A couple thermocouple questons.

weo

weo

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Hello all. I hope everyone continues to keep themselves and others healthy and safe.

I noticed my TC is starting to wear out, and am planning on ordering a couple to have on back-up.
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1) Is it possible to cut the corroded portion off, and if there's enough left, bend the remaining portion into a U and use? I don't think so, because I think how they work is having 2 different metals that expand at different rates, and I'm guessing the tip is where the 2 metals meet.

2) Does anyone know what the shortest length needed to protect the wire connections from the forge heat? I'm currently using a 12" TC, but with only 2-3 inches in the forge, I have 6-7 inches hanging out the back of my forge, which is mildly inconvenient.
Thank you.
 
This is interesting question .Two different metal are welded on one end that what is TC .If you cut damaged part and weld them together or spot weld or say stainless welding electrode/rod maybe ? It should work, I don’t see why it wouldn’t .

A thermocouple is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature.
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A Thermocouple is a sensor used to measure temperature. Thermocouples consist of two wire legs made from different metals. The wires legs are welded together at one end, creating a junction. This junction is where the temperature is measured.
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Junctions should be made in a reliable manner, but there are many possible approaches to accomplish this. For low temperatures, junctions can be brazed or soldered; however, it may be difficult to find a suitable flux and this may not be suitable at the sensing junction due to the solder's low melting point. Reference and extension junctions are therefore usually made with screw terminal blocks. For high temperatures, the most common approach is the spot weld or crimp ??using a durable material.

One common myth regarding thermocouples is that junctions must be made cleanly without involving a third metal, to avoid unwanted added EMFs. This may result from another common misunderstanding that the voltage is generated at the junction. In fact, the junctions should in principle have uniform internal temperature; therefore, no voltage is generated at the junction. The voltage is generated in the thermal gradient, along the wire.

A thermocouple produces small signals, often microvolts in magnitude. Precise measurements of this signal require an amplifier with low input offset voltage and with care taken to avoid thermal EMFs from self-heating within the voltmeter itself. If the thermocouple wire has a high resistance for some reason (poor contact at junctions, or very thin wires used for fast thermal response), the measuring instrument should have high input impedance to prevent an offset in the measured voltage. A useful feature in thermocouple instrumentation will simultaneously measure resistance and detect faulty connections in the wiring or at thermocouple junctions.
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Thanks Natlek. I wasn't thinking it would be as easy as merely cutting off the end and re-welding the leads together.
If I have to do this, hopefully I'll remember to post my experience.
 
I don’t know ... in principle thermocouples are incredibly simple things (put two metals with different electronegativities in contact). In practice the junction between the two will likely suffer from oxides, corrosion, and other impurities that will affect the voltage difference between the two sides. I would wonder how easy it is to create a good predictable junction in a home shop. If it were me, I would just buy a new one...
 
You can cut the whole end off completely and weld the two wires together. It should be done with a puer weld, like a hydrogen torch. I think they are laser welded in industry. If you have a friend with a laser welder he could do it.

In the long run you will have a TC that you don't know the exact test data on. It is far better to buy new ones in as heavy gauge as you c an get. 8 gauge is what I prefer.
 
I have done this in the past when I was a just using it in my forge. How I did it was used a tig welder and just fused the 2 ends. Cut the bad end off and butt them togather and use the tig and fuse them together. Once it’s fused all the way around just heat that joint and bend it back into a U. I have found that this is rather accurate and saves quite a bit of money in the long run. In my heat treating ovens I ONLY use certified and calibrated TCs from omega that come with there magic sheath. I don’t know what thy but around them but man do thy last for a long time.
 
JTknives said:
I have done this in the past when I was a just using it in my forge. How I did it was used a tig welder and just fused the 2 ends. Cut the bad end off and butt them togather and use the tig and fuse them together. Once it’s fused all the way around just heat that joint and bend it back into a U. I have found that this is rather accurate and saves quite a bit of money in the long run. In my heat treating ovens I ONLY use certified and calibrated TCs from omega that come with there magic sheath. I don’t know what thy but around them but man do thy last for a long time.
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Well , thanks :thumbsup: I owe you a beer ,now i don t need to try myself that :thumbsup: Wait ,we all owe you beer ...............weo ...two beers :D by repairing the TC we will save some money in the future :thumbsup:
 
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Nice thought, Natlek, but unfortunately I don't have access to a TIG welder. But you're right, the cost of a new TC is about the cost of 2 beers....
 
JTknives said:
certified and calibrated TCs from omega
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Which one? I looked at omega.com and they had 23 pages of options, and the only one I saw that I thought you were talking about was over $500.
 
Instead of trying to figure out which options Omega may have, in the long run it would be less of a headache to order one from the heat treat oven manufacturer.
On another note, Type K thermocouples are not the easiest to weld due to the kinds of dissimilar metals. A lot of times if you look at a new Type K thermocouple the weld will look like it is splattered.

Craig
 
C L Wilkins said:
Instead of trying to figure out which options Omega may have, in the long run it would be less of a headache to order one from the heat treat oven manufacturer.
On another note, Type K thermocouples are not the easiest to weld due to the kinds of dissimilar metals. A lot of times if you look at a new Type K thermocouple the weld will look like it is splattered.

Craig
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I’m not sure on this splattered thing but all I can say is tig welding them works good. Would I put it in a critical heat treat oven, no. But for a forge it was perfectly fine. But that’s the point is to fuse 2 different metals together. Hell if you want to get technical thy do t even need to be welded. The 2 alloys just need to touch each other. I have one such TC that’s just the clipped off ends from a 12” tc that I twisted around each other for a few inches and It works perfectly fine. I use thermocouple wire with the ends twisted together to make little short TCs to measure the temp inside my heat treat oven cabinet. Thy are welded In industry because it creates a much More permanent and application inert joint. But like I said down to brass tacks the 2 mettle’s just have to touch.

Ps if we want to take this further I can toss one in my dry well calibrator and compare it to very expensive calibrated omega TCs.
 
What kind/Brand of heat treatment furnace? Something homemade or purchased?

weo said:
Hello all. I hope everyone continues to keep themselves and others healthy and safe.

I noticed my TC is starting to wear out, and am planning on ordering a couple to have on back-up.
View attachment 1464087
1) Is it possible to cut the corroded portion off, and if there's enough left, bend the remaining portion into a U and use? I don't think so, because I think how they work is having 2 different metals that expand at different rates, and I'm guessing the tip is where the 2 metals meet.

2) Does anyone know what the shortest length needed to protect the wire connections from the forge heat? I'm currently using a 12" TC, but with only 2-3 inches in the forge, I have 6-7 inches hanging out the back of my forge, which is mildly inconvenient.
Thank you.
Click to expand...
 
JTknives said:
I’m not sure on this splattered thing but all I can say is tig welding them works good. Would I put it in a critical heat treat oven, no. But for a forge it was perfectly fine. But that’s the point is to fuse 2 different metals together. Hell if you want to get technical thy do t even need to be welded. The 2 alloys just need to touch each other. I have one such TC that’s just the clipped off ends from a 12” tc that I twisted around each other for a few inches and It works perfectly fine. I use thermocouple wire with the ends twisted together to make little short TCs to measure the temp inside my heat treat oven cabinet. Thy are welded In industry because it creates a much More permanent and application inert joint. But like I said down to brass tacks the 2 mettle’s just have to touch.

Ps if we want to take this further I can toss one in my dry well calibrator and compare it to very expensive calibrated omega TCs.
Click to expand...

Jarod
This is what I was alluding to as far as the weld splattering.

Figure-A11-a-Thermocouple-welder-b-Thermocouple-Push-Posts-that-used-to-couple.png


I don't disagree with anything that you have stated. Type K thermocouples can be troublesome at times at the weld, and yes, all that has to be done is to twist the wires together. Just a little insight, I have seen where the weld would fail at higher temps but "heal" themselves when they cool back down. In industry, this could wreak havoc when this happens on a high limit cutout controller that is configured for upscale burn-out and the thermocouple opens up. We didn't tend to see this issue with Type J, T, R, or S thermocouples.
 
DeadboxHero said:
What kind/Brand of heat treatment furnace? Something homemade or purchased?
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Not a HT furnace, but the TC for my PID controlled forge.
JTknives said:
Hell if you want to get technical thy don't even need to be welded....twisted around each other for a few inches and It works perfectly fine.
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Now that you mention it, I have see TCs like this before. Thanks for reminding me.
 
That picture looks like thy where attached with a stud welder of sorts. I have never seen that kind of temp TC befor but it looks handy for directly measuring vessels or pipes.
 
JTknives said:
That picture looks like thy where attached with a stud welder of sorts. I have never seen that kind of temp TC befor but it looks handy for directly measuring vessels or pipes.
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Well , one TC like that welded to some stainless steel inside oven in place where knife will rest ........is excellent idea , don t you think ?
 
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