This handy little device will come in very handy to double-check your humidity is where it should be! Precision Hygrometers, bi-metal type available in 45MM and 70mm - made by TFA Germany. 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. The 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. 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. The smaller gold one (second image) is the 45cm and the blue one (main image) is the 70cm hygrometer.
When it comes to incubation there are 4 key things to remember: Temperature, Humidity, Turning and Ventilation. Make sure you get all 4 right!