The psycho-meter basically consist of two thermometers -a ‘dry bulb’ and a wet bulb’ – over which air passes at a certain speed. Dry bulb thermometer just a liquid (most frequently used liquid mercury) in glass thermometer. Wet bold thermometer a similar one, but having its bulb covered by a cotton wick. The wick which covers the bulb of one of the thermometers fully wetted with distilled water.The psycho meter placed at the point of measurement and air at a certain speed passed over the bulbs. When air passed over the thermometer bulbs, the reading in the dry bulb remains unchanged. While reading in the wet bulb decreases until equilibrium attained. The air movement which directly influences the evaporation of water from the wick will have a cooling effect, thus decreasing the temperature in the wet bulb, referred to as the depression of the wet bulb the air velocity over the bulbs must sufficient to ensure that equilibrium reached rapidly.The temperature values in both thermometers are read and recorded as I dry bulb’ and wet bulb temperature which are plotted on to a psychometric chart, and the values for relative humidity, few point and absolute humidity can be obtained.