This humidifier filter fits GeneralAire power humidifier (drum type) model 81 and removes all minerals from the water it evaporates! Minerals are left on the evaporator sleeve and are not blown into the airstream to collect in the living area. It measures approximately 7 1/2 H x 26 1/2 L x 1 and for optimum performance, replace filter every two months or, in hard water areas, more frequently if necessary.