The measurement of relative humidity in a climatic environment - The main types of hygrometer for measuring or determining relative humidity and associated uncertainty

Many users of hygrometers encounter problems with the metrological performance required, in industrial or pharmaceutical environments, on measured temperature and relative humidity values, in climatic environments (climatic chamber, air-conditioned room, etc.), compared with the specifications of industry standards (e.g. Maximum Tolerated Errors (MTE): ±1°C and ±2% R.H., at 25°C and 60% RH).

In fact, the accuracy errors announced by most manufacturers for the measured or calculated relative humidity parameter are of the order of: ±1% to ±2% RH, over the entire measurement range, whatever the temperature and relative humidity values.

This requires a thorough understanding of the actual metrological performance of the measured or calculated relative humidity parameter, depending on the type of hygrometer chosen to perform the measurements.

In principle, it is preferable to use a measuring instrument that is directly sensitive to the desired parameter, but for relative humidity, the situation is a little different: hygrometers that are directly sensitive to this parameter, such as impedance variation hygrometers, are not necessarily those that provide the lowest measurement uncertainties, due to their physicochemical measurement principle. In some cases, therefore, it may be preferable to use a hygrometer sensitive to another humidity parameter (dew point temperature, wet bulb temperature) and calculate relative humidity at the temperature of the environment in which the products under test are placed.

This article presents the definitions and main relationships for accessing relative humidity, as well as the sensitivity coefficients and relative humidity uncertainties calculated from the uncertainties of the measured humidity and temperature parameters.

Readers will also find information on the main types of hygrometers used in climatic environments (condensation hygrometers, impedance variation hygrometers, psychrometers).