Electrochemical sensing circuits

Details Electrochemical sensing circuits Electrochemical sensing circuits

1999-12-17

2001-08-21

Tung, T. (Department: 1743)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S401000, C204S412000

Reexamination Certificate

active

06277255

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to electrochemical sensing circuits and in particular to electrical circuits which sense cell current flow from a three terminal electrochemical cell.
Three terminal electrochemical cells are used for a variety of gas monitors and comprise a cell in which a gas to be analysed is introduced and three spaced apart electrodes. The three electrodes comprise a main pair across which the cell current is generated and a reference electrode which enables a potential at a predetermined point in the cell electrolyte measured. The cell current is proportional to the concentration of the compound or element being sensed by the cell, which may, for example, be carbon monoxide.
Known three-terminal electrochemical cells can be stabilised using the circuit shown in FIG.
1
. In order to stabilise the cell, the “working” and “reference” electrodes, labelled “W” and “R” respectively must be brought to the same electrical potential. No current is taken from the reference electrode. Instead, current is injected into the counter electrode, labelled “C”, by the amplifier Al until both the reference and working electrodes, R and W respectively, are the same potential. The current which flows in both the counter and working electrodes is the cell current and due to the internal operation of the cell this is proportional to the concentration of the compound being sensed by the cell.
Referring to
FIG. 1
, the Amplifier, A
1
, maintains the reference electrode at 0V by feeding back current to the counter electrode. Amplifier A
2
maintains the working electrode at 0V, since the negative input of amplifier A
2
is at 0V. The cell current is driven by amplifier A
1
but is sensed by amplifier A
2
, because the cell current passes through resistor R
2
to develop the voltage V out.
A disadvantage of the prior known circuit shown in
FIG. 1
is that it is prone to oscillation, because the virtual earth impedance of each amplifier appears as part of a feedback path of the other amplifier. This can lead to oscillation at high frequencies, where the virtual earth impedances are not well defined.
A second disadvantage, for low cost microcontroller-based applications, is that the output V out is an analogue voltage which must go through an analogue to digital conversion before it can be processed digitally.
A third disadvantage is that, whereas V out is normally positive when gas is being sensed, the counter electrode charges negatively, requiring the output of amplifier A
1
to go negative. Therefore the circuit shown in
FIG. 1
requires both positive and negative supplies (shown as V+ and V−).
A further cell which uses a potentiostat-type circuit is described in U.S. Pat. No. 4,048,041 (U.S. Army). The electronic circuit controls voltage potential applied to working electrodes of a three-electrode electrochemical cell. The electrochemical cell is incorporated into a sensor which operates by measuring the difference current between the cell's grounded anode and a negatively pulsed cathode. This circuit is quite complex, and requires both positive and negative supplies.
BRIEF SUMMARY OF INVENTION
An object of the present invention is to provide a simplified sensing circuit in which at least one of the above mentioned disadvantages is overcome, and which can be powered by an isolated DC supply, such as a battery.
According to one aspect of the present invention there is provided an electrochemical cell sensing circuit comprising an electrochemical cell having a working electrode, a counter electrode and a reference electrode in an electrolyte, the cell being constructed such that in use, when a gas to be analysed is introduced into the cell, a current flows between the counter electrode and the working electrode, and a potential at a position in the electrolyte is sensed by the reference electrode, the circuit further comprising power supply means for applying an offset voltage to the counter electrode relative to the working electrode, an amplifier means for monitoring the voltage difference between the reference electrode and the working electrode and operable in use to feed back a current to the working electrode through a feed back loop and thereby tend to maintain the working electrode at substantially the same potential as the reference electrode, and measuring means for measuring the current feed back by the amplifier to the working electrode as a measure of the cell current flowing between the working electrode and the counter electrode.
Preferably the measuring means comprises a resistor means in series connection between an output of the amplifier and the working electrode, and means are provided for measuring the voltage generated across the resistor.
Alternatively the measuring means comprises a capacitor connected in series between an output of the amplifier and the working electrode, a switch means connected in parallel across the capacitor, said switch being operable in a closed position to short out the capacitor and in an open position to allow the capacitor to be charged by the current feed back to the working electrode by the amplifier and a comparator adapted to receive, at a first input, the output of the amplifier, and at a second input, a reference voltage Vr, said comparator being operable to compare the first and second inputs and produce an output signal indicative of the cell current when said switch is open.
Preferably the power supply means comprises a DC supply which applies a positive potential to the counter electrode.
Preferably the amplifier is connected between a second DC supply which is isolated from the DC supply which applies to the positive potential to the counter electrode.
Preferably the said power supply means comprises a DC supply which applies a positive potential to the counter electrode and the comparator and amplifier are connected between a second DC supply which is isolated from the DC supply which applies the positive potential to the counter electrode.
The output signal from the comparator may be a digital signal. A means may be provided to inject a pulse in the supply to the counter electrode to provide a means of testing the correct operation of the circuit.


REFERENCES:
patent: 4426621 (1984-01-01), Galwey et al.

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