[time-nuts] Chronometer contest sponsored by IEEE Spectrum

Chuck Harris cfharris at erols.com
Sat Dec 1 10:18:43 EST 2007


Twiddling the trimmer on those watches that do have one (really old watches)
is not the right way to do the job.  You will waste an incredible amount of time
moving the trimmer, and waiting only to find that you overshot your mark.  Here
is the correct way to do it:

Before you test, set the watch accurately to your favorite time source, and
wear the watch  normally for one week.  At the end of the week, check the time,
and make note of the offset.  Calculate from the offset in time, the number of
parts per million the watch is in error.

Put the watch in the center of a coil of wire that has a few hundred to a few
thousand turns.  Connect the coil to a reciprocal frequency counter, and count
the crystal's frequency (It is a good idea to add some parallel capacitance
equal to the amount necessary to make your coil resonate at 32KHz.).

Take that measured frequency, and add or subtract the "personal offset" of the
watch, and then using the trimmer adjust so the counter shows the slightly offset
frequency.  Put the watch back together, and wear it for another week.

The thing is watches rates are dependent on how you personally wear them.
Some people's wrists are warmer than others, some people take the watch
off at certain times of day, others don't...  If you wear the watch normally,
and measure your personal offset, you can adjust the crystal to compensate
for that change.  The watch won't be any more accurate at any given instant,
but it will stay spot-on over the course of a week.

-Chuck Harris (amateur watchmaker)

Note:  If you don't have a reciprocal counter, such as a 5370B, you can also use
a good fractional /N synthesizer, such as the 3666[a,b,c] and an oscilloscope.
Put the 3666 on the horizontal axis, and the coil on the vertical axis.  Adjust
the synthesizer until you get a good stable circle.  Add/subtract your personal
offset from the synthesizer's frequency and reset it to the offset frequency.
Then adjust the watch's trimmer for a good stable circle.

Daun Yeagley wrote:
> Yes, they have a trimmer (at least all of mine do). Most backs can be opened by
> either prying, screws, or threaded ring.
> It's very helpful to have good lighting and some magnifiers.  If you've done any
> surface mount work, this would be a piece of cake.
> 
> Daun 



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