Hysteresis and Thermal Inertia of Spheres of Alternative Materials for Black Globe Thermometers

Abstract

This research was conducted with the objective of evaluating the thermal inertia and hysteresis of the following materials in a wind tunnel: hollow copper sphere with 150.0 mm diameter used as standard in globe thermometers, hollow brass sphere with 75.0 mm diameter, and two hollow plastic spheres with 115.5 and 69.7 mm diameter. The results showed that the thermal inertia and hysteresis for the hollow copper sphere were approximately 24 and 28 min, respectively; these values were bigger than those obtained for the other spheres, implying faster responses of these. While there were no statistical differences in the measurements conducted by globe thermometers which are used to indicate the black globe temperature with the different types of spheres mentioned above under indoor environment conditions, i.e., with no wind and solar radiation, the wind causes faster cooling in smaller spheres under outdoor environment conditions, which may denote erroneous interpretations of the measurements at those instants. It was concluded that it is possible to replace the hollow copper sphere of diameter 150 mm by other types of spheres to measure the black globe temperature. However, this requires knowledge of the response times of the spheres as well as corrections to be made in the measurements with respect to the globe thermometer standard.

Mean Radiant Temperature Measurements through Small Black Globes under Forced Convection Conditions

“…As stressed in two recent investigations [25,28], over the past years many studies have been carried out on the mean radiant temperature and globe thermometers [25,29,30]. The most debated issues were the effect of materials on the accuracy [31][32][33][34][35][36], its response time which seems to be acceptable for continuous monitoring only for table tennis balls [20,25,28,32,[36][37][38], and, finally, the correction factors of small globes in case of WBGT measurements [28,[39][40][41][42]. On the contrary, few investigations have been focused on the metrological characterization of globes and their accuracy in the measurement of the mean radiant temperature [25,[43][44][45][46].…”