An expression for the actual water vapor partial pressure in terms of wet-bulb and dry-bulb temperatures, referred to as the Carrier equation, is given as where.

Pv, Pgw, and P are the actual partial pressure, the saturation pressure at the wet-bulb temperature, and the total pressure (an in bars), while Tdb and Twb are the dry- and wet-bulb temperatures in Kelvin's. Consider air at 1 atm and 37.8°C flowing over a wet-bulb thermometer that indicates 21.1oC.

(a) Using Carrier's equation, calculate the partial pressure of the water vapor in the free stream. What is the relative humidity?

(b) Refer to a psychometric chart and obtain the relative humidity directly for the conditions indicated.

Compare the result with part (a). (c) Use Equation 6.65 to determine the relative humidity. Compare the result to parts (a) and (b).

( )(T w) Po = Pgw - 1810 Twb