Master Chronometer is not a COSC standard: it's a separate certification issued by METAS (Swiss Federal Institute of Metrology) that tests the finished, assembled watch, not just the raw movement. This distinction is fundamental and often overlooked in sales materials.
The canonical path follows two stages. First the caliber passes COSC certification, with tolerance between -4 and +6 seconds per day. Then the complete watch enters the METAS protocol: eight tests over ten days, which include accuracy in six positions, two temperatures, two power reserve levels, resistance to magnetic fields up to 15,000 gauss, and verification of the declared water resistance. METAS's accepted rate tolerance drops to 0/+5 seconds per day on some calibers, -2/+4 on others: the exact values depend on the certified caliber and should be read in the certificate, not inferred from the designation alone.
The 15,000 gauss threshold deserves a practical note. A magnetic field of that intensity corresponds to roughly double that produced by a standard hospital MRI machine. To achieve this with conventional movements requires components in non-magnetic materials: balance wheels with silicon springs, co-axial escapements or equivalents, bridges and plates in selected alloys. Simply declaring resistance isn't enough: METAS verifies it instrumentally on the closed watch, with the case mounted.
Omega introduced the certification in 2015 with the Globemaster, and remains the manufacturer who uses it most systematically. Tudor applies it to a growing number of in-house calibers, identifiable by the -U suffix in the caliber code (MT5601-U, MT5602-U, MT5652-U). Tudor's internal TRC system replicates part of these tests autonomously in continuous fashion before movements even reach METAS.
A detail rarely explained: Omega developed Dual Metric Technology to extend certification to two-hand calibers, which lack the seconds hand necessary for the traditional photographic method. The technical solution allows measuring rate variation with alternative methods and obtaining the same validation.
From the bench I've seen watches bearing Master Chronometer markings and watches certified only COSC leave with practically identical rate variation after years of wear. Certification guarantees the condition at the moment of testing; oil degradation and wear of escapement surfaces do the rest over time. That said, the METAS protocol remains the most complete available today for a production watch: it certifies the system, not just the heart.