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    Re: How Many Chronometers?
    From: George Huxtable
    Date: 2009 Sep 22, 09:55 +0100

    Gary wrote-
    
    "Back to using cheap quartz watches for chronometers. I have stopped my 
    experiment since there is a very obvious change in their rates at -5� F 
    (-20�C)compared to my room temperature which is very close to the 25�C at 
    which quartz watches are designed to work. After accounting for the fact 
    that all three watches had been gaining every day, the three watches showed 
    a change of rate -4.6, -3.7 and -3.3 seconds per day when operated 45�C 
    colder than the designed temperature. After running this result through the 
    formula for predicting this change in frequency it appears that the constant 
    in that formula, for these watches, should be .02 ppm rather than the 
    commonly quoted .04 ppm. Using this constant, a temperature change of +/- 
    5�C from a cabin temperature of 25�C would only cause a change of rate of 
    .04 seconds per day."
    
    ==========================
    
    Gary has deduced, from the change in rate between two temperatures, that the 
    coefficient of temperature change of the crystals in his watches is more 
    like .02 parts per million per degree-squared that the commonly-quoted value 
    of .04. And he may well be correct. But he may be jumping to conclusions; 
    there are other possibilities.
    
    John Huth ("Apache Runner") had suggested previously that some degree of 
    temperature compensation can be applied to a crystal oscillator to improve 
    its timekeeping. Which is perfectly true, though I suspect unlikely in the 
    cheap watches that Gary is discussing.
    
    Rather more likely, in my view, is that the crystals in Gary's watches may 
    have been cut so that, intentionally or otherwise, the central temperature 
    at which the rate is greatest is at a rather lower temperature than the 25�C 
    that Gary has asssumed. If the rate peaked around 15�C rather than 25�C, 
    then the rate changes Gary observed between his two trial temperatures would 
    fit in reasonably well with that coefficient of .04 parts per million per 
    degree-squared. That could be determined by another rate check made at a 
    more elevated temperature, perhaps 35 or 40�C.
    
    ====================
    
    As a non-navigational side-issue, I told only half the story about using 
    hot-water-bottles in British winters in my childhood.
    
    Rubber hot-water-bottles were, to us, an innovation. They had recently 
    supplanted stoneware jars: half-gallon vessels with screw caps (returnable 
    to the shop for a refund) that were sold with "pop" (fizzy lemonade) or 
    cider, being bomb-proof against internal pressures.
    
    I come from a large family, with never enough rubber bottles to go round on 
    a cold night, so if you were unlucky you got a stone jar instead. They did 
    their job well enough, but the problem arose when the bottle cooled off. It 
    could be pushed to one side, but woe betide the child who let it slip out of 
    bed. The noise as it crashed to the floor could (and did) wake the entire 
    household.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. 
    
    
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