hygrometer; hygrograph
A device that measures humidity. 湿度計 is the general term; a hygrograph is a recording hygrometer.
この湿度計は正確に湿度を測れます。
This hygrometer can measure humidity accurately.
The museum uses hygrometers to protect the exhibits.
I measured the indoor humidity with a hygrometer.
We installed a hygrometer in the greenhouse.