Wet Bulb Globe Temperature WBGT

Overview

Engineering reference data for Wet Bulb Globe Temperature WBGT in air psychrometrics.

Key Formulas

Humidity Ratio

$\omega = 0.622 \frac{P_v}{P_a}$

Mass of water vapor per mass of dry air.

Relative Humidity

$\phi = \frac{P_v}{P_{vs}} \times 100\%$

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

$T_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}$

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

$h = c_p T + \omega h_g$

Sensible + latent heat per unit mass of dry air.

Variables

WBGT Formulas

Outdoor conditions (with solar radiation):

$$WBGT = 0.7 T_{nwb} + 0.2 T_g + 0.1 T_a$$

Indoor conditions (negligible solar radiation):

$$WBGT = 0.7 T_{nwb} + 0.3 T_g$$

Measurement Components

• Black Globe Temperature ($T_g$): Measured by a 150 mm (6-inch) black copper sphere containing a thermometer at its center. It represents the integrated effect of radiation and wind.
• Natural Wet-Bulb Temperature ($T_{nwb}$): Measured by a thermometer with its bulb covered by a wet cotton wick, exposed to ambient wind and radiation. It represents the integrated effect of humidity, wind, and radiation.
• (Shade) Air Temperature ($T_a$): Measured by a thermometer shielded from direct radiation, typically in a Stevenson screen or similar shelter.

References

• Original Source