Method and means for compensating for IR voltage drop in electrochemical cells

• US 4,459,180 A
• Filed: 05/16/1983
• Issued: 07/10/1984
• Est. Priority Date: 05/16/1983
• Status: Expired due to Term

• Current Assignee: General Signal Corporation (SPX Corporation)
• Original Assignee: General Signal Corporation (SPX Corporation)
• Inventors: Fogel, John D.
• Primary Examiner(s): Kaplan, G. L.
• Assistant Examiner(s): Nguyen, Nam X.
• Application Number:
  US06/495,322
• Time in Patent Office: 421 Days
• Field of Search: 204/400, 204/401, 204/406, 204/415, 204/412, 204/1 P, 204/1 T, 73/1 G, 364/571
• US Class Current: 205/782.5
• CPC Class Codes: G01N 27/404 Cells with anode, cathode and cell electrolyte on the same side of a permeable membrane which separates them from the sample fluid , e.g. Clark-type oxygen sensors
• CPC Class Codes: G01N 27/4163 checking the operation of, or calibrating, the measuring apparatus G01N27/3274, G01N27/4175 and G01N33/0006 take precedence

Abstract

Apparatus for compensating for the IR drop in a polarographic type electrochemical cell which utilizes an injected current for in situ calibration includes means operable for a brief period of the current injection to change the cell bias so as to maintain the total cell current for that period at the value existing prior to current injection as well as means for maintaining throughout the remainder of the injection period the cell bias which exists at the end of the brief period.

Claims

1. In a system for measuring the concentration of an electrochemically active species by measuring the current through a polarographic type cell, wherein means are provided for recalibration of said system by injecting additional current into said cell and measuring the factor by which the cell'"'"'s transient response to said current injection has changed since the last calibration as an indication of the factor by which the calibration has changed, apparatus for compensating for the IR drop in said cell due to said injected current, comprising:

means operable for a brief initial part of the period of said current injection for changing the cell bias so as to maintain the total cell current for said brief initial part of that period at the value it had just before said current injection, said brief initial part of the period being of duration sufficient for the effect of the IR drop to be compensated by said change in bias but not sufficient to extend into that part of the injection period when electrochemical action due to the injection affects the total cell current; and

means for maintaining throughout the remainder of said injection period the cell bias which exists at the end of said brief initial part of said injection period.

2. A method for automatically compensating for IR drops in a polarographic type electrochemical cell caused by additional cell current not required by the measurement being made with the cell comprising the steps of:

adjusting the bias of said cell during a brief initial part of the period when said additional cell current is present so as to maintain the total cell current during said initial part of the period at the value it had just before said additional current, said brief initial part of the period being of duration sufficient for the change in bias to be effected but not sufficient to extend into that part of the period when electrochemical action due to the injection affects the total cell current; and

maintaining throughout the remainder of said period the cells bias which exists at the end of said brief part of said period.

3. In a system for measuring the concentration of an electrochemically active species by measuring the current through a polarographic type cell which includes a cathode, an anode, a reference electrode and a fourth electrode, wherein a current amplifier connected to said cathode produces a voltage representing the total cell current, a control amplifier is operable to control the normal electrode current so as to produce a desired bias between said reference electrode and said cathode, a bias summing amplifier produces an input to said control amplifier to establish said desired bias as the sum of a preset bias and a compensating bias, and wherein means are provided for recalibration of said system by injecting additional current into said cell and measuring the factor by which the cell'"'"'s transient response to said current injection has changed since the last calibration as an indication of the factor by which the calibration has changed;

apparatus for automatically adjusting said compensating bias to compensate for the IR drop in said cell due to said injected current, comprising;

a first sample and hold circuit having unity gain and connected to receive as an input said voltage representing the total cell current;

a first switch means operable to automatically disconnect the input to said first sample and hold circuit for the duration of said injection period;

a high gain amplifier for comparing the output of said first sample and hold circuit with the said voltage representing the total cell current;

a second sample and hold circuit;

a second switch means operable to connect the output of said high gain amplifier to the input of said second sample and hold circuit for a brief initial part of each injection period;

a third switch means for connecting the output of said second sample and hold to the bias summing amplifier as said compensating bias so that for said brief initial part of the period of said current injection the compensating cell bias changes so as to maintain the total cell current for said brief initial part of that period at the value it had just before said current injection and for the remainder of said injection period the cell bias which exists at the end of said brief initial part of said injection period is maintained in compensation of said IR drop.