How Accurate can we expect our Accutrons to be?

The advertizing literature from Bulova emphasized the accuracy of the Accutron watch compared to a traditional mechanical balance wheel watch. They stated that the Accutron was accurate to one minute a month, or 99.9977% accurate, which was considerably better than most mechanical watches of the day. Bulova stated that to achieve that accuracy, the watch may need to be adjusted for the individual user, a service they offered free for new watches. Out of the box, Accutrons are set to run at -2 seconds a day dial-up, as that was found in practise to be the optimum setting for a watch that is worn on the left wrist and dial out, the most common way a watch is worn. That is also how I set Accutrons after I have serviced them, unless otherwise requested.

It is often mentioned that the daily accuracy of the Accutron, by extrapolating the minute/month accuracy, should be +/- 2 seconds a day. Can we expect this to be a valid way to look at it? Well, the answer to that is No! you can't say that, because the advertizing clearly states 1 minute per month, a length of time that takes into consideration the many factors that affect the instantaneous or daily rate of the watch. The main factors that affect rate are Positional error caused by the design of the tuning fork, Isochronism (how the battery voltage affects the rate), Temperature in the range of 0 to 60C (which mostly affects the transistor, less so the fork as it is made from a special temperature stable alloy (NiSpan-C), Altitude and exposure to external magnetic fields. Note that the temperature variation of an Accutron that is on the wrist is quite small, just a few degrees, as the body temperature stabilizes the watch temperature quite well.

More specifically:

• Positional error of 4 seconds per day from tuning fork tines up to tuning fork tines down
• Isochronism of 1.5 sec/day for a battery voltage variation of 10%
• Temperature variation of approximately 0.25 secs/day/Degree C (for later watches (after 1967 approx) that have a silicon transistor)
• Approximately 0.8 secs/day variance per 1000 ft altitude change
• Variation in rate due to mechanical shock over 1000G, dependant on the direction of the shock
• Approximately -10 sec/day due to exposure to a DC magnetic field of 20 oersteds, which reduces back to 0 when the field is removed
• Percentage of time the watch is worn vs. sitting on the nightstand.

Most of the same factors above apply to the Esa "mosaba" movements as they apply to Accutrons. However there is one major difference. The Esa movements do not have the positional error that an Accutron does (you can see my page on the 9162/4 which gives a detailed explanation why). So that means that the monthly accuracy of one of these can be easily much better than an Accutron. At least twice as good and possibly a bit more too. Positional error is probably the main contributing factor to the monthly inaccuracy of an Accutron. In fact, this is the reason why no Accutrons were ever certified as Chronometers, whereas many ESA movement were official chronometers, especially many of the Omega versions of the Esa movement. You will no doubt have seen the dials of many of the Omega versions specifically marked as "chronometers".

I should add one other note here. When servicing these movements, the rate on the bench is adjusted to 0 secs/day, not the -2 secs/day dial-up as we do with Accutrons.

So, you can see why it is not valid to state the accuracy in seconds per day, as there could be many variables the movement is exposed to over a period of one month, the one month period having been found long enough to give a good average of all the factors.