One of the biggest factors in achieving a successful hatch is maintaining the correct temperature. Most incubator thermometers measure the air temperature, but with the Thermo-Egg thermometer, you will receive an accurate temperature reading at the egg level. Place the Thermo-Egg in the incubator next to the other eggs and the display will show you both the egg sensor temperature and also the ambient air temperature where the control unit is located. Comes with 2 egg sizes: small (quail) and medium (other poultry). The thermometer can be switched from Fahrenheit to Celsius. One AAA battery is included.