Method of and system for real time differential pulse detection

• US 4,500,391 A
• Filed: 10/13/1983
• Issued: 02/19/1985
• Est. Priority Date: 10/13/1983
• Status: Expired due to Fees

• Current Assignee: Environmental Technologies Group, Inc.
• Original Assignee: Allied Corp. (Honeywell International Inc.)
• Inventors: Schmidt, John C., Campbell, Donald N., Sands, Clifton A.
• Primary Examiner(s): NOT, DEFINED
• Application Number:
  US06/541,631
• Time in Patent Office: 495 Days
• Field of Search: 204/406, 204/411, 204/412, 204/431, 204/432, 204/1 T, 204/415, 340/632
• US Class Current: 205/783
• CPC Class Codes: G01N 27/48 using polarography, i.e. measuring changes in current under a slowly-varying voltage

Abstract

An electrochemical detection cell which includes, in the order named, a stack of components constituted by a first membrane, an electrode carrier having reference and counter electrodes thereon, a membrane support, an electrode carrier having a sensing electrode thereon, and a permselective membrane. Whatman filter papers are positioned between the sensing electrode and both the counter electrode and reference electrode, these being positionable in a central rectangular opening of the membrane support. The cell allows air or gas ambient to address the membranes and effects application of an electrolyte to space between the electrodes by, for example, a wick which extends between an electrolyte-containing reservoir and the Whatman filter papers. A system for detecting a gaseous agent uses the aforementioned detection cell coupled with circuitry capable of applying a fixed DC voltage bias to the reference electrode and superimposing a train of DC voltage pulses on the fixed bias. Further circuitry is coupled to and responsive to signals from the sensing electrode. This circuitry determines the difference between the sensing electrode signal at the end of each pulse minus that just before each pulse. The difference is proportional to the concentration of the test gas in the atmosphere adjacent to the sensor, and is a more specific and sensitive indication of the test gas concentration than prior art sensors.

Claims

1. A system for detecting a gas in an environment comprising an electrochemical detection cell having a reference electrode, a counter electrode and a sensing electrode;

said reference electrode and said counter electrode spaced apart from each other and from said sensing electrode, an electrolyte in contact with said reference electrode, counter electrode and sensing electrode, a membrane having a first surface exposed to said environment for passing said gas in said environment through said membrane to said sensing electrode, an operational amplifier having an output terminal, an inverting input terminal and a non-inverting input terminal, said output terminal being coupled to said counter electrode, said reference electrode being coupled to one of said input terminals;

means for applying a fixed D.C. voltage bias to the other of said input terminals;

means for applying at least one differential D.C. voltage pulse to said other input terminal to superimpose same on said fixed D.C. voltage bias to provide a corresponding differential D.C. voltage pulse on its output terminal, and means coupled to said sensing electrode and responsive to current therefrom for determining difference in Faradaic currents produced during a period before application of the differential D.C. voltage pulse and a period before termination thereof as an indication of presence of a given gas.

2. A system according to claim 1, wherein said reference electrode is coupled to said inverting input terminal of said operational amplifier, and said means for applying a fixed D.C. voltage bias and said means for applying a differential D.C. voltage pulse are coupled to said non-inverting input terminal.

3. A system according to claim 2, further including means for adjustably setting the fixed D.C. voltage bias.

4. A system according to claim 3, including means for adjustably setting amplitude of said differential D.C. voltage pulse.

5. A system according to claim 2, further including means for adjustably setting the amplitude of said differential D.C. voltage pulse.

6. A system according to claim 1, wherein said means coupled to said sensing electrode and responsive to current therefrom includes first storage means for storing a signal representative of current from said sensing electrode during a period before application of said differential D.C. voltage pulse and second storage means for storing a signal representative of current from said sensing electrode during a period before termination of said D.C. voltage pulse.

7. A system according to claim 6, wherein said first storage means and said second storage means comprise respective first capacitance means and second capacitance means, the signals being voltage signals.

8. A system according to claim 6, including means coupled to said first storage means and to said second storage means and responsive to the signals therefrom for determining the difference therebetween.

9. A system according to claim 8, wherein said first storage means and said second storage means comprise respective first capacitance means and second capacitance means, the signals being voltage signals.

10. A system according to claim 8, including alarm means, said alarm means being coupled to said means coupled to said first storage means and to said second storage means.

11. A system according to claim 10, including sample-and-hold circuit means for holding the highest signal representing the difference current, said sample-and-hold circuit means being coupled between said means coupled to said first storage means and to said second storage means, and said alarm means.

12. A system according to claim 10, wherein said alarm means comprises means for producing an audible signal.

13. A system according to claim 12, wherein said alarm means comprises means for producing a visual signal.

14. A system according to claim 10, wherein said alarm means comprises means for producing a visual signal.

15. A system according to claim 1, wherein said electrochemical detection cell comprises at least one further membrane positioned between said counter electrode and a gas-containing chamber and between said reference electrode and the gas-containing chamber.

16. A system according to claim 1, wherein said reference electrode, said counter electrode and said sensing electrode are meshes.

17. A system according to claim 1, wherein said sensing electrode is a mesh and said reference electrode and said counter electrode are composite electrodes.

18. A system according to claim 17, wherein said composite electrodes are of finely divided conductive powder and an inert plastic binder.

19. A system for detecting a gas in an environment comprising an electrochemical detection cell having a reference electrode, a counter electrode and a sensing electrode, said reference electrode and said counter electrode spaced apart from each other and from said sensing electrode an electrolyte in contact with said reference electrode, counter electrode and sensing electrode, a membrane having a first surface exposed to said environment for passing said gas in said environment through said membrane to said sensing electrode, means for applying a fixed D.C. voltage bias to said counter electrode for supplying electrical current through said electrolyte to said sensing electrode to maintain a predetermined potential at said reference electrode, means for superimposing at least one differential D.C. voltage pulse on said fixed D.C. voltage bias, and means coupled to and responsive to Faradaic signals from said sensing electrode for determining the difference between Faradaic signals produced during a period before the at least one differential D.C. voltage pulse is superimposed on the fixed D.C. voltage bias and during a period before termination of the at least one differential D.C. voltage pulse.

20. A system according to claim 19, wherein said means for superimposing at least one differential D.C. voltage pulse comprises means for superimposing a first train of differential D.C. voltage pulses on the fixed D.C. voltage bias, and said means responsive to signals from the sensing electrode includes:

(1) means for producing a further train of pulses and an additional train of pulses both synchronized with the first train of pulses and having the same repetition rate and being timed to effect sampling of signals from the sensing electrode in periods before start and before termination of each differential D.C. pulse of the first train and (2) means responsive to the sampled signals for determining the difference between those produced during the periods before the respective differential D.C. voltage pulse and those produced during the periods before termination of the respective differential D.C. voltage pulses.

21. A system according to claim 20, including sample-and hold circuit means for storing the difference signal.

22. A system according to claim 20, including at least one alarm circuit responsive to the difference signal.

23. A method for detecting a gaseous agent in an ambient with an electrochemical detection cell having a sensing electrode, a counter electrode and a reference electrode, said reference electrode and said counter electrode spaced apart from each other and from said sensing electrode and having an electrolyte in contact with said reference electrode, counter electrode and sensing electrode, comprising the steps of exposing said ambient to one side of a membrane capable of passing said gaseous agent, exposing said sensing electrode with the gas passing through said membrane, applying a fixed D.C. voltage bias to the counter electrode for supplying electrical current through said electrolyte to said sensing electrode to maintain a predetermined potential at said reference electrode, superimposing at least one differential D.C. voltage pulse on the fixed D.C. voltage bias and determining the difference between Faradaic signals produced from the sensing electrode during a period before superimposing the differential D.C. voltage pulse and in a period before termination of the differential D.C. voltage pulse.

24. A method according to claim 23, wherein the step of superimposing at least one differential D.C. voltage pulse on the fixed D.C. voltage bias involves superimposing a first train of differential D.C. voltage pulses thereon, supplying a second train of pulses and a third train of pulses both synchronized with the first train of pulses and having the same repetition rate, and using respectively the second and the third train of pulses to affect sampling of signals from the sensing electrode in periods before start and before termination of each differential D.C. pulse of the first train, and determining the difference between the signals produced during the periods before the respective differential D.C. voltage pulses and those produced during the periods before termination of the respective differential D.C. voltage pulses.