National Physical Laboratory

Humidity calibration in non-air gases at above-atmospheric pressures

NPL provides humidity calibrations in a range of gases at a range of pressures up to 3 MPa (30 bar).

The NPL multi-gas multi-pressure humidity calibration facility
The NPL multi-gas multi-pressure humidity
calibration facility

Many laboratories can only perform humidity calibrations in air at atmospheric pressure. However, industrial humidity measurements may be at high pressures, or in other gases. For best validity, humidity calibrations should ideally be in the gas medium of use, because the performance of some hygrometers can depend on gas species and pressure.

Calibration against the NPL multi-gas multi-pressure humidity calibration facility, uncertainties in applied conditions
Dew-point range -60 °C to 0 °C at pressures from 0.1 MPa to 3 MPa (1 bar to 30 bar), uncertainty ± 0.12 °C (coverage factor k = 2, coverage probability 95 %)

Calibrations can be performed in air, inert gases, methane and pre-made cylinder gas blends.

Where gas species are not suitable for direct saturation in the generator (corrosion risk, hydrate formation), humidification is possible by blending the gas in appropriate flow ratios with nitrogen or another suitable inert gas that has been passed through the saturator.

Dew-point probes, condensation hygrometers, water vapour spectrometers and other hygrometers can be calibrated.

Calibrations can cover other humidity units directly related to dew point, e.g. vapour pressure, amount fraction, vapour density (g/kg), etc.

For extended ranges of dew point and higher pressures, and for other carrier gases, please enquire using the contact details below.


Customer Service tel: +44 20 8943 8681
E-mail: or

Last Updated: 4 Aug 2015
Created: 4 Aug 2015


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