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Sunday, October 31, 2010

Let's Learn about: Wet Bulb Globe Temperature (WBGT)

Definition
The Wet Bulb Globe Temperature (WBGT) is a composite temperature used to estimate the effect of temperature, humidity, and solar radiation on humans.


History

In the late 1950s, at the US Marine Corps Recruit Depot on Parris Island in South Carolina, there was particularly high humidity and the Marines were required to perform high intensity exercise in uniform, and subsequently there was a significant number casualties due to heat stroke. Prompted by this, the Department of the Navy commissioned studies on the effects of heat on exercise performance. These studies resulted in a heat index called the Wet Bulb Globe Temperature (WBGT). The WBGT was later used by researchers as an easily measured general heat-stress index, and over time its use has become more widespread in workplaces and sporting situations. In 1989, WBGT was suggested as an international standard (ISO 7243).


Elements of WBGT
The WBGT is measured by a simple three-temperature element device:

1. The natural wet-bulb temperature (Tw), which consists of a thermometer with its bulb covered with a wettened cotton wick supplied with distilled water from a reservoir. The cotton wick will always be wet, allowing continuous evaporative cooling of the thermometer's bulb, simulating the evaporation of sweat. The natural wet-bulb thermometer, like the black globe thermometer is not shielded from wind or radiation. This thermometer represents the integrated effect of humidity, wind and radiation.
2. The black globe temperature (Tg), which usually consists of a 150 mm (6 inch) black globe with a thermometer located at the center. The black globe temperature represen
ts the integrated effects of radiation and wind.
3. The (shade) air temperature (Ta), which consists of a thermometer shielded from radiation - generally by being placed in a weather screen. It is the standard temperature normally quoted in weather observations and forecasts.


Calculations
The three elements Tw, Tg, and Ta are combined into a weighted average to produce the WBGT.

WBGT = (0.7 × Tw) + (0.2 × Tg) + (0.1 × Ta)

Indoors, or when solar radiation is negligible, the following formula is used:

WBGT = (0.7 x Tw) + (0.3 x Tg)

The temperatures may be in either Celsius or Fahrenheit.


Classification Index
In hot areas, some US military installations display a flag to indicate the heat category based on the WBGT. The military publishes guidelines for water intake and physical activity level for acclimated and unacclimated individuals in different uniforms based on the heat category.

Category

WBGT (oF)

WBGT (oC)

Flag Color

1

≤ 79.9

≤ 26.6

No flag

2

80 – 84.9

26.7 – 29.3

Green

3

85 – 87.9

29.4 – 31

Yellow

4

88 – 89.9

31.1 – 32.1

Red

5

≥ 90

≥ 32.2

Black



Closing
WBGT instruments are available commercially, but they are fairly expensive, requiring regular maintenance if they are to produce accurate values.

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