The core working principle of a combustible gas alarm is to detect the concentration of combustible gases in the environment through sensors, and trigger an alarm after signal processing. Its technical process can be divided into four key steps: sensor detection, signal conversion, threshold comparison, and alarm output.
The sensor detection process adopts catalytic combustion, semiconductor, or infrared absorption technologies. Catalytic combustion sensors use platinum wire catalysts to cause flameless combustion of combustible gases, generating heat that changes the resistance value of the platinum wire, thereby breaking the balance of the Wheatstone bridge and outputting a voltage signal proportional to the gas concentration. For example, for every 1% increase in methane concentration, the resistance change rate of platinum wire can reach 0.1 Ω/℃. Semiconductor sensors use metal oxide materials such as tin dioxide. When combustible gases are adsorbed, the electronic state on the surface of the material changes, resulting in a decrease in resistance value. The rate of resistance change is linearly related to the gas concentration. Infrared absorption sensors calculate concentration based on Lambert Beer's law by measuring the degree of absorption of specific wavelength infrared light by gases, and are suitable for detecting hydrocarbon gases such as methane and propane.
In the signal conversion and processing stage, the weak electrical signal output by the sensor is processed by a preamplifier circuit and converted into a standard signal of 0-5V or 4-20mA. For example, the μ V level voltage signal output by a catalytic combustion sensor needs to be amplified to V level by an instrument amplifier, and then converted into a digital signal by an A/D converter for processing by the controller.
The threshold comparison and alarm triggering process are completed by the controller. When the detected concentration exceeds the preset threshold, the controller immediately drives the sound and light alarm device, while displaying the leakage location and concentration value. For example, household alarms are usually set with a low alarm threshold of 25% LEL and a high alarm threshold of 50% LEL, while industrial settings are adjusted based on the lower explosive limit of gas. Some devices can also be linked to the exhaust system or cut off the gas source, such as the gas pipeline solenoid valve completing the closing action within 0.5 seconds after receiving the alarm signal.
In practical applications, sensors need to be calibrated regularly to maintain accuracy. Catalytic combustion sensors require calibration with standard gases every 6 months, while semiconductor sensors require cleaning of surface adsorbates every 3 months. The installation position should consider the gas density. For gas probes that are lighter than air (such as natural gas), they should be 0.3m away from the ceiling, and for gas probes that are heavier than air (such as liquefied gas), they should be 0.3m away from the ground.