Humidity measurement

Hair hygrometer, principle operation and application

Hygrometers are humidity sensors, The hair hydrometer is a type of absorption hydrometer and uses the technique of mechanical moisture detection.

Principle of Hair hygrometer

Due to humidity, several materials undergo a change in physical, chemical and electrical properties. This property is used in a transducer designed and calibrated to directly read the relative humidity.

Certain hygroscopic materials, such as human hair, animal membranes, wood, paper, etc., undergo changes in the linear dimensions when they absorb moisture from the surrounding air. This change in the linear dimension is used as the measurement of the humidity present in the air.

Construction of Hair hygrometer

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

This hair arrangement is placed under a 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 rotated at one end. The pointer sweeps over a calibrated scale of humidity

Working of hair hygrometer:

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

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

Application of Hair hygrometer

  • These hydrometers are used in the temperature range of 0’C to 75’C.
  • These hydrometers are used in the range of relative humidity (relative humidity) from 30 to 95%.
  • Limitations of the hydrometer for the hair
  • These hydrometers are slow in response
  • If the hair hydrometer is used constantly, its calibration tends to change.


Instrumentation Engineer

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