The WMO International Hygrometer Intercomparison which was held at the Norwegian Meteorological Institute (DNMI) from February 1987 to March 1989, included 20 different humidity sensors, some with 2 or 3 equivalent units. All together 34 sensors were compared, representing products from 15 manufacturing firms. Eleven Member countries participated in the project. The instruments were exposed in a grass-covered area to the south of the institute buildings, -some sensors in open air, some inside an instrument screen specially constructed for the purpose. The instruments were exposed in accordance with instructions received and/or decisions made by an International Organizing Committee (QC). Sampling of data was accomplished by an EDPsystem at the site. Logging took place every 6th second during last minute of every 10 minutes interval, continuously. The 10 signals from each of the sensors thus received during each 10 minutes interval were linearized, averaged and stored in a raw data file.Other meteorological variables were also monitored simultanously, averaged and stored. Since no absolute reference was available, the hygrometers were compared with Relative Reference Values (RRV), constructed in accordance with methods established by the QC, and described in the Report. For analysis of the data, accomplished by a separate EDP-system, several methods were employed, as, for example, comparison with RRV in relation to temperature, to relative humidity, to certain weather phenomena and time (for possible drift investigation) etc. In addition, intercomparison between sensors of same type, between reference values and between values recorded inside and outside the instrument screen have been carried out. Graphs and tables have been prepared for all the sensors included. The Report also contains short descriptions and general performance of the instruments included, and summary tables of result.
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