PPM is an absolute measurement, whereas mg/m3 varies depending on pressure and temperature.
Handheld monitors measure in ppm and apply a conversion factor if you want to measure in mg/m3. The following table shows the conversion factor for each sensor head. A conversion factor is required as the different gases have different masses. The conversion factor is also dependent on the standard temperature and pressure. Aeroqual uses 0°C and 101.325kPa (1 atm).
| Sensor | PPM conversion to mg/m3 |
|---|---|
| Ammonia | 0.7 |
| Carbon dioxide | 1.96 |
| Carbon monoxide | 1.25 |
| Chlorine | 3.16 |
| Formaldehyde | 1.34 |
| Hydrogen | 0.082 |
| Hydrogen sulphide | 1.52 |
| Methane | 0.72 |
| Nitrogen dioxide | 2.05 |
| Non-methane hydrocarbon | 2.5 |
| Ozone | 2.14 |
| Particulate matter | N/a* |
| Perchloroethylene | 6.78 |
| Sulphur dioxide | 2.86 |
| Volatile organic compounds | 2.5 |
Because particulate matter varies in size (unlike a gas molecule), ppm isn’t a valid unit of measurement. Therefore, the true unit for the PM measurement is always in mg/m3.
Conversion factor
You can’t change the conversion factor on the sensor head or within the Series 500 software. To apply a different conversion factor, log your data in ppm, use the Series 500 software to download and export the data to an Excel spreadsheet, and then calculate mg/m3 readings using a different conversion factor.