The official Swiss chronometer test is an independent certification of the precision of a movement. It was created in 1973.

Two conditions are necessary to be able to deposit a piece at the control. The movement must have an individual number and its dial must be equipped with a second hand. The control being done in a visual way with the help of a camera this hand of the seconds is essential. This is why there is no watch certified by the COSC that does not have a seconds hand.

The control is done during fifteen days, in five different positions and at three different temperatures (8°C, 23°C and 38°C). The chronographs are tested with the chronograph stopped and running. The tolerances for obtaining the certificate vary according to the size of the movement and the type of resonator. The mechanical movements of smaller dimensions have the right to slightly wider criteria.

The automatic movements are controlled without automatic systems, which could distort the control.

The COSC has sometimes been questioned because it does not control nested watches. Since the controls are done at different temperatures, the fact of controlling a naked movement can be considered as an asset. The certification criteria have become a little more complex to understand in recent years on the COSC website but can be summarized as follows:

Mechanical movement of a Ø > 20mm or a surface > 314 mm²: -4 to +6 s/d

Mechanical movement of a Ø ≤ to 20mm or an area ≤ to 314 mm²: -5 to +8 s/d

Quartz movement: +/- 0.07 s/d

The tolerances for mechanical movements may seem large but these are the extreme results allowed per position and not just an average. To obtain the average rate, these figures can be approximately divided by two. For these tolerances, as is generally done in watchmaking, we consider that a quality watch should have a slight advance. This means that the wearer will be earlier than later for appointments.

The COSC communicated its detailed figures by brand until 2015. It now communicates the global figures. The last six annual reports can be downloaded from this link.