The sensors of the air-Q usually measure the respective concentration of the substance to be detected in the unit ppm ( parts per million), which indicates a volume ratio. However, since the respective specified exposure limits and maximum workplace concentrations are given in mg per m³, which is a mass ratio, conversion is necessary here. In the case of measured values in ppm, the inclusion of temperature and pressure is also missing, as these are fixed at the time of measurement, i.e. actual values supplied by the corresponding sensors. The equation for calculating the mass concentration at arbitrarily variable temperature and pressure ratios can now be derived with the help of the general gas equation or the gas constant (product of Avogadro’s and Boltzmann’s constant), since according to Avogadro, the same number of particles/molecules is present in two different ideal gases that have the same volume (at the same pressure and temperature):
Now a conversion equation can be created by including the sensor measurement value and a conversion factor, depending on which unit is to be specified:
The following values are used here: βi = concentration in mg/m³ M = molar mass in g/mol p = reference pressure in mbar R = gas constant from NA and kB T = temperature in °C Xi = concentration in ppm The equation can then be converted to other units such as µg/m³ by changing the conversion factor (for mg/m³ it is 0.1) and it can also be converted to Xi in order to be able to conclude the volume percentage concentration at certain pressure and temperature conditions from a known mass concentration.