The principle of toxic and harmful gas detector
Combustible gases are the most common hazardous gases encountered in industrial settings such as petrochemicals. They are mainly organic gases such as alkanes and certain inorganic gases such as carbon monoxide. The explosion of combustible gases requires certain conditions, which are: a certain concentration of combustible gas, a certain amount of oxygen, and a source of fire with sufficient heat to ignite them. These are the three essential elements of an explosion, and none of them are indispensable. In other words, the absence of any of these conditions will not cause a fire or explosion. When combustible gases (steam, dust) and oxygen are mixed and reach a certain concentration, they will explode when exposed to a fire source with a certain temperature. We refer to the concentration at which combustible gases explode when exposed to a source of fire as the explosive concentration limit, abbreviated as the explosive limit, which is generally expressed in%. In fact, this mixture does not necessarily explode at any mixing ratio and requires a concentration range. When the concentration of combustible gas is below LEL (* low explosive limit) (insufficient combustible gas concentration) and above UEL (* high explosive limit) (insufficient oxygen), no explosion will occur. The LEL and UEL of different combustible gases are different (see the introduction in the eighth issue),
This should be taken very seriously when calibrating instruments. For safety reasons, we should generally issue an alarm when the concentration of combustible gas is at 10% and 20% of the LEL, where 10% LEL is referred to. Make a warning alert, while 20% LEL is called a danger alert. That's why we call the combustible gas detector LEL detector.
It should be noted that the 100% displayed on the LEL detector does not indicate that the concentration of combustible gas reaches 100% of the gas volume, but rather reaches 100% of LEL, which is equivalent to the lower explosive limit of combustible gas. If it is methane, 100% LEL=4% volume concentration (VOL). In operation, the detector that measures these gases in LEL mode is a common catalytic combustion detector. Its principle is a dual bridge (commonly known as a Wheatstone bridge) detection unit. A catalytic combustion substance is coated on one of the platinum wire bridges. Regardless of the flammable gas, as long as it can be ignited by the electrode, the resistance of the platinum wire bridge will change due to temperature changes. This resistance change is proportional to the concentration of the flammable gas, and the concentration of the flammable gas can be calculated through the instrument's circuit system and microprocessor. Thermal conductivity VOL detectors that directly measure the volume concentration of combustible gases can also be obtained on the market, and there are already detectors that combine LEL/VOL. VOL combustible detector is particularly suitable for measuring combustibles in oxygen deficient environments
