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Tempreture indicator reading unstable with burner running


jsurgeson

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I know this is partly a electronic design issue but I am sure that there are many here that are knowledgeable or have experience with regards digital temp indicators or controllers on forges.

I have built a type k temp indicator for my 2 burner blown forge, here is the thing, when the forges get up to 600+ deg C the reading starts to jump around 600 - 1200 erratically different values.

I thought this was a electronic issue however when I turned the burners and blower off, the reading immediately stabilized and slowly started to drop as it cooled.

I assume from that, that it is not the electronics or the tc.
Also when I tested outside of the forge, using a gas torch to apply heat directly to the tc, it remained stable all the way up to 800 odd degrees, the hottest i could get it with this little torch.

What do you guys think?

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.

I have built a type k temp indicator for my 2 burner blown forge, here is the thing, when the forges get up to 600+ deg C the reading starts to jump around 600 - 1200 erratically different values.



I seem to recall K types suffer from one of the elements going none magnetic around the curie point which is somewhere around the 600 mark and this affects the output voltage. This might be related to what you're seeing. I'm sure type N don't have this issue
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I think the probe has a short in it. The curie point is down around 354C (I looked it up). I also looked up a chart of millivolt outputs, and 600C to 1200C is approximately doubling the output.

Been a while since I played with thermocouples, and I am not an expert.

Phil

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My guess is that whatever the problem is, it's related to the turbulence caused by the burners. (Turbulence that isn't a present out of the forge, in a torch flame.) You might try rigging up some kind of shield to help protect the probe, and see if that makes a difference.

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Kinda of what I thought, just did not think that flame/air movement would cause the tc input to vary that much.
I think I need to rig it so i can put a meter in the box to see if the tc output is actually changing as much as that.

And a shield as you suggest might be a answer, do you know if commercial tc's are shielded from the turbulence or not?

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http://www.coleparmer.com/catalog/product_view.asp?sku=9043386&pfx=

just an SS tube with a plugged end really


Yep. Depending on the temps you'll be running, ceramic might be better than stainless. (Even good stainless will scale away at high forging temps.)
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Makes sense, however priced a 50mm one yesterday and found that they cost more than a complete type k tc with leads and fittings.

Going to have to find another supplier or a cheaper solution.

I am also going to try pulling the tc back into the tube in the wall, and see if thats does the trick.

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Makes sense, however priced a 50mm one yesterday and found that they cost more than a complete type k tc with leads and fittings.


I realize you're in a different country, but keep looking. I have to believe it must be possible to find a small ceramic tube for a reasonable price somewhere in South Africa.
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