I have built a kiln for fusing glass and using arduino nano and max 6675 to control temperaure, ramp, soak and annealing cycles.
At first i used that cr@ppy cheap k type thermocouple that i got with max 6675, supposedly it measured up to 1200C.
Well, that turned out not to be the case. It stopped working at roughly 700C (temperature says: nan).
Now i got a proper ceramic insulated k type thermocouple but i cant get a reading off of it. It simply says: nan.
I use the exact same code and max 6675 library that used to work with that cheap k thermocouple i got off ebay. Why doesnt it work now with this new, but still K type thermocouple?
Is it possible that max6675 died? I am leaning towards the fact that the max6675 library isnt suited for this thermocouppe, althoug that is weird since it is k type thermocouple after all.
At the bottom i attached a picture of an old, cheap thermocouple that worked, the new ceramic insulated thermocouple and the reading i get on the screen with the new thermocouple.
I tried running the basic max6675 serial example with the new thermocouple but it doesnt work.
Help would be appreciated. I really need to get the kiln up and running asap.
Is the TC the grounded type? Did you try reversing the TC leads? Are you using TC extension wire? Where did you get the TC? Post a link to the seller's webpage.
BTW: What temperature is required to fuse glass?
How is the TC connected to the MAX? Picture looks like you tried to solder it, that won't work well, TC should be connected to MAX using TC extension wire and screw terminals, using copper will introduce errors caused by the TC to copper wire junctions.
JCA34F:
How is the TC connected to the MAX? Picture looks like you tried to solder it, that won't work well, TC should be connected to MAX using TC extension wire and screw terminals, using copper will introduce errors caused by the TC to copper wire junctions.
i did not solder the wires are screwed into place.
unfortunately i have to use copper wire because the leads on the TC are way too short.
i used the old TC this way without any issues.
maybe the heat damages max6675, i dunno, it was sittinh cloe to the kiln, but then again, it is pretty efficient (lots of hollows) and the outside doesnt get real hot.
i am 99% sure it is the max6675 that stopped working
unfortunately i have to use copper wire because the leads on the TC are way too short.
Then your super-sensitive thermometer is actually measuring a complex combination of temperatures from inside the kiln and wherever the joint between the K-type metals and copper metal is.
It is very important to use compatible K-type wire. "Thermocouple extension wire" is not insulated for high temperatures because it's supposed to be outside the kiln. But it's actually the same identical Chromel and Alumel metals inside.
The MAX6675 is useful because it is able to compensate for the "cold junction" that's formed between the K-type metals and the copper circuit board. That's why all the breakout boards have the connectors right there next to the chip - the chip's internal thermometer must measure the temperature of the cold junction. It actually doesn't matter how much copper you use; you could have 100m of wire, but you (or the chip) must know the temperature of the joint between K-type and copper to get any useful numbers out of the thermocouple.
jremington:
That won't work, because it creates several different thermocouples at the junctions.
You must use thermocouple extension wire of the correct type.
For this sort of work you just need to ensure the junctions are basically all at room temperature except the TC itself, you can afford a few degrees of error in 930C. In effect they are all cold junctions. If you want precision
you need a platinum resistance sensor anyway.
Well since i cant get temperature reading at all despite the fact that TC gives off votage i am sure max 6675 failed.
I didnt have issues with copper junction before. However, when copper wires are added, reading tends to oscillate +-2C, which is not the case when not using copper extension wires.
I am going to try to get temperature reading first, and then worry about finding K type compatible extension wires.
