[time-nuts] My Favorite Watch
Bob kb8tq
kb8tq at n1k.org
Tue Mar 6 21:20:15 UTC 2018
Hi
Most (as in the center of the distribution) afordable watches are calibrated to run fast.
There is info on this in the archives. A pretty typical target is 10 seconds a month. Yes,
we used that as a target back in the 70’s but it’s still pretty common. You have to get
lucky to find one that is dead on time.
Some math:
2,592,000 seconds in a month. 1 second is about 0.4 ppm.
Your typical watch crystal moves about 20 PPM over 0 to 50 C.
The curve is a parabola with a tolerance on the inflection point.
So 10 seconds a month comes out to around 4 ppm. With the temperature out around
20 ppm, you need a fairly stable / consistent environment to hit even that sort of number.
Yes, you could play compensation games, but doing so only works to the degree that
the crystal matches some ideal curve. ( = you get a pretty limited gain with low cost crystals
and no testing).
Bob
> On Mar 6, 2018, at 3:02 PM, al wolfe <alw.k9si at gmail.com> wrote:
>
> My Timex Indiglo looses about a second a month. I think that's pretty good
> for a <$30 watch I got at Walmart.
>
> Al
>
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