Okay good, you can keep the 12% adjustment as-is. The purpose of this tool is to eliminate any discrepancies in the water sample tube volume and pump head calibration. As long as the results are consistent, that is what matters most. The end result can always be adjusted. If you were seeing wildly fluctuating numbers from test-to-test, that would be an issue in itself, but you're not seeing that so the KHD is working within the specified tolerances.

You can add a very small amount of vaseline to the 3 white prongs which hold the 3 rollers in-place. This will make the heads quieter, however it does not mean one should go longer between cleaning intervals. Depending on how often you're testing, once every couple months may be enough for you.

Regarding the air bubble, if the tubing is not pushed up enough on the tubing adapters is can allow air to make its way inside the tube. Make sure you have it pushed partly over the second step of the adapter.