2000

Forsen, Holger; Tarvainen, Veikko

The main task of VTT in this EU project was to test and improve the reliability and performance of moisture content meters. A total of 16 resistance type and 6 capacitance type hand-held moisture meters were included in the test series. Test samples of the most important European species (pine, spruce, birch, oak, beech, alder, larch) were obtained from all over Europe. The total survey included about 2,700 pieces for comparative testing. The test material was conditioned to three different moisture contents (8 - 10%, 12 - 14% and 16 - 18%). The moisture gradients in the test specimens were small due to the long conditioning time lasting at least 1 year. The effects of various factors, such as moisture content, species, and temperature, on the electrical resistance of conditioned wood were studied. In the laboratory tests, the resistance - moisture content curves for different species from different countries were determined using conditioned wood material. The species corrections (resistance curves) are quite similar for different counties. Only the resistance curve for Maritime Pine differs clearly from the other resistance curves for the pine species originating from the different countries. The wood temperature corrections are about 0.1 - 0.15% units/°C which has to be considered when the moisture content of wood is measured at temperatures other than 20 °C. The other properties of wood, such as as sapwood/heartwood and density, do not have a significant effect on resistance values. There were no significant resistance differences related to the type of electrodes, distances between electrodes, and different measuring direction. The commercial instruments for the determination of wood moisture content were tested with respect to accuracy, reliability and ergonomy. The moisture meters were tested both under laboratory and industrial conditions. Most of the resistance meters show a systematic deviation from the actual moisture content because of incorrect MC resistance curves. When the MC was lower than 10%, the readings of all the resistance meters tended to creep. The measuring accuracy of the MC meters (95 % confidence interval) in laboratory tests with well conditioned material was about ±1.5 . ±2.5% units for the resistance meters and about ±2.5% . ±4.0% units for the capacitance meters. The corresponding accuracy of MCmeters in industry test was about 2.0 % . ±5.0% units for the resistance meters and about ±3.0% . ±5.0% units for the capacitance meters.