Humidity and moisture measurement

Developing humidity, moisture and air temperature measurement solutions

Humidity in gases and moisture in materials affect a vast range of processes, and can be critical to product quality, lifetime and cost, in wide-ranging areas such as fuel cells, energy gases, electronics, pharmaceuticals, weather and climate.

Measured humidity quantities include relative humidity, dew-point and frost-point temperature, water vapour partial pressure, fraction and ratio. Hygrometers are used to measure the amount of water vapour, or humidity, and there are at least twenty measuring principles for hygrometers, whose applicability depends on the measurement need and conditions.

Moisture in materials is also measured using a variety of techniques, targeting either water content specifically or total moisture content, including other volatiles. Measurement and calibration approaches also depend on the background, or matrix, material.

NPL is continually developing techniques in order to meet user needs for a growing variety of humidity and moisture calibrations and related measurements, over a wide range of conditions. We have experience in designing and building humidity generators and sub-chambers for the calibration of hygrometers.

Humidity in industrial conditions

Industrial processes often involve gases other than air, and pressures other than atmospheric pressure. This can include inert blanketing process gases, natural gas and alternative fuel gases, such as biogas and hydrogen. In these 'non-ideal' gases, the behaviour of water vapour is less well-known than in air. The more different a gas is from standard air, the more its electrical, optical, thermal and transport properties will differ, with the possibility that sensors will perform differently. So conventional humidity calibrations performed in air at atmospheric pressure do not accurately address the gas-sensitivity of some hygrometers in industrial conditions. This calls for gas-specific calibrations.

We have developed a versatile multi-gas, multi-pressure humidity facility to generate defined values of humidity in various gases and conditions. This uses a combination of techniques to generate chosen values of humidity according to the required humidity range, pressure, background gas and humidity quantity to be reported. This validated primary standard enables humidity calibrations that are fully traceable to the International System of Units (the SI). It also provides a flexible test facility for generating temperature- and humidity-controlled gas environments for research studies and investigations under a wide range of simulated industrial conditions. 

Air temperature

Relative humidity varies strongly with air temperature. At room temperature an error of 1 °C can affect the relative humidity value by more than 6 % of value. So accurate air temperature measurements are essential for reliable measurement and control of relative humidity, and nowhere more so than in humidity standards and calibrations. NPL is investigating novel air temperature measurements and uncertainty of temperature sensors in air, to benefit humidity science, meteorology, and other users needing accurate gas temperature measurements.