable while the associated shield electrode will be connected by the shield conductor of the same coaxial cableO This ensures that the only capacitive effects are at the sense electrodes. Parallel lines or twisted pairs would have stray capacitances which might well exceed the small capacitance effects at the sense electrodes so that the apparatus would not function properly.
Turning now to Figures 3, a master oscillator 40 produces a sinusoidal signal of, for example, 10 kHz, which is fed to three phase shifters 42, 43 and 44. The phase shifters are adjusted to produce three signals of identical frequency but 120 of phase apart. The outputs of the phase shifters are connected to shield d~iving amplifiers 47, 48 and 49 having output lines 32a to 32c connected to the shield electrodes 20 (not shown in Figure 3). The outputs of the phase shi~ters 42 to 44 are also connected via matched resistors 50a to 50c and lines 51a to 51c (actually coaxial shields for lines 32a to 32c) to the sense electrodes 16 (Figure 2.) ~early the same voltage is applied to the sense and shield electrodes resulting in a rotating electric field. Charging curren-ts to the sense electrodes, dependent on the permittivity of the material in the sample chamber, develop reading voltages or sense voltages across resis-tors 50a to 50c which are amplified by amplifiers 54 to 56 and conver-ted to DC
signals by signal converters (rectifiers) 60-62. The DC signals æ~
from converters 60-62 are summed by summation stage 65 to produce an output Vout which is a voltage proportional to the -true RMS value oE the charging currents applied to each of the three sense electrodes.
The use of helical electrodes results in good linearity of measurements and reduces the sensitivity to orientation for nonhomogeneous flowing systems.
The sample chamber could be a section of tubing with fluid material flowing through it rather than a "static" sample chamber as described above.
The driven cable shields and guard electrodes maximize sensitivity by minimizing current in the sense lines and electrodes required to polarize materials other than those within the sense chamber.
Although Figure 1 shows single turn helical electrodes they could be, for example, double turn helices. More than three sets of electrodes can be used and the driving signals need not be sinusoidal.
Claims (4)
THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for measuring the mean permittivity of a sample of material comprising a sense chamber for containing said sample, said sense chamber comprising a cylindrical tube of low permittivity, low conductivity material and having an outer surface provided with at least three helical foil electrode struc-tures extending along substantially the entire length of said tube, each electrode structure comprising an inner sense electrode in contact with said tube and an outer shield electrode, said sense electrode and said shield electrode being separated by a thin barrier of insulating material, said apparatus further comprising means for applying out of phase alternating voltage signals to said electrode structures to produce a rotating electrical field in said sense chamber and for deriving a voltage read-out signal proportional to the sum of the charging currents applied to the sense electrodes of said electrode structures, said read-out signal being directly related to the mean permittivity of said sample.
2. Apparatus as claimed in claim 1 wherein there are three electrode structures and said alternating voltage signals are sinusoidal signals 120° out of phase with each other.
3. Apparatus as claimed in claim 2 wherein said alternating voltage signals are produced by three phase shifters connected to the output of an oscillator, each phase shifter having an output connected via an amplifier to a shield electrode and via a resistor to a sense electrode.
4. Apparatus as claimed in claim 3 wherein each said resistor is connected across the input of an associated signal sensing amplifier having an output connected to a converter for converting the alternating signal to a DC signal, the outputs of the converters being connected to a summation device for producing said read-out signal.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000537062A CA1271524A (en) | 1987-05-15 | 1987-05-15 | Dielectric constant measuring apparatus |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000537062A CA1271524A (en) | 1987-05-15 | 1987-05-15 | Dielectric constant measuring apparatus |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1271524A true CA1271524A (en) | 1990-07-10 |
Family
ID=4135655
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000537062A Expired - Fee Related CA1271524A (en) | 1987-05-15 | 1987-05-15 | Dielectric constant measuring apparatus |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1271524A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2266777A (en) * | 1992-05-07 | 1993-11-10 | Seiko Instr Inc | Measuring dielectric constants |
| US5945831A (en) * | 1997-06-10 | 1999-08-31 | Sargent; John S. | Volume charge density measuring system |
-
1987
- 1987-05-15 CA CA000537062A patent/CA1271524A/en not_active Expired - Fee Related
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2266777A (en) * | 1992-05-07 | 1993-11-10 | Seiko Instr Inc | Measuring dielectric constants |
| US5389884A (en) * | 1992-05-07 | 1995-02-14 | Seiko Instruments Inc. | Parallel plate dielectric constant measuring apparatus having means for preventing sample deformation |
| GB2266777B (en) * | 1992-05-07 | 1996-06-12 | Seiko Instr Inc | Apparatus for measuring a dielectric constant |
| US5945831A (en) * | 1997-06-10 | 1999-08-31 | Sargent; John S. | Volume charge density measuring system |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKLA | Lapsed |