In order to find a suitable instrument to replace the current humidity sensor used in KNMI’s measurement network, a project for testing has been started. Following a market research, four likely candidates have been found. These sensors are: Vaisala HMT337, Vaisala HMT317, Rotronic HygroClip and EplusE EE31. All these sensors are capacitive hygrometers and the first two are heated.
The test has two clear phases. First, all the instruments are thoroughly tested in the laboratory. In a climate chamber, measurements of humidity or dew point temperature (and, if applicable, air temperature) are taken for 9 values of humidity (17 %RH – 97 %RH) and for temperatures from -20 °C to + 30 °C. Also, response times are measured for the sensors. After all laboratory tests are done, an evaluation takes place and if a sensor does not meet the KNMI criteria, it will be omitted from further testing.
The second phase of the test consists of field tests. The purpose of these is to find the effects of the outdoor environment on the sensors, and to have an indication of future calibration intervals. These tests will take one year. The instruments are placed in KNMI radiation screens at two locations, one in the centre of the country and one near the coast to find the effect of sea salt on the sensors. During this year, the sensors will be regularly checked in the climate chamber to monitor their behaviour over time. After the field tests, the sensors will again be thoroughly tested in the climate chamber.
The progress of these tests is shown. The results of the first phase are also shown, and first results from the field tests are expected.
HI Bloemink, R van Krimpen. KNMI Humidity Sensor Test
2006, 2006, WMO