Gas concentrations can be expressed in two ways: mass concentration and volume concentration. Gas detection instruments measure gas volume concentration, with%Vol, ppm, and%LEL being commonly used as concentration units.
%vol is a volume percentage unit, while ppm is a parts-per-million volume unit. The conversion relationship between the two is as follows:
1%Vol=10⁴ppm
%LEL is a commonly used concentration unit for combustible gases; it involves dividing the lower explosive limit (LEL) volume concentration of a combustible gas (usually denoted as%Vol) into 100 equal parts, with each part corresponding to 1% LEL. For example, the lower explosive limit of methane is 5% Vol, so the 100% LEL of methane is 5% Vol.
Additionally, mg/m³ is a common unit for expressing gas concentrations, utilizing mass concentration as its representation method. The conversion between mg/m³ and ppm can be performed using the following formula:
1 mg/m³ = M / 22.4 × 1 ppm × [273 / (273 + T)] × [Ba / 101325],
where M is the molar mass of the gas, T is the temperature, and Ba is the pressure.
