There are three types of sensors available and they vary in cost, accuracy and speed of response.
All three types of sensor elements typically last from 3 to 5 years. At least one CO detector is available which includes a battery and sensor in a replaceable module. Most CO detectors do not have replaceable sensors.
A biomimetic (chem-optical or gel cell) sensor works with a form of synthetic hemoglobin
which darkens in the presence of CO, and lightens without it. This can either be seen directly or connected to a light sensor and alarm.
A type of fuel cell
that instead of being designed to produce power, is designed to produce a current that is precisely related to the amount of the target gas (in this case carbon monoxide) in the atmosphere. Measurement of the current gives a measure of the concentration of carbon monoxide in the atmosphere. Essentially the electrochemical cell consists of a container, 2 electrodes, connection wires and an electrolyte - typically sulfuric acid. Carbon monoxide is oxidised at one electrode to carbon dioxide whilst oxygen is consumed at the other electrode. For carbon monoxide detection, the electrochemical cell has advantages over other technologies in that it has a highly accurate and linear output to carbon monoxide concentration, requires minimal power as it is operated at room temperature, and has a long lifetime (typically commercial available cells now have lifetimes of 5 years or greater). Until recently, the cost of these cells and concerns about their long term reliability had limited uptake of this technology in the marketplace, although these concerns are now largely overcome.
Thin wires of the semiconductor tin dioxide
on an insulating ceramic base provide a sensor monitored by an integrated circuit. CO reduces resistance and so allows a greater current which if high enough will lead to the monitor triggering an alarm. The power demands of this sensor means that these devices can only be mains powered.