What is the meaning of LEL% / VOL% and ppm in a gas detector?
1, VOL% (gas volume percentage)
VOL is a physical unit that describes the volume of a gas and is expressed as a percentage. It is the percentage of the volume of a specific gas in the air. 5% VOL methane represents 5% of the volume of methane in the air. Our gas detector's detection range is often expressed as VOL%, for example, the detection range is 0-100% VOL, which means that when detecting a certain gas, this gas detector can detect its proportion in the air in the range of 0-100%. We can also set a certain percentage value of VOL as the alarm point. When the content of a certain gas reaches or exceeds this set value, the gas detector will sound an alarm. This involves another unit, LEL%.
2, LEL% (lower explosive limit)
We have previously introduced combustible gases. The concept of combustible gases refers to the ability to mix uniformly with air (or oxygen) within a certain concentration range to form a premixed gas. When encountering a fire source, it will explode. Therefore, the minimum volume percentage concentration at which this combustible gas can detonate in air, which is what we call the lower explosive limit concentration of gas, LEL%, abbreviated as the lower explosive limit. His unit is also a percentage, which divides the lower explosive limit into 100 parts, with each unit being 1LEL%. The concentration of gas volume inside the lower explosive limit is expressed in VOL%.
3, PPM (parts per million of gas volume percentage)
The concept of PPM is similar to VOL, but PPM represents one millionth of a gas volume. For example, 10ppm carbon dioxide refers to carbon dioxide in the air that contains 10 parts per million. Since PPM units are dimensionless, most gas detectors that can detect PPM level leaks are used to detect gas micro leaks in the working environment. Because gas micro leaks are very dangerous, long-term gas micro leaks may cause major accidents, so we need to use PPM level gas detectors to promptly eliminate the location of micro leaks.
4, Conversion of VOL%, LEL%, and PPM
Firstly, there is the conversion between VOL and PPM. The conversion between these two units is relatively simple, as% VOL is the percentage of volume, while PPM is the parts per million of volume. Therefore, 1% (VOL)=10000PPM.
To convert VOL and LEL, we need to first find out the lower explosive limit of the combustible gas. When the concentration of combustible gas in the air reaches its lower explosive limit, we call the explosion hazard level of the combustible gas environment in this place 100%. For example, the LEL of hydrogen is 4% VOL, which means that when its volume percentage in the air reaches 4% VOL, it will explode when exposed to an open flame. Therefore, considering 4% VOL as 100% hazard, it is called 100% LEL, which means 4% VOL=100% LEL, and 1% VOL=25% LEL.
The conversion between PPM and LEL cannot be directly converted. LEL needs to be converted to VOL first, and then VOL needs to be converted to PPM. Here is a formula: PPM=% LEL x LEL (vol%) x 100. Taking methane as an example, what is the PPM of 20% LEL methane? According to the calculation formula, 20 (% LEL) x 1 (% VOL) x 100=2000PPm. Generally speaking, gas detectors that can detect PPM are very accurate, while LEL gas detectors are generally used in explosion detection situations, and the order of magnitude of VOL is the largest among them, which is also quite commonly used. We are familiar with what these three units represent and their conversions, which can help us better choose and use a gas detector.
