According to the analysis of Threlkeld (1970), for a wet-bulb diameter of 0.1 in. (2.5 mm) and an air velocity flowing over the wet bulb of 400 fpm (2 m / s), if the dry-bulb temperature is 90°F (32.2°C) and the wet-bulb temperature is 70°F (21.1°C ), then (T T *)/(T T ) is about 2.5 percent.