Convert dew point to ppmv residual moisture and back - pressure-corrected using the Magnus formula, with mg/Nm³ and purity grade classification.
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Dew point and ppm: the two languages of gas moisture
Moisture analysers read dew point in °C, gas specifications state ppmv – anyone running high-purity gas systems translates between both constantly. The physics: the dew point fixes the water vapour partial pressure e(Td) (Magnus formula, over ice below 0 °C), and the mole fraction follows as ppmv = e(Td) / (P − e(Td)) × 10⁶.
Key anchor values
At atmospheric pressure: −70 °C ≙ 2.6 ppmv (nitrogen 5.0 grade) · −80 °C ≙ 0.54 ppmv (6.0 grade) · −60 °C ≙ 10.7 ppmv · −40 °C ≙ 127 ppmv.
Atmospheric vs. pressure dew point
The dew point depends on total pressure: compressing a gas raises the water partial pressure proportionally – the pressure dew point is therefore higher than the atmospheric one. Example: 3 ppmv at 1.013 bar(a) equals −69.3 °C; the same gas at 10 bar(a) has a pressure dew point of about −49 °C. Always state the reference pressure – this calculator takes it as a direct input.
Worked example
After purging a new stainless steel line, the chilled-mirror hygrometer reads −65 °C at atmospheric pressure. The calculator returns 5.4 ppmv – still too wet for a 5.0 specification (≤ 3 ppm H₂O); purging must continue until about −69 °C.
FAQ
Why calculate over ice? Below 0 °C water deposits as frost; analysers show the frost point. The ice formula deviates increasingly from the water formula – several ppm at −70 °C.
Conversion to mg/Nm³? 1 ppmv H₂O = 0.804 mg/Nm³ (ideal gas, 0 °C, 1.013 bar) – both values are shown.
