The Replacement USB humidifier is a replacement part for the Exo Terra Precision Incubator PRO with humidity control. Connect the USB plug of the Replacement USB humidifier to the USB port located at the back of the LED display next to the power cable of the Exo Terra Precision Incubator PRO. To prime the Replacement USB humidifier, pour a bit of water in the middle of the device, then turn it upside-down and lightly shake to remove excess water. Then place the humidifier flat on the water so it will float and starts humidifying.