Get $50 Discount On PureGuardian Humidifier At Bestbuy.com. Guard against problems caused by dry air with this Pure Guardian Elite ultrasonic humidifier that features a Smart Mist sensor for automatic humidity-level adjustments. The Silver Clean protection hinders growth of slimy mold and mildew. It holds up to 1.3 gal. of water and runs up to 90 hours per tank filling. It utilizes a sensor that monitors the humidity in the room and makes automatic adjustments to the output to maintain the desired humidity level.