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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: How Many Chronometers?
From: Gary LaPook
Date: 2009 Sep 26, 02:43 -0700
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. > > > > > > --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---