Fig. 3— From top to bottom: apparatus 1 (A1), from left to right: mouthpiece, elbow connector, coupling device; apparatus 2 (A2), from left to right: mouthpiece, in-line filter, elbow connector, coupling device; apparatus 3 (A3), from left to right: mouthpiece, elbow connector, pneumotachograph, coupling device; and apparatus 4 (A4), from left to right: mouthpiece, in-line filter, elbow connector, pneumotachograph, coupling device. The same lid is used for apparatus 1 and 2, and the same lid for 3 and 4. Despite different connector diameters for the two lids, the inside diameter of the four connections to the coupling device is nearly the same, being represented by the inside diameter (26 mm) of the lid connector in apparatus 1 and 2 and by the inside diameter of the pneumotachograph in apparatus 3 and 4 (25 mm).