The paper describes a method and an instrument to measure the thermal conductivity and the thermal diffusivity of liquids simultaneously by the transient hot-wire method. Though the principle of this method has formerly been known, the practical application to fluids has been enabled only by high-precision measurement. In the present method, the thermal conductivity can be determined from the slope of the temperature rise against time of electrically-heated wire and the thermal diffusivity can be obtained from the slope and the absolute temperature rise. In this instrument, the absolute temperature rise, which is detected as voltage change of an unbalanced bridge, is recorded by a high-precision digital system. This method has been tested by measuring toluene in the temperature range 0°-80°C at atmospheric pressure. The accuracy of the thermal conductivity measurement is estimated as ±0.5%, whereas that of the thermal diffusivity measurement is ±5%.
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