In many industries, monitoring air quality is mandatory for security reasons. For these reasons, different gas sensor technologies exist. Of these, the electrochemical sensing technique has the advantage of having a linear output and operating with a low consumption. Consequently, it operates on batteries for a long period of time (particularly true for the toxic gas sensor which delivers almost no current in ambient air), and therefore is used for portable personal protection equipment.

The gases most frequently monitored by handheld safety detectors which use electrochemical sensors are oxygen (O₂), carbon monoxide (CO), hydrogen sulfide (H₂S) and nitrogen dioxide (NO₂). However, many other gases can also be monitored with electrochemical sensors.

An electrochemical gas sensor contains a gas membrane and two or three electrodes in contact with an electrolyte. The sensor is impermeable to the electrolyte. Gas enters the sensor through the gas membrane which limits the rate of gas diffusion (therefore impacting the sensitivity of the sensor). When gas reaches the working electrode (WE), a chemical reaction occurs: either oxidation (loss of electrons) occurs from the gases CO, H₂S, sulfur dioxide (SO₂), and nitrogen monoxide (NO), or reduction (gain of electrons) occurs from the gases O₂, NO₂, and chlorine (Cl₂). The reaction depends on the sensor. If oxidation occurs at the WE, the complimentary reaction (reduction) occurs at its counter electrode (CE).

Electrochemical sensors are widely used. They require specific op-amps to drive them and to amplify the signal (current) they generate which is proportional to the concentration of the gas being measured. The best choice of op-amp is generally a low-power, rail-to-rail output, CMOS device.

Read more: Download the full application note