Martti Heinonen, Centre for Metrology and Accreditation, Finland
Olev Saks, University of Tartu
Ivo Leito, University of Tartu
Vito Fernicola, National Institute of Metrological Research, Italy
Air humidity is a physical quantity that affects climate, human thermal comfort and properties of materials. Air humidity should be measured and controlled in museums, libraries, for manufacturing semiconductor materials, etc.
It is necessary to periodically compare the readings of air humidity measuring devices to the readings of standard (etalon) in order to get to know how well the readings agree with the so-called "true" values of air humidity. No physical measurement is entirely accurate. Each measurement is accompanied by some uncertainty. Therefore true values are experimentally inaccessible and reference values of measurement standards are used instead. This dissertation is dedicated to developing Estonian air humidity reference measurement standard further. The measurement uncertainty of the reference values of the reference standard has been assessed using three different methods. The results agree well to each other.
In order to check the readings of the reference standard on site, a simplified humidity generator has been constructed. It generates air with wished humidity in a narrow range of air humidity values.
It was found that the natural presence of different ions in the humidity generator causes humidity generation error that can be considered insignificant. It was also found experimentally and theoretically that moderate overpressure in the tubing of a humidity generator practically stops escaping of humid air into the tubing through small holes inside the wall of the tubing even if very dry air is flowing in it. However, the escaping of humid air into the tubing through slightly loosened connectors is possible if there is moderate overpressure in the tubing.
The results of the current dissertation have been taken into account for enhancing the reliability of Estonian air humidity reference standard and enable to develop the standard further in future.