Effect of radiation on psychometric measurements, suppose that a wet-bulb and dry-bulb thermometer are installed in a long duct with constant inside surface temperature T_s and that the gas velocity is small. Then the dry-bulb temperature T_db and the wet-bulb temperature T_wb should be corrected for radiation effects. We assume, as in Example 22.3-2, that the thermometers are so installed that the heat conduction along the glass stems can be neglected. 

Make an energy balance on a unit area of the dry bulb to obtain an equation for the gas temperature T_∞ in terms of T_db, T_s, h_db, e_db, and a_db (these last two are the emissivity and absorptivity of the dry bulb).