Dew-point temperature in air at above-atmospheric pressures
Calibration of dew-point measuring hygrometers against an NPL primary standard or a transfer standard in the range –60 °C to +10 °C, at pressures up to 1 MPa.
NPL provides dew-point calibrations at a range of pressures up to 1 MPa (10 bar), and facilities are in further development for humidity calibrations at higher pressures and in a range of gases.
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 hygrometers commonly depends on gas species and pressure.
Calibration against an NPL primary standard, uncertainties in applied condition
Dew-point range -60 °C to +10 °C at pressures from 0.1 MPa to 1 MPa (1 bar to 10 bar), uncertainty ± 0.08 °C (coverage factor k = 2)
Dew-point probes, condensation hygrometers and other hygrometers operating at elevated pressure can be calibrated.
Calibrations are in air (unless otherwise agreed) at pressures from nominally 0.1 MPa (atmospheric pressure) to 1 MPa, controlled to within 0.2 kPa.
Uncertainties stated on certificates are the combination of the values due to the applied condition together with a contribution from the hygrometer resolution and stability during calibration.
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 see here
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