A transient hot-wire method for measuring the thermal conductivity of gases and liquids

In this paper we describe a version of a transient hot-wire apparatus which employs an integrating digital voltmeter to measure the bridge out-of-balance signal. The integrating period of the voltmeter is variable and is routinely set equal to one 60-Hz power-line cycle, 16.67 ms. Use of measurement or integration periods less than an integral multiple of the power-line period results in substantially more electronic noise and a significant degradation in experimental precision. A correction to the working equation which accounts for the integration of the out-of-balance signal is also presented. The precision of the digital voltmeter used with the apparatus is ±0.1 V, which translates into an ultimate precision of ±0.03 mK in the measured temperature rise. In practice the precision in the temperature rise is typically ±0.3 mK, which represents a moderate improvement over the precision generally obtained with transient techniques employing automatic bridge balancing schemes. Although the current apparatus is designed principally for measurements of the thermal conductivity of liquids, it can been used for gas-phase measurements, with some decrease in accuracy due to the somewhat larger heat capacity correction which must be applied to the temperature rise measurements. The operation of the instrument was verified by measuring the thermal conductivities of toluene and nitrogen. Preliminary data are presented for the new environmentally acceptable fluorocarbons such as R-134a (CF₃CH₂F), R-123 (CHCl₂CF₃), and R-141b (CCl₂FCH₃).Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.