What are "PPM" and "LEL" in gas detectors
On gas detectors, we often see labels such as "PPM" and "LEL". So what are "PPM" and "LEL"? Perhaps many people are not very clear. Below, the four in one gas detector will introduce the specific meanings of these two words in detail.
PPM is the volumetric concentration. PPM is a method of representing the concentration of a solution, where PPM represents parts per million. For a solution: that is, if there is 1/1000 milliliter of solute in 1 liter of aqueous solution, its concentration is 1 PPM. For gases: one of the methods for representing the concentration of pollutants in the ambient atmosphere. Volume concentration representation method: The volume of pollutants contained in one million volumes of air, also known as PPM. Most gas detection instruments measure gas concentration as volume concentration (PPM), but according to Chinese regulations, especially the environmental protection department, gas concentration is required to be expressed in units of mass concentration. Our national standards and regulations also use units of mass concentration to represent gas concentration. It is manufactured using the principle that the conductivity of some metal oxide semiconductor materials changes with the composition of environmental gases at a certain temperature.
"LEL" refers to the lower explosive limit. The minimum concentration at which a combustible gas can explode when exposed to an open flame in the air is called the lower explosive limit - abbreviated as% LEL.
Fixed VOC gas detector
The maximum concentration at which combustible gases can explode when exposed to an open flame in the air is called the upper explosive limit, abbreviated as% UEL. So what is the lower explosive limit? If the concentration of combustible gases is too low or too high, it is not dangerous. It only burns or explodes when it mixes with air to form a mixture or, more precisely, when it encounters oxygen to form a certain proportion of the mixture. Authoritative departments and experts have conducted combustion and explosion analysis on the currently discovered combustible gases, and have established an explosion limit for combustible gases below the lower explosive limit. The combustible gas content in the mixture is insufficient to cause combustion or explosion, and the oxygen content in the mixture 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, ignition energy, etc. The explosion limit is generally expressed as a volume percentage concentration. The explosive limit is a general term for the lower and upper explosive limits, and the concentration of combustible gases in the air will only explode between the lower and upper explosive limits. No explosion will occur below or above the lower explosive limit.
Therefore, when conducting explosion measurements, the alarm concentration is generally set below 25% LEL of the lower explosive 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.
For example, the lower explosive limit of methane is 5% by volume ratio. That is to say, divide this 5% by volume ratio into 100 equal parts, so that the 5% by volume ratio corresponds to 100% LEL. That is to say, when the detector value reaches the 10% LEL alarm point, it is equivalent to a methane content of 0.5% by volume ratio at this time. When the value of the detector reaches the 25% LEL alarm point, it is equivalent to a methane content of 1.25% by volume at this time.
So, you don't have to worry about whether there is any danger at any time after the alarm. At this point, it is a reminder to take corresponding measures immediately, such as opening the exhaust fan or cutting off some valves. There is still a long way to go from the lower limit of the explosion that may actually cause danger, which will serve as an alarm prompt. Of course, it is still necessary to take protective measures and check the instrument in a timely manner afterwards to see if calibration is necessary.
