What are "PPM" and "LEL" in gas detectors?
We often see "PPM" and "LEL" marked on gas detectors. So what are "PPM" and "LEL"? Maybe many people are not very clear. The following is a four-in-one gas detector for everyone. Let’s introduce the specific meaning of these two words in detail.
"PPM" is a volume specific concentration. "PPM" is a way to express the concentration of a solution, and PPM means one part per million. For solutions: that is, if there is 1/1000 ml of solute in 1 liter of aqueous solution, its concentration is 1 PPM. For gases: one of the expression methods for the concentration of pollutants in the ambient atmosphere. Volume concentration expression: the volume of pollutants contained in one million volumes of air, that is, PPM. The gas concentrations measured by most gas detection instruments are It is the volume concentration (PPM). According to our country's regulations, especially the environmental protection department, gas concentration is required to be expressed in units of mass concentration. Our country's standards and specifications are also expressed in units of mass concentration. It uses some metal oxide semiconductors The material is made on the principle that at a certain temperature, the electrical conductivity changes with changes in the composition of the ambient gas.
"LEL" refers to lower explosive limit. The lowest concentration of flammable gas that explodes when encountering an open fire in the air is called the lower explosion limit—abbreviated as %LEL.
Fixed VOC gas detector
The highest concentration of flammable gas that explodes when encountering an open fire in the air is called the upper limit of explosion—abbreviated as %UEL. So what is the lower explosive limit? There is no danger if the concentration of flammable gas is too low or too high. It will burn or explode only when it is mixed with air to form a mixture, or more precisely, when it encounters oxygen to form a mixture of a certain proportion. Relevant authoritative departments and experts have conducted combustion and explosion analysis on the currently discovered combustible gases, and have formulated the explosion limit of flammable gases. If it is lower than the lower explosion limit, the content of combustible gas in the mixed gas is insufficient to cause combustion or explosion. High If the oxygen content in the upper limit mixture is insufficient, it cannot cause combustion or explosion.
In addition, the combustion and explosion of combustible gas are also related to factors such as gas pressure, temperature, and ignition energy. Explosion limits are generally expressed in terms of volume percentage concentration. The explosion limit is a general term for the lower explosion limit and the upper explosion limit. An explosion will occur only when the concentration of flammable gas in the air is between the lower explosion limit and the upper explosion limit. There will be no explosion below the lower explosion limit or above the upper explosion limit.
Therefore, when conducting explosion measurements, the alarm concentration is generally set below 25% LEL of the lower explosion limit. The measurement range of various combustible gas detectors is 0-100% LEL. Fixed combustible gas detectors usually have two alarm points: 10% LEL is a first-level alarm, and 25% LEL is a second-level alarm. Portable combustible gas detectors usually have an alarm point: 25% LEL is the alarm point.
