Why is calibration necessary?
Every instrument is subject to aging, wear and dirt. In order to ensure the instruments measure values representing the truth, a periodic check is often recommended. For some applications a periodic check is even required due to legislation, norms or directives.
Minor deviations can be caused by aging of mechanical and analog electric components, this is almost unavoidable. If the deviation is more than a few percent, this is usually caused by dirt or wear. In this case full service, repair and new adjustments are recommended for the instrument.