Determine the relative humidity by using the sling type psychrometer.

Apparatus:-

Sling type psychrometer, dew point calculator and stop watch.

Theory:-

Dry-bulb temperature:-

The dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. Dry bulb temperature is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It is the temperature measured by a regular thermometer exposed to the airstream.

Wet-bulb temperature:-

The wet-bulb temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor. Wet bulb temperature is the lowest temperature that can be reached by the evaporation of water only. It is the temperature one feels when one's skin is wet and is exposed to moving air.

Relative humidity:-

The relative humidity  of an air-water mixture is defined as the ratio of the partial pressure of water vapor (H₂O) (e_w) in the mixture to the saturated vapor pressure of water (e^*_w) at a prescribed temperature.

Relative humidity is normally expressed as a percentage and is calculated by using the following equation:

=e_w/e^*_w *100

Construction of the sling psychrometer:-

It consists of two identical thermometers; one measures the dry-bulb temperature and the other with a silk or cotton sleeve measures the wet-bulb temperature. Both thermometers are mounted side by side in a rigid frame; they are generally 6 to 10 in. (152.4 to 254.0 mm) in length, graduated from 0 to 120⁰F (-180 to 49⁰C). A water reservoir, filled with distilled or tap water, keeps the wet-bulb sleeve wet at all the times during measurement. The frame is attached by a swivel connection to a handle for whirling.

Procedure:-

• First of all revolve the psychrometer at 2 to 3 revolutions per second by keeping the handle in hand by keeping the instrument at the distance of arm length.
• Do this for some reasonable time and after that stop this and note the time.
• Also note the wet-bulb and dry-bulb temperatures quickly.
• By using the Dew point calculator find out the relative humidity.
  
  

  Supposed Observation:-

  
  

Wet bulb temperature

91⁰F (32.77⁰C)

Dry bulb temperature

75⁰F (23.89⁰ C)

Time

2 mint 15 sec

Relative humidity

48%