Wet Bulb Temperature vs Wet Bulb Globe Temperature - What's the Difference?

Wet Bulb Temperature

Before the arrival of digital meters, wet bulb temperature (WBT) was determined on analogue thermometers using a damp wick covering a thermometer. The purpose of using a wet wick is to compare the drop in temperature against a dry bulb thermometer displaying the actual air temperature. Comparison of the two temperatures allows for the straightforward calculation of relative humidity. An example of a meter which using a wet and dry bulb to take these measurements is the Sling Psychrometer -5- 50 Celsius - IC736700.

Sling Psychrometer -5-50 Celsius
Product Code: IC736700

The IC736700 allows users to obtain quick and precise relative humidity readings anywhere, without the need for batteries. The sling psychrometer has a recessed dry and wet bulb thermometer with a reservoir to keep the wet bulb wick hydrated.

It's important to note that wet bulb can still be determined and displayed on digital meters without the use of the wick. Some meters have internal processors which use the measured humidity, dry bulb temperature and pressure measurements to calculate a wet bulb temperature reading. An example of a digital meter which calculates wet bulb using the three parameters mentioned above is the Kestrel 5400 Heat Stress Tracker.

Kestrel 5400 Heat Stress WBGT Meter
Product Code: IC-0854ORA

The Kestrel 5400 Heat Stress Meter accurately measures and displays the wet bulb temperature, wet bulb globe temperature, thermal work limit, and globe temperature to flag when outdoor working conditions become unsafe.

Wet Bulb Globe Temperature

Wet Bulb Globe Temperature (WBGT) is a specialised heat stress index which considers several environmental and personal factors. Wet bulb globe temperature is used across several professions and industries, including for athletes, the military, and industrial hygienists to determine heat risks associated with persons working in adverse conditions. Here is the calculation employed by WBGT meters to determine a measurement:

WBGT=0.7T_w+0.2T_g+0.1T_d

Where;
T_{w = Wet bulb Temperature}T_{g = Globe thermometer temperature (measured with a globe
thermometer)}T_{d = Dry Bulb Temperature (actual air temperature)}

WBGT Meter SD Card with Data Logger
Product Code: IC800037

The IC800037 meter records wet bulb globe temperature, black globe temperature, humidity, air temperature and dewpoint. These recordings give a comprehensive insight into the heat risks of the current environmental conditions.

Conclusion

To summarise, wet bulb temperature is the lowest temperature that the evaporation of water can cool air. The wet bulb temperature will always be lesser than the dry-bulb temperature, except when there is 100% relative humidity.

Wet bulb globe temperature is a type of apparent temperature used to estimate the effect of climatic conditions and is used to assess potential heat-stress on humans.

Want more information on wet bulb temperature or wet bulb globe temperature meters? Speak with an Instrument Choice Scientist! We're here to help! Call 1300 737 871 or email customer-service@instrumentchoice.com.au.

Country/region

Wet Bulb Temperature vs Wet Bulb Globe Temperature - What's the Difference?