A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: How Many Chronometers?
From: George Huxtable
Date: 2009 Sep 16, 08:37 +0100
From: George Huxtable
Date: 2009 Sep 16, 08:37 +0100
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 -~----------~----~----~----~------~----~------~--~---