With more and more complex parts coming out, Testers used to test these parts have to be calibrated periodically against a known standard and preventive mantenance must be done to ensure Quality. Calibration period may be once a month, once in 6 months or may be once a year or so depending upon the manufacturer's specifications. Usually the tester companies provide CAL boards and associated software to the customer ( companies ) to run CAL and checkers/diagnostics on the tester.
When time came for Calibration we had a test station sent to us from the NRC, (National Research Council) in Ottawa, Canada. This units calibration was tracable to the international standard wherever on earth that was. If anyone who was our client, ISO Auditor or similar wanted, he/she could question the reality distorted bureaucratic bubble that is Ottawa. Most clients and auditors choose to be happy with our record keeping rather than risk being lost in that alternate reality. Kidding aside, the calibration of some 200 DMMs, 50 Scopes and the various ATE's on the production floor only took a week of a technician's time( no enginneer required) on a part time basis as production coudln't be halted. The only problems we had were the engineers in the labs when we walked into their lair and said we were taking their test gear. Of course we were in riot gear and had pepper spray.