Moisture analysers display dew point in °C, gas specifications demand ppm – and an exponential function lies in between. The Dew Point to ppm Calculator (Moisture in Gases) translates in both directions and accounts for the often forgotten influence of pressure.
The physics behind it
The basis is the Magnus formula for saturation vapour pressure – below 0 °C correctly calculated over ice, not over supercooled water. The moisture content in ppmv then results as the ratio of water vapour partial pressure to total pressure: ppmv = e(Td) ÷ p × 10⁶. Because total pressure sits in the denominator, the pressure entry is decisive.
Step by step
- Choose the direction: dew point → ppm or ppm → dew point
- Enter the value: e.g. the dew point from your trace moisture analyser
- Specify the pressure: 1.013 bar(a) for atmospheric dew point – or the line pressure for the pressure dew point
- Read the result: ppmv, mg/Nm³ and the gas grade classification
Worked example
Dew point −70 °C at atmospheric pressure: saturation vapour pressure over ice ≈ 0.26 Pa, hence 2.57 ppmv – matching the residual moisture requirement of grade 5.0 (≤ 5 ppm). For 6.0 (≤ 0.5 ppm), the dew point must be below about −80 °C.
The classic pitfall: pressure dew point vs. atmospheric dew point
A dew point of −40 °C at 7 bar line pressure is not the same as −40 °C atmospheric: expand the gas and the dew point drops by around 20 K. Anyone comparing readings from the pressurised network with specifications at standard conditions without converting assesses their gas quality systematically wrong – in practice the most common source of error in moisture acceptance tests. For binding acceptance, the measurement conditions of the test protocol apply.
