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 of Hair Hygrometer
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 vapour pressure in atmosphere. Due to this, extension or elongation of hairs is a function of relative humidity. Fig. 1 shows an arrangement, where expansion and contraction of human hair may be reflected through a mechanical linkage (i.e. an arm with pivot joints) to an indicating pointer.
Fig. 1: Hair Hygrometer
Working of Hair Hygrometer
The air sample, whose humidity is to be measured, is made to pass over the strands of hair (also called as hair strands). These hair strands are arranged parallel to each other with enough gap or space between them, for giving free access to air samples. The hair strand is rigidly fixed at one end and at the other end, it is connected to arm maintained under tension by a spring. The pointer moves over a calibrated scale through pivot to give direct indication of humidity. As relative humidity of surrounding air increases or decreases, human hair becomes longer and shorter respectively. These changes in length are used to operate the pointer to display the value of relative humidity measured.
Advantages of Hair Hygrometer
- Low cost.
- Available in pocket size shapes, hence convenient.
- No need for precision.
- It is useful at room temperature.
Disadvantages of Hair Hygrometer
- Hair hygrometers are recommended, only when temperature ranges from 5°C to 35°C and relative humidity ranges from 40 % to 95%.
- Less accuracy (3 to 4%).
- Needs periodic calibration. Also, time interval between recalibrations is too short.
- Slow in response.
Applications of Hair Hygrometer
- To measure relative humidity at offices, process plants and air-conditioned rooms.
- To measure humidity in residences and greenhouses.
