DIFFERENTIAL METHODS AND APPARATUS FOR MEASURING DIELECTRIC CONSTANT TO DETERMINE A CONDITION OF A MATERIAL

• US 3,774,237 A
• Filed: 10/08/1971
• Issued: 11/20/1973
• Est. Priority Date: 10/08/1971
• Status: Expired due to Term

• Current Assignee: Edward V. Hardway Jr
• Original Assignee: Edward V. Hardway Jr
• Inventors: Hardway, Edward V. Jr.
• Primary Examiner(s): Krawczewicz, Stanley T.
• Application Number:
  US05/187,793
• Time in Patent Office: 774 Days
• Field of Search: 324/61R,61P 73/34C
• US Class Current: 324/663
• CPC Class Codes: G01N 27/223 for determining moisture content, e.g. humidity rain detectors on vehicle windows B60S1/0825

Abstract

Method and apparatus responsive to conditions such as the moisture content or ingredients ratio of a material are disclosed in which the dielectric constant of a standard sample of such material of a reference moisture content or ingredients ratio is compared in a three-terminal differential capacitive device including two three-terminal capacitor probes with the dielectric constant of a test sample of such material of unknown moisture content or ingredients ratio. The comparison provides an electrical output signal proportional to the differences in moisture content or ingredients ratio of the test sample and the standard sample. For example, the differential capacitive device may be mounted in a differential capacitive cell which includes a standard sample cavity in which the standard sample is placed around the electrodes or conductive elements of one threeterminal capacitive probe and a test cavity in which the test sample is placed around the electrodes or conductive elements of a second three-terminal capacitive probe. Each probe of the differential capacitive device includes a conductive driven element, a conductive receptor element connected to an amplifier and maintained at virtual ground, and suitable shielding connected to a ground terminal. The test cavity is maintained at the same temperature as the standard sample cavity and the feed through capacity of each probe is a function of the dielectric constant of the material between their electrodes.

Claims

1. A three-terminal differential capacitive device for providing an electrical output signal proportional to the difference in dielectric constant of a first sample of material having a reference dielectric constant, and a second sample of material having a dielectric constant to be measured, wherein the measured dielectric constant is a function of a condition of said second sample with respect to said first sample, said device comprising in combination:

a first three-terminal capacitor probe including a first electrically driven element, a first receptor element, first support means for supporting said first driven and receptor elements spaced from and adjacent to each other and in the proximity of said first sample so that the dielectric properties of said first sample affect the capacitive coupling between said first driven and receptor elements;

a second three-terminal capacitor probe including a second electrically driven element, a second receptor element and second support means foR supporting said second driven and receptor elements spaced from and adjacent to each other and in the proximity of said second sample so that the dielectric properties of said second sample affect the capacitive coupling between said second driven and receptor elements;

a source of input electrical signals of predetermined amplitude with respect to circuit ground;

first circuit means coupling said source to said first and second driven elements;

second circuit means coupled to said first and second receptor elements for maintaining said receptor elements substantially at virtual ground while providing said electrical output signal proportional to the difference in dielectric constant of said first and second samples; and

conductive means maintained at substantially zero potential with respect to circuit ground for shielding said second circuit means from said first circuit means except between said first driven and receptor elements and between said second driven and receptor elements.

2. The device of claim 1 further including converting means coupled to the output of said second circuit means for converting said electrical output signal to a direct current signal proportional to the difference in dielectric constant of said first and second samples.

3. The device of claim 2 wherein said converting means includes a demodulator connected to said source of input electrical signals and synchronized therewith to materially reduce the effect of resistive leakage and conductance of said first and second samples on said direct current signal.

4. The device of claim 1 wherein said source of input electrical signals provides first electrical input signals of a predetermined amplitude and second electrical input signals of substantially the same amplitude, but opposite phase, and said first circuit means includes first conducting means for conducting said first input electrical signals to one of said first and second electrically driven elements and second conductive means for conducting said second electrical input signals to said other electrically driven element, and said second circuit means includes an amplifier having a capacitive feedback circuit and connected at one input to said first and second receptor elements to form a null junction where a zero signal level is provided when the feed through capacities of said first and second three-terminal capacitors probes are equal.

5. The device of claim 1 further including a differential cell having a thermally insulated housing, and means for maintaining substantially temperature equilibrium in said housing, and wherein said first and second three-terminal capacitor probes are mounted in said housing.

6. The device of claim 5 wherein said first and second receptor elements are formed by a common receptor element and said first and second support means supports said common receptor element in said housing spaced apart and between said first and second electrically driven elements.

7. The device of claim 5 wherein said housing includes two cavities, one being a standard sample cavity for receipt of said first sample and the other a test sample cavity for receipt of said second sample, and said first three-terminal capacitor probe is mounted in said standard sample cavity and said second three-terminal capacitor probe is mounted in said test sample cavity.

8. The device of claim 7 wherein said source of input electrical signals provides first alternating current electrical input signals of a predetermined amplitude and second alternating current electrical input signals of substantially the same amplitude, but of opposite phase, and said first circuit means includes first conducting means for conducting said first input signals to one of said first and second electrically driven elements, and second conducting means for conducting said second input signals to the other electrically driven element, and wherein said second circuit means includes an amplifier having a capacitive feedback circuit and a demodulator connectEd to the output of said amplifier for converting the output thereof to a direct current signal proportional to the difference in dielectric constant of said first and second samples.

9. The device of claim 8 wherein said demodulator is connected to said source of input electrical signals and synchronized therewith to materially reduce the effect of resistance leakage and conductance of said first and second samples on said direct current signal.

10. The apparatus of claim 2 wherein said second circuit means further includes means for providing a quantitative indication proportional to the amplitude of said direct current signal.

11. The apparatus of claim 2 wherein said second circuit means includes means for providing a control signal when the amplitude of said direct current signal deviates from a preselected amplitude by a predetermined amount.

12. The device of claim 1 wherein said second circuit means includes a first amplifier connected at an input to one of said first and second receptor elements, and a second amplifier connected at an input to the other of said first and second receptor elements, each of said amplifiers including a capacitive negative feedback circuit, and wherein said second circuit further includes combining means connected to the output of each of said first and second amplifiers for combining the output signals therefrom to provide said electrical output signal.

13. The device of claim 12 wherein said source of electrical input signals provides electrical input signals of substantially the same amplitude and phase to said first and second electrically driven elements, and said second circuit means further includes means connected between the output of one of said first and second amplifiers and said combining means for inverting the output signals from said one amplifier.

14. The device of claim 12 wherein said source of electrical input signals provides first electrical input signals of predetermined amplitude to said first electrically driven element, and second electrical input signals of substantially the same amplitude, but of opposite phase, to said second electrically driven element.

15. The device of claim 1 wherein said second sample is a material in a container and further including means for mounting said second three-terminal capacitor probe in said container to be responsive to the dielectric constant of the material in said container.

16. The device of claim 15 wherein said first three-terminal capacitor probe is disposed remotely from said container and further including means connected between said container and said first three-terminal capacitor probe to establish and maintain substantially temperature equilibrium between said first and second three-terminal capacitor probes.

17. The device of claim 15 wherein said container is a pipeline which provides for the flow of a plurality of different materials with flow interfaces between them and said second three-terminal capacitor probe has a different feed through capacity when adjacent to a different one of said interfaces, and wherein a plurality of said first three-terminal capacitor probes are provided each including a different first sample simulating the dielectric of said second three-terminal capacitor probe when adjacent a different one of said interfaces, and further including switch means connected to each of said first three-terminal capacitor probes and said second circuit means to selectively connect a desired one of said first three-terminal capacitor probes to said second circuit means.

18. The device of claim 15 wherein said container is a pipeline and said first and second samples are materials flowing in said pipeline and further including means for mounting said first three-terminal capacitor probe in said pipeline spaced from said second three-terminal capacitor probe, to be responsive to the dielectric constant of the material in the pipeline, whereby the arrival at one of said three-terminal capacitor probes of an interface betwEen two materials in the pipeline can be detected.

19. A differential capacitive cell for permitting determination of the difference in dielectric constant of a standard sample of a material to be tested and a test sample of such material while providing substantially temperature equilibrium between the samples, comprising, in combination:

a housing;

a first conductive driven element mounted in said housing and adapted to be connected to a source of electrical input signals;

a second conductive driven element mounted in said housing and spaced from said first conductive driven element and adapted to be connected to a source of electrical input signals;

conductive receptor means mounted in said housing and adapted to be connected to an electrical output circuit to provide an electrical output signal which is a function of the difference in feed through capacity between each of said conductive driven elements and said receptor means, said receptor means being aligned with respect to said first conductive driven element to provide a first three-terminal capacitor in which said standard sample can be placed between said first conductive driven element and said receptor means so that the feed through capacity of said first three-terminal capacitor is a function of the dielectric constant of said standard sample, and said receptor means also being aligned with respect to said second conductive driven element to provide a second three-terminal capacitor in which said test sample can be placed between said second conductive driven element and said receptor means so that the feed through capacity of said second three-terminal capacitor is a function of the dielectric constant of said test sample.

20. The differential capacitive cell of claim 19 wherein said receptor means is a single receptor element mounted in said housing between said first and second conductive driven elements, and provides a common receptor element for each of said three-terminal capacitors.

21. The differential capacitive cell of claim 19 wherein said housing includes two cavities, one being a standard sample cavity for receipt of said standard sample and the other a test cavity for receipt of said test sample, and said first conductive driven element is mounted in said standard sample cavity and said second conductive driven element is mounted in said test cavity, and wherein said receptor means includes a first receptor element mounted in said standard sample cavity and spaced from said first conductive driven element to provide one of said three-terminal capacitors, and a second receptor element mounted in said test cavity and spaced from said second conductive driven element to provide the other of said three-terminal capacitors.

22. The differential capacitive cell of claim 21 wherein said test cell is adapted to be connected to a process flow line to provide on-stream monitoring of a condition of the material in the flow process.

23. A method of comparing the dielectric constant of a first sample of material having a reference dielectric constant and the dielectric constant of a second sample of material having a dielectric constant to be measured, comprising the steps of:

placing a first three-terminal capacitor having a first electrically driven element and a first receptor element in close proximity to said first sample so that said first sample affects the capacitive coupling between said first driven and receptor elements;

placing a second three-terminal capacitor probe having a second electrically driven element and a second receptor element in close proximity to said second sample so that said second sample affects the capacitive coupling between said second driven and receptor elements;

establishing a temperature equilibrium between said first and second samples;

driving said first and second electrically driven elements with alternating current input signals of predetermined amplitude with respect to circuit ground;

clamping the potential on said first and second receptor elements at virtuAl ground while conducting said potential to a differential output null junction to provide an electrical output signal responsive to the difference in the dielectric constant of said first and second samples; and

converting said electrical output signal to a measurable direct current signal.

24. The method of claim 23 wherein said first sample is a standard sample of a material having a desired moisture content and said second sample is a test sample of the same material having an unknown moisture content, and further including the step of scaling said direct current signal to provide a quantitative representation of the percentage of moisture in said second sample.

25. The method of claim 23 wherein said first sample is a standard sample of a material having a desired ingredients ratio and said second sample is a test sample of the same material having an unknown ingredients ratio, and further including the step of scaling said direct current signal to provide a quantitative representation of the percentage of an ingredient in said second sample.

26. The method of claim 23 wherein said driving step includes driving said first electrically driven element with a first input electrical signal of one phase and amplitude and driving said second electrically driven element with a second electrical input signal of substantially the same amplitude as said first input electrical signal, but of opposite phase, and wherein said clamping step is provided by connecting said first and second receptor elements together to form said null junction and utilizing an integrating amplifier having a capacitive feedback loop to clamp said null junction to virtual ground while providing said electrical output signal, and wherein said converting step is synchronized by one of said first and second input signals to effectively cancel the effect of conductance in said first and second samples from said direct current signal.

27. The method of claim 23 wherein said first and second three-terminal capacitor probes are utilized to determine the interface of pipeline products.

28. The method of claim 23 wherein said first and second three-terminal capacitor probes are utilized to determine a material level in a container.