On Tue, 18 Aug 2009 22:08:44 -0700, isw wrote:

In article ,
Jeff Liebermann wrote:

On Mon, 17 Aug 2009 07:41:19 +0100, "Dave Plowman (News)"
wrote:

In article ,
David Nebenzahl wrote:
I agree that for most a minute per month is reasonable but I would
expect the same accuracy as my $29.99 Timex wris****ch which is more
like a second a month.

So that kinda begs the question of why computer mfrs. can't (or won't)
include clocks that are at *least* as accurate as a Timex, no? Wouldn't
a computah be a more compelling reason for a more accurate clock? (I
know, $$$ bottom line, right?)

Wonder if it's because a wrist watch is kept at a pretty constant
temperature via the skin?

Do you really expect people to wear a watch when they sleep just to
maintain accuracy? There's quite a difference in temperature between
skin temp (about 37C) and room temperature (about 25C). The same for
a computah. When turned off or in standby, the clock is slightly
above room temperature. When running, it might be as warm as 75C.

Yup, but the long-term average will be pretty good -- gain a little in
the daytime, lose a bit at night (or the other way around; could be
either one depending on how the circuit was set up).

Maybe, if the wearer maintains a regular schedule. That's a fair
assumption, until the wearer changes their usage pattern, such as
going on a ski trip.

Also, please note that the original discussion was over the accuracy
of a computah clock, not a wrist watch. Unless left on continuously,
computers don't maintain a set schedule. Even so, their internal
temperature is affected by the building environment.

Remember the old "Accutron" watches -- the ones with a tuning fork
inside? You could adjust those by deciding which way to lay them on the
table when you went to bed. "12 up" would run at a different rate than
"12 down" because of the effects o gravity on the fork. Also, they ran
noticeably fast on airplane trips, due to thinner air.

http://members.iinet.net.au/~fotoplot/acc.htm

I have a 1965 Accutron 214 Space View wrist watch in poor condition.
The specs offered 1 or 2 seconds per day, but only for the first year.
After about 30 years (the last time it ran) and zero service, my guess
is that it was off about 60 seconds per day. I forgot if it was a
gain or loss. The mercury battery leaked inside and it's
unfortunately not currently running. (Yet another project).

You might also be refering to the problem caused by the original steel
watch hands. When they were near the tuning fork coils, the frequency
would lower slightly. The effect was not very big, but still and
error.

The position problem is also not 12 o'clock up versus down. It's 12
o'clock verus 90 degree rotation which is 3 or 9 o'clock. The problem
stems from the tuning fork being vertical or horizontal. The
recommended solution is to lay the watch flat at night. I don't think
it was ever a major problem, just an interesting curiousity for
accuracy fanatics.

A more interesting problem was mechanical vibrations in the 360Hz
range. (the frequency of the tuning fork). When my watch was working,
it would tend to run quite fast if I was working near big synchronous
or induction motors driven by 60Hz such as in my fathers clothing
factory. It was not unusual to gain about a minute, after spending an
hour pushing cloth through an industrial sewing machine (with my hands
on the table). I suspect (guess) that vibration was also the problem
in airplanes, not thin air.

Temperature is of course a problem:
http://bmumford.com/mset/tech/accutron/index.html

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558