In order to determine the loading on people from the thermal climate, various types of temperature indices can be used. Different climatic indices are used depending on whether one is determining the dangers to health or the comfort levels. The index most commonly used to determine the effects on health is the wet bulb globe temperature (WBGT) index:
WBGT = 0.3 Tg + 0.7 Tw (7.1)
where Tg is the globe temperature and Tw is the wet bulb temperature.
Globe temperature is measured using either an electronic or a common mercury-in-glass thermometer mounted inside a black globe 150 mm in diameter, and it gives an estimation of the radiant temperature; it is affected by the air temperature, the temperatures of the surrounding surfaces, and the air speed. Tw is the wet bulb temperature, which is measured with an ordinary thermometer whose bulb is surrounded by a wetted ‘sock’. This should give the ‘natural’ wet bulb temperature.
Where a fan is used to force air past the temperature-sensitive wet bulb, the ‘psychrometric’ wet bulb temperature is obtained. The natural wet bulb temperature represents the air temperature corrected for the relative humidity in the room, and with a certain effect from the air movement where this is below about 2 ms-1. Lower relative humidities and higher air movements give a lower temperature on the natural wet bulb thermometer compared with the ordinary air temperature. Figure 7.1 shows the acceptable WBGT values for different time periods and levels of activity.
The area of particular interest in the control room is the ‘comfort region’, and a special comfort index may be used to determine this. Many comfort indices have been produced. The one with the best foundation is that produced by P. O. Fanger (1970), called the Fanger index.
