Water vapour in natural gas can affect the safety of transmission of the gas in pipelines, and affects its price at custody transfer when pipeline gas is bought and sold. Water vapour must be removed to a regulated level. Therefore, accurate measurement of the humidity of the gas is important to ensure the gas meets pipeline specifications.
Hygrometers, including a variety of water-vapour spectrometers, are used to make these measurements and offer a fast response. However, some of these instruments can be sensitive to the composition of the natural gas, so gas-specific calibration is needed to avoid errors in measuring in these gases.
NPL has developed a capability to generate precisely humidified gas blends at a range of pressures. This allows calibration of water vapour spectrometers and a wide range of other hygrometers in gas conditions relevant to use. It also enables studies of hygrometer sensitivity to both pressure and gas composition.
The NPL Multi-gas Multi-pressure Humidity Facility provides a significant step forward from past techniques of humidity calibration only in air and at atmospheric pressure.
Safe transport and fair trade of fuel gases are now underpinned by more accurate measurements of water vapour, because instruments can be provided with humidity calibrations appropriate to use in fuel gas mixtures.
Using the NPL facility, water vapour spectrometers and other hygrometers can be tested for their sensitivity to components of the gas mixture.
This allows measurement errors to be identified and corrected, and supports improvement and innovation in gas monitoring.