【Gas Concentrations in 2026】%VOL, %LEL, ppm, μmol/mol, ppb, mg/m³ – Application scenarios for each unit and key conversion formulas
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Two ways of representing gas concentration: Essentially, there are only two categories of gas concentrations, plus a unit specific to safety considerations: – Volume concentration: Indicates the proportion of volume occupied by the gas in the air → %VOL, ppm, μmol/mol, ppb – Mass concentration: Indicates the weight of the gas per unit volume → mg/m³ – Explosive hazard level: Does not represent the actual concentration; it merely indicates how close the environment is to an explosion → %LEL
I. %VOL: Volume percentage, allowing for a “visual” assessment of high-concentration gases. 1. What is %VOL? Volume percent concentration, simply put: the volume of the target gas in every 100 volumes of the mixed gas (air). 2. Examples – Oxygen in air: 20.9% VOL (20.9 liters of oxygen in 100 liters of air) – Methane in pipes: 95% VOL (95 liters of methane and 5 liters of other gases in 100 liters of air). 3. Applicable scenarios ✅ Measurement of high-concentration gases: the methane content in pipes.
II. ppm: Parts per million, a “special unit” for trace amounts of toxic gases. 1. What is ppm? Parts per million, one part in a million, is the standard unit for extremely low concentrations of gases. Core conversion: 1%VOL = 10,000 ppm; 1 ppm = 0.0001%VOL. 2. Example – 1 ppm of methane: a very low concentration, with no explosion risk. 3. Applicable scenarios: ✅ Detection of low-concentration gases: used for checking for leaks in gas pipelines and measuring the level of methane leakage.
III. μmol/mol: The unit for the mole fraction, also known as the fraction of moles; it is read as ‘micromoles per mole’. 1. What is ppm? When describing the methane content in air, the units μmol/mol and ppm are commonly used. μmol/mol is a way of expressing the mole fraction, used to describe the proportion of a gas in a mixture. Specifically, 1 μmol/mol means that out of every million gas molecules, ten thousand are carbon monoxide molecules. PPM, or parts per million, is another way of expressing concentration, indicating how many volume units of carbon monoxide are present in each million volume units. 1μmol/mol=1ppm. 2. Applicable scenarios: ✅ Detection of low-concentration gases: detecting leaks in gas pipelines and measuring the level of methane leakage.
IV. %LEL: Percentage of the lower explosive limit; the \"lifeline\" for the safe use of flammable gases. 1. What is %LEL? Lower Explosive Level, the percentage of the lower explosive limit. It is not the actual concentration of the gas, but rather a relative value indicating the level of danger: Current gas concentration ÷ Lower explosive limit of that gas × 100%. 2. Core safety logic: Every flammable gas has a lower explosive limit; below this concentration it will not ignite, but at or above this level it can explode easily in the presence of an open flame. - 0% LEL: No flammable gases present, absolutely safe – 100% LEL: At the lower explosive limit, extremely dangerous – ＞100% LEL: In the explosive range; operations are prohibited!
3. Example (methane): The lower explosion limit for methane is 5% VOL – 1% VOL = 20% LEL; 2.5% VOL = 50% LEL; 5% VOL = 100% LEL (reaching the lower explosion limit). 4. Applicable scenarios: ✅ Detection of flammable gases: natural gas, liquefied gas, methane, hydrogen, gasoline vapor, etc. ✅ Primary use: early warning of gas leaks. ⚠️ Important note: The %VOL corresponding to 100% LEL varies greatly depending on the gas, so direct conversion between different gases is not possible!
V. mg/m³: milligrams per cubic meter, the “standard unit” in environmental protection/occupational health. 1. What is mg/m³? Milligrams per cubic meter, a unit of mass concentration that indicates how many milligrams of a gas or particulate matter are present in 1 cubic meter of air. The main difference from volume concentration is that it does not take into account the amount of space occupied, but only the weight. 2. Applicable scenarios ✅ Environmental monitoring: dust, SO₂, NOx, TVOC, odors ✅ Occupational health: toxic vapors, heavy metals, chemical mist droplets
VI. PPB (part per billion) is a dimensionless unit used to express concentration; in Chinese it is referred to as billionth part concentration, meaning that the mass of the solute constitutes one billionth of the mass of the solution (10⁻⁹). It is commonly used for measuring the concentration of trace substances. 1. Its conversion relationship to ppm and ppt is as follows: 1 ppb = 1/1000 ppm. 2. Applicable scenarios: ✅ High-precision detection of gas leaks, as well as high-sensitivity monitoring for ensuring the safety of pipeline networks.
1. Core conversion formulas for the 4 major units: This is used frequently in work! 1.%VOL ↔ ppm (the simplest method): 1%VOL = 10,000 ppm. Taking methane as an example: 2%VOL = 20,000 ppm; 500 ppm = 0.05%VOL
2. %VOL ↔ %LEL (it is necessary to determine the lower explosive limit of the gas). %LEL = Current concentration (%VOL) ÷ The LEL value of that gas (%VOL) × 100%. There is no fixed coefficient; the corresponding LEL value must be looked up for each type of gas. Taking methane as an example: 5%VOL = 100%LEL, 2.5%VOL = 50%LEL
3. ppm ↔ μmol/mol (most common); 1 μmol/mol = 1 ppm