The Effect of Thermal Factors on the Measurement of Wet Bulb Globe Temperature

Abstract

The wet bulb globe temperature (WBGT) can be evaluated by measuring the globe temperature (Tg), the natural wet-bulb temperature (Tnwb), and the air temperature (Ta, only with solar radiation outdoors) to provide a fast and useful index of heat stress in hot environments. For the past few decades, the WBGT has been used as the standard for protecting workers in hot environments in Taiwan. The aim of this paper is to investigate measurement errors in Tnwb and Tg. All the tests were conducted in a climate chamber where the environmental factors (Ta, RH, Va) were controlled in order to study the effect of globe size and radiator temperature (Tp) on Tg, Tnwb, and WBGT. A greater globe size yielded higher Tg measurements for different Va and Tp with significant radiation. The response time of Tg increased with globe size and decreased with air velocity for all test conditions with radiant heat. The measured values of Tnwb under all experimental conditions was greater than the psychrometric wet bulb temperature (Tpwb) estimated by the ambient vapor pressure and air temperature in different combined conditions. Air velocity (Va) is critical for the globe temperature and natural wet-bulb temperature measurement, which can cause errors in heat stress evaluation when the air temperature is greater than skin temperature with significant radiant heat.