Types of Hygrometers
Metal-Paper Coil Hygrometer
In this hygrometer, water vapor absorbed by a salted paper strip attached to a metal coil causes the coil to change shape. These changes (analogous to those in a bimetallic thermometer) cause an arrow to indicator on a dial.
Pros: Gives a dial indication of humidity and is relatively inexpensive and simple.
Cons: Limited accuracy. Errors could read ±10% or more.
Hair tension Hygrometer
These devices use animal or human hair under tension. The length of the hair changes with humidity and the length change may be magnified by a mechanism and/or indicated on a dial or scale.
Pros: Very simple and inexpensive.