Guarded Hot-Plate (GHP) Method: Uncertainty Assessment

In civil engineering, the thermal conductivity is the most important quantity for thermal insulations and, as such, is in most cases determined with the guarded hot plate instrument in accordance with the applicable standards. These standards are to assure that uniform and reliable measurements will lead to comparable results. In addition, the quality of a measurement result essentially depends on its measurement uncertainty which, for reasons of acceptance, should also be determined according to a uniform guideline. For some time now, such a standard has been available: The ISO ‘‘Guide to the Expression of Uncertainty in Measurement,’’ often abbreviated to GUM. Its application will be demonstrated comprehensively, and in detail, by the example of a guarded hot plate instrument which can be used at working temperatures from −70 to 200°C. The terms and definitions of the GUM required for this purpose will be extended by the instrument-and-sample-specific corrections and used as a basis for establishing the uncertainty budget. As far as possible, both alternatives offered by the GUM—type A and type B methods—are used in parallel. The combination of the sensitivity coefficients and variances of the budget yields the expanded standard uncertainty of the specific guarded hot plate instrument examined. In this case, it is 1.9% at 20°C.