We often see "PPM" and "LEL" on the gas detector, so what is "PPM" and "LEL"?
"PPM" is the specific concentration by volume, "PPM" is a representation of the concentration of the solution; PPM is one part per million.
For a solution: that is 1/1000ml of solute in a solution of 1 liter of water, the concentration is 1 PPM.
For gases: a representation of the concentration of pollutants in the ambient atmosphere. Volume concentration: the number of pollutants in a million volumes of air by volume, or parts per million. The gas concentration measured by most of the gas detection instruments is volume concentration (PPM), and according to regulations in China, especially the environmental protection department, the gas concentration is required to be expressed in the unit of mass concentration, our national standards are also expressed in the unit of mass concentration. It is using some metal oxide semiconductor materials, at a certain temperature, conductivity changes with the composition of the ambient gas changes.
LEL" is the lower bound. The lowest concentration at which a mixture of combustible vapor, gas, or dust and air can explode when ignited is called the lower limit of explosion, or LEL. Similarly, the maximum concentration at which a mixture of combustible vapor, gas or dust and air can explode when it encounters a ignition source is called the upper limit of explosion, or UEL for short. So what is the lower limit?
It is not dangerous if the concentration of combustible gas is too low or too high. It can only burn or explode when it is mixed with air to form a mixture, more specifically, when it is mixed in a certain proportion with oxygen. The explosion limit of combustible gas has been set by the state based on the relevant test data. When the combustible gas is below the lower limit of the explosion, there is not enough combustible gas in the mixture to cause combustion or explosion, and there is not enough oxygen in the mixture above the upper limit to cause combustion or explosion. In addition, combustion and explosion of combustible gas are also related to gas pressure, temperature, ignition energy and other factors. The explosion limit is usually expressed as a percentage concentration of volume.
The explosion limit is that combustible material (combustible gas, vapor and dust) and air (or oxygen) must be evenly mixed within a certain concentration range to form premixed gas, which can only occur in the event of a fire source. This concentration range is called the explosion limit, or the explosion concentration limit. The upper limit of explosion plus the lower limit of explosion is the explosion limit. The concentration of combustible gas in the air can only occur between the lower limit of explosion and the upper limit of explosion. No explosion will occur below the lower limit or above the upper limit. Therefore, in the explosion measurement, the alarm concentration is generally set below 25%LEL of the lower limit of explosion, and the measurement range of combustible gas detector is 0-100%LEL.
For example, the lower explosion threshold for methane is a 5% volume ratio. That is, if you divide the 5% volume ratio into 100 equal parts, the 5% volume ratio corresponds to 100%LEL. When the detector value reaches the 10%LEL alarm point, the methane content is equal to 0.5% volume ratio. When the detector value reached the 25%LEL alarm point, the methane content was equivalent to 1.25% volume ratio. So, you do not need to worry about the danger after alarms, this is a prompt to you, to take corresponding measures immediately, such as open the exhaust fan or cut off some valves, from the really dangerous lower limit of the explosion is still a long distance, so as to play the role of alarm prompt.