What Does "PPM" Mean in Gas Detectors and Equipment Maintenance
What does "PPM" stand for in gas detectors? PPM refers to volume‑ratio concentration, meaning one part per million.
For liquid solutions: If 1/1000 mL of solute is contained in 1 liter of aqueous solution, the concentration is 1 PPM.
For gases: It is one of the notations for pollutant concentration in ambient air.
Volume‑concentration notation: PPM stands for the volume of pollutants contained in one million volumes of air. Most gas detectors output readings in volume‑based PPM. However, according to Chinese regulations, especially for environmental‑protection authorities, gas concentrations shall be expressed in mass‑concentration units, which are also adopted in national standards. Some gas detectors are manufactured based on the principle that the conductivity of metal‑oxide semiconductor materials changes with gas composition at a certain temperature.
A reading of 200 ppm for a certain gas means 200 volume units of such gas exist in one million volume units of air. PPM is a common unit for gas detectors and alarms, generally applied for non‑combustible‑explosive gases. Combustible‑explosive gases adopt %LEL for indication.
Mass‑concentration unit (mg/m³) facilitates calculation of actual pollutant mass. Nevertheless, mass concentration is affected by temperature and atmospheric pressure, and its value varies with environmental conditions. Actual measurement requires simultaneous acquisition of gas temperature and pressure. By contrast, PPM is a volume ratio and free from such influence.
Most gas‑detection instruments measure gas concentration in volume‑based PPM. According to Chinese requirements, especially for environmental‑protection administration, gas concentrations shall be stated in mass‑concentration units such as mg/m³, which are specified in national standards. What is the relationship between mg/m³ and PPM, and how to convert between them?
Conversion formula between ppm and mg/m³ mg/m³ = M/22.4 · ppm · [273/(273+T)] · (Pa/101325)
Where: M - molecular weight of gas ppm - measured volume concentration T - temperature Pa - atmospheric pressure
With this formula, conversion from ppm to mg/m³ and reverse calculation can be completed conveniently.
