Next to maintaining proper temperature in an incubator, humidity level is the most important factor of a successful hatch. Eggs in too dry of an environment lose their moisture and fail to develop. Eggs kept too moist can't take in oxygen or release carbon dioxide buildup, killing the unborn chicks. A few basic tools will help you monitor the humidity and keep it at the right level for optimal hatching conditions.
Preparing the Incubator
Before putting eggs in the incubator, you'll need to get the temperature and stabilized. Follow your incubator model's directions for adding water to the reservoir, and turn on the incubator. Set the temperature at 100 degrees Fahrenheit for a forced-air incubator or 102 degrees for a still air incubator. Allow the temperature to stabilize before you read the humidity level on your hygrometer. It should be beween 55 and 60 percent humidity. In most instances, adding water regularly to the water tray during the 21-day incubation period keeps the humidity at a proper level.
Use a wet-bulb thermometer or hygrometer to check the humidity regularly throughout the incubation period. Some commercial incubators come with built-in thermometers and hygrometers. You can purchase a digital hygrometer-thermometer combo for a few dollars online, at a pet store or at a hardware store. These tell you relative humidity at a glance. If you use a wet-bulb thermometer, a reading of 85 to 87 degrees Fahrenheit at 100 degree incubator temperature is the same as 55 to 60 percent humidity. You can double-check conditions by shining a bright flashlight beneath the egg in a darkened room once a week. The size of the air cell in the egg increases noticeably each time.
If the humidity in your incubator is fluctuating, the temperature in it may be fluctuating as well. Make sure your incubator is not close to a window where direct sunlight hits it, or is in a drafty area. If the temperature is constant and the humidity is too low, you may need to add an extra pan of water or a wet sponge. If your incubator is homemade, make sure the surface of the water in the container is at least half as large as the egg tray. A long, shallow pan provides more surface area and more humidity. If your incubator is too humid, try a pan with less surface area, or increase ventilation.
The Last Three Days
The last three days before hatching is when chicks move into position to hatch. Increase the humidity to about 65 percent during this time. The best way to do this is to add an extra pan of water or a wet sponge. Do not decrease ventilation, as the egg needs to take oxygen in through its shell and release carbon dioxide. Condensation on the incubator's viewing window is normal during this time. Open the incubator only long enough to add water to the tray during the last three days to maintain the humidity level.