Setting Heights for Apparatus 1 and 2

Paddle and basket height is a critical component for having a properly aligned dissolution apparatus, and small differences can result in very different hydrodynamics. There are many ways to set the height of paddles and baskets, and some options are better than others. When setting height, there are a few things to consider - accuracy, ease of use, and preventing damages to the accessories. Proper height setting can be done quickly, easily, and without potential damages.

For most dissolution methods, the USP height of 25 +/- 2mm should be set to ensure proper alignment of the system. There are some rare occasions where you may have a larger height differences for a larger dosage form such as an antibiotic - but only a handful of methods call for this. Setting the height within 2mm is a small target, and you must have the appropriate tools to have do this.

Above are some of the various height tools that are available. Of these, my preferred height tools are the ones shown in the header photo and in the top right of this photo. The height ball for paddle height setting and the clip on basket gauge. These tools offer the best way to set height in my opinion.

Height balls for paddles are simple to use. They are dropped into each vessel, and the paddle is then lowered onto them and height is set where the paddle rests on the top of the ball. These balls should never be used to set the height of baskets, as the balls will dent the bottom of the basket. Height balls are the most accurate way to set the height, as they will settle in the bottom of the vessel, which isn't always exactly centered. For paddles, these balls offer an easy way to set all heights at one time. In addition, they don't have any chance of damage to the paddle or vessel.

Clip-on basket gauges replace using the basket for height setting, and are the height of the basket plus a 25mm peak. This is my preferred tool because it prevents damage to the baskets during height setting activities, and still allows for good accuracy. Setting height of baskets with some of the other tools can result in potential damages.

Height balls and Clip-on basket gauges both come standard with a new 708-DS Dissolution unit from Agilent, as we find these are the best tools to protect customers' investment and help keep the system working in optimal condition.

The other tools shown in the photograph as options fall into 2 basic categories, a hockey stick tool or a gauge that is clipped to the leg of the dissolution unit (or is built onto the dissolution unit).

The Hockey stick tools aren't ideal for a few reasons. The major issue with these is that they are technique dependent. If the stick isn't properly used and is angled when setting the height, it can result in heights being less than the 25mm setting. The other issue that comes with their use is that you can scratch the bottom of the vessels with them, or even the bottom of the paddle blade (especially with PTFE coated paddles).

The gauges that are clipped to the legs of the dissolution apparatus are a little better than the hockey sticks - they allow for setting all heights at the same time, and don't have as much potential of damage to the paddles and baskets. These gauges can be misused though and put too much pressure between the shaft and the vessel. I have seen several labs where these were used and paddles were forced to the bottom of the vessel and then rotated in this position to set the height. This leads to circular scratches in the vessel bottom about an inch from the center. Baskets can be smashed with this set up as well, so one must be very careful with this.

I recommend using the tools supplied with the instruments, and you can always purchase new ones if you need additional or have dissolution units from other vendors. The height balls for paddles, and clip on basket gauge for baskets offer the best option for best alignment and maintaining your accessories for a long time.