K tej presnosti:
It is important to remember that even when a mechanical watch is allowed to vary +6/-4 seconds per day, that does not mean it will consistently vary by that high an amount each day. Mechanical movements--except the very rare 'turbillon' movements that correct for it--are noticably affected by the gravitational pull of the Earth. It only takes a performance distortion of 1/1000th of a percent for a watch movement to be one second less accurate in a day. This causes the performance of mechanical movements to be somewhat different from day to day when not stored in a fixed position. The good news is that the actual variations of a mechanical watch will often cancel each other out. This means a mechanical watch will tend to be more accurate over a longer period than the single-day COSC measurement may imply.
The day-to-day performance of quartz is much more consistent than mechanical under identical conditions. Quartz performance is affected mainly by temperature changes and weakened batteries. So a quartz watch that you measured to gains 0.5 second yesterday will be consistently increasingly off correct time by about that amount. You can be pretty certain that in 60 days, it will be about 30 seconds off. At the end of a year, it would be likely be over 180 seconds off.
Compare that to a mechanical watch that you measured to gain 2 seconds yesterday. It would seem that our example quartz watch is 4 times more accurate than this. But while the daily measured daily variations seem much higher, they are not likely to be as consistent, so will have a dampening effect. You cannot accurately predict that this mechanical would therefore be off by 120 seconds at the end of the same 60 days. It might be right on time, or it may be 200 seconds off. That broader range of variations allows most mechanical watches to stay closer to correct time than the daily variation rate implies. Over a year, some mechanicals can on average stay closer to correct time without having to be reset than a quartz watch might.
zdroj:
http://chronocentric.com/watches/accuracy.shtml