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    Re: How Many Chronometers?
    From: Gary LaPook
    Date: 2009 Sep 26, 02:43 -0700

    George stated that a thermos bottle would not protect a watch from long 
    term temperature changes so I decided to do an experiment to see. I put 
    one liter of water at 23.4�C (actually it was at 74.1�F but I converted 
    all readings because the math is easier to do with centigrade, oops, 
    Celsius degrees) in my thermos bottle and placed it in my freezer which 
    maintained a temperature of -20.5�C. I then recorded the temperature of 
    the water in the bottle every hour. For the first six hours the average 
    temperature change per hour was .481�C per hour. This means that 481 
    calories were sneaking into the bottle every hour, a rate of .134 cal 
    per second. Since there are 4.2 J per calorie this means that .56 J per 
    second were getting into the bottle which is the same as .56 watts. The 
    average temperature difference was 41�C.
    
    Forty eight hour later the water was down to 3.5�C and the temperature 
    difference was only 24�C. Since my thermos bottle had to obey the laws 
    of physics, the rate of temperature change was now down to only .286�C 
    per hour, a rate of 286 cal/hr; .079 cal/sec; or .33W. Since the rate of 
    heat flow is proportional to the temperature difference it works out to 
    be about 12 calories per hour per degree Celsius; .014 W /�C; or .3 
    degrees per day per degree Celsius with one liter of water in the bottle 
    along with the watches.
    
    Now applying this information to the example I gave of a 24 day cruise 
    from New Zealand to Tahiti as the ambient temperature changed from 16� C 
    to 28�C (#9894), keeping the watches in the thermos with a liter of 
    water, the watches would end up at 26.7� C instead of 28�C for an 
    unprotected watch so George appears to be right, the thermos will not 
    make a significant difference in the final temperature of the watches. 
    When the ambient temperature change is slow the temperature inside the 
    thermos has a chance to "catch up" with the ambient. But if you made an 
    eight day voyage in which the ambient changed 5�C per day then the 
    thermos would restrict the temperature change to about 2/3 of the change 
    of the ambient.
    
    But I was right in that the thermos will restrict changes due to diurnal 
    temperature swings. Looking at at example in which the the temperature 
    climbs 12�C in 6 hours, stays constant for 6 hours, goes down 12�C in 6 
    hours and then stays constant for the last 6 hours of a daily cycle the 
    temperature inside the thermos varies only .7�C instead of the 12�C 
    temperature swing.
    
    For someone who want to make an "oven" to maintain the watches at a 
    constant temperature, putting the watches along with the oven circuit in 
    a thermos will limit the power needed to run the oven to about one half 
    of a watt requiring only 41 ma current from the battery, only one 
    amp-hour per day from the ship's battery.
    
    gl
    
    
    
    George Huxtable wrote:
    > In [9766], Gary recorded a disagreement with my suggestion that one can do 
    > little to protect a sensitive object, such as a crystal oscillator, from 
    > environmental changes in ambient temperature, by such means as wrapping it 
    > in blankets. I claimed that this could have only a short-term effect, and 
    > that the ambient changes would always get through in the end.
    >
    > He wrote-
    >
    > "So I disagree with George to the extent that if the watches are kept in
    > an insulated box, to limit the effect of diurnal changes in cabin
    > temperatures, then the change in rate will only happen based on long
    > term changes in ambient temperature, say on a cruise from the Caribbean
    > to England. But, if the cabin is kept in a range of temperatures which
    > are habitable for humans then the change of rates can be kept to a small
    > number."
    >
    > I recall that as a child, British winters, in a house with only local 
    > heating, were made tolerable at night only by using a hot-water-bottle (such 
    > aids may be unknown and unnecessary in the USA). And I recall how rapidly 
    > such a bottle would lose its heat, though wrapped in blankets with me, so 
    > that before very long it was a pleasure to kick it out. I also recall the 
    > use of a vacuum "Thermos" flask to keep cups of tea hot, and their failure 
    > to do so for more than a very few hours. This in spite of the fact that some 
    > pints of water were involved, a substance which has the highest 
    > specific-heat of any, which means that it holds more heat, and keeps its 
    > temperature, longer than anything else. Gary's crystal oscillator in a 
    > blanket, without such heat-ballast, would not keep the outside temperature 
    > at bay for long, but with a bottle of water to keep it company, he might 
    > increase its thermal time-constant to a few hours; not for days, however.
    >
    > As for his "range of temperatures which are habitable for humans", there 
    > might well be different views on that from different societies in different 
    > eras. A week ago, I was looking over HMS Cavalier, a World war 2 destroyer 
    > built in 1944; the sort of ship that escorted Arctic convoys to Murmansk and 
    > Archangel in Northern Russia, Summer and Winter. She was refitted in 1957 
    > and retired in 1974. Yet still, she had a completely open bridge, without 
    > windows, and only a dodger to keep the weather off. And I was rather 
    > surprised to find, tucked away on that open bridge, sheltered from rain but 
    > from nothing else, electronic gear such as a radar display.
    >
    > How, I wonder, would modern American bridge-teams, sheltered from the world 
    > in their air-conditioned cocoons, without even an open bridge-wing to walk 
    > out on to, have fared under such conditions? Or in the environment accepted 
    > by some present-day small-craft sailors, the ones that venture away from 
    > tropical trade-wind sailing?
    >
    > The "bridge" got its name from an open lattice structure, that bridged the 
    > gap between the top of the paddle-covers, to provide an all-round view. From 
    > such a perch, steamers in their thousands were conducted across the North 
    > Atlantic, the year round.. In thick weather, a lookout was expected to be 
    > stationed at the fo'c'sle head; no shelter provided. These were considered 
    > to be habitable conditions then, but I doubt whether Gary would describe 
    > them so now.
    >
    > Polar explorers, by the way, would carry sledge chronometers: pocket 
    > watches, slung round their necks inside their furs, essential for guiding 
    > them back to base along the correct longitude. When Shackleton's Endurance 
    > went down in the Antartic ice in 1915, the ship's box-chronometers were 
    > abandoned with the ship, and from then on the expedition relied on four such 
    > pocket-chronometers. Only one of those remained in working order throughout 
    > that remarkable journey from Elephant Island to South Georgia, being 
    > essential for Worsley's feat of small-craft navigation.
    >
    > Together with Brad, I'm intrigued by the type of yoga that Gary adopts to 
    > enable him to read a chronometer that's duct-taped to his belly.
    >
    > 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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