When using gas detectors in daily life, the detection range on the LCD label often displays terms like 0-100LEL% or 0-2000ppm, or VOL% or ppm. What do these three units mean, and how are they converted?
VOL% (Volume Percentage)
VOL is a physical unit describing the volume of a gas, expressed as a percentage. It represents the percentage of a specific gas's volume in the air. For example, 5%VOL for methane means that methane occupies 5% of the volume in the air.
Many gas detectors use VOL% to describe their detection range. For example, a detection range of 0-100% VOL means that the gas detector can detect a gas within a range of 0-100% of its concentration in the air.
We can also set a specific percentage value for VOL as an alarm point. When the concentration of a gas reaches or exceeds this set value, the gas detector will sound an alarm. This involves another unit, LEL%.
LEL% (Lower Explosive Limit)
A combustible gas is a gas that can be uniformly mixed with air (or oxygen) within a certain concentration range to form a premixed gas. When it encounters an ignition source, it will explode. The lowest volume percentage concentration of this combustible gas in air that can ignite it is called the lower explosive limit (LEL%), or simply "lower explosive limit." The volume concentration of the gas at the lower explosive limit is expressed as LEL%, and its unit is a percentage; that is, the lower explosive limit is divided into one hundred parts, and one unit is 1 LEL%.
PPM (Parts per Million by Volume)
The concept of PPM is similar to VOL, except that PPM represents one millionth of a gas volume.
For example, 10 ppm carbon dioxide means that there are 10 parts per million of carbon dioxide in the air. Since PPM is a dimensionless unit, Gas detectors capable of detecting PPM levels are used to detect micro-leaks in the working environment. Micro-leaks are very dangerous; prolonged micro-leaks can cause major accidents. Therefore, PPM-level gas detectors are needed to detect and eliminate micro-leaks promptly.
Conversion between VOL%, LEL%, and PPM
First, the conversion between VOL and PPM is relatively simple. %VOL is a volume percentage, while PPM is a volume fraction (parts per million). Therefore, 1%VOL = 10,000 PPM.
Second, the conversion between VOL and LEL requires determining the lower explosive limit (LEL) of the flammable gas. When the concentration of a flammable gas in the air reaches its LEL, the explosion hazard level of the flammable atmosphere is considered 100%. For example, hydrogen has an LEL of 4%VOL, meaning it will explode upon contact with an open flame when it reaches 4%VOL by volume. Therefore, 4%VOL is considered 100% hazard, referred to as 100%LEL, i.e., 4%VOL = 100%LEL. Thus, 1%VOL = 25%LEL.
Third, the conversion between PPM and LEL cannot be directly converted. LEL must first be converted to VOL, and then VOL to PPM. Here's a formula for PPM: PPM = %LEL × LEL (vol%) × 100. Taking methane as an example, how many PPM is 20% LEL methane? The formula is: 20 (%LEL) × 1 (%VOL) × 100 = 2000 PPM.
Generally, gas detectors capable of detecting PPM are very accurate, while LEL gas detectors are typically used in explosion detection situations. VOL detectors have the highest order of magnitude and are the most commonly used.
Share
