Influence of heat flowmeters on thermal measurements

The measurement of heat flows in the engineering field is relevant to thermophysical characteristics of materials in the study of the behaviour of a wall or the achievement of an energy balance in a building. Nevertheless, the utilisation of heat flowmeters in a thermal circuit has to be accompanied by some precautions, notably for materials which are slightly heat conducting. The perturbation induced by the presence of the heat flowmeter is in fact often difficult to evaluate. An analysis method is proposed that allows the definition of optimum conditions of use of two sensor types in the case of conductive exchanges. This analysis is based on a simplified model considering the heat flowmeter as a resistive and capacitive thermal layer placed in series in the studied system. The modelling of these perturbations demonstrates three influential factors: the type of heat flowmeter, the ratio of the temperature of the surface of the studied system to the outgoing heat flow, and the time scale characterising the studied phenomena. An example of the use of heat flowmeters to measure the thermal effusivity of building materials and the measurement of the heat flow inside walls of building is presented.