NC Filtration's™ Second Stage (Secondary) Ring Panel Filter features no by-pass, self sealing fail-sale filtration. They are U.L. Class 2 rated and constructed with three (3) distinct layers of Deniers (15/40/6) allowing for high dust holding capacity and low initial pressure drop. Heavy internal wire support provides a sturdy filter ready to tackle your toughest jobs. The filter may be used in a varierty of negative air machines and air scrubbers, including Aerospace America Aeroclean™ Negative Air Machines #9100, #9103, #9143, & #9180.