Further examination of the transient hot-wire method for the simultaneous measurement of thermal conductivity and thermal diffusivity

Abstract

The transient hot-wire technique is re-investigated in order to measure, simultaneously and accurately, the thermal conductivity λ and thermal diffusivity κ of a liquid. Newly derived effective temperatures Tλ and Tκ that are associated with the measured values of the thermal conductivity and thermal diffusivity are introduced and replace the previously used temperatures T0 and Tm. Higher-order corrections due to the thermal properties of the wire material are also re-investigated; these may be significant for experiments on lower-density substances or when using a hot-wire sensor with a thicker wire. The radiation (photons) emitted from or absorbed into the medium is discussed, and its contribution is taken to be part of the ordinary heat-conduction phenomenon exhibited by carriers such as phonons, electrons and molecules. The new system was tested by measuring the thermal conductivity and thermal diffusivity of n-heptane, n-pentane and iso-pentane, and values for their volumic heat capacities were calculated from the ratios λ/κ. The relative combined expanded uncertainties of the measured values (coverage factor k = 2) were 0.38 % for λ, and 1.7 % for κ and cpρ.