From: Dr. David Kirkby <david.kir...@onetel.net>
Subject: [time-nuts] Can a quartz crystal go off by 2% ?
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Date: Monday, 28 December, 2009, 23:22
I'm on the so-called 'Economy 7'
electric in the UK, where I'm supposed to get cheap electric
from 0030 to 0730 - i.e. a 7 hour period when electricity
demand is low. I'm no longer heating by electric, but do run
some computers 24/7. It's not totally clear whether this
saves me money or costs me money, as I pay a higher price
per unit during the expensive period, to compensate for the
fact I get it cheap for 7 hours. But I run some computers
24/7. I guess I should do the maths and work it out. Apart
from some heaters in the garage, which are very rarely used,
I no longer heat with it.
The time when the electric is cheap is set by a clock,
which rotates once/day. It says on it "quartz" somewhere, so
it must be regulated by a crystal and not from the 50 Hz
supply, which would be pretty useless, as the clock would go
wrong if there was ever a power failure. The clock has not
been changed in the 17 years I've lived at my house, though
the meter has on a couple of occasions.
The clock used to keep accurate, but now it looses time
about 30 minutes/day. I wrote a computer program to predict
when the electric is cheap, so we can schedule when things
like the washing machine, dishwasher, Hoover etc are used.
Even cooking to a certain extent, if it's convenient, though
our life does not revolve around the cheap electric.
I'm wondering if this is a mechanical fault in the clock,
or whether the crystal has developed a fault. It's clearly
well outside any tolerance or aging process of any crystal -
even the cheapest ones.
I've not done any very extensive tests, but the error does
not appear to be constant. Hence every month or so I need to
produce a new table, as my predictions get less accurate
with time. Since one can only read the clock to an accuracy
of about 15 minutes, it's not easy to know how far it is
out. Sometimes we hear the contactor go over, as this is
supposed to then power the storage heaters, which we no long
use.
Dave
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