What are "PPM" and "LEL" in gas detectors?
PPM "is the volume to concentration ratio. PPM "is a method of representing solution concentration, where PPM stands for 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 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, is measured by most gas detection instruments as volume concentration (PPM). However, according to China'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 also use mass concentration units to represent gas concentration. It is manufactured using the principle that the conductivity of some metal oxide semiconductor materials changes with the composition of the ambient gas at a certain temperature.
LEL "refers to the lower explosive limit. The lowest concentration of combustible gas that can explode when exposed to a spark in air is called the lower explosive limit, abbreviated as% LEL.
Fixed VOC gas detector
The highest concentration of combustible gas that can explode when exposed to a spark in air is called the upper limit of explosion, abbreviated as% UEL. So what is the lower explosive limit? The concentration of flammable 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. Authoritative departments and experts have conducted combustion and explosion analysis on the currently discovered combustible gases, and have formulated explosion limits for combustible gases. If the content of combustible gases in the mixture is below the lower explosive limit, it cannot cause combustion or explosion. If the content of oxygen gas in the mixture is above the upper limit, it cannot cause combustion or explosion.
In addition, the combustion and explosion of combustible gases are also related to factors such as gas pressure, temperature, ignition energy, etc. The explosion limit is generally expressed in terms of volume percentage concentration. Explosion limit refers to the general term for the lower and upper explosive limits. The concentration of combustible gases in the air will only explode when it is between the lower and upper explosive limits. No explosion will occur below the lower explosive limit or above the upper 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 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.
You don't have to worry about whether there is any danger at any time after the alarm is triggered. At this time, it is reminding you to take corresponding measures immediately, such as opening the exhaust fan or cutting off some valves. There is still a long way to go before the actual lower explosive limit where danger may occur. Only in this way can the alarm prompt be effective. Of course, protective measures should be taken, and the instrument should be checked in a timely manner after the event to see if calibration is needed.
