Measurement system for long-term recording of wood moisture content with internal conductively glued electrodes

Moisture as an important influence factor on fungal growth needs to be considered for service life prediction of wood and wood-based products. Therefore, a long-term moisture measuring and data logging method for wood in weathered conditions was developed. The method is based on measuring the electrical resistance with glued electrodes for sustainable connection. The measuring point at the tip of the electrodes was glued conductively into the wood, the remaining outer part of the electrodes was glued with an isolating glue. For this purpose, special conductive and isolating glues and electrodes were developed and comparatively evaluated in laboratory tests. The most suitable system consisted of a 2k-epoxy resin, serving for the isolating glue and also as conductive glue (when mixed with graphite powder and ethanol) in combination with a partly isolated stainless steel cable, acting as both, electrode and cable. This system was further tested in combination with mobile mini data logger at 29 different exposure sites in Europe and the United States. After 4–6 years of natural weathering with many extreme climatic and moisture changes, no loosening or other detectable abnormality in 541 pairs of electrodes was observed. The data logging systems were working without any problems for 5 years with the first and only battery, and without any additional maintenance. For the calibration of the measuring system, resistance characteristics were determined for different provenances of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies Karst.), and Douglas fir (Pseudotsuga menziesii Franco).