[geeks] Watch prescision

[Chris Byrne]

All,

Someone off the list emailed me asking about the precision of waches. I
was going to just write him back personally, but I thought you all might
be interested.

The minimum standard for precision in any modern watch movement is +-5
seconds per day or less (almost 200 seconds per year). This isn't really
very impressive. Even the first precision chronometers (developed around
1800 or so) were able to meet a +-240 seconds per year standard. That is
the precision necessary to determine position within 1 mile using
astrogation. This is also often called the "naval standard". 

Most manufacturers rate every crystal controlled watch they make at +-20
seconds per month (240 seconds per year) because that is the standard
precision of a quartz movement. With a crystal controlled watch its
actually quite easy to achieve astonishing precision if multiple
oscilators and temperature, vibration, and G compensation systems are
used.

The theoretical limit for crystal controlled timers in general is
something like 1000th of a second per century (atomic clocks are about
1000 times more precise than that, and the most precise clocks ever
devised are 1000 times more precise than that as in a billionth of a
second per century. These clocks are quantum based.). The natural
entropy of the universe, cosmic rays, special particles, and local
variations in gravity and space time which may effect the speed of light
are more of a factor on these things than anything else. 

That being said a wristwatch has to exist in a tough environment with
radical G loadings and extreme changes in temperature and vibration.
Under these circumstances the maximum achievable precision is probably
something like 1/1000th of a second per year, but there is no wristwatch
ever devloped that is that precise.

Just about any  of the "very good" crystal controlled watches will be
precise to 60 seconds per year or less, even if they aren't rated at
that. There is a higher standard for special "super precise" or "high
precision" movements of +-10 seconds per year. The highest standard is
for precision crystal controlled chronometers. In order to be certified
as a precision crystal controlled chronometer it must be certified
precise to about 5 seconds a year. The maximum precision available in a
production watch is something like 1 second every two years. There arent
a lot of them on the market, but they are available. 

The best quartz movements offered by Breitling (and others) are rated at
+-10 seconds a year and are substantially more precise than that. I know
personally of watches that hanevt deviated more than one second per year
in the five years their owners have had them. 

Even a lower class of watch can often meet or exceed these "super
precise" standards. I would estimate that my navihawk is precise to
about +-10 seconds per year based on the fact that during the year I
would reset it a total of about 5-10 seconds and assuming that some
error is self cancelling. The watch is only rated at the standard 20
seconds per month.

With mechanical watches precision is far more difficult and more
expensive to achieve, however they can still be amazingly precise. 

Most modern mechanical movements are precise to within about 1-2 seconds
per day at the worst. This is distinctly unimpressive compared to even
cheap quartz watches. But that's nowhere near the whole story. The
highest grade mechanical movements are capable of meeting the 10 seconds
per year mark if they include temperature and position compensation
mechanisms. That's a pretty significant difference. The Chronometer
certification so highly touted by manufactureres is pretty much
meaningless because it was created long before todays materials and
prduction methods made more precise watches economically viable.

There are several brands marketing multi movement systems where two (or
more) extremely precise movements are averaged together, along with
their compensating mechanisms, and they can reach the one second per
year mark. There are also combined electromechanical watches that have
an independent thermally compensated crystal controlled correction
system along with an automatic mechanical movement that are capable of
achieving a theoretical precision the same as the best crystal
controlled movements, which means that in the real world they can
probably match the 1 second every two or so years precision of the best
quartz watches.

The most precise mechanical movements are hybrid turbillons with
multiple compensation and adjustment mechanisms. This is an extremely
rare movement these days, and is generally only offered by custom makers
at extremely high cost. If perfectly executed they are able to sustain a
precision of better than 1 second per year. 

The theoretical limit (based on the number of beats per minute we are
able to achive and the compensating mechanisms we can currently build)
for a purely mechanical watch up to this point is something like 1
second every three years. As far as I know no-one has done so. 

No manufacturer I know of has ever credibly claimed an acuracy greater
than 1/20th of a second per year for any wristwatch movement that I know
of, mechaical, electrical, or otherwise. There have been several
manufacturers over the years who have claimed 1/10th but I don't know if
htose claims have been verified. I do know that the 1 second every two
years for a quartz and one seocnd a year for mechanical have been
verified.

For more information check this page out
http://www.chronocentric.com/watches/accuracy.shtml 

In general the chronocentric page is good, but I disagree with a lot of
what they say. Its really a matter of opinion more than anything else.
For one thing they rate Raymond Weil higher than Baume et Mercier, which
I disagree with. They also insist on using the term accuracy when they
mean precision.

Chris Byrne