5.1 The application of HFTs and temperature sensors to building envelopes provide in-situ data for evaluating the thermal performance of an opaque building component under actual environmental conditions, as described in Practices C1046 and C1155. These applications require calibration of the HFTs at levels of heat flux and temperature consistent with end-use conditions.

5.2 This practice provides calibration procedures for the determination of the heat flux transducer sensitivity, S, that relates the HFT voltage output, E, to a known input value of heat flux, q.

5.2.1 The applied heat flux, q, shall be obtained from steady-state tests conducted in accordance with either Test Method C177, C518, C1363, or C1114.

5.2.2 The resulting voltage output, E, of the heat flux transducer is measured directly using (auxiliary) readout instrumentation connected to the electrical output leads of the sensor.

Note 1: A heat flux transducer (see also Terminology C168) is a thin stable substrate having a low mass in which a temperature difference across the thickness of the device is measured with thermocouples connected electrically in series (that is, a thermopile). Commercial HFTs typically have a central sensing region, a surrounding guard, and an integral temperature sensor that are contained in a thin durable enclosure. Practice C1046, Appendix X2 includes detailed descriptions of the internal constructions of two types of HFTs.

5.3 The HFT sensitivity depends on several factors including, but not limited to, size, thickness, construction, temperature, applied heat flux, and application conditions including adjacent material characteristics and environmental effects.

5.4 The subsequent conversion of the HFT voltage output to heat flux under application conditions requires (1) a standardized technique for determining the HFT sensitivity for the application of interest; and, (2) a comprehensive understanding of the factors affecting its output as described in Practice C1046.

5.5 The installation of a HFT potentially changes the local thermal resistance of the test artifact and the resulting heat flow differs from that for the undisturbed building component. The following techniques have been used to compensate for this effect.

5.5.1 Ensure that the installation is adequately guarded (3). In some cases, an assumption is made that the change in thermal resistance is negligible, particularly for very thin HFTs with a large surrounding guard, or is incalculable (1).

5.5.2 For the embedded configuration, analytical and numerical methods have been used to account for the disturbance of the heat flux due to the presence of the HFT. Such analyses are outside the scope of this practice but details are available in Refs (4-8).

5.5.3 For the surface-mounted configuration, measurement errors have been quantified by Trethowen (9). Empirical calibrations have also been determined by conducting a series of field calibrations or measurements. Such procedures are outside the scope of this practice but details are available in Orlandi et al. (10) and Desjarlais and Tye (11).