[time-nuts] Can a quartz crystal go off by 2% ?

Dr. David Kirkby david.kirkby at onetel.net
Mon Dec 28 23:22:26 UTC 2009


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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