First, know what's normal. A certified chronometer is tested to stay within −4 and +6 seconds a day, and other movements have wider tolerances: Seiko's 4R36, for example, is specified at −35 to +45 seconds a day. A watch that stays within its tolerance is a healthy watch. A change in how it runs is what deserves a closer look.
1. Magnetism
The most common cause of a watch that suddenly gains a lot, often tens of seconds or even minutes a day. Magnetized coils of the hairspring cling to each other, so the balance swings faster. Speakers, laptop and tablet covers, magnetic bag clasps and phone accessories can all do it. A watchmaker can demagnetize a watch in minutes, and movements built to resist magnetism are far less affected.
2. Position
A watch runs at slightly different rates dial up, dial down and crown in each direction. If it lies the same way every night, that position's rate adds up. Some owners use this: if a watch runs fast, they rest it overnight in the position where it runs slowest. A six-position test shows which is which.
3. Low power
Near the end of its power reserve, a movement's amplitude falls and its rate changes. A watch worn only briefly, or left to run down every night, keeps less regular time than one that stays well wound.
4. Shocks and temperature
A hard knock can disturb the balance or the hairspring and change the rate or the beat error. Modern hairspring alloys keep the effect of temperature small, but a watch can still run a little differently in a hot car than on a cold wrist.
5. A movement due for service
Over the years oils age, amplitude falls and the rate becomes less stable. A slow decline over months, rather than a sudden change, points to a service.
6. Regulation
Some watches simply run a little fast or slow within their tolerance from the start. A watchmaker can regulate the movement to bring its rate closer to zero.
Finding out which
- Measure the daily rate over a few days with Time Check, noting how you wore or rested the watch.
- If it suddenly gains a lot, suspect magnetism first.
- With a timegrapher, check the amplitude fully wound. Low amplitude points to power or a service; healthy amplitude with a large rate error points to magnetism or regulation.
- Run a six-position test to see whether one position is far from the others.