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

NPL operates a Multi-Gas, Multi-Pressure primary standard humidity calibration facility which has been developed in-house. This is a primary standard for calibration of hygrometers, traceable to the SI, as well as providing a versatile test capability for different aspects of hygrometer performance.

Industrial need

Industrial humidity measurements are made in a wide variety of conditions – sometimes at high pressures, or in gases other than air. However, the performance of some hygrometers can depend on background gas composition or pressure. These sensitivities are not revealed in typical calibrations carried out in air, near atmospheric pressure. Thus, for best validity, humidity calibrations should ideally be in the gas medium of use. However, such specialised calibrations are not widely available.

Calibration facility

In response to the need, NPL has developed the Multi-Gas, Multi-Pressure primary standard humidity generator, working closely with leading companies to ensure that the capability is industrially relevant. The facility incorporates a versatile combination of techniques to generate chosen values of humidity according to the required dew-point range, pressure, background gas, and humidity quantity to be reported. This is a validated primary realisation, and calibrations are fully traceable to the International System of Units (the SI). Thus a wide variety of industrial environments can now be simulated and calibrations of hygrometers can be made in relevant conditions. In this way, the problematic dependence of the hygrometer on the measurement conditions can be tested or compensated successfully, in a way not enabled by conventional calibration in air at atmospheric pressure.

Defined values of humidity can be generated at a range of pressures, for a variety of single gas species and gas mixtures. This allows hygrometers to be calibrated or tested in conditions relevant to their real-world use.

  • Dew-point temperature range: -60 °C to +15 °C with an uncertainty of ± 0.12 °C
  • Water vapour fraction: from 1 μmol/mol to 100 μmol/mol (known informally as ppmv), with an uncertainty ± 0.6 μmol/mol to 1.8 μmol/mol
  • Pressures: from atmospheric pressure up to 3 MPa (30 bar)
  • Gases: air, inert gases, methane, hydrogen, natural gas components, including mixtures blended in the laboratory or pre-made cylinder gas mixtures

Dew-point probes, condensation hygrometers, water vapour spectrometers and other hygrometers can be calibrated.Calibrations within the above ranges are performed with UKAS (ISO 17025) accreditation, with the above uncertainties specified with a coverage factor of k = 2 corresponding to a coverage probability of 95 %. Calibrations can cover other humidity units directly related to dew point, e.g. vapour pressure, amount fraction, vapour density (g/kg), etc.

Where gas species are not suitable for direct saturation in the generator, 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.

This service offers the traceable calibration of humidity instruments in the gases and pressures in which they are used, eliminating errors associated with interference of the gas species and pressure dependence. The resulting confidence in the instrument response improves process control, saving money and energy, and improving product quality and safety.

Wider uses

In addition to UKAS accredited calibration, the facility has wider uses, providing specialised atmospheres, e.g. humidity- and temperature-controlled inert blanketing gas, and climate-controlled trace gas mixtures.

NPL has provided independent assessment of performance of hygrometers and other instruments, working with manufacturers, suppliers and users of hygrometers for a number of applications:

  • Natural gas mixtures
  • Hydrogen-fuel for vehicles
  • Biogas
  • Nuclear blanketing gases

Other industries which could benefit from this facility include: specialised gases for manufacturing, energy, oil and gas, and even food.

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

To read about some of the work that has recently been completed using the Multi-Gas, Multi-Pressure generator, see:

Read more about our world-leading research and recent developments in the fields of humidity and moisture measurement

Download the service flyer Adobe Acrobat PDF file


Customer Service tel: +44 20 8943 8681

Last Updated: 26 Jun 2018
Created: 4 Aug 2015


Please note that the information will not be divulged to third parties, or used without your permission