Centering of Stirring Element
USP Toolkit: ≤2.0 mm, not more than 2 cm below the flange of the vessel
ASTM: ≤1.0 mm, at 2 mm and 60 mm above basket or at 2 mm and 80 mm above paddle
Centering measures whether the paddle or basket is exactly in the centre of the vessel. If they are not then irregular media flow patterns can result.
A variety of tools exist to perform this test, but the best fit on the end of the rotating shaft, or replace the shaft altogether.
The shaft should not be off-centre by more than 2mm total, and the location of the test in the vessel is important. Too close to the vessel flange and errors around the flange can contribute to the readings, and so the best place is around 2cm inside the vessel where only the wall can contribute. (80mm above the paddle position also for ASTM).
The centering test relies on the fact that the vessels are good, and so only fully dimensionally certified vessels should be used.