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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Casio F-91W watches

Author Topic: Casio F-91W watches
John Pinto

 - posted June 10, 2021 01:06 PM      Profile for John Pinto           Edit/Delete Post 
Does Starpath sell the rated quartz watches separately from the backup kit? Is Starpath still using the Casio F-91W watches?

I picked up one of these Casio watches recently and the rate is terrible compared to the ones shown on David's video on these watches. Instead of the 1 or 2 (or less) seconds error per 10 days this Casio F-91W I picked up recently has an error of 5 seconds per 10 days.

Is Starpath still getting these F-91W watches with decent error rates? I'd rather purchase a rated watch from Starpath if they are sold separately.

From: Orlando
David Burch

 - posted June 10, 2021 01:28 PM      Profile for David Burch           Edit/Delete Post 
The short answer is No, Yes, Yes.

But you bring up several points that deserve more discussion. I will come back to this as soon as I can.

Looking ahead: "5s per 10 days" is not specific enough. We need more decimals. How long was the test, etc.

Back within a couple days at latest.

From: Starpath, Seattle, WA
John Pinto

 - posted June 10, 2021 01:33 PM      Profile for John Pinto           Edit/Delete Post 
David I am just beginning to rate this watch. I usually wait a month or two before setting a specific watch error rate but after just 10 days it is already 5 seconds slow. I was surprised it had that much error already.

I noticed this one is made in Thailand. Maybe their quality control in tuning the quartz crystals is not as good there in Thailand as opposed to those made for Casio in Japan or China?

From: Orlando
David Burch

 - posted June 17, 2021 09:44 PM      Profile for David Burch           Edit/Delete Post 
To follow up. First it is crucial to get more precision on the rate, which takes enough time to actually learn what it is. The videos show how we get this to the tenth of a second; there may be other ways.

Also I must note that 5.0 s per 10 days is actually within the realm of typical quartz watches. Our text quotes 3s per 10 days as a typical good quartz watch (no knowledge of brands in those days), from a study or research we must have done 30 years ago. I would get permission from several large outlets in a shopping mall to set all the ones they had in stock and then i came back and checked them in a couple weeks.

Here are what we have done over the past few years for the remarkable F-91W. The one listed as 0.08 is just nominal. It has in effect no measurable loss in over one year! These are rates in seconds per 10 days.

This has RMS average of 1.28

So it seems if yours is really 5 after a careful study, then it is not within F91 standards we know. But note we have had 4 approaching or over 3.0. we buy them in lots of 3 and cannot really identify one lot better than others.

The most important discovery to me for this process was learning an Excel function =_xlfn.DAYS(A15,A1) which just counts days between A15 and A1 regardless of year or dates.

From: Starpath, Seattle, WA
John Pinto

 - posted June 18, 2021 10:03 AM      Profile for John Pinto           Edit/Delete Post 
Are the ones you have rated lately being made in Thailand? I’m wondering if that is why this F91W I just purchased so far appears to be nowhere near as accurate as those you’ve listed. However I am still in the early stages of rating this watch.

I don’t do the videos like Starpath does as I’m not looking to rate it in just a few weeks. My procedure, after setting it to current NTP time, is to log each day when it switches to the next second of being fast or slow and after 2 months I do my graph in Excel and get the rate. It’s not quick but I’m in no hurry either.

Thanks for the Excel tip! That is VERY handy.

From: Orlando
David Burch

 - posted June 18, 2021 10:25 AM      Profile for David Burch           Edit/Delete Post 
All of our units are from Thailand, including the one with no error over more than a year!

My guess is to get better data before making judgements on that.

From: Starpath, Seattle, WA
David Burch

 - posted June 25, 2021 03:01 PM      Profile for David Burch           Edit/Delete Post 
The last 3 we purchased have these 10-day rates evaluated over 21 days


From: Starpath, Seattle, WA

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