A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: How Many Chronometers?
From: Greg R_
Date: 2009 Sep 16, 13:14 -0700
From: Greg R_
Date: 2009 Sep 16, 13:14 -0700
LOL - this is why our spouses/significant others/etc. think we're half-crazy a lot of the time. Or maybe they're on to something... ;-) Thanks for a good read, Gary. :-) -- GregR --- On Wed, 9/16/09, Gary LaPook
wrote: > From: Gary LaPook > Subject: [NavList 9766] Re: How Many Chronometers? > To: email@example.com > Date: Wednesday, September 16, 2009, 10:37 AM > > > > > > > > As usual, George is right again (mostly.)� I will keep > the watches is > the freezer for a while longer but even the� short > time period so far > has shown a considerable change in the rates of the > watches. The > temperature in the freezer has been in the range of 1� > F to - 10�F, say > an average of about -5� about 80�F (45� C) > lower than before. In the > 10.3 hours that they have been in the freezer so far they > have lost > 1.5, 1.5 and 1.0 seconds corresponding to daily rates of > -3.5, -3.5 and > -2.3 seconds per day. Based on their performance in the > past they > should have each gained some small amount but that is lost > in the > imprecision of my reading of the watches compared to the > WWV time > signals, about one half second resolution. George predicted > a change of > rate of 7 or 8 seconds per day with this change in average > temperature > but the observed change in the short period so far is only > about half > of that but this might be masked by the imprecision of the > readings > also. > > > > Apache Runner provided a formula for the change in > frequency of the > watch crystal as: > > > > "Quartz crystals have the great advantage that they > have very little > temperature dependence.�� Typically, they're > fabricated to have a > minimum sensitivity to temperature around 25 degrees C. > > > > As I > recall, the dependence is roughly a quadratic, and goes > like the square > of the difference in temperatures - departures from 25 > degrees C.�� The > coefficient is something like 0.04 ppm/(degrees C)**2 > > > > So, at freezing, one might expect 25 ppm shift, which is 2 > seconds > per day - pretty significant, if I consider that my typical > systematic > drift is 0.1 seconds per day at standard temp's." > > > > Using this formula would also predict a change of rate of 7 > seconds per > day which hasn't happened so for but we will follow it > for a while > longer. Assuming that this formula is approximately > correct, a change > of average temperature of 5�C would predict a change > of rate of .09 > seconds per day and a change of 10�C would cause a > change of .35 > seconds per day. > > > > 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. > > > > And my advice for anyone wanting to use these watches for > celestial > navigation on an expedition across Antarctica is to duct > tape them to > your stomach under all of your clothes which will turn you > into a > temperature stabilizing "oven" for the crystals. > Off course, prior to > your expedition, you must determine their rates by wearing > them taped > to your stomach for some reasonable period of time. > > > > gl > > > > George Huxtable wrote: > > Gary wrote- > > "So I have decided to extend my experiment. I have > just placed all thee > watches in my freezer which is at -7� right now (along > with the recording > thermometer) and will see what the rates are after three > weeks and I will > report back then." > > Let me predict that Gary will then see all three watches > losing about 7 or 8 > seconds a day (if he's talking about temperatures > measured in Fahrenheit > degrees). > > Quartz crystal frequencies do change with temperature, but > not necessarily > in a linear way. By choosing the way that the crystal is > cut, it's possible > to make its resonant frequency change parabolically with > temperature, such > that it's a maximum at a convenient ambient temperature > (such as 25� C) and > falls away either side, at temperatures that are higher or > lower. This means > that it's most constant over the range of ambient > temperatures that a watch > has to live in. (I understand that in some circumstances > crystal oscillators > can be made to give a point-of-inflection rather that a > maximum frequency at > that temperature, which can extend the useful temperature > range somewhat > further.) > > But, as with any such parabolic variaition, once you get > away from the > optimum temperature, the dependence on temperature becomes > more severe. > > In the freezer, Gary will be operating his watches at about > 47�C below their > optimum temperature of 25�C. Similarly, I would expect > that if he operated > them at 47�C, above it, at 72�C, if they will > stand that (he may be > understandably reluctant to try), then I would expect them > to run similarly > slow, 7 or 8 seconds a day. > > Wearing a watch on the wrist well help to keep its > temperature up in the > daytime, but won't help much if it's taken off at > night, in many > environments (such as small craft) that don't expect > central heating. Nor > will "wrapping the watch in blankets"; an > inanimate object will derive > little benefit from such attentions, much less than Gary > or I would. They > will only delay changes in ambient temperature reaching the > watch; but they > will get to it in the end. > > Gary refers to the use of a "crystal oven", to > compensate for changes in > ambient temperature. Indeed, that's a viable > technology, that I was using > for precise time measurement, 40 years ago. The crystal is > put into a little > insulated housing containing a heating element and a > temperature sensor, > with feedback to keep the crystal's temperature > constant. It's done that > way, because it's so much easier to heat things above > ambient temperature > than to cool them below it. An operating temperature is > chosen that's > higher than the environment is ever expected to reach > (40�C, say) and a > crystal is chosen which has its optimum temperature to > correspond. Such an > oscillator has its own "warm-up" period, after > switch-on, until the oven > stabilises. This technique is seldom used for anything > portable, unless > unavoidable, because of the power consumption by the oven. > > George. > > contact George Huxtable, at firstname.lastname@example.org > or at +44 1865 820222 (from UK, 01865 820222) > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. > > > ===================== > > ----- Original Message ----- > From: "Gary LaPook" > To: > Sent: Wednesday, September 16, 2009 1:34 AM > Subject: [NavList 9757] Re: How Many Chronometers? > > > I remember when I first got involved with radios back in > the '60s that I > coveted a high end radio that had an "oven" to > keep the oscillator crystal > at a constant temperature to keep the radio frequency from > drifting as the > crystal changed temperature. I now think, however, that > that was mainly "a > self inflicted wound" due to the tubes (valves) in the > radios having > "heaters" to "boil off" electrons from > the cathodes in order to make the > tubes function which caused the radios to change > temperature a lot and to > run quite hot. The young guys won't remember waiting > for a radio to "warm > up" before it would start working but us old timers > will remember the orange > glow coming out of the back of the radio from the glow of > the "heaters" in > each tube. I clearly remember warming my hands on cold > nights over the hot > radio. > > I now wonder if the much less extreme swings of temperature > that would be > expected in a wrist watch, or by a watch kept in an > insulated box below > decks, would make a large change in the watch crystals' > resonant frequency > affecting their rates in any significant way. > > So I have decided to extend my experiment. I have just > placed all thee > watches in my freezer which is at -7� right now (along > with the recording > thermometer) and will see what the rates are after three > weeks and I will > report back then. > > gl > > --- On Tue, 9/15/09, Werner Luehmann > wrote: > > From: Werner Luehmann > Subject: [NavList 9737] Re: How Many Chronometers? > To: email@example.com > Date: Tuesday, September 15, 2009, 10:21 AM > > > Sorry Gary, wrong conclusion. The problem with quartz > watches (or any quartz > driven oscillator) is their temperature dependance. Only > under a constant > temperature you would get constant "rates". For > example, in high class > radios the quartz is kept at a constant temperature higher > than the ambient > temperature in order to ensure frequency stabilty. In wrist > watches > compensating electronic devices can be used. But this is > expensive and not > found in 17 Dollars pieces, if at all. > So unfortunately this cheap solution doesn't work for > us. > B.T.W.: I have some nice digital (and not too cheap) > stopwatches (made by > the > German manufacturer "Hanhart") that elected to > adjust their rates according > to the year's season ;-) > > Werner > > Am Dienstag, 15. September 2009 11:22:33 schrieb Gary > LaPook: > > > Based on our discussion, I became curious about > the accuracy of digital > watches and their suitability for use as chronometers so I > went to my > local TARGET store and purchased three identical watches > for $17.00 > each, the cheapest that they had. I set them and let them > run for a few > days and, as I expected, they each had different rates. > Based on this I > labeled them "A", "B", and > "C" in the order of their rates starting with > the slowest. I then reset them to UTC at 0121 Z on May 28, > 2009. I > checked them against UTC from WWV eleven days later on June > 8th and > found that they were all running fast by 2, 4 and 7 seconds > respectively > and I worked out their daily rates as .1818, .3636, and > .6363 seconds > per day, respectively. > > On July 11th, 44 days after starting the test, the watches > were fast by > 9, 17 and 28 seconds. Using the rates determined in the > first 11 days > the predicted errors would have been 8, 16 and 28 amounting > to errors in > prediction of 1, 1, and 0 seconds. If using these three > watches for a > chronometer we could average the three errors and end up > with only a .66 > second error in the UTC determined by applying the daily > rates to the > three displayed times after 33 days from the last check > against WWV > which took place on June 8th. > > I determined new rates now based on the longer 44 day > period of .2045, > .3864 and .6363 seconds per day, respectively. > > On September 15th at 0800 Z (per WWV), 110 days after > starting the > test, I took a photo of the watches which I have attached. > The photo > shows the watches fast by 21, 41 and 69 seconds but by > carefully > comparing them individually with the ticks from WWV the > estimated actual > errors are 21.5, 41.8 and 69.0 seconds. Using the 44 day > rates, the > predicted errors are 22.5, 42.5, and 70 seconds giving the > errors in the > predictions of 1.0, 0.7 and 1.0 seconds which, if averaged, > would have > caused a 0.9 second error in the computed UTC after 66 days > from the > last check against WWV on July 11th. > > If, instead, I used the 11 day rates then the predicted > errors would > have been 20.0, 40.0, and 70.0 seconds which would result > in errors of > prediction of -1.5, -1.8, and 1.0 which, if averaged, would > cause and > error in the computed UTC of -0.6 seconds after 99 days > from the last > check against WWV which would have been on June 8th in this > example. > > From this experiment it appears that fifty one dollars > worth of cheap > watches would give you a perfectly adequate chronometer. > > gl > > > > > > > > > > > > > > > > > > > > > > > > > > --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To unsubscribe, email NavListfirstname.lastname@example.org -~----------~----~----~----~------~----~------~--~---