Heat Stroke Prevention Meters –
Measuring Principle

This section contains some information about the basic principles of  Wet Bulb Globe Temperature measurements.  The information is divided in several chapters. Please click on the desired section in the menu.



What is Wet Bulb Globe Temperature (WBGT)

The Wet Bulb Globe Temperature (WBGT) is a composite temperature. It is used to estimate the effect of temperature, solar radiation and wind speed on the human body. The WBGT is not the same as the ambient (dry) temperature, as it takes into account the levels of radiation, wind movement, humidity and the ambient temperature.

During during training activities at Parris Island (South Carolina) in the early 1950, the US Marine Corps suffered significant casualties due to heat stroke. The WBGT was the outcome of the study commissioned by the US Department of the Navy because of this incident, aiming to find an index which allows to estimate the effects of heat on exercise performance. In 1989, WBGT was suggested as an international standard (ISO 7243).

The WBGT is expressed in °C or in °F. Many public institutions like the American College of Sports Medicine, the US Army, the Japanese Amateur Sports Association, the Japan Society for Occupational Health and the American Conference of Governmental Industrial Hygienists recommend the monitoring of on-field WBGT to prevent heat strokes.

The tables below contain examples of such recommendations.

Japan Society for Occupational Health

WBGT [°C]Recommended maximum workload
32.5 Very light
30.5

Light

29.0

Moderate

27.5

Medium

26.5 Heavy

American Conference of Governmental Industrial Hygienists

Work and rest regimen  Permissible Limits (WBGT [°F]) 
Light work Moderate work Heavy work
8 h continuous work

86.0

80.1 77.0
75% work – 25% rest

87.1

82.4 78.6
50% work – 50% rest

88.5

84.9 82.2
25% work – 75% rest 90.0 88.0 86.0

How is the Wet Bulb Globe Temperature (WBGT) measured?

Originally, the WBGT was determined determined based on the measurement of three temperatures with a standard (bulb) thermometer:

  1. Temperature measured with a normal (bulb) thermometer (Dry Bulb Temperature, DB).
  2. Temperature measured by thermometer whose bulb is wrapped with a wet cotton sleeve. The continuous evaporation of water has a cooling effect, simulating the evaporation of sweat (Wet Bulb Temperature, WB). Sweat evaporates more quickly if the humidity of the air is low. Consequence: Sweat cools the body more efficiently in dry than humid conditions.
  3. Temperature measured with a thermometer, which is located inside a large (6 inches) black globe (Globe Temperature, GT). This temperature allows to estimate the effects of direct solar radiation.


The WBGT can then be calculated as follows:

WBGT = 0.7 WB + 0.2 GT + 0.1 DB

If the WBGT is measured inside a building, there is no direct solar radiation. When doing indoor measurements the WBGT is therefore calculated as follows:

WBGT = 0.7 WB + 0.3 GT

Portable WBGT Heat Stroke Prevention Meters

Modern portable WBGT Heat Stroke Prevention Meters

  • are equipped with a much smaller globe to measure GT. The measured temperature is converted from the 24 mm diameter globe to a standard 150 mm diameter black globe.
  • do not measure WB directly: This temperature is calculated based on the relative humidity (measured with a built-in sensor) and the ambient temperature.

 

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