Development of a strain measurement system for the study of effect of relative humidity on wood

Wood is a widely used material in various applications where its dimensional stability is of practical interest in the design and performance of wooden materials. The change in geometry of wood depends upon the environmental conditions (such as relative humidity) as well as internal structure and composition of wood. This work presents a measurement technique and development of the associated system for the measurement of strain changes of wood samples with relative humidity. The developed system is capable of measuring the strain change and relative humidity (RH) with temperature compensation. The system comprises of strain gauge based strain measurement unit and RH sensor with its related signal conditioning circuit along with temperature sensor. The strain gauge signal conditioning is based on quarter bridge method with high precision resistors which is excited by an AC source. The whole system is centered on an 8-bit RISC microcontroller (PIC18F43K22). The built in 10-bit analog to digital converter (ADC) is used to read the strain and ambient RH. The temperature is directly read from temperature to digital converter using ZACwire™ interface. The measurement system is calibrated using a cantilever of stainless steel and is used for collecting and analyzing data of four wood samples. The uncertainties associated with the measurements are reported in the paper. Experimental results obtained for a few wood samples are presented.