![]() 2 proposed a complete reappraisal of reference values for thermal conductivity. In 1986, in view of the rapid developments in the measurement of the thermal conductivity, primarily of fluids in the liquid phase, Nieto de Castro et al. The absence of a gravitational field (e.g., spacebased measurements) can mitigate convective heat flow, and radiative heat flow is generally less of a problem at low temperatures. The inherent difficulty in the measurement of the thermal conductivity for both liquids and gases arises from the impossibility of decoupling the processes associated with heat transfer: conduction, convection, and radiation. ![]() At that time Riedel 1 suggested that the thermal conductivity of liquid toluene (a liquid that can easily be obtained at high purity) be adopted as a reference value at 293.15 K and 0.1 MPa. It was not until 1951 that any proposal was made for standard reference values for this fluid property. It is important to recall that the thermal conductivity, λ( T, P), is the state-dependent proportionality constant in Fourier’s Law relating heat flow to an infinitesimally small temperature gradient. The thermal conductivity of a fluid, λ, has proven to be one of the most difficult thermophysical properties to measure accurately. These three organizations often collaborate on both reference data and correlations for transport properties. Finally, we should also mention the International Association for the Properties of Water and Steam that since 1929 has been the body that proposes the reference correlations and values for the properties of water and steam, including transport properties. The International Association for Transport Properties (IATP), formerly known as the Subcommittee on Transport Properties of IUPAC, has been proposing mostly reference values. The National Institute of Standards and Technology (NIST) in Boulder, CO, has been involved in the development of wide-range reference correlations for thermal conductivity and viscosity to extend the capabilities of the reference software they develop. The current paper emphasizes the work of three main bodies that remain active in the field of reference values and correlations for transport properties. When appropriate, the reference correlations or restricted reference correlations are constrained to agree with any reference values that may have been established for the fluid of interest. In between these two categories, there exist “restricted reference correlations” that refer to a limited range of conditions, often with lower uncertainty than wide-range reference correlations, and may be of specific industrial or scientific interest. “Reference correlations” for pure-fluid transport properties often cover a wide range of conditions - typically from the triple-point temperature to 1000 K, and up to 100 MPa pressure - and are developed to achieve the lowest possible uncertainties (although perhaps higher than those of reference values). These values are often characterized by the lowest uncertainty possible at the time of their acceptance. Reference values refer to the properties specified at a fixed state condition (specific temperature, pressure and composition) or at a small number of such states. Second, in the case of instruments operating in a relative way, they provide the basis to calibrate one or more unknown constants in the working equation. Internationally accepted “reference values” (known also as “standard reference values”) serve two primary purposes: first, they can provide a means of confirming the operation and experimental uncertainty of any new absolute apparatus and the stability and reproducibility of existing absolute measurement equipment. ![]() In this work, we review reference values and correlations for two important fluid transport properties: thermal conductivity and viscosity.
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