There are three principal methods of detecting the presence of carbon monoxide (CO) in air. The first method of detection uses a plug-in detector having a periodically-heated semi-conductor that exhibits a change in conductivity when CO is present. However, this type of detector requires AC-power, and ceases to function when electricity to the unit fails. The detector tends to be sensitive to changes in humidity, and is cross-sensitive to the presence of other combustible gases e.g. alcohols, including materials containing alcohols, examples of which include hairspray.
The second type of detector uses a translucent gel disk that darkens on prolonged exposure to CO. The change in translucency is detected by an infrared sensor within the unit. Detection tends to be less responsive than for other detectors, taking hours rather than minutes to recover after the ambient air has become free of CO. Consequently, it becomes necessary to remove the battery-sensor pack in order to silence the alarm that sounds when CO is detected. In addition, the gel tends to accumulate CO over a period of time, resulting in a tendency for false alarms after prolonged exposure to urban pollution.
The third type of detector uses a fuel-cell type electrochemical sensor. These detectors are battery-powered and are much more accurate and responsive to the presence of CO.
The electrolytic cell of an electrochemical sensor must have at least two electrodes. One electrode is the electrode that comes in contact with the gas that is to be detected, and is usually referred to as the sensing electrode. A second electrode is known as the counter electrode or auxiliary electrode. When the gas to be detected comes in contact with the sensing electrode, an oxidation or reduction reaction takes place at the sensing electrode, with a corresponding reduction or oxidation reaction occurring at the counter electrode.
In order to detect CO, the potential of the sensing electrode must be sufficiently positive so that CO will be oxidized. However, the potential of the sensing electrode is subject to change, because the use of a fixed external voltage bias inter-relates the potential of the sensing electrode to the potential of the counter electrode. The potential of the counter electrode is unstable if the electrode material is not electrochemically reversible, i.e. its exchange current is not high enough compared with the current passing through the cell. Consequently, it is possible that the potential of the sensing electrode will shift to a value where CO is not fully oxidised at the sensing electrode.
Thus, it can be important to have an electrode with a constant or almost constant potential throughout the reaction. Such an electrode is called the reference electrode and its main role is to stabilize the potential of the sensing electrode. In that event, the potential of the sensing electrode will remain relatively stable so that CO may be quantitatively oxidized.
An example of a two-electrode sensor is described in U.S. Pat. No. 3,755,125 and examples of three-electrode sensors are described in U.S. Pat. Nos. 4,587,003, 5,284,566 and 5,338,429. In all of these sensors, a platinum/air/water electrode was used as reference electrode. However, such sensors have a number of disadvantages, including (a) high cost due to the use of precious metals e.g. platinum foils and wires, (b) the requirement of strict performance criteria in contact between electrodes and precious metals, and high failure rates due to poor contact, (c) leakage of electrolyte through the interface of metal and plastic housing when subjected to temperature shock or after long periods of operation, (d) costs of assembly of numerous parts of the sensor, and (e) large piece-to-piece variations in sensor output.