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The object of this research was to study the effect of various design parameters of a guarded hot plate (GHP) on the error of measurement due to gap unbalance without having to resort to simplifying assumptions as has been the case in existing analytical studies. A better knowledge of this effect could lead to better national standards. It could explain some of the discrepancies often found between the results of round-robin tests. The method used is the finite element (FE) method applied to an axisymmetric GHP. The latter choice is justified by the fine discretization of the domain which is achievable without requiring a huge computer memory. This feature allows us to study the influence of thermopile wires crossing the gap.
The results show that for a given gap unbalance, one is not free to arbitrarily choose the thermopile wire diameter, nor the temperature drop across a given specimen thickness, if one seeks accurate measurements. One cannot achieve precise measurements with one apparatus and only one set of operating conditions that would apply to a broad range of thermal conductivities.
The value of the FE method to solve the differential equation of heat conduction while satisfying the operating boundary conditions of the GHP apparatus is also demonstrated.
guarded hot plate, errors, finite element, thermopile
Director of Manufacturing and Process Development, Pittsburgh Corning Europe N. V., Tessenderlo,