PSYCHROMETER - A psychrometer is simply two ordinary thermometers mounted together on a single strip of material. The bulb thermometer is covered by a water-soaked wick from which the water evaporates rapidly slowly, depending on the amount of water in the surrounding atmosphere. Evaporation of water around the wet thermometer cools it. The amout of cooling depends on the rate of evaporation. The reading on the wet bulb is lower than the reading on the dry bulb except when the humidity is 100%, at which time both readings are the same. The difference between the wet-bulb and dry-bulb readings, when applied to tables developed for that purpose, results in relative humidity and dewpoint temperature. The dewpoint is the temperature to which air must be cooled at constant pressure and constant water vapor content to reach saturation (100% relative humidity). When air is cooled to dewpoint temperature, small water droplets condense on objects and dew forms.
SLING PSYCHROMETER - A sling psychrometer sometimes is used to speed up the process of getting accurate wet- and dry-bulb readings. The sling psychrometer can be whirled around to rapidly bring the wet bulb into contact with a great volume of air. This contact with air accelerates the evaporation rate. The person using the sling psychrometer should face the wind and should shield the instrument as much
possible from the direct rays of the sun. Whirling should not be too rapid because it force might displace the mercury columns in the thermometers. The whirling should be repeated until no further change can be detected in the wet-bulb reading.
DEWPOINT
The dewpoint is computed by using the psychrometer table. For example, a dry-bulb temperature of 60 F and a wet-bulb temperature of 50.5 F. The difference between the two readings is 9.5 F. This difference is called the depression of the wet bulb.
To compute the dewpoint, enter the table with the wet-bulb reading (50.5 F). Read across the top of the table to the proper depression column (9.5 F). Read the dewpoint temperature (42 F) directly from the intersection of the temperature row and the depression column.
