DEVELOPMENT OF DYNAMIC MOISTURE CONTENT MEASUREMENT SYSTEM FOR WOODEN MEMBERS EXPOSED TO FIRE HEATING

A small sensor for the dynamic measurement of local moisture content within a wooden member exposed to fire heating is developed. Time variation of local moisture content of a wooden member during fire tests has not been grasped for the lack of non-destructive measurement system suitable for high temperature. This work intends to develop a heat resistive small sensor applicable to wooden specimen for fire tests to make it possible to monitor the distribution and time variation of moisture content within a wooden specimen just as thermocouples commonly used for the measurement of the distribution and time variation of temperature of a specimen. Importance of the measurement of local moisture content of a wooden specimen during fire tests comes from the recent advance in the recognition of the importance of moisture content in the high temperature performance of wood, e.g. significant influence of moisture content on the mechanical properties especially at high temperature and on the thermal decomposition as well as for the possible significant transfer of water within heated porous material such as wood due to the vaporization and re-condensation during fire. The possible influence of moisture content of wood may affect not only the charring rate of load bearing members but ignition and flame spread above wood based interior surface. The proposed measurement system is based on the unique dependence of electric resistance of wood on the moisture content and temperature. The present work consists of the design of a small heat resistant electrical resistance sensor and the development of calibration curves by experiments for the correlation of electrical resistance against temperature and moisture content for most typical tree species for building construction: Cryptomeria japonica, Larix leptolepis, Pseudotsuga menziesii and Zelkova serrata are chosen. Main results as follows. First, the specifications of the heat resistant sensor are shown in Fig. 1, which essentially monitor the electric resistance between the couples of the wires. Copper wire reported in a previous work for the measurement at room temperature ( Fredriksson et al, 2013 ) is replaced by electrically insulated nickel wire for the improvement of heat resistance. The upper temperature limit of electrodes the proposed sensor is 150℃; electrically insulated wires need to be wired not to be exposed to temperature higher than 150℃ during measurement. Second, the proposed sensors are applied to wood