Wet Bulb Globe Thermometer- How does it work?
A Wet Bulb Globe Thermometer (WBGT) Heat Index Monitor is a hand-held thermometer that measures heat in three different ways to produce a reading referred to as the heat stress index. It measures air temperature, globe temperature, and relative humidity. This type of combined measurement is more accurate than any of the single temperatures alone because it measures how hot the weather will feel to the human body. For athletes, manual laborers, farmers or anyone who works outside in the summer months, is essential information for heat safety. The WBGT reading informs decisions for determining a safe ratio of work to rest. As the heat stress index number rises it is important to allow adequate time for the body to hydrate and cool down.
The first temperature is measured by a dry bulb thermometer. This reading is the ambient air temperature (DB) that indicates the hotness of the day without taking into consideration the amount of humidity in the air. This reading alone will not accurately predict how hot it will feel to the body.
The human body sweats as the core body temperature rises on a hot day. The body's capacity to cool itself depends on how fast the sweat can evaporate on the skin to keep the body temperature regulated. A slight wind on a hot, dry day can keep the body regulated by drying the sweat on the body's surface. On particularly humid days sweat is less able to evaporate posing a danger for the body's cooling system. The Web Bulb temperature (WB) measures the humidity in the air, and how this might affect a person based on the evaporation rate. The Wet Bulb is wrapped in a wet cotton sleeve that stays wet in a small pool of distilled water. This second temperature reading simulates the evaporation of sweat due to the evaporative cooling of the thermometer's bulb.
The third temperature reading is recorded by the Black Globe (GT). A standard dry bulb thermometer is inserted into a black ball that measures how much sunshine, wind, and radiant heat that may be absorbed on the surface. This reading simulates how heat might feel on dark colored clothing or working on asphalt.
The three temperatures are calculated by the hand held thermometer using the following formula: WBGT = 0.7 WB + 0.2 GT + 0.1 DB. The reading is given as a WBGT temperature.
