[time-nuts] Oscilloquartz 8601 Rebuild Question

ed palmer ed_palmer at sasktel.net
Sat Jul 30 23:25:23 UTC 2011


On 7/30/2011 2:01 PM, J. Forster wrote:
> I would avoid steel. It usually has higher thermal leakage than glass.
>
True, but at least I wouldn't have to worry about the stupid thing 
breaking again!

The designers were concerned about size so they were limited in the 
thickness of the foam plug that sealed their Dewar.  The thermal leakage 
through the steel plus a better plug looks like it might balance out to 
something near the original.  Also, the steel thermos I have is the best 
physical fit.  The glass ones are either much bigger or smaller than I 
want.  I also like the idea of the electrical shielding that the steel 
thermos gives.
>> I'm rebuilding an Oscilloquartz 8601 oscillator that has a smashed
>> Dewar.  The 8601 is basically the same as an 8600, but with a different
>> connector arrangement.
>>
>> Since I can't replace the original Dewar, I'm trying various vacuum
>> flasks to see which one works best.  The differences are related to the
>> size (how much room for a foam plug) and material (glass vs. steel).  Am
>> I correct in thinking that 'best' means minimizing the current drain?
> Yes, but you maqy not want to cut the leakage too much below what was
> originally there. It could cause loop overshoot or instability.
>
I'm starting to realize that.  While poking my fingers near the cable 
that connects the oven to the controller I saw the oven current bounce 
and ring by many milliamps just because of the capacitance of my 
fingers.  That's what started me thinking about matching the original 
rather than going for lowest current.
>> I'm thinking that minimizing the current drain means minimizing heat
>> loss to the external environment and therefore most stable performance.
> Only if the loop can handle it.
>
>> Or should I try to match the thermal characteristics of the original
>> flask and plug?  I could approximate this by adjusting the thermal
>> characteristics to obtain the same current drain as stated in the
>> specifications.
> That will give you a static match, not necessarily a dynamic match.
> Consider if you filled the Dewar with mercury.
>
Certainly a thermos of mercury would have much different thermal 
behaviour than an oscillator, but I don't have a thermos of mercury.  I 
think what you're getting at is whether my replacement thermal system 
has similar dynamic thermal characteristics to the original.  
Unfortunately, there's no way for me to test that since the original is 
smashed.  Static comparison is all I can do.
>> Of course, if you go too far in reducing the current, the oven could
>> overheat.  The oscillator includes a precision thermistor to monitor the
>> temperature of the oven to prevent this.
>>
>> Any comments?
>>
>> Ed
> Good luck.
>
> -John
>
Thanks.  This is turning into an interesting project.

Ed




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