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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: How Many Chronometers?
From: Douglas Denny
Date: 2009 Sep 26, 03:51 -0700
From: Douglas Denny
Date: 2009 Sep 26, 03:51 -0700
Gary,
You are right.
Constancy of temperature is the important thing.
The original Essen quartz ring oscillator at Dollis Hill London was put 20
Metres underground in the London clay to keep at a constant temperature.
My Racal MA259 prescision 5MHz standard has a double-ovened crystal
arrangement, with the crystal and first oven in a silvered glass vacuum Dewar
vessel ('Thermos' flask).
In spite of this, all crystals still have the problem of 'ageing' whereby they
change frequency slowly and unpredicably over time. This is thought to be due
to re-arrangement of the crystline molecular structure over time.
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Modern crystals can be cut to give a turnover point of the parabolic
tempertaure curve at a required temperature within 'normal' limits, though 25
degrees C is normally chosen.
I have included some information from my old IQD Crystals catalogue for watch
crystals which you might find interesting. The standard ones have a Parabolic
Curvature constant of minus 0.035ppm/degree C.
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Ultra high stability crystals are available with special GT cut giving +or-
5ppm stability over -30C to +70 C. and an almost flat temperature
charateristic: but these are not used in watches. I include teh information
sheet on these too.
Note ageing is still around 3ppm per annum with them all.
Douglas Denny.
Chichester. England.
File:
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