Humidity measurement

What is a Sling Psychrometer?


Sling Psychrometer is used to measure both the dry bulb and wet bulb temperatures at time. These temperatures are a measure of humidity content in air.

Description of Sling Psychrometer

The main parts of the instrument are

There are 2 thermometers in this instrument.

1.Dry bulb thermometer, whose sensor bulb is in direct contact with air (mercury is the sensing element)

2.Wet bulb thermometer: the sensor bulb of this thermometer is covered with a wick of cotton or muslin moistened with pure water
A glass frame is provided as the housing for the instrument.

A rotating handle is attached to the glass frame housing – the arrangement of the thermometer to ensure that the air in the wet bulb always in immediate contact with the wet wick.

When a thermometer bulb is directly exposed to a mixture of air-water vapor, the temperature indicated by the thermometer is the dry bulb temperature.

When the bulb of a thermometer is covered by a constantly wet wick and if the bulb covered by the wet wick is exposed to the mixture of air vapor and water, the temperature indicated by the thermometer is the temperature of the wet bulb.


Operation of Sling Psychrometer.

In order to measure the dry bulb and the temperature of the wet bulb, the disposition of the thermometer of the psychrometer- glass cover – is rotated from 5 m / s to 10 m / s to obtain the necessary air movement.

Note: An important condition is that the correct / accurate measurement of the wet bulb temperature is obtained only if the air moves with the speed around the wet wick. To get this air velocity, the psychrometer is spinning.

The thermometer whose bulb is empty contacts the air indicates the temperature of the dry bulb.

At the same time, the thermometer whose bulb is covered with the wet wick comes in contact with the air and when this passes the wet wick present in the thermometer bulb, the moisture present in the wick begins to evaporate and a cooling effect occurs produced in bulb. Now the temperature indicated by the thermometer is the wet bulb thermometer that will naturally be lower than the temperature of the dry bulb.

Note: If the psychrometer is rotated for a short period, then the registered wet bulb temperature will not be adequate.

Note: If the psychrometer is turned for a longer period, the wick will dry soon and the temperature of the wet bulb will not be at its minimum value.

Application of Sling Psychrometer

  1. It is used for checking humidity level in air-conditioned rooms and installations.
  2. It is used to set and check hair hygrometer.
  3. It is used in the measurement range of 0 to 100% RH.
  4. It is used for measuring wet bulb temperature between 0’C to 180’C.

Limitation of Sling Psychrometer

1. The measured medium is altered due to the act of measurement. The evaporation process in the wet bulb will add moisture to the air.
2. Can not be used in situations of automation requirements.
3. Can not be used for continuous recording purposes.
4. If the wick is covered with dirt, the wick will harden and its water absorption capacity will be reduced; however, a stiff / dirty wick will return to normal when boiling in hot water

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