What is calibration?
calibration is a comparison of two measurement devices against each other and the documentation of the comparison. The device to be calibrated is compared against a more accurate, traceable reference standard, often referred to as a calibrator.
It is pretty obvious that the fundamental reason for calibrating any process instrument is to test and assure that it is measuring correctly.
It is a fact that all measurement instruments drift over time.
Some instruments drift more, some drift less. The most common procedure is to check instruments periodically to see if they have drifted and make adjustments as necessary to bring them into the required accuracy.