Small globes and pocket heat stress meters for WBGT and PHS evaluations. A critical analysis under controlled conditions

This paper is aimed at quantifying the effects of the well-known criticalities related to the use of small globes in the assessment of extreme hot thermal environments. Based on the experimental measurements carried out through a special apparatus able to reproduce a black enclosure under controlled conditions, the analysis has been focused on both screening (WBGT) and rational (PHS) assessment methods. Obtained results showed the underestimation of the WBGT values within 1 °C using small globes for measuring the WBGT index according to the Standard ISO 7243 and no relevant errors occur also in the presence of high radiative loads. These errors can be even eliminated if specific correction coefficients for the heat transfer by convection are used. On the other hand, pocket WBGT meters based upon small globes and psychrometric (shielded) sensors – instead of the wet wick – are affected by an unacceptable underestimation (by more than 10 °C) of the WBGT that is particularly enhanced at low air velocity and high radiative loads. Finally, the systematic underestimation of the mean radiant temperature measured through small globes can lead the allowable exposure times predicted by the PHS model to be underestimated by more than 5 h when a 38 mm table-tennis ball is used.