Bimetal coil hygrometer is very useful for giving a dial indication of humidity changes. It appears most often in very inexpensive devices, and its accuracy is limited, with variations of 8% or more. In these devices, water vapor is absorbed by a salt-impregnated paper strip attached to a metal coil, causing the coil to change shape. These changes (analogous to those in a bimetallic thermometer) cause an indication on a dial.
Shanghai QualityWell Industrial Co.,Ltd. can supply bimetal coil hygrometers to customers with higher accuracy +/-6% & many kinds shapes, they can be stand on the desk, also can be hung on the wall.