The gas sensor is the core of the gas detection system and is usually installed inside the detector head. Essentially, a gas sensor is a converter that transforms the volume fraction of a certain gas into the corresponding electrical signal. The probe adjusts the gas sample through a gas sensor, typically including filtering out impurities and interfering gases, drying or cooling treatment, sample suction, and even chemical treatment of the sample to enable the chemical sensor to measure more quickly.

The sampling method of gas directly affects the response time of the sensor. At present, the main methods for gas sampling are simple diffusion or drawing gas into detectors.

Simple diffusion takes advantage of the characteristic that gases naturally spread in all directions. The target gas passes through the sensor inside the probe, generating a signal proportional to the volume fraction of the gas. As the diffusion process gradually slows down, the diffusion method requires the position of the probe to be very close to the measurement point. One advantage of the diffusion method is that it directly introduces gas samples into the sensor without the need for physical and chemical changes. Sample suction probes are typically used for sampling positions close to processing instruments or exhaust pipes.

This technology can provide the sensor with a stable airflow with controllable speed. Therefore, in cases where the magnitude and flow rate of the airflow frequently change, this method is more recommended. The gas sample from the measurement point may be led to the measurement probe over a certain distance. The length of the distance mainly depends on the design of the sensor. However, a longer sampling line will increase the measurement lag time, which is a function of the length of the sampling line and the flow velocity of the gas from the leakage point to the sensor. For certain target gases and vapors, such as SiH4 and most biological solvents, the sample volume of gases and vapors may decrease due to their adsorption effect or even condensation on the wall of the sampling tube.

Gas sensors are a major category of chemical sensors. From working principles, characteristic analysis to measurement techniques, from the materials used to manufacturing processes, from the objects to be inspected to application fields, all can form independent classification standards, giving rise to a series of complex classification systems. Especially in the issue of classification standards, there is currently no unified one, making it quite difficult to conduct a strict and systematic classification.