DESIGN OF A CALIBRATION SYSTEM FOR HEAT FLUX METERS

Accurate heat flux measurements are needed to gain a better knowledge of the thermal performance of buildings and to evaluate the heat exchange among various parts of a building envelope. Heat flux meters (HFMs) are commonly used both in laboratory applications and in situ for measuring one-dimensional heat fluxes and, thus, estimating the thermal transmittance of material samples and existing buildings components. Building applications often requires heat flux measurements below 100 W•m-2. However, a standard reference system generating such low heat flux is available only in few NMIs. In this work, a numerical study aimed at designing a HFM calibration apparatus operating in the heat flux range from 5 W•m-2 to 100 W•m-2 is presented. Predictions about the metrological performance of such a calibration system were estimated by numerical modelling exploiting a commercial FEM code (COMSOL®). On the basis of the modelling results, an engineered design of such apparatus was developed and discussed in detail. The system was designed for two different purposes: i) for measuring the thermal conductivity of insulators, ii) for calibrating a HFM with an absolute method (i.e., by measuring the applied power from the heater and its active cross section) or by a relative method (i.e., by measuring the temperature drop across a reference material of known thickness and thermal conductivity). The numerical investigations show that in order to minimise the uncertainty of the generated heat flux, a fine temperature control on the thermal guard is needed. The predicted standard uncertainty is to within 2 % at 10 W•m-2 and to within 0.5 % at 100 W•m-2