Estimating wet bulb globe temperature using standard meteorological measurements

Workplace heat stress management programs throughout government and industry utilize American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit values (TLVs) for heat exposure. The ACGIH TLVs are based on measured values of the wet bulb globe temperature (WBGT). The WBGT is given by:

WBGT=0.7 Tn + 0.2Tg + Ta,

Where Tn is the 'natural' (static) wet bulb temperature (the temperature achieved by a thermometer covered with a moistened white wick and left exposed to the ambient environment), Tg is the globe temperature (the temperature inside a blackened hollow copper sphere exposed to the ambient environment), and Ta is the ambient temperature. Medical research indicates that WBGT provides good correlation to deep body temperature and other physiological responses to heat. Instrumentation is available for direct measurement of WBGT; however, these measurements must be taken manually using a custom hand-held device.

At the DOE's Savannah River Site (SRS), a computer algorithm was developed which calculates estimates of WBGT from standard meteorological measurements. Tg is calculated explictly from a heat balance equation using measured values of solar radiation, wind speed, and ambient temperature. Tn is approximated from an expression that adjusts that standard (psychometric) wet bulb as a function of wind speed and solar radiation. Furthermore, scripts were developed to automatically execute a calculation every 15 minutes and post the results to an Intranet web site for access by the general SRS workforce. Although manual measurement of WBGT continues to be necessary to quantify heat stress for specific microscale work environments, experience over the last two years has shown that the automated WBGT calculation provides a highly effective means of identifying and communicating a general need for controls on outdoor work to a large workforce.

The proposed paper will describe the development and operational implementation of the WBGT algorithm and discuss comparisons of calculated WBGT values with field measurements.