A small sensor is an electrode wetted by a water-borne gel electrolyte (usually sulfuric acid: H2SO4). When the detected gases (such as carbon monoxide: CO or hydrogen sulfide: H2S) enter the sensor and oxidize or change the concentration of the electrolyte, the working electrode produces a weak current under the action of the catalyst. The current is amplified by an amplifier connected to the sensor to display the gas concentration in the target area.
Typical structure diagram of electrochemical sensor
Most electrochemical gas sensors are used in diffusion mode, in which the gas sample in the surrounding environment enters the sensor through a small hole in the front of the sensor (through the natural flow of gas molecules). Some devices use an air pump to pump air / gas samples into the sensor. A polytetrafluoroethylene film is installed at the air hole to prevent water or oil from entering the sensor. The measurement range and sensitivity of the sensor can be changed by adjusting the size of the air inlet during design. The larger air inlet can improve the sensitivity and resolution of the equipment, while the smaller air inlet can reduce the sensitivity and resolution, but it can increase the measurement range.
The working principle of the oxygen sensor is similar to that of the electrochemical oxygen sensor described before, but the service life of the oxygen sensor is predictable, so the replacement cycle can also be preset – generally 2-3 years. Unlike toxic gas sensors, oxygen sensors are continuously exposed to the target gas for a long time. In the common application of oxygen consumption monitoring, the oxygen concentration of the sensor working environment is 20.9%, which will cause chemical reaction on the lead anode, resulting in the gradual consumption of the anode. Therefore, the ability of the sensor to continuously generate current by reacting with oxygen depends on the amount of lead in the electrolyte.
By adding the key mechanism of “temperature compensation”, the gas detection equipment manufacturer ensures the performance of the sensor. The gas sensitivity (and zero baseline signal) often changes with temperature, so when the temperature rises or falls, the gas sensitivity changes nonlinearly.
In the process of developing gas detection equipment, people spend a lot of time placing the same gas sensor in different temperatures and concentrations of gas (temperature between – 30 ℃ and + 50 ℃). The collected data is processed to generate a temperature compensation algorithm for the gas detector to ensure that the sensor readings are consistent throughout the operating range.