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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  george@hux.me.uk
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: navlist@fer3.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
>
>







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