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I just got my new high temperature thermocouple. Any ideas?

 
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Location: Dirtling Farm, Jackson County, Oregon
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I ordered a heavy duty furnace thermocouple to try to stick in my 8" CFB RMH core.

Not entirely sure where to stick it. The probe length is about 6.5" long and can be trimmed. One option I thought of was to screw it into the side of the burn chamber somewhere. The length could put the sensor right in the fire path.  I could put it just about anywhere as my core and riser are 2" thick ceramic fiber.

The problem comes with the connecting wiring. It can only handle 750 degrees and so that limits it pretty much to the bottom of the barrel to be any kind of safe. I'm led to believe temps down there should be in the 300F range.

Perhaps I could stick it in the back wall of the core or riser where the flame splashes against the back wall before travelling up the riser? I'm looking for the hottest possible spot. If a J tube core can make 2300+ temperatures, I'll happily sacrifice the thermocouple to find out. The base of the riser also seems like a likely candidate.

First experiment, I just tested it with my soldering torch with MAP gas (which should be capable of 2000C). I read max of over 2000F and over 1300C, which doesn't make any sense because that's not the same temperature. Or maybe I misread it. My MicroSD card took a crap, so I lost the video. But according to the color chart, I would say the 2000F (1093C) is more or less accurate.

Was glowing bright orange. The sensor itself seems like it was fine. One of the ceramic insulators cracked on reheat. That kind of thermal shock I don't think is possible in a RMH.

Anyway, how do you think I should mount it? Any other ideas? How much should it stick through?


Let's burn this thing up.
 
Rocket Scientist
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Location: latitude 47 N.W. montana zone 6A
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Hi Solomon;
I suggest mid point in the riser.  Your wires should be safe. I predict 1800F in the riser.
Insert the probe to the middle of the gas flow.
 
Solomon Parker
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I found some wire on Amazon that is higher temp resistant.

I like that idea. My riser is made of two 24" pieces, so I could waller out a channel in one or drill a hole and stick it in there.

To make the middle of the gas flow I'll need the full 6" of the probe.

I think a four thermocouple display would be good. One for the core, perhaps the bottom of the barrel, somewhere in the mass, and definitely the exhaust before it goes up the insulated chimney.
 
Solomon Parker
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Location: Dirtling Farm, Jackson County, Oregon
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On second thought, those risers were very expensive. I think I would rather like to put the thermocouple through the ceramic fiber board. That's replaceable and much cheaper. Perhaps right at the back of the burn tunnel right below the riser.

Probably won't be the very hottest point in the system, but those risers are expensive.

I don't know, I'm torn. Probably won't hurt anything to drill a hole in one of them. If I decide to take it out, I can seal it up with high temperature mortar or ceramic caulk or maybe just a piece of kaowool.
 
thomas rubino
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Hi Solomon;
Your riser must be a ceramic fiber sleeve. They very nice & work well, but they are expensive!
Drilling a small hole to insert a temp probe won't hurt the riser at all.
And as you say you can plug that hole easily if you remove the probe.
The hottest point tested is 1/3 the way up  the riser.  It is the best place to see how hot your stove is burning.
For your second stove I recommend a five minute riser using Morgan super wool.
Just as good as a sleeve but much cheaper and extremely easy to make.
20181112_104340.jpg
5 minute riser
5 minute riser
 
Solomon Parker
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I've seen 5 minute risers. They're pretty good, but not what I'm looking for. Price per performance is surely very good. But they're not "just as good," I mean not on the level I'm thinking about this stuff.

For one thing, they're not smooth or consistent on the inside, which means their performance in terms of friction loss is not equivalent. Perhaps if one were oversized a bit to compensate for inefficiencies, they could approach equivalence.

In terms of thermal conductivity, CFB is a tiny fraction as conductive. ~0.2 vs ~2.5 BTU•in./hr•ft²•°F. And since I'm working with a 2" thick riser, vs 1" thick 5-minute-riser, the difference is double. That's a huge difference, a 25X difference in this case. Now whether or not that makes any sort of difference in a RMH remains to be seen. We're just trying to keep super hot gases away from very hot gases. It may not matter that much.

But again, back to dollars per performance, 5-minute-risers are quite good. But they are not the same thing.

CFB risers are expensive. Just FYI for people wondering, $225 for a 2 foot section of 2" thick 8" ID riser.  I would think if someone like me who just can't live with good enough, a better option might be to use 1 or 2 inch thick ceramic fiber board and build a square riser. For friction loss, you can oversize it a little (which Matt Walker's design does to compensate for being square) to maintain effective CSA, but it will be still be as insulative and you'll pay $153 for a full riser rather than $450 ($93 vs $318 for 1").

You know, you've inspired me. I'm building this system such that the risers can be switched out in minutes. And it will have full temperature instrumentation, and one day I hope gas instrumentation as well. So I might as well do some tests and figure out just exactly what the difference is. I have no doubt that all of them will perform acceptably in a well designed system.

I will say in terms of the original goal of creating inexpensive wood heaters with commonly available materials, the 5-minute-riser surely succeeds whereas the full CFB core with ceramic fiber riser sleeve riser does not.
 
Solomon Parker
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Location: Dirtling Farm, Jackson County, Oregon
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Update:

Thermocouple read 2446.7 before it quit working. Temp was rising at 2-3 degrees per half second when it read this.

Seems to have survived no problem.

Until I see other numbers posted, I'm claiming the record.
 
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