I’ve been playing with home made thermocouples. My best results are made using 1/8’ nickel welding rod, for the core and 1/4" copper tubing for the outside casing.
For insulation I used fiber glass, good to about 900 deg.F or teflon tape good to about 500 deg. F.
Voltage out put is over 25 MV using a propane torch I have not calibrated for temp yet. I am still looking for better insulation to match with the copper tube which melts at 1800 deg. F
Dave, a roll of Scotch/3M 69 woven glass tape might be worth a try. The adhesive breaks down around 400F but the glass should maintain electrical isolation well beyond that.
I have already pre fit my Weatherhead 1/4 inch SS thermocouple at 12 inch long so that it is centered above the grate and in the center of the unit.
Of many times that I witnessed up close and in video, the poker rod is slamming down for the funnel effect. Is there a gentle way for not damaging the probe installed or do I need a sheath to the last inch as protection? I worry about everything on this project, but I am a machinist and can whip one up by the end of next week. What say you?
I’ve got no sheath on mine, and I poke at it all the time. Never had a problem in two years of running.
it is round and pretty small. You’re not too likely to hit it , and if you do the poker rod will probably roll off one side or the other.
Still, I would avoid beating the crap out of it. just to be safe. 
The end broke out of my 12" TC, I don’t know if I hit it with the poker rod or it just broke. Cheap one from ebay.
So replaced with a 6" one…That is all I had. It seems to run about 300* cooler than the long one. Still a good indicator in case it goes into heater mode.
I removed my grate TC. It is great for learning the system, and I kept it on and off for over 2 years. The drawback I saw was a 1/4" TC extending 6" into and 1" above the grate produced a “tee pee” effect, and effectively blocked a 1" wide, by 6" long portion of the grate. Every time I cleaned down to the grate, there it was, a line of fine hard ash under the TC. in a tee pee 1" wide x 6" long.
Carl, would there be any reason that I shouldn’t put my TC just touching the top surface of the grate?
Hi William, My first guess would be the SS grate will pull some heat off the TC. Raising it up an inch will get it “insulated” so to speak, and hot gas will be the main influence. I feel confident now in suggesting a TC in the 8" cross over pipe as a good way to monitor reactor temp, and it is a lot easier to install and maintain. It reacts very quickly, and gives a good indicator of what is going on. When learning to drive, keep it below 700-800F When I had both TC’s in place the crossover read about 400F less than the grate.
Thanks Carl. I haven’t even started with the 75% yet. So, I need all the info I can get. I have 2 TC ports in the crosover AND an aircraft pyrometer with 2 readouts (analog). This gives me an excuse to hook up 2 , for extra safety 
Hi William, Mine is about the same as carls, When the crossover TC was at 800 the TC under the grate would be in the fourteens. The grate TC burned out so I put a 6’’ TC in its place. It goes to the edge of the grate and reads about 300* less. I never see it over 1200s it should last a long time and still give a good reading on the bottom of the gasifier. Four readouts covers about everthing… one in the hopper…one in the cooling rails…one in the crossover…one in the bottom of the gasifier.
Hi there, does anyone know how high can a thermocuple go?
It depends on the type, and how well it is constructed. Approx F temps are: J=1382 K=2282 T=662 E=1652 N=2372 R or S=2642 B=3092
Higher temps = higher $$$ except type K which we use, very common very rea$onable.
That is in *F right? What does the type tell you?
Yes that’s Fahrenheit. The different types are different bi-metal combinations. For example Type K is Chromel Alumel.
A couple more important factors in choosing a thermocouple:
Insulation quality: You can buy a type K thermocouple that is capable of reading up to about 2500 degrees F, but it may only have insulation that is only good for 500 to 800 or 900 degrees F. So you need to make sure that the insulation matches the application. Sometimes those Chinese vendors just plain get it wrong, too.
Sheathing:
Mild steel or stainless steel. Even some stainless steels won’t stand up forever in a “glowing” envirionment, particularly if there’s an oxidizing environment.
So---- if you are looking in places like Ebay, be careful what you buy. Don’t be afraid to ask the vendor a question if the description isn’t clear. Also, don’t be tooooo surprised if the answer you get doesn’t totally answer your question.
Inconel sheathing is more expensive, but makes the best quality thermocouple. I’ve got around 7,000 miles on mine at the grate, full char contact. No signs of failure.
Thanks for the help! What do you conect the thermocuple to? Just a voltmerer or multimeter or do you guys have some special contraption?
You need a display made for thermocouples. I’ll let Carl explain.
Here’s some more thermocouple info:
http://www.spaco.org/Woodgas/thermocouple%20calibration/thermocouple%20calibration%20for%20woodgas%20reactor.htm
