Apparatus 2, commonly known as the paddle method, has similar physical specifications to those of Apparatus 1 with a paddle substituted for the rotating basket.
Some of the dimensions and tolerances for the paddle are critical if consistent results are to be obtained from vessel to vessel.
The USP specifies that the paddle must rotate smoothly without significant wobble. The arc of the paddle blade creates considerable flow and any wobble will have an exaggerated effect on the flow profile.
The contours of the paddle blade must not include any sharp edges - at the tips for instance - that could produce turbulent instead of laminar flow patterns.