The Slug Calorimeter (thermal capacitance) Method was recently developed for the evaluation of the apparent thermal conductivity of fire resistive materials at temperatures up to 1100 K [1, 2]. The experimental setup consists of a metal plate (made of material with a known specific heat) symmetrically "sandwiched" between two flat nominally identical samples of low thermal conductivity. If the temperature of the samples' outer surfaces is slowly increasing or decreasing at a constant rate, then the ratio of the slug's temperature rate to the difference between the samples outer surface's and the slug's temperatures becomes approximately proportional to the apparent thermal conductivity of the samples. Specific heat of the samples has to be known in order to calculate the apparent thermal conductivity. Now, more accurate values of thermal conductivity λ, and theoretically of other thermal properties - thermal diffusivity k, volumetric specific heat C_pρ, and thermal effusivity ε (two of them are independent) - can be obtained using a revised theory along with a new procedure for the Slug Calorimeter Method presented in this work. A more accurate formula for the temperature difference vs. time dependence was derived by using a cubic polynomial, instead of a quadratic one, as was previously used for the approximation of the temperature distribution inside the samples. To find three unknown coefficients of the cubic polynomial the boundary conditions on the sample's two surfaces were used. The formula contains an unknown ratio of the volumetric specific heats of the slug's metal and of the tested material. This problem in general can be solved by recording the slug's temperature when the outer temperature is maintained constant. An exact analytical solution of this thermal problem was found which contains a dimensionless thermal similarity parameter. The first (the slowest) relaxation time used for calculations can be found from the slope of the graph of the logarithm of the difference between the outer (constant) and the slug's temperatures vs. time (which produces a straight line) during the system's exponential relaxation toward the final thermal equilibrium, after reaching the so-called "regular regime". This relaxation time and the problem's solution substituted into the formula for thermal conductivity allows converting it into a numerically solvable transcendental equation with only one unknown - the dimensionless thermal similarity parameter, so after finding it, the ratio of the specific heats can be determined. Correctness of the relaxation time calculations was checked experimentally. Thus, all four thermal properties listed above can be measured using the new formulas and a two-step procedure: first maintaining the outer temperature constant, and then, changing the outer temperature at a constant rate, and so on, if the relaxation time is measured with sufficient accuracy, i.e. no preliminary knowledge of the specific heat of the tested material will be necessary. A numerical experiment using Finite Element Analysis showed that the new formula is noticeably more accurate than the old one - especially during the early moments.
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