typical sensors found in gas detectors
The core part of the gas detector is the gas sensor, which varies according to different gas detection principles. Common gas sensors include: PID photoion sensor, infrared sensor, electrochemical sensor, catalytic combustion sensor and semiconductor sensor. The following Honey Iger Technology will introduce you in detail the working principle and advantages and disadvantages of each sensor.
1. Infrared principle of gas detector
Principle: The non-spectral infrared principle NDIR sensor is based on the Beer-Lambert infrared absorption law, that is, different gases absorb light of a specific wavelength, and the intensity of absorption is proportional to the concentration of the gas to achieve detection. It uses a filter to divide the infrared light into a small band of spectral lines required, and the gas to be detected absorbs the spectral lines of this small band.
Advantages: high reliability, good selectivity, high precision, no toxicity, less interference from the environment, long life, and no dependence on oxygen.
Disadvantages: It is greatly affected by humidity, and the types of detection gases are limited. At present, it is mainly used in methane, carbon dioxide, carbon monoxide, sulfur hexafluoride, sulfur dioxide, hydrocarbons and other gases.
2. Semiconductor principle of gas detector
Principle: The semiconductor gas sensor is made by using some metal oxide semiconductor materials, and at a certain temperature, the resistance changes with the composition of the ambient gas. For example, the alcohol sensor is based on the principle that when tin dioxide encounters alcohol gas at high temperature, the resistance will decrease sharply.
Advantages: It has the advantages of low cost, simple manufacture, high sensitivity, fast response, long life, low humidity sensitivity and simple circuit.
Disadvantages: poor stability, greatly affected by the environment, especially the selectivity of each sensor is not unique, and the output parameters cannot be determined. Therefore, it is not suitable to be used in places where accurate measurement is required, and it is mainly used for civil purposes.
3. The principle of catalytic combustion of gas detector
Principle: The catalytic combustion sensor is to prepare a high-temperature resistant catalyst layer on the surface of the platinum resistor. At a certain temperature, the flammable gas is catalytically burned on the surface. The combustion causes the temperature of the platinum resistor to rise and the resistance to change. The change value is a function of the concentration of the flammable gas.
Advantages: The catalytic combustion gas sensor selectively detects combustible gases: the sensor does not respond to anything that cannot be combusted. Quick response, long life, less affected by temperature, humidity and pressure. The output of the sensor is directly related to the explosion hazard of the environment, and it is a dominant type of sensor in the field of safety detection.
Disadvantages: In the range of flammable gases, there is no selectivity. The sensor is easily poisoned, and most elemental organic vapors have a poisoning effect on the sensor.
Note: The realization of catalytic combustion detection is conditional. It must be ensured that the detection environment contains enough oxygen. In an oxygen-free environment, this detection method may not be able to detect any flammable gases. Certain lead-containing compounds (especially tetraethyl lead), sulfur compounds, silicon, phosphorus compounds, hydrogen sulfide, and halogenated hydrocarbons may poison or inhibit the sensor.
4. PID principle of gas detector
Principle: PID is composed of ultraviolet light source and ion chamber and other main parts. There are positive and negative electrodes in the ion chamber to form an electric field. The gas to be measured is ionized under the irradiation of ultraviolet lamp to generate positive and negative ions, and a current is formed between the electrodes. Amplify the output signal
Advantages: high sensitivity, no poisoning problem.
Disadvantages: non-selective, highly affected by humidity, short life of UV lamp, expensive.
5. Electrochemical principle of gas detector
Principle: It works by reacting the electrolyte inside the sensor with the target gas and generating an electrical signal proportional to the gas concentration.
Advantages: wide operating temperature range, multiple measuring ranges, high sensitivity, linear output, good selectivity
Disadvantages: short life, limited storage period, short life in extremely dry or high-concentration gas environments, non-specific, susceptible to interference, humidity affects accuracy.
NOTE: Most toxic gas sensors require small amounts of oxygen to function properly. There is a vent hole on the back of the sensor for this purpose. High humidity and high drought will affect the service life of the sensor. Momentary pressure changes may produce a transient sensor output, and may also reach a false alarm state.
