Humidity is a measure of how much water vapor is in the air. Relative humidity, given as a percentage, tells you the ratio of water vapor to the maximum amount that a given volume of air can hold. Maintaining the humidity within an acceptable range is close to as important as maintaining the right temperature inside an incubator, so your eggs will mature and hatch. A hygrometer measures humidity. A simple homemade hygrometer serves the purpose of incubating poultry or other bird eggs with reasonable accuracy.

Making a Wet/Dry Hygrometer

One of the simplest types is a wet/dry hygrometer, which relies on the difference in the temperature showing on a pair of thermometers that are mounted next to each other. The bulb of the dry thermometer is exposed directly to the air. The second thermometer bulb is kept wet and the difference in the readings is caused by the rate that the water is evaporating. The higher the relative humidity, the less evaporation can occur, so the difference between the two readings is less.