4-23. ADVANTAGES AND DISADVANTAGES OF EACH MEASUREMENT
Because of the difficulty in recording all three of these measurements and combining
the readings into a single index as a measure of overall heat stress, usually only the dry
or wet bulb temperatures are used.
a. Limitation of Dry Bulb Temperature Measurement.
The dry bulb temperature is, in general, a poor indication of thermal
stress because it is not affected by humidity, air movement, or radiation.
The wet bulb temperature is a better index since it includes the humidity
factor as well as the air temperature.
b. Example. For example, work that is relatively easy at a dry bulb temperature
of 96 F with a relative humidity of 50 percent (80 F wet bulb) becomes almost
impossible at the same dry bulb temperature when the relative humidity approaches
100 percent (96 F wet bulb). At a given wet bulb temperature, the dry bulb
temperature can vary upward over a wide range without much physiologic or
psychological effect on an individual.
c. Limitations of Wet Bulb Temperature Measurement. However, the wet
bulb temperature is not a good index of the heat stress imposed upon individuals
exposed to the sun. Air movement also is not taken into account in these
d. A Single Index. Many attempts have been made to devise a single index
combining all four of these factors, an index that would be simple and could be used
without elaborate equipment or training of personnel. The wet bulb globe temperature
(WBGT) index provides such a standard.
4-24. WET BULB GLOBE TEMPERATURE (WBGT) APPARATUS
a. Parts of the Apparatus. The WBGT apparatus consists of:
A stationary wet bulb thermometer exposed to the sun and to the
A black globe thermometer similarly exposed.
A dry bulb thermometer shielded from the direct rays of the sun.
b. Location and Setup. The apparatus is set up at a location representative of
the conditions to which soldiers are exposed. The wet bulb and globe thermometers
are suspended in the sun at a height of 4 feet above ground for at least 20 minutes
before readings are taken.