I used to work for a company that manufactured top of the line power monitors.

It's true that the frequency of the mains power in the U.S. is very
accurate.  I've got the numbers from a recent survey somewhere but I think
it was +/-0.0001 Hz.  It has to accurate so that the power companies can buy
power from each other.  Imagine what would happen if the 59Hz power company
tries to connect to the 61Hz power company.  What is not constant is the
electrical noise present on the mains supply.  Dropouts of a several cycles
may be common in a house (just watch the lights when the washer changes
cycles) causing you to lose time.  Phase disturbances caused by switching
inductive loads (or the power company switching grids) can cause multiple
zero crossings which can cause your time base to appear to speed up.  Light
dimmers and mains based remote controls can cause a lot of noise right at
the zero crossing.  These problems are not trivial and they can cause
repetitive problems that are hard to track down.

Now if I were in sales I'd try to sell you a $10,000 power line monitor!

Check my math but I think 60 +/-0.0001Hz works out to +/-25sec. a month.  Is
that accurate enough?

At 09:10 AM 3/11/97 -1000, you wrote:
>Thanks to all who answered my question.
>
>I think I'll stick with the mains frequency as a time base and use
>some of Eric's ideas for powerfail.
>
>Regards
>Tony
>
>
>Just when I thought I knew it all,
>I learned that I didn't.
>
>
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bryan@wllink.com
Wireless Link Corporation
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