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    Re: How Many Chronometers?
    From: Gary LaPook
    Date: 2009 Sep 22, 02:39 -0700

    George wrote:
    
    "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 assumed."
    
    I had recognized that was a problem with my experiment. With only two data 
    points you can draw any curve, including a straight line, that you want 
    through those points. But it made sense to me to use the standard formula. In 
    this case, with only two points, it could be a different rate or a different 
    zero point as you point out. But I actually have six data points, two for 
    each watch. I simplified my analysis and combined all three to determine an 
    average rate. The change in rates were -4.6, -3.7 and -3.3 seconds per day. 
    These equate to .026, .021 and .019 ppm the average of which is .022 ppm. I 
    just rounded off the the precision given by the normal value given of .04 
    ppm. It doesn't seem likely that all three crystals would be cut for a 
    different central temperature so is seems more likely to me that they have 
    improved the rates to .02 ppm.
    
    gl
    
    
    
    
    
    
    George Huxtable wrote:
    > 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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