Relative humidity: Measuring with a dewpoint hygrometer (NT BUILD 474)

  • Report #: NT BUILD 474
  • Approved: November 1996
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This test method is used for measurement of the moisture content of air by determination of the dewpoint temperature, using a dewpoint hygrometer. The results may be recalculated and expressed as the total moisture content of the air or as partial water vapour pressure. If the air temperature is measured at the same time, the method can be used for determination of the relative humidity of the air.


Dewpoint hygrometers are used for measurement of the dewpoint temperature of ambient air. As most dewpoint hygrometers have good accuracy, they are often used as references (normals) for calibration of other instruments. If the air temperature is measured simultaneously with the dewpoint temperature, then the relative humidity can be calculated. The method can be used for determination of the relative humidity of air in equilibrium with the pores of building materials stored in a closed air volume. Some instruments have a probe, with the sensor element being mounted in an open enclosure in such a way as to allow air to diffuse inside to the sensor. Probes of this kind are often sufficiently small to allow them to be placed inside confined spaces, such as drillholes. Some instruments have a probe with a sealed enclosure to which tubes are connected, with air being brought to the sensor by a small pump. Both methods can be used to measure the humidity of various building materials on site, as well as that of samples maintained in closed air volumes. Long-term measurements can be made by most instruments: the maximum sampling rate varies for different instruments between 5 and 50 samples/hour.

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