Moisture content measurements in wood and wood-based materials - advancements in sensor calibration and low-moisture-content regime

Wood is a major structural material used in the construction of residential and commercial buildings. In situ and continuous measurement of moisture in a wood element of the building envelope is critical in preventing the occurrence of moisture related damages to the building structure, and also helps in determining the thermal and energy performance of the system. Moisture content of a wood assembly may serve as an indicator of the effectiveness and potential condensation-related damage to the adjoining thermal insulation layers in the building envelope. Typically, the moisture content in a wood component is determined by measuring the electrical resistance across a pair of metallic pin sensors, with correlation or calibration between resistance and moisture content already established for the studied wood species. As the wood dries from fiber saturation point (FSP) to equilibrium moisture content (EMC), the resistance typically increases by 4 orders of magnitude from hundreds of k to G. Typical resistance measurement instrumentation schemes such as voltage divider and multimeters are often incapable to cover this wide dynamic range of resistance. To further complicate the measurements, low EMC values of 5 %-10 % range correspond to very high resistances in the range of M-G, a regime that has remained challenging to measure because of lower current and leakage issues. To circumvent these issues, we developed an instrumentation methodology based on a simple voltage divider circuit in combination with a data-logger with reference resistors selected in such a manner to maximize the dynamic range of the measurement with sufficient accuracy and resolution for higher resistance range. The results showed significant improvements in the dynamic range and resolution for the MC measurements.