The new air tightness testing device MEZ DUCT TESTER 321 is used to determine the amount of air leakage and the air tightness class achieved in accordance with DIN EN 16798 3 or DIN EN 1507 and DIN EN 12237, especially in ventilation and air conditioning systems. An air tightness test on air handling systems is necessary to verify compliance with the required air tightness class and thus the air tightness of the duct system. Tighter air ducts enable energy savings. Furthermore, leaky systems can lead to hygienic impairments. Missing air volumes at the destination (due to leakages) can have a negative effect on the room conditions.