Background: 

Electrochemical gas sensors—sometimes called electrochemical analyzers or electrochemical toxic gas detectors—are designed to measure the concentration of a specific gas, such as oxygen or carbon monoxide, in an external circuit.

The working principle of electrochemical gas sensor

Electrochemical gas sensors typically consist of a working (i.e., sensing), counter, and reference electrode, all contained within a housing with a gas-permeable membrane. During sensing operation, it is submerged in electrolyte. Combined, these components allow it to perform its function.

The housing membrane allows gas (but not liquid) to enter the sensor. When the gas reaches the working electrode, an electrochemical reaction (oxidation or reduction, depending on the type of gas) occurs. This reaction initiates the flow of electrons (ie, current) between the working and counter electrodes. Oxidation reactions move electrons from the working electrode to the counter electrode, while reduction reactions move electrons from the counter electrode to the working electrode. In either case, the resulting current is proportional to the concentration of the target gas. This current is then amplified and processed according to the calibration to provide the user with a parts per million (PPM) or percent by volume reading. Most electrochemical sensors indicate a reading of zero if the target gas is not detected. However, unlike catalytic bead sensors, this sensor does require balancing or zeroing.

Note on electrochemical gas sensors

Although electrochemical sensors are designed to recognize specific gases, most exhibit some degree of cross-sensitivity. This phenomenon refers to the response of the sensor to a gas other than the target gas, usually due to higher chemical reactivity in the non-target gas than in the target gas. In some cases, non-target gases can mask the presence of target gases. For these reasons, filters and bias voltages are used during operation. It is important to minimize the impact of cross sensitivity on reading accuracy.

The rate of the chemical processes on which electrochemical sensors rely is directly proportional to temperature. Therefore, temperature changes can affect the sensing performance. For better reading accuracy in a wider range of environmental conditions. Some form of temperature compensation is recommended.