PRODUCTION • PROCESSING • HANDLING
THE SOLUTION: VALIDITY IN THE GAS MEDIUM OF USE Although industrial humidity measurements may be at high pressures, or in other gases, many laboratories can only perform humidity calibrations in air at atmospheric pressure. Te UK’s National Physical Laboratory (NPL) has developed a novel facility to enable humidity calibrations at the highest levels of accuracy in a range of real-world energy gases and gas mixtures at an extended pressure range.
Te NPL multi-gas multi-pressure humidity generator is a primary standard based on the saturation of gas with water vapour under controlled conditions. Te facility covers a wide humidity and pressure range (with plans to extend these further), and results are obtained in terms of typically either dew point (frost point below 0°C) or volume fraction (ppmv). 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. Calibrations at NPL can be performed at pressures up to 3 MPa (30 bar), in air, inert gases, methane and pre-made cylinder gas blends. Where gas species are not suitable for direct saturation in the generator (due to corrosion risk or 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), and others.
WHAT’S NEXT? Te NPL facility provides a unique capability to generate defined water content. Plans are being devised for future research that could provide new measurements of non-ideal behaviour for a variety of gases and gas mixtures. By allowing users to obtain calibrations that are appropriate to the conditions of use, this facility supports the accurate use of industrial equipment in process conditions, making sure industry needs are met l
TOP: Schematic diagram of the NPL multi- gas multi-pressure humidity generator, a hybrid instrument with three modes of operation that gives flexibility in different regimes and gas compositions
ABOVE: Graph for one sensor type (polymer impedance sensor) illustrating dependence of sensor reading on gas pressure, and on composition (here in methane with varying small amounts of carbon dioxide)
Dr Stephanie Bell is lead scientist – Humidity and Moisture at the National Physical Laboratory.
www.npl.co.uk
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