LEL%, VOL% and ppm: Definitions in Gas Detectors
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 air. For example, 5% VOL methane means that the volume of methane in the air is 5%. Our gas detector's detection range is often expressed in terms of VOL%, for example, the detection range is 0-100% VOL, which means that when this gas detector detects a certain gas, it can detect its proportion range in the air from 0-100%. We can also set a certain percentage value of VOL as the alarm point, and 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 introduced combustible gases before. The concept of combustible gas refers to the ability to mix uniformly with air (or oxygen) within a certain concentration range to form a premixed gas. When exposed to a fire source, it will explode. Therefore, the lowest volume percentage concentration that this combustible gas can ignite in air, which is what we call the lower explosive limit concentration of gas, LEL%, abbreviated as the lower explosive limit concentration. His unit is also a percentage, which divides the lower explosive limit into one hundred parts, with one unit being 1LEL%. The concentration of gas volume in 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, except that PPM represents one millionth of the volume of gas. For example, 10ppm carbon dioxide refers to the presence of 10 millionths of carbon dioxide in the air. As PPM units are dimensionless, most gas detectors that can detect PPM level 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%, PPM
Firstly, the conversion between VOL and PPM is relatively simple, as% VOL represents the percentage of volume, while PPM represents the parts per million of volume. Therefore, 1% (VOL)=10000PPM.
The conversion between VOL and LEL requires us to first determine 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 location 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% LEL, that is, 4% VOL=100% LEL, then 1% VOL=25% LEL.
The conversion between PPM and LEL cannot be directly done. LEL needs to be converted to VOL first, and then VOL needs to be converted to PPM. Here is a formula for everyone, PPM=% LEL × LEL (vol%) × 100. Taking methane as an example, what is the PPM of methane in 20% LEL? According to the calculation formula, it can be obtained as: 20 (% LEL) × 1 (% VOL) × 100=2000PPm.
Generally speaking, gas detectors that can detect PPM are very accurate, while LEL gas detectors are generally used in explosive testing, and VOL is the largest and most commonly used among them. We are familiar with what these three units represent and their conversions, which can help us better choose and use a gas detector.
