Nice.
Constantan with Cu makes a type T thermocouple, of course, so it generates a relatively large potential of about 40uV/K difference near room temperature. Manganin is about 40 times better (but still quite noticable in some situations). IIRC, Phosphor-bronze 510 is about 20 times better (~2uV/K).
Constantan and (especially) Manganin have the advantage of low TCR.
Yeah, that clean-room assembly slows things down.
Most pure metals run about 150,000 k/w per ohm. Constantan is actually worse, only about 102K. Manganin's about the same. But their thermal conductivity is 20 times worse than copper, so they are handy for stuff like this.
Some brasses apparently do better, around 250K, but "brass" is poorly defined.
rBeforeAss...
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Nice. Is the idea that you put the thermal control circuitry right on the cold plate so that you (at least) don't have to worry about it's TC?
George H.
The little board on the cold plate will be the integrating servo amp, so the TC offsets will get tracked out eventually.
Cheers
Phil Hobbs
d r
us
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I like Phosphor bronze (PB) too. (it's non-magnetic to boot.) I can't find constantan in my MWS wire catalog, but Manganin has ~30 times the resistivity of Cu whereas PB is only 5 or 6. (And of course we all know that thermal resistivity follows with electrical... at least apporoximately. (for metals))
George H.
jig:
Styrofoam)
offsets:
loop
It gets rid of drift and thermocouple offsets to leading order, at least if you put the integrating servo on the cold plate.
That's probably overkill for this application, but I had to put it someplace, and that was the best place available.
Cheers
Phil Hobbs
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Copper is actually the champion for electrical vs thermal conductivity, but it's bad enough working with 3 mil wire, and getting down to sizes where copper's heat leak is sufficiently small is a problem.
The big advantage of constantan for this job is that it's solderable.
Cheers
Phil Hobbs
Yes, and too much R and our old friends Johnson and Nyquist may come to visit and make too much noise.
Oh, I was thinking Phil wanted lots of R, it's only a little bit of wire. (I hadn't thought about noise.) I wish there was a copper alloy with more resistivity. With Phosphur bronze I sometimes feel it's a tug-of-war between strength and resistance.
Manganin is solderable too, just not as easily. It's nothing like as bad as nichrome.
In another age, I wound hundreds (maybe a couple thousand in total) custom manganin resistors to calibrate precision instruments (each one different!). Soldered with Kester 44, IIRC. These days, I'd probably try one of the agressive stainless steel fluxes if the wire gave any trouble.
Best regards, Spehro Pefhany
Hmm.. copper-plated 316 SS? No idea if it's actually available, but I've used Cu-plated Fe wire a fair bit.
Best regards, Spehro Pefhany
I've not seen copper plated stainless, copper plated steel wire is used for MIG welding, also for co-ax inners.
rse
"The Journey is the reward"
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Well I do have an old roll of SS coax from lakeshore. The shield is ~5 ohms/foot and the center ~30 ohms per foot. So maybe just some thin stainelss. PITA to solder.
George H.
Grin... I found out about copper plated steel coax the hard way. (A 'screwed up' magnetic field.)
George H.
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