Lower explosive limit in gas detectors
PPM "is the volume specific concentration. PPM "is a representation of solution concentration, where PPM represents parts per million. For a solution: if there is 1/1000 milliliter of solute in 1 liter of aqueous solution, the concentration is 1 PPM. For gases: One of the methods for expressing the concentration of pollutants in the ambient atmosphere. Volume concentration representation: The volume of pollutants contained in one million volumes of air, known as PPM. Most gas detection instruments measure gas concentrations as volume concentration (PPM). According to Chinese regulations, especially environmental protection departments, gas concentrations are required to be expressed in units of mass concentration. Our national standards and regulations also use mass concentration units to represent gas concentrations. It is manufactured using the principle that the conductivity of some metal oxide semiconductor materials changes with the composition of the environmental gas at a certain temperature.
LEL "refers to the lower explosive limit. The minimum concentration of combustible gases that can explode when exposed to an open flame in the air is called the lower explosive limit - referred to as% LEL.
Fixed VOC gas detector
The maximum concentration of combustible gases that can explode when exposed to an open flame in the air is called the upper explosion limit - referred to as% UEL. So what is the lower explosive limit? The concentration of combustible gases is too low or too high, and 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. Relevant authoritative departments and experts have conducted combustion and explosion analysis on the currently discovered combustible gas, and established an explosion limit for combustible gas below the lower explosion limit. The content of combustible gas 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, and ignition energy. The explosion limit is generally expressed as a volume percentage concentration. The explosion limit is a general term for the lower and upper explosion limits. The concentration of combustible gases in the air will only explode between the lower and upper explosion limits. Explosion will not 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 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 the first level alarm, and 25% LEL is the 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 the methane content being 0.5% by volume ratio at this time. When the detector value 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 a 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 explosion limit where there is a real possibility of danger. This will serve as an alarm prompt. Of course, protective measures should be taken and the instrument should be checked promptly afterwards to see if calibration is necessary.
