Calibration involves comparing measurements from a standard device of known correctness to measurements from a second device, called the unit under test. Calibrations may be needed for a new instrument, after repairs or modifications, when a specified time or usage period has elapsed, before or after a critical measurement, or after events that could impact accuracy like shocks or adverse conditions. Calibrations also occur when observations seem questionable or readings don't match surrogate instruments, or as required by specifications.

1. CALIBRATION ROOM
Calibrationisa comparison between measurements – one of
known magnitude or correctness made or set with one
device and another measurement made in as similara way as
possible with a second device. The device with the known or
assigned correctness is calledthe standard. The second
device is the unit under test, test instrument, or any of
several other names for the device being calibrated.
Calibrationmay be called for:
 a new instrument
 after an instrument has been repaired or modified
 when a specified time period has elapsed
 when a specified usage (operating hours) has elapsed
 before and/or after a critical measurement
 after an event, for example
 after an instrument has had a shock, vibration,or has
been exposed to an adverse condition which potentially
may have put it out of calibrationor damage it
 sudden changes in weather
 whenever observationsappearquestionableor
instrument indicationsdo not match the output of
surrogate instruments
 as specified by a requirement, e.g., customer
specification,instrument manufacturer
recommendation.