by İpek Yavuz Konak
on 11 July 2018

Toxic and flammable gases can be seen in daily life frequently. For example, carbon monoxide is a very toxic gas and carbon monoxide gas emission is common in parking garages. Also methane is a very flammable gas, and it is used in kitchens. Therefore almost everyone is under the threat of toxic or flammable gas leakage.

In order to avoid devastating effects of toxic or flammable gases, gas detectors should be used. Gas detectors determine the existance of target gas and activates relevant automations in order to save lives and properties.

Gas sensor is the fundamental component of a gas detector because it is the main part that detects the target gas. There are different types of gas sensors which vary according to detection mechanism and target gas.

Catalytic Sensor

Catalytic sensors are commonly used for detection of flammable gases. There are two pellistors in a catalytic sensor. The pellistor is covered with catalyst layer. Also there is platinum wire coil inside. When flammable gas reaches the sensor, it reacts with catalyst layer and oxidation occurs. As a result of this reaction, the temperature of pellistor and consequently resistance of wire coil increases.

There are two pellistors in the sensor but only one of them is covered with catalyst layer. The other pellistor is used as reference so that there won’t be false alarms when ambient temperature increases. If the temperature of the place increases, both pellistors’ temperature will increase. However, when there is a flammable gas leakage, only one pellistor’s temperature will increase and sensor will trigger alarm signal. This mechanism is provided by catalytic sensor wheatstone bridge.

Catalytic sensor can detect flammable gases up to 100% LEL. However, catalytic sensors should not be exposed to high concentration of flammable gases for a long time. Because oxygen levels are quite low in places where the flammable gas concentration is high. When the oxygen is not enough for oxidation of the pellistor, the sensor may not work efficiently and this will lead to faulty measurement.

Average life of a catalytic sensor is 5 years. 5-10% dilution of precision occurs every year. Lifetime and sensitivity of the sensor may vary according to level, frequency and duration of gas exposure.

Infrared (IR) Sensor

Infrared sensors are used for flammable gas detection. These sensors are generally used for detection of hydrocarbon gases. Hydrocarbon gases alter the wavelength of Infrared light beams. IR sensors use this feature of hydrocarbon gases for detection.

The structure of Infrared sensors are stated below. IR lamp sends infrared light beam to optical filter. Hydrocarbon gas that enters the sensor absorbs infrared light beams. The difference between reference light and detection light causes alarm signal.