An apparatus for the routine measurement of thermal conductivity of materials for building application based on a transient hot-wire method

Abstract

An apparatus has been designed and developed for the measurement of the thermal conductivity of samples of non-metallic materials whose thermal conductivity is quite low (in the range between 0.2 and 4 W/m K) by the transient hot-wire method. It is especially conceived to ensure short time consuming and economic measurement of the thermal conductivity of mortar and lateritic bricks for building application. Thermal conductivity is measured by tracking the thermal pulse propagation induced in the sample by a heating source consisting of a Nickel alloy wire. The temperature is measured on the wire by means of two T type (Copper–Constantan) thermocouples. The heat impulse transferred to the wire between two observed times gives a temperature increment of 2–10 °C, depending on the thermal conductivity of the material tested and on the preselected level of the heating power supplied. The thermal conductivity of the materials can be obtained in a comparative way or in a semi-absolute way. In both cases a preliminary calibration of the instrument, obtained with a reference sample whose thermal conductivity previously certified by absolute methods, is in the range required. In the second case, the calibration is necessary to obtain the characteristic curves of the instrument. The paper shows the measurement obtained using materials with thermal conductivity between 0.2 and 1.5 W/m K. In good experimental conditions the accuracy of the measurements is within 5%. The proposed apparatus offers significant advantages, mostly in terms of economy and flexibility, over systems currently in use and over similar systems based on transient methods.