Wobble

The USP limit for wobble is ±2mm. If there is visible wobble it can increase the dissolution rate significantly depending on the dosage form being tested. Some tablets, such a Predisone are particulartly sensitive to media flow variations and can be badly effected.

Wobble can be caused by a bent or loose shaft or a misshapen basket. The easiest way to check for wobble is by eye (2mm will be easily seen) but it should be quantified by a suitable tool such as a wobble meter. Wobble MeterTo visually check shaft straightness simply roll the shaft over a bench top — it should roll smoothly along it's entire length, but this is not measurable.

A wobble meter, such as the one shown here can be used on all open design baths (not baths where the vessel plate is closed). A caliper can also be used but calibrated devices sucg as this are very easy to use and extremely accurate.

Measurements should be taken approx 1cm above the paddle blade for each paddle. For basket tests, the measurement should be taken for the basket shaft approx 1cm above the basket, but also on the rim of the basket itself. This means that any bent or mis-shapen baskets or poor fixing clips, are likley to cause a reading outside the desired range. In this case remdial action must be taken which might require replacement of the baskets.

Wobble can be kept to a minimum by handling the baskets and shafts with care — they should be kept in proper holders rather than just collectively in a drawer.