Dew Point to ppm Moisture Calculator
Convert the dew point (or frost point) of a gas to water content in ppm by volume at the line pressure, and back, for compressed air, natural gas and process gases.
- e = 6.112 exp(22.46 t ÷ (272.62 + t)) hPa (Magnus, WMO)
- ppmv = e ÷ P × 10⁶
Dew Point and Moisture
over water e = 6.112 exp(17.62 t ÷ (243.12 + t)) hPa · over ice e = 6.112 exp(22.46 t ÷ (272.62 + t)) hPa
ppmv = e ÷ P × 10⁶
Below 0 °C the frost point over ice is used. Accuracy is about 1 to 2 % in vapour pressure between −45 and +60 °C; at very low dew points use a dedicated humidity standard.
Worked example
A frost point of −40 °C at atmospheric pressure is about 127 ppmv. The same gas compressed to 7 bar a has a pressure frost point of about −22 °C.
Frequently Asked Questions
What is −40 °C dew point in ppm?
About 127 ppmv at atmospheric pressure.
Why does dew point change with pressure?
Compressing a gas raises the partial pressure of water, so it condenses at a higher temperature.
What is the difference between dew point and frost point?
Below 0 °C water vapour deposits as frost; the frost point is the temperature at which that happens.
Learn more on the blog
Instrumentation and control standards
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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