Tolleranza

Rate tolerance is the maximum permissible deviation between the time shown by a movement and actual time, expressed in seconds per day. It is not a defect: it is a specification declared by the manufacturer or verified by a third party. Understanding how to read it changes everything in evaluating a watch.

A standard automatic without certification typically falls between -15 and +25 seconds per day, sometimes worse. The COSC, the independent body certifying Swiss chronometers, imposes a tolerance of -4/+6 seconds per day on movements tested disassembled for 15 days in five positions and at three temperatures. Since 2023 there is also the COSC Excellence Chronometer, more rigorous: -2/+4 seconds, magnetic tests up to 200 gauss, five additional days with a wrist-wearing robot simulating real use. The difference between the two standards is substantial on the bench.

METAS, the Swiss Federal Institute of Metrology, introduced in 2015 with Omega the Master Chronometer: tolerance 0/+5 seconds per day, but tested on the movement already mounted in the case, not on the bare calibre. It includes verification of magnetic resistance certified up to 15,000 gauss and control of stated water resistance. The fact that METAS audits COSC processes as well says something about the level of rigour. Rolex chose an internal path with the Superlative Chronometer: -2/+2 seconds per day, tighter than any COSC standard, verified on the finished movement. The Patek Philippe Seal brings tolerance to -3/+2 on the certified calibre.

There are also asymmetrical tolerances declared by manufacturers without external certification. The Soprod P024 mounted on certain Hanhart is regulated 0/+8: it never loses time, but can gain up to eight seconds. Precise technical choice, not approximation. Miyota on the Caliber 82 works the barrel spring with dimensional tolerance of 0.01 mm in the winding process: upstream mechanical tolerance affects the consistency of energy delivered and thus the stability of rate over time.

Grand Seiko with MEMS technology takes the matter to another level: the escape wheel is shaped with tolerances on the order of a millionth of a gram, making components 5% lighter and reducing escapement energy consumption. Here tolerance is no longer just a rate specification: it is a geometric design parameter. ETA distinguishes its movements in quality grades — Standard, Elaboré, Top Grade, Chronometer — where each level carries increasingly tighter working tolerances, finishes and precision on the same mechanical base.

On the bench, declared tolerance means little without knowing the conditions under which it was measured. A COSC tested in the laboratory on a bare movement does not guarantee the same performance once mounted in a case, worn and aged. METAS has a point here.