What is the difference between functional test and calibration of an instrument?

Calibration is when you adjust on a test bench the values of the instrument or in the field.

Functional testing is when you check calibration and operation of all the components of an instrument or loop.

Functional testing is verification that a device responds properly to process inputs only.

Calibration is adjusting the transmitter or equipment to KNOWN and CERTIFIED values. The difference is simply no touching to settings in the equipment for testing and making adjustments.