WO2003001195A1

WO2003001195A1 - Monitoring of gas sensors

Info

Publication number: WO2003001195A1
Application number: PCT/GB2002/002939
Authority: WO
Inventors: John Chapples; Martin Legg
Original assignee: Zellweger Analytics Limited
Priority date: 2001-06-26
Filing date: 2002-06-26
Publication date: 2003-01-03
Also published as: GB0115586D0

Abstract

A monitor is disclosed for monitoring an atmosphere for the presence of a target gas having: a) a two electrode electrochemical gas sensor (11) having a working (sensing) electrode (11a) and a counter electrode (11b), the sensor providing a current between the electrodes that is indicative of the amount of the target gas in the atmosphere; b) a circuit (24, 25, 28) applying successively a pulse of positive potential between the electrode terminals and a pulse of negative potential to a point (19) causing a change of potential between the electrode terminals; and c) an operational amplifier (12) connected between the sensor electrode terminals to generate an output signal according to the current flowing between the electrodes, including pertubations therein causes by the applied pulses. A detector monitors the output of the amplifier and detects the changes in the current over the course of the pluses and generates an alarm signal when the value of the said change is less than a threshold value.

Description

Monitoring of Gas Sensors

Technical Field

The present invention relates to the monitoring of sensors for detecting and measuring quantities of gases or vapours in an ambient atmosphere. The present specification will refer to such sensors as "gas sensors", although throughout the present specification that term also applies to the measurement of vapours. The present invention is especially concerned with the monitoring of electrochemical gas sensors, e.g. for measuring toxic gases such as carbon monoxide.

State of the Art

Gas sensors are incorporated within gas sensing equipment (called herein "monitors") which include circuitry for analysing the sensor output and, for example, displaying the level of gas in an atmosphere and/or triggering alarms if excessive levels of gas are detected (either instantaneously or when integrated over time). One use of gas sensors is to monitor the level of carbon monoxide in a dwelling, particularly in temporary accommodation. Various national and international standards apply to the detection of carbon monoxide in dwellings, including a requirement to measure with reasonable accurately up to 600 parts per million (ppm) of carbon monoxide. It is also a requirement that the monitor tests each sensor within the monitor to ensure that it is operational (viable) for the whole of its expected lifetime, generally 5 years.

It is known to test the viability of an electrochemical gas sensor by imposing an electric pulse across it. EP-0 039 549 discloses the imposition of a pulse of potential across the electrolyte of an electrochemical cell, the potential being sufficient to electrolyse the electrolyte. The resulting current is observed and taken to be an indication of the viability of the sensor. The potential may be applied as an alternating or direct current, although the latter is very much preferred. US-5,202,637 discloses a method of monitoring three electrode sensors by applying a pulse of potential between the reference electrode and the sensing electrode (also known as the working electrode) of a sensor. Although current does not flow at a significant level between the electrodes, the pulse charges up the double ionic layer at the sensing electrode and this results in a current flow in external circuitry, which can be detected to show that the sensor is operational. Obviously, if the sensor has dried out (i.e. lost a sufficient volume of electrolyte) or if there is a poor connection between the sensor and the circuit, no current will flow and an "error" signal is generated. US-5,024,637 is, according to its teaching, applicable only to three electrode sensors (containing working, counter and reference electrodes) and not to two electrode sensors (containing working and counter electrodes only).

In EP-0840 112, the working electrode of a two-electrode sensor is connected to the inverting terminal of an operational amplifier while a current pulse is applied periodically to the non-inverting terminal of the amplifier. The application of such a pulse can cause a bias voltage to be built up and the readings to drift.

Commercial pressures require carbon monoxide sensors to be relatively inexpensive; however, their electrodes contain expensive catalyst, which is usually a metal from the platinum group (Group VIII metal). In order to minimise the cost, the amount of catalyst used in each electrode is restricted. The problem of drift is particularly acute when the sensor uses a relatively small amount of catalyst.

Drift affects the accuracy of the readings and can cause a false alarm to be generated if especially severe. Recalibration is rarely possible with domestic monitors.

The present invention is based on the realisation that the long term drift in the output of two electrode sensors can be at least reduced.

Disclosure of Invention

According to the present invention, there is provided a monitor for monitoring an atmosphere for the presence of a target gas, the monitor comprising: a) two terminals for connection, respectively, to the working (sensing) electrode and the counter electrode of a two electrode electrochemical gas sensor, the sensor providing a current between the terminals that is indicative of the amount of target gas in the atmosphere; b) a circuit applying successively a pulse of positive potential between the electrode terminals and a pulse of negative potential between the electrode terminals (the pulses can be applied in either order); and c) a detector for monitoring the current of the sensor, including perturbations therein caused by the pulses applied in step b) and generating an alarm signal when the perturbations are less than a predetermined threshold, which is indicative of a malfunctioning sensor or the absence of a sensor.

The application of a negative and a positive pulse causes the net potential across the sensor to be zero hence reducing the likelihood of a bias voltage building up, which causes the readings to drift in sensors, which is a problem when only a single pulse is applied.

A stream of negative and positive pulses may be applied but they should be limited so that they are not, in effect, the application of a steady state alternating current. Thus a number of pulses, e.g. up to about 10 (5 positive and 5 negative) may be applied successively. However, it is preferred that only one such pair of pulses is applied at any instance.

The perturbation measured is preferably the change in the output signal during the course of each positive pulse and each negative pulse.

An operational amplifier is preferably connected between the sensor electrode terminals to generate an output signal according to the current flowing between the terminals, including perturbations therein caused by the applied pulses; in these circumstances, the detector can monitor the output of the amplifier for detecting the said perturbations. Except for their opposite polarity, the pulses are advantageously substantially identical. The duration of each pulse may be between 100 and 1000 milliseconds, e.g. 300 to 800 milliseconds and preferably about 600 milliseconds.

The potential of each pulse should be insufficient to electrolyse the sensor electrolyte.

The present invention also provides a method of monitoring an atmosphere for the presence of a target gas using the monitor as described above.

The application of both positive and negative pulse also provides a method of testing the sensor while measuring gas within the normal range of the instrument. Typically low cost domestic gas sensors operate from a single ended supply rail (typically 0V to 4.5V) When no gas is present and hence output is 0V, one of the current pulses cannot be measured since it would tend to produce a negative signal and the amplifier cannot output a values below 0V. However, the other pulse can be measured since it would tend to produce a signal greater than 0V. Equally at maximum output (typically corresponding to a gas concentration of 600ppm), the second type of pulse will not be detected since the output is already saturated and so the imposition of the second type of pulse will not affect the output signal but the first type of pulse can be measured since it would tend to lower the output signal below the maximum and so can be detected. Hence by using both positive and negative pulse the presence of at least one of the pulses can be seen.

Description of Drawings

The present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic circuit diagram showing a circuit of the present invention;

Figure 2 is a detailed circuit diagram of the circuit of Figure 1;

Figure 3 shows the change of current flowing through point 19 of the circuit of Figure 2 when a double pulse is imposed on that point; Figure 4 shows the change of output of the circuit of Figure 2 when a double pulse is imposed on the point 19;

Figure 5 shows the change of output of the circuit of Figure 2 when a double pulse is imposed on the point 19 and there is no operational sensor included in the circuit;

Figure 6 shows the change of output of the circuit of Figure 2 when a double pulse is imposed on the point 19 and the sensor is short circuited; and

Figure 7 shows the change of output of the circuit of Figure 2 when a single pulse is imposed on the point 19.

Best Method for Carrying out the Invention

Referring initially to Figure 1, there is shown a known gas monitoring circuit having a two-electrode gas sensor 10, the structure of which sensor is well known, see for example EP-0,840,112. Essentially, the sensor includes a sensing electrode 11a and a counter electrode 1 lb separated by an intervening body of electrolyte. The sensing electrode 11a is exposed to the atmosphere being monitored and accordingly any toxic gas (in this case carbon monoxide) in the atmosphere comes into contact with the sensing electrode 11a. The sensing electrode 11a is an anode and oxidises the carbon monoxide to carbon dioxide. This oxidation causes a current to flow through the sensor between the working electrode 11a and the counter electrode lib. A pair of resistors 16a and 16b is connected between the sensing electrode 11a and the inverting input of an operational amplifier 12. The non-inverting input of the amplifier is connected to the earth 14.

The operational amplifier 12 is acting as a transimpedance amplifier, the gain of which is defined by V_out / I^_m and given by the value of the feedback resistor between the negative input and output of the operational amplifier. The amplifier has a negative feedback including a resistor 17 such that the gain on the operational amplifier 12 is about 125,000 fold.

The presence of carbon monoxide at the sensing electrode 11a causes the sensing electrode to generate a current proportional to the amount of gas present. The potential difference between the electrodes floats until it reaches a level that is sufficient to generate the current concerned between the sensing and counter electrodes. Such current causes a potential difference across the resistors 16a and 16b, causing a change of potential at the inverting input of the amplifier 12. The operational amplifier 12 generates a signal at its output 15 that is proportional to the potential between its inputs and so the output signal is proportional to the current flowing in the sensor 10 and hence the amount of gas in the atmosphere being monitored. The output signal can be fed to a display and an alarm (neither shown) to display the concentration of carbon monoxide in the atmosphere and to generate an alarm if the concentration exceeds a pre-set threshold. Alternatively, the integrated concentration can be computed over different time periods to generate alarms based on the rate that the human body absorbs a particular concentration of gas.

In order to test that the sensor is functioning correctly, i.e. is viable, the monitor includes two sources of current 24 and 25 of opposite polarity. A switch 28 controlled by a microprocessor 22 periodically connects the sources of current 24 and 25 successively to point 19 which changes the potential between the working and the counter electrodes 11a and l ib of the sensor and alters the signal on output 15.

A working circuit corresponding to the schematic circuit of Figure 1 will now be described in connection with Figure 2. The components shown in both Figures 1 and 2 are indicated by the same reference numbers.

The microprocessor 22 includes a square wave generator 22' (PWM Output) that is connected to the base of transistor Ql that is connected to a supply rail 21. Resistor Rl and capacitor C3 provide a low frequency filter that filters out the frequency of the square wave and so a voltage is applied to the base of the transistor that is the weighted average of the peaks and troughs of the applied square wave, i.e. if the peaks and troughs are of equal duration, the voltage applied to the base will be half that voltage of the peak voltage and if the peaks are of much longer than the troughs, the voltage supplied to the base is a little less than the voltage of the supply. The transistor Ql acts as an emitter follower so that the voltage of the emitter is 0.7 volts less than that applied to the base of Ql. In this way, the voltage applied by the transistor Ql to a diode D3 can be set by altering the ratio of the durations of the peaks and troughs from the square wave generator 22'. The emitter of transistor Ql is connected via diode D3 and a resistor R3 to a point 33 and so the current supplied to the point 33 from the transistor Ql can be controlled by adjusting the ratio of the peaks of the square wave from generator 22' to the troughs.

Section 32 of the circuit is a charge pump deriving power from the square wave generator and providing a constant voltage of approximately -3 volts at point 31 of the circuit. Section 34 containing transistors Q2 and Q3 is a constant current source providing a current of -30μA to point 33 at the voltage of point 31, i.e. -3V, irrespective of the ratio of peaks to troughs of the square wave generator 22'.

When the square wave generator 22 is dormant, i.e. does not provide a square wave signal for an extended period, then the charge pump section will not be charged and so no voltage is applied to point 31 and no current is supplied to point 33. However, the charge pump section 32 is not discharged immediately and so, if the square wave generator 22 produces no peaks for a short period, then a current of -30μA is still provided to point 33 at a voltage of -3V for a time.

By controlling the square wave generator to produce no peaks for a short period, no current is applied to point 33 by transistor Ql but a current of -30μA is supplied by sections 32 and 34 and so the current flowing at point 33 and hence at point 19 will be -30μA. By controlling the square wave generator to provide an appropriate proportion of peaks, the transistor Ql can be made to supply a current of +60μA at point 33, which results in a net current of +30 μA being supplied to the point 19. In this way, successive positive and negative pulses of +30 μA and -30μA can be applied to point 19.

In the case of a CO sensor 10, the sensor is only periodically tested for viability and between the tests, the normal state of affairs will be for the square wave generator to be set to be dormant and produce no peaks, whereupon the current supplied by sections 32 and 34 to point 33 is zero; the transistor Ql will also not supply any current to point 33 and the overall effect is that no current is supplied at point 19. However, periodically the sensor will undergo testing. Once the charge pump section 32 is charged, the microprocessor 22 produces no square waves and so the transistor Ql will supply no current to point 33 causing a pulse of -30μA to be applied to point 19 from the charge pump and the constant current source 32,34. After about 600 milliseconds, a current of +60μA is supplied by transistor Ql, under the influence of the square wave generator, to point 33 causing a pulse of +30μA to be applied to point 19. After a further 600 milliseconds, the square wave generator is set to be dormant as described above. The double pulses can be applied periodically, e.g. every minute, to ensure that the sensor is functioning properly.

The change in current at point 19 is shown in Figure 3. The two pulses will impose a positive and a negative current on the normal current A flowing though point 19. However, the sensor acts a capacitor and the pulses will change the potential across the sensor 10, causing current to flow at the sensing electrode as the sensor charges or discharge. Such charging or discharging current will change the potential at the sensing electrode 11a over the course of a positive or negative pulse and will also change the current applied to the operational amplifier over the course of each pulse.

When a negative current is applied at point 19, see Figure 3, the current flowing at the sensing electrode will become more negative than its normal value A, (the value A results from the detection of gas at the sensing electrode but is zero if there is no gas detected). The resulting current supplied to the amplifier 12 will consequently increase (become more negative). Over the duration of the negative current pulse, the amount of current flowing will tend to the value A as the sensor charges up. This effect coupled with the direct effect that the imposition of the negative pulse has on the amplifier means that the output 15 increases from Vi to V₂ over the course of the pulse (see Figure 4).

When a positive current pulse is applied at point 19, the current flow at the sensing electrode immediately decreases (becomes less negative), see Figure 3, but again tends to value A over the course of the pulse as the sensor adjusts to the change in potential resulting from the imposed positive pulse. This effect coupled with the direct effect that the imposition of the positive pulse has on the amplifier results in the output 15 increasing from V₃ to V over the course of the pulse (see Figure 4). When the double pulse has finished, the current resumes its normal reading V₀.

The increase in the signal from Vi to V₂ and from V₃ to V over the duration of the two pulses indicates that the sensor is correctly operational since it indicates that charge is flowing at the sensing electrode as a result of the imposed pulses. If (a) the electrolyte in the sensor has dried up e.g. due to leakage, or (b) the sensor is absent or (c) the sensor terminals are not correctly connected to the circuit, or (d) the sensor is short circuited, the sensing electrode will not accept charge and the above change of current over the course of a pulse will not occur or will occur to a much lower extent, indicating that the sensor is no longer correctly functioning. This state of affairs will be detected by the microprocessor 20 and an appropriate error signal generated.

Figure 5 shows the change in amplifier output 15 when there is no operational sensor present. The whole of the pulses are applied to the inverting input of the amplifier and there will be no change in the signal over the duration of a pulse. On the other hand, if there is a short circuit across the sensor, part of the pulse current flows via the short circuit to earth and the output from the operational amplifier will be smaller (Figure 6) than in the case of the situation described in connection with Figure 5. Again there is no change in the signal over the duration of a pulse. Thus the present invention can distinguish between a fault occurring when there is no operational sensor present and a fault occurring when there is a short circuit across the sensor.

After a double pulse has been applied, the signal 15 returns to its normal value according to the amount of gas in the atmosphere. However, if only a single pulse is applied, there is an offset in the signal once the pulse has finished (see Figure 7).

One advantage of the present invention is that the double pulse will still indicate the viability of the sensor even when the operational amplifier is saturated since the positive current pulse will decrease the current flowing to the amplifier input and will cause a change in the output. On the other hand, the negative current pulse will not be observed since the operational amplifier is already saturated and the increase in the signal at the amplifier input will not alter the amplifier output. Likewise, when there is no gas in the atmosphere being sensed, the sensor produces no current. In cheap monitors that have only one charge rail, the amplifier cannot output a negative signal and so only the application of the negative current to the inverting input of the amplifier will change the current. Hence by using both positive and negative pulse the presence of at least one of the pulses can be seen.

When there is target CO gas in the atmosphere being monitored, classical current flows into the sensing electrode (electrons flow out of the sensing electrode).

However, following exposure to high gas concentrations and subsequent removal of the gas, there is a tendency for the sensor to produce a current in the opposite direction, i.e. current flows out of the sensing electrode (electrons flow towards the sensing electrode). By injecting short pulses of +30 μA current into the cell to oppose this current, the time for the sensor cell to recover normal operation is decreased. Thus if the output of the sensor cell is negative, which in a single rail monitor means that the output of the operational amplifier is zero, such short pulses of current will assist in bringing the sensor back to its normal operational state.

When the microprocessor 22 detects a reversed current, it can apply such voltage pulses to bring the sensor back to its normal operational state more quickly.

By applying both positive and negative potential pulses, the long term drift of the sensor output is reduced and the lifetime of the sensor is improved.

In practice, the square wave generator 22' is part of the microprocessor chip 22.

Claims

1 A monitor for monitoring an atmosphere for the presence of a target gas, the monitor comprising: a) two terminals for connection, respectively, to the working (sensing) electrode and the counter electrode of a two electrode electrochemical gas sensor, the sensor providing a current between the terminals that is indicative of the amount of target gas in the atmosphere; b) a circuit applying successively a pulse of positive potential between the electrode terminals and a pulse of negative potential between the electrode terminals; and c) a detector for monitoring the current of the sensor, including perturbations therein caused by the pulses applied in step b) and generating an alarm signal when the perturbations are less than a predetermined threshold, which is indicative of a malfunctioning sensor or the absence of a sensor.

2 A monitor as claimed in claim 1, wherein the pulse of negative potential is applied between the electrode terminals before the pulse of positive potential.

3 A monitor as claimed in claim 1, wherein an operational amplifier is connected between the sensor electrode terminals to generate an output signal according to the current flowing between the terminals, including perturbations therein caused by the applied pulses and wherein the detector monitors the output of the amplifier for detecting the said perturbations.

4 A monitor as claimed in any of claims 1 to 3, wherein the detector detects the changes in the current over the course of the pulses and generates an alarm signal when the value of the said change is less than a threshold value.

A monitor as claimed in any of claims 1 to 4, wherein the positive and negative pulses are substantially identical except for their opposite polarities. A monitor as claimed in any of claims 1 to 5, wherein the duration of each pulse is between 100 and 1000 milliseconds, e.g. 300 to 800 milliseconds and preferably about 600 milliseconds.

A monitor as claimed in any one of claims 1 to 6, which includes an electrochemical gas sensor connected to the terminals.

A method of monitoring an atmosphere for the presence of a target gas by means of a two electrode electrochemical gas sensor having a working (sensing) electrode and a counter electrode, the sensor providing a current between the electrodes that is indicative of the amount of target gas in the atmosphere, the method comprising: a) applying successively a pulse of positive potential between the electrodes of the sensor and a pulse of negative potential between the sensor electrodes; and b) revealing malfunctions in the sensor by detecting perturbations in the current of the sensor caused by the pulses applied in step b) and generating an alarm signal when the perturbations are less than a predetermined threshold.

A monitor as claimed in claim 1, wherein the pulse of negative potential is applied between the electrode terminals before the pulse of positive potential.

A method as claimed in claim 8 or claim 9, wherein the step of detecting the perturbations comprises detecting the changes in the current over the course of the pulses and generating an alarm signal when the value of the said change is less than a threshold value.

A method as claimed in claim 8 or claim 9, which comprises connecting the inputs of an operational amplifier to the sensor electrodes and wherein step b) is performed by monitoring the output of the amplifier. A method as claimed in claim 11, wherein the step of detecting the perturbations comprises detecting the changes in the signal over the course of the pulses and generating an alarm signal when the value of the said change is less than a threshold value.

A method as claimed in any of claims 8 to 12, wherein the pulses are substantially identical except for their opposite polarity.

A method as claimed in any of claims 8 to 13, wherein the duration of each pulse is between 100 and 1000 milliseconds, e.g. 300 to 800 milliseconds and preferably about 600 milliseconds.