Accuracy of Fine Wristwatches
Dispelling myths and misconceptions about how well a good watch tells time
- An expensive watch is more accurate, right?
- So why would anyone want a less accurate watch?
- So what makes a mechanical watch a "Chronometer" or "Certified Chronometer?"
- Aren't some quartz watches certified as Chronometers, too?
- How is a "Superlative Chronometer" different from a "Certified Chronometer" or "Chronometer?"
- Are quartz watches always more accurate than mechanical ones?
An expensive watch is more accurate, right?
"Excellence is achievable, perfection is much more elusive."
If this is your first time buying an expensive wristwatch, there is one very important
fact you need to know in advance. A $25 Timex or Casio digital watch will keep time just
as well as, and possibly better than, a $20,000 solid gold mechanical Omega, Rolex, or
other very fine watch.
If that last statement surprised you, read the rest of this section carefully.
All watches tend to gain or lose a few seconds over a period of time. These are small mechanical
or electro-mechanical devices that are counting out 86,400 seconds per day. Even if a watch is 99.9%
accurate, it will still be off by a minute and a half in only 24 hours! So even a mediocre wristwatch
has to be well over 99.9% accurate to even begin to be useful on an ongoing basis.
So, what is a reasonable expectation of accuracy from a wristwatch?
|Reasonable Accuracy Expectations
by Type of Watch
|Seconds gain/loss per day
|Vintage mechanical watch
in good repair
|Modern mechanical watch
|Modern mechanical watch
|Modern quartz watch
|Modern quartz watch
chronometer certified (rare)
While some people desire wristwatches with extremely high accuracy
over long periods of time, it is seldom for any reason besides personal satisfaction. The few
professions that depend on precision time synchronization (such as astronomy, global
navigation, train scheduling, and broadcasting) base their operations on high precision time
sources, not consumer wristwatches.
Ultimately, if you are living so close to the edge that having your watch off perfect time
by less than a minute bothers you or otherwise throws your life into disarray, you probably need less
caffeine and a vacation!
So why would anyone want a less accurate watch?
The short answer is that pretty much any modern wristwatch from a reputable brand is more than
accurate enough for normal use. So some people choose to enjoy the esoteric, emotional and
jewelry-value advantages of older mechanical watch technologies over the small accuracy advantages
of quartz watches.
For more detail, see our Buyer's Guide article: Why would I want a mechanical/automatic watch when quartz is more accurate?
So what makes a mechanical watch a "Chronometer" or "Certified Chronometer?"
Fine watchmakers often have their mechanical watch movements individually certified by
the Contrôle Officiel Suisse des Chronomètres. COSC is the official Swiss institute
responsible for certification of wristwatch movements. Only watch movements certified with a
COSC 'bulletin de marche' (certificate of watch performance) are allowed to bear the
internationally protected label "Official Swiss Chronometer" or even use the word "Chronometer"
anywhere on the product, packaging or advertising.
The standard used by COSC is to test the accuracy of a mechanical wristwatch
movement--before it is assembled into a watch--for consistent accuracy under a range
of position and temperatures. COSC actually peforms seven tests as part of the certification.
But the most commonly mentioned is the "mean daily rate" test for which a standard men's
watch size mechanical movement, the watch must maintain an accuracy within -4 to +6 seconds
of variation per day (that's +99.994% accuracy!).
The other six less mentioned measurements are: mean variation in rate, greatest
variation in rate, horizontal and vertical difference, greatest deviation in rates,
rate variation due to temperature and resumption of rate. Overall, these tests
measure not only the overall daily accuracy but also the consistency under various
normal ranges of conditions.
It is also important to note that a "COSC certified chronometer" is not the Holy
Grail of watchmaking. With the high quality of modern manufacturing, this test is nowhere
near as important as it was several decades ago. Most decent modern watches, when
adequately adjusted, should be able to match the performance specified by COSC.
A chronometer certificate is not a guarantee of future accuracy. Watch movements that
have been certified can get out of adjustment and perform poorly. Movements that were
not certified may still exceed the COSC standards--the manufacturer may simply have simply
chosen to bypass the expense of the certification process.
Aren't some quartz watches certified as Chronometers, too?
Yes, but they are very rare. Even a very cheap quartz watch can easily exceed the COSC
requirements for a mechanical chronometer. So COSC had created a separate series of tests
and standards to certify the truly superior quartz movements. They are tested under several
measurements, the most important being a 'mean daily rate' of +/-0.2 seconds per day.
Almost no watch manufacturers bother to have their quartz movements certified by COSC,
though. This is primarily because the general public are seldom willing to pay extra for a
certified accurate quartz watch--they expect quartz watches to be well more accurate than
needed for normal life. The few companies that offer Quartz Chronometers do so mainly
to appeal to niche market of super-high-accuracy wristwatch enthusiasts.
How is a "Superlative Chronometer" different from a "Certified Chronometer" or just "Chronometer?"
The term "Superlative Chronometer" is a trademark of Rolex. The addition of the word
"Superlative" in front of the official designation of Chronometer is merely a Rolex
marketing angle to give a more distinguished sound to the chronometer status of their
products. All watches that have earned the privilege of bearing the official Swiss
designation of "Chronometer" have been held to and met the exact same C.O.S.C. standards.
Any words added before or after the official designation of "Chronometer" are merely
fluff--there are not any different grades or levels of chronometer certification.
Likewise, "Certified Chronometer" also means nothing different than just "Chronometer."
It is a redundant phrase--since Chronometer status is a certification--that is used like
"verde green" or "hot water heater" to ensure that the reader clearly and quickly
understands the point.
Are quartz watches always more accurate than mechanical ones?
Typically they are, but not always. Accuracy and precision are not exactly the same
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.