What principle does hair hygrometer based?

Answer 1

The predominant proteins in hair are from the family of keratins. Protein molecules are built from amino acids. In a hair strand, the keratin molecules contain a large number of a particular amino acid called cysteine. Each cysteine in the keratin molecule is a potential attachment point, where the keratin molecule can be tightly connected to another cysteine, forming a chemical bond called a cross-link. The keratins in hair have many such cross-links, making a hair strand strong and flexible. There are also many weaker bonds, called hydrogen bonds between the keratin molecules. Hydrogen bonds can be disrupted by water, including water vapor in the air. In fact, this is what causes hair strands to expand in moist air and to contract in dry air.

A single lock will stretch about 2.5% percent as the humidity goes from 0 to 100 percent. While that's only a smidgen of length, the rate of this change is very dependable, so it's possible to obtain highly accurate humidity readings by measuring these tiny shifts.

So the Principle in question is the chemical property of hair protein molecules (or disruption of hydrogen bonds between keratin molecules by water)

Answer 2

Hair hygrometers are based on the principle that human hair expands or contracts with changes in humidity, which in turn affects its length. As the hair absorbs or releases moisture from the surrounding air, it changes in length, allowing the hygrometer to measure the humidity level by tracking these length variations.