Principle Analysis and Troubleshooting of Combustible Gas Detectors
① Combustible gas detectors are installed in industrial and civil buildings to respond to the concentration of single or multiple combustible gases. The two most commonly used types in daily application are catalytic combustible gas detectors and semiconductor combustible gas detectors. Semiconductor detectors are mainly deployed in restaurants, hotels, home kitchens and other locations consuming coal gas, natural gas and liquefied petroleum gas. Catalytic detectors are widely adopted in industrial sites where combustible gases and vapors are released.
② A catalytic combustible gas detector measures gas concentration based on the resistance change of heated refractory platinum wire. When combustible gas enters the detector, an oxidation reaction (flameless combustion) occurs on the surface of the platinum wire. The heat generated raises the wire temperature and alters its resistivity. Similarly, high ambient temperature and other external factors will also change the platinum wire resistivity and lead to deviation in measured readings.
③ A semiconductor combustible gas detector determines gas concentration by detecting surface resistance variation of semiconductor materials. It adopts highly sensitive gas-sensitive semiconductor components. Under operating conditions, the resistance of the semiconductor drops upon contact with combustible gas, and the magnitude of resistance reduction corresponds proportionally to the gas concentration.
④ A combustible gas detector consists of a measuring unit and an alarm unit with respective independent functions. Working principle of the measuring unit: The sensor forms a measuring bridge circuit with the sensing element, fixed resistors and zero-adjustment potentiometer. The bridge uses platinum wire as the carrier of the catalytic element. After power-on, the platinum wire heats up to its operating temperature, and ambient air reaches the element surface via natural diffusion or other sampling methods. When no combustible gas is present in the air, the bridge outputs zero voltage. If combustible gas diffuses onto the sensing element, flameless combustion takes place through catalytic reaction, elevating the element temperature and increasing platinum wire resistance. This unbalances the bridge and generates an output voltage proportional to the gas concentration. The voltage signal is amplified, converted via analog-to-digital module, and the concentration value is displayed on the LCD screen.
Working principle of the alarm unit: Once the measured combustible gas concentration exceeds the threshold, the amplified bridge output voltage is compared with the preset reference voltage by a voltage comparator. A square wave generator then outputs pulse signals to drive the audible and visual alarm circuit: the buzzer emits continuous sound and the LED flashes to trigger the alarm.
It can be concluded from the operating principle that several factors may cause detector malfunctions: electromagnetic interference distorts detection signals and results in data errors; physical impact or vibration may cause open circuits and complete detection failure; excessive humidity or water ingress into the device can lead to short circuits or changed line resistance, further triggering abnormal detection faults.
