Investigation of Different Temperature Measurement Designs and Installations in Timber Members as Low Conductive Material

An accurate measurement of the temperature during fire tests is of utmost importance. The measuring device used therefore (1) shall indicate the exact temperatures in the specimen and (2) shall not disturb its temperature field. For timber as a low conductive material this is particularly difficult. The paper presents five standard fire tests (four cross laminated timber panels and one solid timber panel), in which eight different thermocouple designs and installations were investigated. The tests showed that any thermocouple design installed in boreholes perpendicular to the isotherms leads to considerably lower measurements than wire thermocouples placed parallel to the isotherms. Referring to the time when the measurement indicates 300°C the difference is between 10 and 35 min. An installation parallel to the isotherms is found to be accurate, while perpendicular to them suffers from significant heat loss through conduction in the thermocouple. Additionally, missing contact pressure between the thermocouple and the sample increases the temperature discrepancy. Convection in the tight gaps between the thermocouple and the borehole does not happen in usual configuration and hence has no effect on the measured temperature. For proper temperature measurements in timber, it is thus recommended to install thermocouples parallel to the isotherms, to use thermocouples with fewest possible conductive material and a high contact pressure.
Structures in Fire SiF'2018 : 10th International Conference on Structures in Fire, Belfast, United Kingdom, 6-8 June 2018
ISBN:978-1-85923-274-3