Optimus 1.5 Gallon Cool Mist Ultrasonic Humidifier Combat dry air in any living or working space with the Optimus Ultrasonic Cool Mist Humidifier. This device features an auto shut off function once the water has been depleted as an added safety feature. With an extremely quiet operation and no bubbling or gurgling sounds, this machine will not disturb your sleep and you can enjoy undisturbed concentration when trying to work, read or perform other tasks. The Optimus humidifier also contains a removable transparent tank so that it is easy to refill. It boasts a handy refill light so that you will know exactly when it is time to add more water.