For this homemade hygrometer, I used a slice of wood as the sensing component. The slice is hung on one end of a fiberglass rod, and the rod is suspended as shown in the picture.
The wood has been equilibrated to a moisture content of about 8 percent, which is normal for this facility. As the relative humidity (RH) goes up, the wood becomes heavier and moves down. For example, after days of rain which brought the humidity up to 80 percent RH, it indicates we are well outside of the ideal relative humidity range for my area. It is not an exact science, but I have found that it seems to have a time constant of about an hour or less, where you can determine whether the ambient conditions are within range or out of range. The idea came from Bruce Hoadley and Alan Crivellaro, who both have produced this type of device.
For additional information about relative humidity, check out the NWFA’s Installation Guidelines at NWFA.ORG/TECHNICALGUIDELINES/.