What are "PPM" and "LEL" in gas detector?
We often see "PPM" and "LEL" on the gas detector, so what are "PPM" and "LEL"? Perhaps many people are not very clear. The following four-in-one gas detector will introduce the specific meanings of these two words in detail.
PPM "is the specific concentration by volume. "PPM" is a representation of solution concentration, and PPM means one in a million. For solution: that is, there is 1/1000 ml solute in 1 liter of water solution, and its concentration is 1 PPM. For gas: one of the methods to express the concentration of pollutants in the ambient atmosphere. Volume concentration representation: The volume number of pollutants contained in one million volumes of air is PPM, and the gas concentration measured by most gas detection instruments is PPM. However, according to China's regulations, especially the environmental protection department, the gas concentration is required to be expressed in units of mass concentration, and our national standards and specifications are also expressed in units of mass concentration. It is made by using the principle that the conductivity of some metal oxide semiconductor materials changes with the change of environmental gas composition at a certain temperature.
"LEL" refers to the lower explosion limit. The lowest concentration of combustible gas that explodes when exposed to fire in air is called the lower explosion limit-%LEL for short.
Fixed VOC gas detector
The highest concentration of combustible gas that explodes when it meets the fire in the air is called the upper limit of explosion-%UEL for short. So what is the lower explosion limit? If the concentration of combustible gas is too low or too high, it is not dangerous. It will burn or explode only when it is mixed with air to form a mixture or, more precisely, when it meets oxygen to form a certain proportion of mixture. Relevant authorities and experts have made an analysis of the combustion and explosion of combustible gas found at present, and worked out the explosion limit of combustible gas. Below the lower explosion limit, the combustible gas content in the mixed gas is insufficient to cause combustion or explosion, and the oxygen content in the mixed gas above the upper limit is insufficient to 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. The explosion limit is generally expressed by volume percentage concentration. The explosion limit is the floorboard of the lower explosion limit and the upper explosion limit. The concentration of combustible gas in the air will only explode between the lower explosion limit and the upper explosion limit. No explosion will occur below the lower explosion limit or above the upper explosion limit.
Therefore, in the explosion measurement, the alarm concentration is generally set below 25%LEL of the lower explosion limit. The measuring range of various combustible gas detectors is 0-100%LEL. The fixed combustible gas detector usually has two alarm points: 10%LEL is the first alarm and 25%LEL is the second alarm. Portable combustible gas detector usually has an alarm point: 25%LEL is the alarm point.
For example, the lower explosion limit of methane is 5% by volume, that is to say, divide this 5% by volume into 100 equal parts, so that the 5% by volume corresponds to 100%LEL, that is, when the detector value reaches the alarm point of 10%LEL, it is equivalent to the methane content of 0.5% by volume at this time. When the detector value reaches the alarm point of 25%LEL, it is equivalent to the methane content of 1.25% by volume.
Therefore, you don't have to worry about whether there is danger at any time after the alarm. At this time, you are prompted to take corresponding measures immediately, such as turning on the exhaust fan or cutting off some valves. There is still a big gap from the lower explosion limit where there is a real possibility of danger, so as to play the role of alarm prompt. Of course, you should take protective measures and check the instrument in time afterwards to see if it needs calibration.
