Sling psychrometer | Hair hygrometer | recording psychrometer

Here we learn about Sling psychrometer, and their advantages of sling psychrometer with fig., Hair hygrometer, recording psychrometer.

A ) Sling psychrometer :

~ The sling psychrometer consists of handle of two mercury thermometers mounted on frame, which has handle provided for rotation of psychrometer as shown in Fig. 5.2.2.

~ For accurate measurement temperature, it is an essential condition that the air should move with velocity around the wet wick.

~ The sling psychrometer is a specific form of wet and dry bulb psychrometer.
~ It uses glass thermometers in a frame designed to be swing rapidly to obtain adequate air velocity.
~ This is suitable for measurement in the range of- 0 to 100 % RH.

Fig. Sling psychrometer

Applications of sling psychrometer :

For setting and checking hair hydrometer.
For routine meteorological observations.

B) Hair hygrometer :

~ Hair hygrometer is the simplest and oldest type of hygrometer.
~ It is made using hair as shown in Fig. 5.2.1.
~ Human hair lengthens by 3 percent when the humidity changes from 0 to 100 percent, this property of hair can be used to operate a pointer or recording pen through a system of mechanical linkage.
~ The transducer element consists of strands of hair which are generally arranged parallel to each other.
~ It is not a precision instrument and not recommended where high degree of accuracy is equired.
~ The change in length can be used to control a pointer for visual readings or a transducer such as linear variable differential transformers (LVDT) for an electrical output.
~ The hair hygrometer has an accuracy of about 5 percent for the humidity range 20 to 90 percent over the temperature range 5°C to 40°C.

Fig. Hair hygrometer

C ) Recording Psychrometer :

~ A set of dry and wet bulb thermometer is used for recording psychrometer.
~ These thermometers are mounted in duct through which air flows with velocity 5 to 8 m/min.
~ If instead of duct humidity is to be measured in open atmospheric air, a blower is provided, which over provides the necessary draft of air thermometer.
~ The instrument is provided with pen recording system to continuously record dry and wet bulb temperature.
~ Now when dry and wet bulb temperature is plotted on psychometric chart we get humidity.