Sling psychrometer consists of a dry bulb thermometer and a wet bulb thermometer for measuring dry bulb temperature of surrounding air and wet bulb temperature of same air respectively. They are mounted side by side in a protective case (Figure 1). Top side of sling psychrometer carries a handle with arrangement of swivel connection, so that, sling psychrometer can be manually rotated in atmospheric air. Sensing bulb of dry bulb thermometer is directly exposed to ambient air to measure dry bulb temperature, whereas, sensing bulb of wet bulb thermometer is covered by a wick thoroughly wetted by distilled water. Temperature measured by this wick covered bulb of wet bulb thermometer is the temperature of liquid water in the wick and is called as wet bulb temperature. Evaporation of water around the bulb lowers the temperature. Therefore, wet bulb thermometer usually shows a lower temperature (wet bulb temperature) than dry bulb temperature. Wet bulb temperature is also referred as temperature reduced due to evaporative cooling.
Fig. 1: Sling psychrometer
Working of Sling Psychrometer
Sling psychrometer is rotated in the air for approximately one minute. And then, readings of both thermometers are noted. This process is repeated several times to assure that, the lowest possible wet bulb temperature is recorded. Relative humidity is determined from these two temperature readings by plotting them on psychrometric chart. At any ambient condition, relative humidity decreases, if temperature difference between DBT and WBT increases.
Advantages of Sling psychrometer
- Simple construction.
- Low cost.
Disadvantages of Sling Psychrometer
- Continuous measurement of relative humidity is not possible.
- Use of psychrometric chart and subsequent calculation part makes the job tedious and time consuming.
- Due to glass bulb, construction of sling psychrometer becomes delicate. Hence, much care and precautions are needed during its use.
Applications of Sling psychrometer
- Sling or motorized psychrometer is used in meteorological observations.
- It is used in the HVAC (Heating, Ventilation and Air Conditioning) industry for selecting and charging proper refrigerant in residential and commercial air conditioning systems.
- Sling psychrometers are also used in the coating industry.
- For checking and setting hair hygrometers.