Welded my thermocouple. It works, but want to find a generic replacement

So I got a thermocouple error on my Quadrafire 1200i pellet stove. When I was taking it out of the ceramic tube the twisted end busted off. Probably had enough of 7 years worth of heat cycles. I had nothing to loose so I cut off some of the sheath and twisted a new section. I then tried some solder and my stainless glass soldering iron on it, no dice wouldn't get hot enough. But I decided to measure the mV with a lighter and it got to 10mv with just barely any solder sticking to it. But I didn't trust it, so I hit it with my little mig welder and some steel wire. I don't know how to weld very well so the outcome was not very pretty and it doesn't look like it penatrated one of the wires very much. I decided to hook it back up and well it's working and measures 28mV when the stove is hot. I am not sure how good my weld is going to hold up and not sure how many more trimmings and if it would start changing the mV the shorter it gets. Got me looking and they sure do want a bunch of money for new ones. Anyone know where to get one that is rated the same and is long enough? Just thinking pellet stoves cant be the only thing using themocouples, anyone know another source?

Harvey Schneider said:

By the way, If it is a type K thermocouple, you can get a welded but unterminated (bare wire ends) thermocouple on ebay for about $5

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Thanks, tonight looking I thought I had narrowed it down to a type K looking at the charts. Just couldn't find anything as think, I was guessing 14 gauge going off of memory last night when welding. I'll look on ebay, not as critical tonight as it's still working fine. Last night I was a little flustered, and didn't really expect to see it working again tonight. I guess that weld isn't really going anywhere. I might hit you up for a link if I can't find what it on ebay, I did a quick search and see mostly test probes, nothing as heavy duty or thick of gauge. Thanks for the info though.

Harvey Schneider said:

Welding thermocouples isn't that difficult. The one thing that is important is not to add any other metals to the wires.The typical way to do it is to make one tight twist of the wires to hold them together and then arc against a graphite block with a small depression in it. The wires are connected together and tied to the hot electrode and the graphite block is tied to the return. What you want to aim for is a small bead at the tip with no other parts of the thermocouple wire touching.
this may help:
http://www.instructables.com/id/Making-A-Thermocouple/

A mechanical bond made by just twisting the wires tightly together will work, but it is more prone to fail due to exposure to gases from the fire.

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I used plain steel wire on my mig and it seems to be working fine so far. Only on second night so who knows. I know it would be best your way and that's the right way, but hey in a pinch it seems to work with steel wire on my welder. When looking at the charts tonight, I saw type K had roughly the temp/mv values and it is made of Chromel-Alumel, guessing I was probably lucky to get the steel wire to fuse to it. It sure didn't want to stick, but I've never welded small wire and was half guessing on my welders amperage and wire feed setting.

Oh yeah, thanks for all the replies, didn't expect to get the attention. Much appreciated.

pelletstove93 said:

Having same issue on my 1100i , regular solder doesn't work what was your final fix? Or is it still working? $112.00 is what they want for mine around here. Also I'm curious how your stove acted with it broken just to compare my issues, some say that fixing the thermocouple wire won't fix my stove that I need a control box.

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My mig welded original thermocouple is still going strong, knock on wood. I can't recall the symptoms when I found out it was bad. I do recall after working on it the original twisted end had become so brittle it all broke off. I trimmed it back and re-twisted it a bit and welded the end. I think I was somewhat lucky the weld actually took to the disimilar metals, so it's kind of a barnyard fix. I ended up ordering a cheapo $5 one off of ebay for when this one finally goes. That terminal block that the wires connect to below the logic board are about the cheapest thing I have seen, I remember when I re-attached the wires, the screws didn't go in correctly as they were half stripped from the factory. Can't wait till I have to fix that.

woodmakesheat said:

I was going to post similar. I have a large capacitor that I charge to some relatively low voltage, say 12V (basically you connect a power supply or 12V battery to the +/- terminals of the capacitor for those unfamiliar), then have a wire with an alligator clip connected to one terminal of the capacitor that I use to hold the two thermocouple wires side-by-side (touching each other.) On the other terminal of the capacitor I use a motor brush (easiest way to find a block of graphite with a wire attached to it for connection) and quickly "spark" the ends of the thermocouple wire pair against the graphite block. If all goes well, you'll get a nice ball on the end that looks like it did before it broke. I often have to cut and re-weld until I get a clean ball connection. I then separate the wires so they only touch at the weld.

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That's some pretty cool info, was wondering what made it so easy, I couldn't for the life of me imagine where/how to weld it using a chunk of graphite. Guessing you wouldn't want to pre-twist the ends of the thermocouple wires or it might arc before touching the graphite?