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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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