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    Re: How Many Chronometers?
    From: Douglas Denny
    Date: 2009 Sep 26, 03:51 -0700

    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.
    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.
    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.

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