Hair Hygrometer

Hair hygrometer 

Hair hygrometer are called membrane hygrometer when the sensing element is a membrane.

Hair hygrometer is a type of absorption hygrometer and uses the mechanical humidity sensing technique.

Hair hygrometer is a type of absorption hygrometer, Which works on the principle that, "Change in moisture content causes change in physical and chemical properties of certain materials", Such as, human hair, animal membrane, Wood, Paper etc.. As human hairs are used in this hygrometer, this type is called as hair hygrometer.

Construction 

Working of hair hygrometer depends upon the physical changes in human hair i.e. tendency to lengthen, when they absorb moisture and shorten, after losing moisture and getting dry. This absorption depends upon temperature and partial water vapor pressure in atmosphere. Due to this, extension or elongation of hairs is a function of relative humidity 

fig shows an arrangement, where expansion and contraction of human hair may be reflected through a mechanical linkage ta an indicating pointer 

Human hair is used as the humidity sensor. The hair is arranged in parallel beam and they are separated from one another to expose them to the surrounding air/atmosphere. Number of hairs are placed in parallel to increase mechanical strength.

This hair arrangement is placed under small tension by the use of a tension spring to ensure proper functioning.

The hair arrangement is connected to an arm and a link arrangement and the link is attached to a pointer pivoted at one end. The pointer sweeps over a humidity calibrated scale.

When the humidity of air is to be measured, this air is made to surround the hair arrangement and the hair arrangement absorbs the humidity from the surrounding air and expands or contracts in the linear direction.

This expansion or contraction of the hair arrangement moves the arm & link and thus the pointer to a suitable position on the calibrated scale and thus indicating the humidity present in the air/atmosphere.

Advantages

• Low cost 
• Available in pocket size shapes
• No need for precision
• It is useful at room temperature 

Disadvantages

• Hair hygrometers are recommended, only when temperature ranges from 0 to 75 degree celsius and relative humidity ranges from 30% to 95%
• Less accuracy(3 to 4%)
• Slow in response 

Application 

• To measure relative humidity at offices, process plants and air conditioned rooms 
• To measure humidity in residences and greenhouses