TOXIC SENSORS

Electrochemical toxic gas sensors are micro fuel cells, designed to be maintenance-free and stable for long periods. They have a direct response to volume concentration of gas rather than partial pressure.

The simplest form of electro-chemical toxic sensor comprises two electrodes: sensing and counter, separated by a thin layer of electrolyte. This is enclosed in a plastic housing that has a small capillary to allow gas entry to the sensing electrode and includes pins which are electrically attached to both electrodes and allow easy external interface. These pins may be connected to a simple resistor circuit that allows the voltage drop resulting from any current flow to be measured (figure 1). Gas diffusing into the sensor is either oxidised or reduced at the sensing electrode and, coupled with a corresponding (but converse) counter reaction at the other electrode, a current is generated through the external circuit. Since the rate of gas entry into the sensor is controlled by the capillary diffusion barrier, the current generated is proportional to the concentration of gas present outside the sensor and gives a direct measure of the toxic gas present.

The central feature of the design is the gaseous diffusion barrier, which limits the flow of gas to the Sensing electrode. The electrode is therefore able to react all target gas as it reaches its surface, and still has electrochemical activity in reserve. This high activity reserve ensures each CiTiceL has a long life and excellent temperature stability.

The reactions that take place at the electrodes in a carbon monoxide sensor are:

Similar reactions take place for all other toxic gases that are capable of being electrochemically oxidised or reduced.

From the reaction at the counter electrode, it is evident that oxygen is required for the current generation process to take place. This is usually provided in the sample stream by air diffusing to the front of the sensor, or by diffusion through the sides of the sensor (a few thousand ppm is normally sufficient). However, continuous exposure to an anaerobic sample gas may result in signal drift, despite the oxygen access paths and so we recommend that toxic sensors are never potted with resin or completely immersed in an anaerobic gas mixture.

For certain demanding applications where the sensors are frequently exposed to very high concentrations of the analyte, for example in flue gas analysis, it may be necessary to ensure there is an additional source of oxygen access to the counter electrode. In the 5 Series flue gas CiTiceL® range additional access is achieved through the side of the sensor.

ACTIVITY RESERVE

It is important in the design of any electrochemical gas sensor that the rate limiting step should be the diffusion of gas through the barrier (capillary) and all other stages should have rates which are significantly faster. To achieve this it is important that the electrode material has high catalytic activity for the electrochemical reactions of the sensor.

All CiTiceLs® have highly active electrodes resulting in sensors with very high activity reserves. This is an important factor in ensuring the long- term stability of the sensor and the low levels of drift.

3 AND 4 ELECTRODE SENSORS

2-Electrode sensors are the simplest form of toxic gas sensors. However they have limited measuring range due to polarisation of the counter electrode. This polarisation effect can be eliminated by using a third, reference, electrode with a stable potential in the sensor design. In these sensors the sensing electrode is held at a fixed potential relative to the reference electrode (from which no current is drawn) so both maintain a constant potential. The counter electrode is still free to polarise, but has no effect on the sensing electrode and does not limit the sensor in any way.

3-Electrode sensors are the most widely used design of electrochemical sensors for detecting toxic gases. Despite this there are some applications where the 3-electrode design proves inadequate. For example cross-interfering gases or zero-offset changes with temperature can compromise their overall performance. By introducing a fourth 'auxiliary' sensor accurate sensor performance can be maintained while also allowing the simultaneous measurement of two gases.

OVERCOMING CROSS INTERFERENCE

A 4th auxiliary electrode can assist in overcoming cross interference from other gases. Typically carbon monoxide sensors show a significant response to hydrogen which can make the accurate measurement of CO difficult when hydrogen is present. However, using a sensor with a 4th auxiliary electrode all of the CO and some of the H₂ reacts on the sensing electrode leaving only H₂ to react with the auxiliary electrode. Once the ratio of the responses on each electrode in known, a H₂ -compensated signal can be obtained by subtracting the auxiliary signal from the sensing electrode signal with an analogue circuit or using a microprocessor with appropriate software.

OVERCOMING TEMPERATURE EFFECTS

The baseline signal of most electrochemical sensors tends to increase exponentially with temperature, approximately doubling for every 10°C rise in temperature. For the majority of applications this does not normally present problems but for applications involving very low concentrations of gases, such as ambient O₃ or CO monitoring, any baseline shift with temperature could seriously affect the ability to measure these gases accurately. The signals from both the sensing electrode and auxiliary electrode will both show similar responses to changes in temperature but because the auxiliary electrode is not exposed to reactive gas, its signal can simply be subtracted from that of sensing electrode. This is a useful method of compensating for any baseline shifts that would normally occur as a result of changes in temperature, but is not ideal.

DUAL GAS SENSOR

This 4-electrode technology allows CO and H₂S to be measured using just one sensor. With space at a premium inside portable safety instruments, this is a significant advantage for instrument designers. The 4COSH sensor operates in a similar way to other standard sensors except that it comprises two sensing electrodes: one for CO and the other for H₂S.The first sensing electrode oxidises the H₂S completely while the CO diffuses through to be oxidised by the second electrode. This 4-electrode design is able to produce two separate signals which allow two gases to be measured with one sensor.