Breathe in filtered air with the Optimus U-31001 1.5-Gallon Cool Mist Ultrasonic Humidifier. This compact device operates for up to 18 hours at a time. Designed for peace of mind as well as ease of use, the Optimus humidifier features an automatic shut-off option. This allows it to stop when it becomes empty. It also has a handy refill light. The whisper-quiet operation means you can even leave this machine running at night, and it won't disturb your sleep. The moisture output is more than 230 milliliters per hour.