Hygrometer Bi-metal TFA 45mm

These hygrometers have a bimetallic mechanism and it's possible to calibrate it completely.

You can use it as a rough indication of relative humidity in Incubators and Intensive care brooders.

Relative humidity is indicated as a percentage from 0-100. The humidity in an incubator required depends on the kind of eggs and the extent to which the shell is porous. An egg should lose approx. 11% to 13% of its weight during incubation. By calculating how much weight the egg should lose throughout incubation, you can determine whether you should adjust the humidity in the incubator. 
You can use this hygrometer as an indicator and then by weighing the eggs regularly (see our Incubation Tools for our range of precise scales).  This combination is the best method to determine the humidity very accurately.

These are one of the best quality hygrometers available, made by TFA in Germany.

Also available is the 70mm Hygrometer.