METAS is not a laboratory of the watch industry: it is the Swiss Federal Institute of Metrology, an independent public body. This distinction matters. When a movement bears the Master Chronometer hallmark, it has passed tests conducted by a third-party state authority, not by an internal department of the manufacturer.
The protocol is structured in two phases. First, the bare caliber obtains COSC certification, which verifies the movement's precision within a tolerance of -4/+6 seconds per day in five positions and at three temperatures. Then the watch is assembled in its final case and subjected to eight METAS tests over ten days. It is this step that changes the nature of the certification: the finished product is certified, not the raw component.
METAS parameters are more stringent on multiple fronts. The rate tolerance narrows to 0/+5 seconds per day — asymmetrical toward losing time, which is the hardest condition to control. Magnetic resistance is raised to 15,000 gauss, a threshold covering exposure to MRI devices in clinics. The declared power reserve is verified on the assembled watch, not calculated at the bench. Water resistance is also rechecked after casing, because the gasket may have undergone stress during assembly.
From my workbench, I often see a source of confusion: numerical tolerances. Some sources cite 0/+5, others 0/+6, still others -2/+4. The explanation is simple. The official Master Chronometer tolerance is 0/+5 seconds per day. The value -2/+4 appears in Tudor's data for some MT5601 calibers and comes from the manufacturer's internal measurements, more restrictive than the METAS standard required. The two numbers do not contradict each other: one is the certified minimum, the other is the performance declared by the maker.
The program was born in 2015 with the Omega Globemaster, the first watch to be released with this certification. Tudor subsequently adopted METAS for most of its in-house calibers, identifying certified movements with the letter U in the caliber code — MT5601-U, MT5602-U, MT5612-U. Not all Tudor watches carry the U: some models remain COSC-certified without the second METAS level, and the difference is indicated in the caliber name.
What I cannot speak with certainty about: whether other brands have adopted or are systematically adopting METAS outside the Omega-Tudor ecosystem; the information I possess is not sufficiently verified to make definitive names.