Measurement of Apparent Thermal Conductivity by the Thermal Probe Method

Three thermal probes were constructed in accordance with ASTM D 5334 and calibrated using heat-flow metre data. The temperature-time response of the thermal probes for determining apparent thermal conductivity λ under transient state conditions was logged at 1 s intervals. The instrumentation used reduced the determinate error associated with voltage and current measurements to a negligibly small value that made the uncertainty in λ dependent on the uncertainty of the slope dT/dInt. A test method was run in 1000 s, in which a criterion of 2.5% spread among three consecutive slope values was used to determine the extent of the linear segment of the T-In t curve. The probes demonstrated repeatability within ±3.5% but had definite individual bias indicating a need for individual calibration. Using individual probe calibration factors, the experimentally determined As for all-purpose sand, sifted sand, and soil were determined to be 0.520, 0.445, and 2.11 W/m.K, respectively.