How is the LEL (Lower Explosive Limit) of combustible gases detected?
When detecting combustible gases the concentration of the gas is not of interest. What you want to know, is if the ambient atmosphere is explosive or not. There for a new variable is nominated: LEL (Lower Explosion Limit). 100 % of the LEL define the concentration in air at which a certain gas starts being explosive. This parameter is gas – specific. In most cases it is at the low % volume range.
As the LEL is a gas – specific parameter, it is very important to know which gas must be expected, when designing a gas detection system. Surely, sensor for combustible gases must not be as specific as sensors for toxic gases, but as different gases are detected with different sensitivity, it is essential to calibrate the detector to the right gas.
Catalytic vs. Infrared Sernsors
Catalytic sensors, also referred to as pellistors consist of a pair ceramic pearls. Both are one half of a Wheatstone Bridge, and both are heated with an internal platinum coil up to roughly 400°C. One of these pearls is doped with a catalyst. As soon as a combustible gas comes its way, it will be burned at the catalytic surface of the active pearl. The combustion heat of this chemical reaction heats the internal platinum coil further up and thus increases its electric resistance, which is accurately measured by the Wheatstone bridge.
Infrared sensors use the fact that many gases absorb light at a very specific wavelength. Of course this physical fact can be used to detect gases. Infrared sensors consist of a cuvette, in which a light source and two light detectors are installed. The light is focused into two beams with different wavelengths. The detector beam has wavelength of 3,4 mm. At htis wavelength the C – H bond of hydrocarbons absorbs energy. The reference beam does not absorb light. Each beam is led to one of the detectors. If there is gas present, the signal of the measuring beam is weaker, as the gas has absorbed energy. The more gas the stronger the absorption the weaker the signal of the light sensor.
Both methods do have pros and cons. While catalytic sensors detect more or less everything combustible, infrared sensors are restricted to a certain group of substances, for instance hydrocarbons. But they are not damaged by catalyst poisons, and they do not need oxygen.