So, you like mechanical watches? I do. But how accurate do you want them to be? There is always somebody on the internet discussion groups, like Timezone who complains about their watch gaining a couple of seconds a day. Here is a little bit about what to expect from your watches.
First, if to-the-second precision is necessary you should wear a quartz watch. Your typical quartz watch is going to be accurate to within 10 seconds a month. Many of them will do better. My Timex runs within a second a month. “Super Quartz” watches are accurate to within a couple of seconds a year. Beware however, quartz watches are very vulnerable to temperature variations. They run very well when they are against your skin and stay at about body temperature, but if they get hot or cold they quickly degrade. Put a hair dryer to your watch and it will quickly lose several seconds a day, the same is true if you put it in a freezer.
With mechanical watches the variations in timing are much greater. Vintage pin lever watches may be accurate to within a minute a day. Modern Rolexes should be within a couple of seconds a day. A couple of seconds may seem like a lot but with 86,400 seconds in a day one second is an incredibly accurate .001 percent error. This also translates to about a minute a month. If you reset your watch a couple of times a year you will always be within 5 minutes of the correct time.
There are two main considerations regarding a watches accuracy. The first is the average daily rate. The second is the watches delta (or maximum variation between positions.) The second criteria is really the more important. These are measured using a timing machine like the one to the right. A watch with a small variance between positions can easily be regulated to tell accurate time. There are lots of different factors which cause a watch to have different rates in different positions, but the important thing for now is that they do. A good quality watch will have a delta less than 15 seconds across 5 positions at full wind and after 24 hours. The perfect watch obviously would have no variation. In the real world a delta of less than 10 seconds is pretty good. The maximum allowable delta for current production Rolexes is 12 seconds. Any modern ETA movement can also be brought to meet these specifications by a talented watchmaker. The average rate is normally calculated across five or six positions at full wind. It is a rough approximation of how a watch will behave in the real world. The smaller the delta, the more accurate the average rate is. I usually shoot for an average rate of about +2 seconds per day, this often corresponds to a real rate of between +0 and +1 seconds per day, but not always. Rolex USA allows for an average rate between +0 and +5, Rolex Geneva will allow -1. All rates should be verified by actual performance over several days.
There are different agencies which certify watches accuracy. COSC is a Swiss agency which certifies watches as “chronometers.” For a mechanical wristwatch movement to be a “Certified Chronometer” it must undergo a series of tests which measures the watches rates in various positions, and at various temperatures. The end result must be a rate between -2 and =4 seconds per day. These tests are however conducted on movements before they are cased up so they are often re-adjusted after being cased up.
I often see a low end watch with a nice ETA movement in it running at +15 seconds per day new in box. This is simply laziness by the assembler. These watches are capable of better.