On the problems related to Natural Wet Bulb Temperature Indirect Evaluation for the Assessment of Hot Thermal Environments by means of WBGT

This paper deals with the indirect evaluation of the natural wet bulb temperature, tnw, one of the two quantities forming the basis of the well known WBGT index, considered worldwide a suitable and user friendly tool for the preliminary assessment of hot thermal environments. This quantity can be measured by a wet bulb thermometer (a temperature sensor covered with a wetted wick naturally ventilated), or, if not available, calculated from others microclimatic parameters (i.e. the air temperature, the globe temperature, the air velocity and the humidity) using a quite trivial energy balance equation. Because of the strong non linear structure of such an equation, the risk of a multiplicity of steady state solutions could result in the failure to obtain a reliable index evaluation. To dispel all doubts, this work carries out an in-depth analysis of the heat balance equation to be solved for the indirect evaluation of the natural wet bulb temperature. A preliminary investigation of each heat flow term involved in the heat balance on the sensor has been carried out; in a second phase a special continuation method has been implemented highlighting the effect of microclimatic parameters on the multiplicity of solutions. Results show that under free convection the evaluation produces a single solution only under uniform conditions, whereas, in the presence of even slight differences between the air temperature and the mean radiant temperature there can be as many as three solutions. This phenomenon, if confirmed by a further experimental investigation, could become a ticklish matter since a sensor, in principle, has to read a sole value of the measurand. In any case, from a numerical point of view, the presence of many values of tnw greatly reduces the possibility of an indirect WBGT calculation from the other involved physical quantities; as a consequence, the indirect evaluation of WBGT should be clearly avoided by ISO 7243 Standard.