Accuracy Verification of the Calculation Formulae of WBGT Using Outdoor Measurement Data of Natural Wet-bulb Temperatures

Abstract

The wet bulb globe temperature (WBGT) is a widely used index correlating with the risk of heat stroke. The IS07243 specifies a calculation formula for WBGT, which relies on natural wet-bulb temperature, air temperature, and black globe temperature. However, obtaining natural wet-bulb temperature using ISO-compliant measuring instruments requires significant effort to manage wick moisture. Therefore, several alternative methods have been proposed as substitutes for measuring natural wet-bulb temperature. This paper examines three major alternatives by utilizing outdoor observation data of natural wet-bulb temperature. Liljergen’s method, which considers the shape of the wick, was found to accurately reproduce WBGT in radiation fields including direct solar radiation. The WBGT calculation formula by ISO conversion was confirmed to be able to approximately reproduce WBGT throughout the daytime period in outdoor environments. Notably, using wet-bulb temperature as a proxy for natural wet-bulb temperature tends to underestimate WBGT compared to the ISO-compliant measuring instruments.