Calibration is an action of comparing instrument output with that of produced by referenced standard instrument to determine the level of instrument accuracy.
The indication produced by standard instrument is a true value therefore standard instrument shall have a better performance compared to instrument to be tested.
The calibration results will show how close the measurement output produced by instrument being tested to true value.
Calibration is needed because instrument device may experiencing the following condition:
There might be a disturbance signal which affecting measurement of true value.
Most process control or measurement require accuracy of process variable measurement.