Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 434

For indoor and outdoor conditions with no solar load, WBGT is calculated using Equation
15.19:

WBGT = 0.7 NWB + 0.3 GT (15.19)

For outdoors with a solar load, WBGT is calculated using Equation 15.20:

WBGT = 0.7 NWB + 0.2 GT + 0.1 DB (15.20)

• Measurement —Portable heat stress meters or monitors are used to measure heat condi-
tions. These instruments can calculate both the indoor and outdoor WBGT index accord-
ing to established ACGIH threshold limit value equations. With this information, in
addition to information on the type of work being performed, heat stress meters can
determine how long a person can safely work or remain in a particular hot environment.
It should be noted that measurement is often required for those environmental factors
that most nearly correlate with deep body temperature and other physiological responses
to heat. The WBGT is the technique used most often for measuring these environmental
factors.

15.8.5.1 Determining WBGT
The determination of WBGT requires the use of a black globe thermometer, a natural (static) wet
bulb thermometer, and a dry bulb thermometer. The measure of environmental factors is performed
as follows:

• The range of the dry and the natural wet bulb thermometers should be within -5°C to
+50°C, with an accuracy of ± 0.5°C. The dry bulb thermometer must be shielded from
the sun and the other radiant surfaces of the environment without restricting the airflow
around the bulb. The wick of the natural wet bulb thermometer should be kept wet with
distilled water for at least one-half hour before the temperature reading is made. It is
not enough to immerse the other end of the wick into a reservoir of distilled water and
wait until the whole wick becomes wet by capillarity. The wick must be wetted by direct
application of water from a syringe one-half hour before each reading. The wick must
cover the bulb of the thermometer and an equal length of additional wick must cover the
stem above the bulb. The wick should always be clean, and new wicks should be washed
before using.
• A globe thermometer, consisting of a 15 cm (6-in.) in diameter hollow copper sphere
painted on the outside with a matte black finish, or equivalent, must be used. The bulb or
sensor of a thermometer (range -5°C to + 100°C with an accuracy of ±0.5°C) must be fixed
in the center of the sphere. The globe thermometer should be exposed at least 25 minutes
before it is read.
• A stand should be used to suspend the three thermometers so air low around the bulbs is
not restricted, and the wet bulb and globe thermometers are not shaded.
• It is permissible to use any other type of temperature sensor that gives a reading similar
to that of a mercury thermometer under the same conditions. The thermometers must be
placed so that the readings are representative of the employee's work or rest areas, as
appropriate.

Once the WBGT has been estimated, environmental professionals can estimate workers' meta-
bolic heat load using the ACGIH method to determine the appropriate work/rest regimen, clothing,
and equipment to use to control the heat exposures of workers in their facilities.