Simultaneous measurement of the thermal conductivity and thermal diffusivity of unconsolidated materials by the transient hot wire method

Abstract

This paper describes a new design for the transient hot wire method that can obtain the thermal conductivity and thermal diffusivity of unconsolidated materials. In this method, the thermal conductivity is determined from the slope of the temperature rise versus time of an electrically heated wire. The temperature rise is detected as the unbalanced voltage of a precision Wheatstone bridge. This voltage is read by a microcomputer via a high-speed analog-to-digital converter. The instrument was designed so that measurements can be taken over a temperature range of 20–200 °C and a pressure range of atmospheric down to 10 mTorr. Tests using glycerin indicate an accuracy of 1% for the conductivity and 6% for the diffusivity and a precision of 0.4% for the conductivity and 4.5% for the diffusivity. Measurements have also been made on materials such as 50-μ glass beads and unconsolidated spent oil shale.