ppmv
part per million by volume (moisture)
Trace moisture is sold by volume, and one part per million by volume converts to a humidity ratio of 0.621945 × 10⁻⁶ kg/kg. The 0.621945 is the ratio of the molar mass of water to that of dry air, the same constant that appears throughout psychrometrics.
Watch out: This is the dilute linearisation. The exact relation is W = 0.621945 x/(1 − x) with x the mole fraction, so the factor here is right to about a tenth of a percent up to 1000 ppmv and progressively wrong above that. Do not use it at room humidity, where x is around 0.01 and the error reaches a full percent; use a psychrometric calculation instead. It also assumes the carrier gas is air, since the 0.621945 carries air's molar mass in it.
| 1 ppmv | 0.000621945 g/kg |
Solvers that use humidity ratio