When and how often is dependent on your Quality Control Manual and what has been stated, which should have been made on monitoring the calibration errors over a period of time.
Calibration picks up errors in the weighing caused by use and will be corrected to within Scale tolerance, plus it produces a record for your Quality Assurance.
If the Scales are not working right, they require repairing and then calibrating.
Stamped Scales are those used for a legally controlled purpose such as selling goods in a retail shop, e.g. fruit and vegetables, or paying charges such as postage, i.e. anything that is being charged for by weight.
Sorting Indicators to Printers is a mine field. I would suggest buy a new system programmed to run together with what you want to do, all but the basic will need a programme.
We need to know what Scale before anything can be done.
There is often a label or rating plate on the back of the Scale, with the manufacturer's name and the model name. Additionally there is usually a brand name by the Indicator Screen and capacity.
This information is needed for us to decide if it is viable to send an Engineer out as the cost can very easily be greater than a new replacement Scale.
More than likely, we need to know what they are and the capacity. There will be a charge.
All Scales from MPM are supplied with Calibration Certificate sheets and are calibration stickered.
Are the Scales the same capacity! With the same division size. Does the package weigh the same if you place it on the same Scale several times? If so calibrate, if not repair and then re-calibrate.