Wet and Dry Bulb Relative Humidity Calculator
Convert dry bulb and wet bulb readings from an aspirated (Assmann type) psychrometer into relative humidity, dew point and vapour pressure, for checking humidity transmitters.
- es(t) = 6.112 × exp[17.62t ÷ (243.12 + t)] hPa
- e = es(Tw) − 6.53 × 10⁻⁴ × (1 + 0.000944Tw) × p × (T − Tw) = 19.96 hPa
- RH = e ÷ es(T) = 63.1 %
Psychrometer Equations (WMO)
es(t) = 6.112 × exp[17.62t ÷ (243.12 + t)]
RH = e ÷ es(T) × 100
T is the dry bulb and Tw the wet bulb temperature in °C, p the barometric pressure in hPa and e the actual vapour pressure. The psychrometer coefficient is the WMO value for an Assmann type aspirated psychrometer with about 2.5 m/s air speed; sling or unventilated psychrometers need different coefficients.
Worked example
A dry bulb of 25 °C and wet bulb of 20 °C at 1013 hPa give about 63 % RH and a dew point of about 17.4 °C.
Frequently Asked Questions
How does a psychrometer work?
Evaporation cools the wet bulb. The drier the air, the faster the evaporation and the larger the difference between dry and wet bulb.
Why must the psychrometer be ventilated?
The coefficient assumes a fixed air speed over the wick. Without ventilation the wet bulb reads high and RH is overestimated.
Can I use this to check a humidity transmitter?
Yes, as a field check. For calibration use a reference hygrometer or a humidity generator.
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