Conductivity measuring method and apparatus utilizing coaxial electrode cells

• US 3,919,627 A
• Filed: 05/31/1973
• Issued: 11/11/1975
• Est. Priority Date: 08/06/1970
• Status: Expired due to Term

• Current Assignee: Gerald F. Allen
• Original Assignee: Gerald F. Allen
• Inventors: Allen, Gerald F.
• Primary Examiner(s): Smith, Alfred E.
• Assistant Examiner(s): Hille, Rolf
• Application Number:
  US05/365,617
• Time in Patent Office: 894 Days
• Field of Search: 324/3R,3B,64,65P 204/195
• US Class Current: 324/443
• CPC Class Codes: G01N 27/06 of a liquid involving electrolysis G01N27/26
• CPC Class Codes: G01R 27/22 Measuring resistance of fluids

Abstract

The invention includes an electrode plate having at least one coaxial electrode positioned in an opening therein and insulated therefrom. The surface of the plate and the coaxial electrode lie in the same plane, at least in the area around the opening. The electrode or a group of four electrodes can be used to measure the conductivity of a liquid sample by either placing a drop of the sample on the coaxial cell or by placing a sorption medium thereon. When the sorption medium technique is used, a large weighted electrode is placed on the sorption medium over the coaxial electrode cell or cells. The cells are in a comparison bridge to which a high square-wave excitation frequency is applied. The signal from the cell passes through a zero crossing or null detector. This signal is fed into a phase detector system and a differential averaging filter and phase threshold bias network to actuate a pair of HI and LO indicators. Alternatively, an analog or digital readout can be obtained from an amplitudeonly detector. Also disclosed in a vacuum funnel arrangement employed with the coaxial electrode cell of the invention.

Claims

1. An apparatus for use in measuring the conductivity of a sample comprising:

a central cylindrical electrode having a smooth, continuous surface for contact with said sample, an annular electrical insulator surrounding said central electrode and in contact therewith, said insulator having a smooth, continuous contact surface in the same plane as the contact surface of said central electrode, and an outer electrode surrounding said insulator and in contact therewith said outer electrode having a smooth, continuous contact surface in the same plane as the contact surfaces of said central electrode and said insulator, said central electrode, insulator and outer electrode forming a continuous, smooth plane surface for contact with said sample.

2. Apparatus as recited in claim 1 including a temperature compensating resistance positioned in said outer electrode.

3. Apparatus as recited in claim 1 including a dielectric sorption medium positioned over said electrodes.

4. Apparatus as recited in claim 3 further comprising a weighted electrode positioned on said dielectric sorption medium over both of said electrodes.

5. Apparatus as recited in claim 1 wherein said continuous outer electrode surrounds a plurality of annular electrical insulators each of said insulators surrounding a central electrode wherein the contact surfaces of all of said central electrodes, insulators and outer electrode form a continuous, Smooth plane surface for contact with said sample.

6. Apparatus as recited in claim 5 wherein said contact surfaces of all of said electrodes are plated with a nonplatinized highly conductive material.

7. Apparatus as recited in claim 1 wherein said contact surfaces of said electrodes are plated with a non-platinized conductive material.

8. Apparatus as recited in claim 1 further comprising means for measuring the conductivity of a sample in contact with said electrodes.

9. Apparatus as recited in claim 1 further comprising:

a bridge network having one arm thereof comprising said electrodes and a second arm thereof comprising variable resistor means, a square wave generator connected to said bridge network for providing square wave output signals, phase detector means connected to said bridge network and said square wave generator for providing output signals indicative of the null of said bridge network relative to said square wave output signals, means connected to said phase detector for providing complementary output signals, a DC averaging network for receiving said output signals and said complementary output signals for providing DC average output signals, and means connected to receive said DC average output signals for actuating indicator means.

10. Apparatus as recited in claim 9 wherein said contact surfaces of said electrodes are non-platinized.

11. Apparatus as recited in claim 1 further comprising:

means for supporting a sample above the surface of said electrodes, filter means positioned between the surface of said electrodes and said sample, and means for pressure forcing a liquid constituent of said sample through said filter means for contact with the surface of said electrodes.

12. Apparatus as recited in claim 11 wherein said pressure forcing means comprises a housing defining a chamber above the surface of said electrodes between said electrode surfaces of said sample, and means for evacuating said chamber.

13. Apparatus for measuring the conductivity of a sample comprising electrode means for contacting said sample, said electrode means comprising:

a central cylindrical electrode having a smooth, continuous surface for contact with said sample, an annular electrical insulator surrounding said central electrode and in contact therewith, said insulator having a smooth, continuous contact surface in the same plane as the contact surface of said central electrode, and an outer electrode surrounding said insulator and in contact therewith said outer electrode having a smooth, continuous contact surface in the same plane as the contact surfaces of said central electrode and said insulator, said central electrode, insulator and outer electrode forming a continuous, smooth plane surface for contact with said sample, said apparatus further comprising;

square wave generator means for producing square wave signals, electrical impedance means, means for connecting said electrical impedance means and said electrode contact surfaces in a bridge circuit having an input and an output, means for connecting said square wave generator means to said bridge circuit, and means for measuring the output of said bridge circuit.

14. Apparatus as recited in claim 13 wherein said square wave generator produces square wave signals having a frequency greater than or equal to 100KHz and said electrode contact surfaces are non-platinized.

15. Apparatus as recited in claim 13 wherein said electrode contact surfaces are non-platinized.

16. Apparatus as recited in claim 13 wherein said means for measuring the output of said bridge circuit comprises means for digitally displaying the conductivity of said test sample.

17. Apparatus as recited in claim 13 wherein said electrical impedance means comprise variable resistors.

18. Apparatus as recited in claim 13 wherein said impedance means comprise a plurality of resistors connected in parallel by the actuation oF a plurality of thumb wheel switches thereby providing direct digital readout of said sample conductivity at the null point of said bridge.