One of the most interesting things about covering one subject for years and years and years (aside from wondering increasingly if writing about wristwatches all day really counts as a suitable job for a responsible adult) is that you start to notice how many gaps there still are in your own knowledge and how little you’ve actually fact-checked things you’ve repeated as conventional wisdom for all those years. Not long ago, I got to talking with someone who in the line of duty hears, much more often than I do, from customers who sometimes feel their expectations for accuracy from their watches are not being met; this person asked me what I thought a reasonable expectation was.
There is not, I think, a single answer to this question; it really does depend on the watch and it also depends on the individual owner. In many cases, whether your expectations are grounded in reality can simply be checked against the manufacturer’s stated spec for the watch in question. Rolex, Grand Seiko, and many other brands routinely publicize their accuracy and precision requirements; if you buy a Rolex today, you should expect it to be accurate to +2/-2 seconds per day, at most, full stop. Grand Seiko Hi-Beat movements are spec’d to +5/-3 seconds per day, and that’s what you should expect.
In both cases you may get better performance; it’s been my experience anecdotally that Grand Seiko routinely underreports the accuracy of their watches but I certainly don’t have a statistically significant sample. A friend who bought a Grand Seiko GMT, non-high beat, a month ago says he hasn’t noticed it being even a minute off; a minute per month maximum gain or loss was once advertised as a major achievement in the 1960s by Girard-Perregaux, for their Chronometer HF (high frequency) watches. A watch with a chronometer certification from the COSC should keep time to COSC specs, obviously; the COSC spec is among other things, +6/-4 seconds per day.
Beyond manufacturers who publish their internal accuracy specs, and those who have their watches chronometer certified, things can get a bit harder to pin down. However, you can get an idea from the specs published by ETA or Sellita for their different grades of movements. Sellita, for instance, for its SW 200-1, has four grades: standard, special (elaboré), premium (top) and chronometer. In the standard grade which is adjusted to two positions, a maximum deviation in rate between positions of 30 sec/day is in spec, with a daily accuracy of ±12 seconds per day. In the top grade the expected maximum variation in rate between positions is 15 seconds per day and the accuracy is ±4 seconds per day (the movement is adjusted to five positions). And of course for both Sellita and ETA, the chronometer grade movements have to fit the criteria given by the COSC.
There are some construction differences between grades, mostly in the escapement and in the antishock system. One essential difference between more and less precise watches is in the balance spring; these are also made in different grades and to produce them in quantity, to the degree of precision necessary for a chronometer grade watch, with any consistency, is something only a few manufacturers can do (the biggest by far being Nivarox-FAR, which is owned by Swatch Group).
I asked one of New York’s most respected watchmakers, Alkis Kotsopoulos, of Swiss Watch Repair Co., whether there is anything to the idea that a new watch needs to settle down on its rate for a few weeks before you can evaluate its accuracy. I mentioned this is something I’ve heard again and again, and said again and again but I didn’t really know if it was true and he laughed. “I’ve said it a lot also,” he said, “but I don’t think it’s really true.” He went on to say that if he has a customer who really wants the best possible accuracy, he asks about wearing habits, including on which wrist the client wears the watch, how it’s stored at night, activity level, and so on.
The takeaway for me from all this is that with a modern movement, made with modern materials and methods, you can get chronometer (in the COSC sense) accuracy or better but it may involve going to a watchmaker and having the watch adjusted and regulated, especially with a view to evaluating your wearing habits. Out of the box, chances are you will get whatever the movement spec and/or manufacturer spec is for that watch, so if you are concerned about accuracy and precision, it makes sense to do your homework.
Remember, consistent performance in mass produced movements doesn’t require just precision manufacturing, it requires best practices quality control, which is part of what you pay for when you get a precision watch. And of course, for a vintage watch with an unknown service history, there simply are no realistic expectations of precision until you get the watch serviced. You might get lucky but you might not, and I’ve always felt that running a watch on a daily basis on dry lubricants is probably not such a hot idea.