01Combustible gas alarms are mainly of catalytic combustion type, semiconductor type, infrared absorption type, etc. Their working principles are as follows:

    Catalytic combustion type

    Combustible gas burns flamelessly on its surface. When the combustible gas enters the detection element of the alarm, an oxidation reaction will occur under the action of the catalyst, causing the temperature of the detection element to rise.

    The temperature change causes the resistance value of the detection element in the Wheatstone bridge (a bridge circuit composed of four resistors) to change, breaking the balance of the bridge and generating an electrical signal corresponding to the concentration of the combustible gas. After the signal is amplified and processed, it can drive the alarm device.

    Semiconductor type

    After the semiconductor gas sensor in the alarm absorbs the combustible gas, the properties such as the affinity of its internal electrons change.

    This will cause the resistance value of the element to change. By detecting this resistance change and converting it into an electrical signal, it will be amplified by the amplifier circuit. When the signal reaches the set threshold, it will trigger the alarm.

    Infrared absorption type

    Different gases absorb infrared light of specific wavelengths. When infrared light passes through an area containing combustible gas, the combustible gas absorbs infrared light of a specific wavelength.

    According to the Lambert-Beer law, the degree of light absorption is proportional to the gas concentration. By detecting the intensity change after the infrared light is absorbed, the concentration of the combustible gas can be calculated. When the concentration exceeds the safety threshold, an alarm will be sounded.

    02Toxic gas alarms are mainly of electrochemical, semiconductor and photoionization types. The working principle is as follows:

    Electrochemical type

    The electrochemical sensor contains electrolyte and electrodes. When toxic gas diffuses into the sensor, oxidation or reduction reaction will occur on the electrode surface. 

    This reaction will generate current, and the current size is proportional to the concentration of toxic gas. For example, after carbon monoxide enters the sensor, it reacts on the working electrode to generate current. After the instrument detects the current, it converts it into digital signal processing, and alarms when the concentration exceeds the threshold.

    Semiconductor type

    The metal oxide semiconductor material in the sensor changes the electronic state of its surface after adsorbing toxic gas.

    This will cause the material resistance to change. The instrument detects the resistance change and converts it into a corresponding electrical signal. After amplification and other processing, the alarm will be triggered when the signal meets the alarm conditions.

    Photoionization type

    The ultraviolet lamp in the instrument emits high-energy ultraviolet light. When toxic gas molecules enter the detection chamber and are irradiated by ultraviolet light, they will be ionized. 

    The positive and negative ions generated during the ionization process will form a current under the action of the electric field, and the current intensity is related to the gas concentration. This current signal is detected, and when the gas concentration reaches the alarm setting value, an alarm will be sounded.

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