Rolf,
Yes, I work on automated blood testing systems and use balances regularly. I also used to work on them directly in a Precision Measurement Laboratory.

The definition of calibration is to simply compare the ability of a measuring device to a known standard. In this case, some known weights. Click the link in my previous post for options on purchasing some. I bet the basic units you use have little or no adjustments. At the price that is OK- you just need to be ready for the day it goes out of spec or quits.