Catalytic combustion gas sensor is currently the most commonly used low-cost combustible gas sensor. It has a pair of small "beads" made of ceramic substrate wrapped in platinum wire coils inside, one of which is a sensitive "bead" used as a detection element. Its surface is covered with a layer of catalyst that reacts with all combustible gases present. The other is an emotional "bead" that does not react with any gas, it only serves as a compensating element for external temperature or humidity changes.

During normal operation, the platinum wire coil is energized and heated to a high temperature of 450 ℃~500 ℃. When it comes into contact with combustible gas, catalytic combustion reaction occurs on the surface of the detection element, and the temperature rises. The internal platinum wire resistance also increases accordingly. The resistance value change of the detection element can be accurately measured by the Wheatstone bridge, and the concentration of the detection gas can be calculated.

2. Semiconductor gas sensor

Semiconductor gas sensors are commonly used to detect low concentrations of flammable and toxic gases. Its gas sensing element is a metal oxide thin film deposited on a silicon wafer. After the gas comes into contact with the metal oxide, it will adsorb on its surface, causing a change in the resistance of the metal oxide, and this change in resistance is related to the change in gas concentration. Usually, the surface of semiconductor sensors is heated to a constant temperature between 200 ℃ and 250 ℃ to increase the reaction rate and reduce the impact of environmental temperature changes.

3. Infrared gas sensor

Infrared gas sensors are mainly used to detect flammable gases and carbon dioxide gas. Compared with catalytic combustion gas sensors, it has the advantages of better stability, high accuracy, strong anti-interference ability, no poisoning, suitable for harsh environments, less maintenance, and long service life. It is usually regarded as a high-end product for detecting combustible gases; Due to price reasons, the application of infrared gas sensors is not as widespread as catalytic combustion gas sensors. However, with the continuous progress and maturity of infrared sensor manufacturing technology, the price gap between the two is narrowing. Currently, infrared gas sensors are gradually replacing catalytic combustion sensors to achieve rapid product upgrades and replacements. There are two common types of infrared gas sensors: point type and open circuit type.

The point type infrared gas sensor consists of an infrared source, an optical focusing lens, a sampling chamber, and a pyroelectric sensor. When the infrared light emitted by the infrared source passes through the sampling chamber, a specific wavelength of infrared light is absorbed by the detection gas in the chamber. By comparing the absorbed infrared light intensity with the reference infrared light intensity, the concentration of the detection gas can be obtained.

An open circuit infrared gas sensor consists of an emitting end and a receiving end, and must be installed in pairs. The transmitting end emits a beam of infrared radiation to the receiving end. When there is a detection gas present, a specific wavelength of infrared radiation in the beam is absorbed by the gas. By comparing the intensity of the absorbed infrared light at the receiving end with the reference infrared light intensity, the concentration of the detection gas can be determined. The measurement unit of an open circuit infrared gas detector is LEL. m, which is the product of the overall gas concentration in the optical path and the distance.

4. PID gas sensor

Photo Ionization Detector (PID) gas sensors can detect extremely low concentrations (in ppm or even ppb levels) of volatile organic compounds (VOCs) and other toxic gases. The PID type gas sensor uses the principle of photoionization to detect gases. It uses a ultraviolet (UV) light source to ionize organic matter into positive and negative ions that can be detected by a detector. The detector detects the ionized gas charge and converts it into a current signal. After amplification, the current is displayed as the ppm concentration value of the gas. After detection, ions will be restored to their original gas or vapor. PID sensors are non destructive sensors that do not "burn" or change the gas being tested for a long time. Therefore, the gas detected by PID can still be collected for further measurement.

5. Electrochemical gas sensor

Electrochemical gas sensors play an irreplaceable role in the detection of toxic and harmful gases. Electrochemical gas sensors consist of electrolytes, electrodes, and filters. By selecting appropriate voltage, electrolyte, and electrode materials, the measured gas undergoes a chemical reaction on the electrode surface and generates a microcurrent, which is proportional to the gas concentration. By analyzing and processing the current, the concentration of the measured gas can be obtained.