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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