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Author: Subject: Passivate Copper for High Temperature Resistance
VeritasC&E
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[*] posted on 13-7-2021 at 06:51
Passivate Copper for High Temperature Resistance



Hi everyone!

I'm in the process of making a beaker heater. I'd like it to heat the beaker content as fast as possible with minimal thermal gradient at the heating interface and at minimal price.

My idea was to use NiCr wire of higher diameter and wrap as many rounds as possible with each wire round just a mm or so from the previous one, but I'm running into two problems (1) I expected NiCr wire to be sold for pennies but it's actually quite expensive (2) Using as long as possible of a resistor means I have to compensate by decreasing the the resistance per meter and thus increasing the diameter, NiCr not being the best conductor this means I'd have to use a much bigger diameter than I'd like to.

Thus I was thinking: Copper wire being both widely available and an excellent conductor, it would allow me to have a cheap, long, and thin resistor (exactly what I want). Now of course NiCr has an advantage when it comes to corrosion.

Can I use copper wire instead of NiCr wire? And if yes do I need to passivate it one way or another? Is there a diameter that's just too thin?

It's for a low temperature application (300-350C Max?).
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[*] posted on 13-7-2021 at 10:02


Another Question: The cheapest copper wire is amnufactured for electronics (and electric motors) and comes with a polyurethane or polyester insulation coating.

I'd be very happy to have such a coating but it's only rated to 150C.

What is best to do with it?

Option A: Mount it as such and let it burn around the wire at first use (is the decomposition gases or product going to cause problems / change the conductivity / risk shorting the resistor?)

Option B: Is there any common & safe solvent I can use to dissolve polyurethane and/or polyester?

Option C: Should I just pyrolize the polyurethane/polyester? If so should is it best to do so in an oxodizing / reducing / inert atmosphere?

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[*] posted on 13-7-2021 at 17:25


Because copper has a high conductivity it will need to be much thinner or much longer than the equivalent nichrome. If its thinner its more likely to corrode and break. Longer can just become unmanageable.
For your temperature range you could use stainless steel wire.





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Vomaturge
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[*] posted on 14-7-2021 at 03:58


if I'm reading this table right, copper is roughly 1/60 the resistance of nichrome at room temperature, and has a resistance that goes up with temperature more than nichrome. At a temperature of 500C (likely beyond the safe operating temperature for copper) it's still around 30x more conductive.

The overall surface area will have to be much larger too, because copper can't be run as hot as nichrome.

Also, the exact size, length, temperature rating, for a given output power and voltage will have to be calculated from scratch, plus trial and error, since there are no recommendations or rating for using copper elements, since this isn't normally done.

The finished heating element will be way more expensive, bulky, and hard to assemble than if it were made of nichrome wire.




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[*] posted on 22-7-2021 at 06:55


Quote: Originally posted by Vomaturge  
if I'm reading this table right, copper is roughly 1/60 the resistance of nichrome at room temperature, and has a resistance that goes up with temperature more than nichrome. At a temperature of 500C (likely beyond the safe operating temperature for copper) it's still around 30x more conductive.

The overall surface area will have to be much larger too, because copper can't be run as hot as nichrome.

Also, the exact size, length, temperature rating, for a given output power and voltage will have to be calculated from scratch, plus trial and error, since there are no recommendations or rating for using copper elements, since this isn't normally done.

The finished heating element will be way more expensive, bulky, and hard to assemble than if it were made of nichrome wire.


That's exactly what I'm looking for: Longer, thinner wire for a greater surface area to minimize thermal gradient on the glass and safely heat up a bit faster.
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[*] posted on 22-7-2021 at 09:54


At 500C you can't rely on glass to be an insulator.
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[*] posted on 22-7-2021 at 14:21


Quote: Originally posted by unionised  
At 500C you can't rely on glass to be an insulator.


are we talking thermal insulation here? (I know nothing)
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[*] posted on 23-7-2021 at 09:16


I meant electrical, though I suspect the thermal insulation might suffer a bit too.
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