Abstract: The invention is characterized by a sealed vessel defining a chamber that is partially filled with a liquid, preferably transparent and safe such as water. The vessel has an inlet for atmospheric air that via a tube is extended to open into the vessel under and into the vessel liquid, and an outlet that is located above the vessel liquid. The vessel outlet is connected to the inlet of the makeup air lumen of a dual lumen medical drainage tube. Negative pressure of a functioning suction/collection system applied to the suction lumen of the dual lumen tube thereby is linked via the makeup air lumen to the vessel chamber, and the resulting flow of makeup atmospheric air is as visible bubbles rising through the vessel liquid. The continuity of the bubble pattern through the vessel liquid thus offers assurance of non-changing air flow rate and assurance of proper functioning of the dual lumen drainage tube.