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rate change when casing

1K views 12 replies 8 participants last post by  watchabit 
#1 · (Edited)
Since my last inquiry regarding rate drift due to changes in current draw generated very interesting feedback (thank you Rob and Chris!), I decided to put out another Accutron mystery for debate.

When closing the case back on an Accutron the rate looses about 2 seconds/day. So when timing I usually set the rate a little higher to compensate for this. The question is - what causes the observed rate change? My best guess would be that the material of the case has an influence on the electromagnetic system of fork and coils. But then the case back is made from non-magnetic materials on all the Accutrons I know. Who knows more?

A great week-end to all,

Al
 
#2 ·
Hi, I definitely see the same thing, but I haven't quantified it like you have. Also, in one of the tech releases I recall reading, Bulova suggests timing Railroad models with their hands at the bottom of the dial, since their big steel hands affect the rate when near the magnets/coils.

After a service, I only regulate them fully cased, with the back on tight, with their new case back gasket and new battery. After service/reassembly I do briefly check them on the timer to make sure something's not grossly wrong with the rate, but I don't bother with any regulation until the next day, because if I do, tomorrow it will be completely different and the time was wasted.

The best way to regulate is with the owner wearing it, the second best way is to run it 24 hours each test, the third best way is on a timing machine. I get them close with the machine and then confirm with the 24 hour run. You need a very good time standard to check them against.
 
#3 ·
I totally agree with Chris in saying that the timing machine is not the best way to regulate an Accutron. I do the long time test as well to get a good result. Still I would be interested to find out the reason behind the observed rate change when closing the case.
 
#4 ·
Likewise, I always time them with the back in place. Note that the Omega manual mentions the change of rate of 1-2 seconds slow by fitting the back. I suspect the cause to be due to eddy currents induced into the back by the tuning fork magnets, which in turn would create small magnetic fields in the case back that would affect the tuning fork. This would occur no matter what metal the back was made of. This principle of eddy currents is what make induction motors work, another example is the watt-hour meter that measures how much power your house uses.

Regards. Rob
 
#12 ·
My first experience with eddy currents was some decades ago when I was gluing a strong rare earth magnet (1" cube would lift ~20# IIRC) onto an aluminum block for an R&D project. Moving the magnet across the block, it felt like it was suspended in molasses. I found that if I dropped the magnet from an inch above the block, it would gently float down to its surface. I thought that was really neat. :001_smile:
 
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