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Context

I'm making an electric furnace for melting metal, and aiming to make my own controller.

Everyone seems to just use thermocouples and a control loop, I thought perhaps I could just measure the resistivity from the heating element. I'm planning on using Kanthal A-1 (rated for 1400°C/2552°F) at temperatures that can reach 1200°C/2192°F. Highest temperature thermocouples I've been able to find were 1250°C/2282°F which is a bit too close for comfort.. a control loop overshoot may kill it and then.. kanthal isn't cheap.


Current Thinking

Now while kanthal has a quite low tempco of ~50ppm/°K, we're talking about a big delta here and precision in the single digits isn't required, so I think it could work. Calibration could be done with the thermocouple.

Since I'm aiming for somewhere around 3-4kw, I was thinking of just rectifying some mains AC, and using 3 ADCs to take the resistivity reading.

Here's the rough idea:
enter image description here
(The switching mechanism for controlling the temperature just isn't drawn, I'm not forgetting it needs to exist).

ADC for this doesn't need to be fast, 100Ksps will give me 1000 readings per every cycle (though I'm thinking 1Msps for switching headroom). And for a 16-bit adc, the 4% change in resistivity over the full temperature swing still gives me a range with 2^16 x 0.04 ≈ 2621 values - about half a degree celsius per increment.
To take the kelvin reading, I'm thinking of using kanthal leads. That way they won't melt. Just tig weld them near the ends of the heating element on the inside of the furnace. The point at which they reach the controller should be ambient temperature, so thermal EMF issues are avoided too.

There are a couple vishay 20ppm/°K 1mΩ shunts for very cheap on mouser, so even though we're talking high currents I think very good precision can be achieved. Could also use multiple, use a heatsink, or even do both.
At 20ppm/°K, in the eggagerated scenario of a 40°C temperature rise we get 20/1,000,000*40 = 0.0008 ≈ 0.1%. That would be 1/40 of that 4% resistivity change.


Wind-Up

I realize this has more possible points of failure than the typical implementation - reading a thermocouple and running a PID loop is tried, tested and rhobust. I'm not asking whether this is the simplest solution to my problem as I know it isn't.
But it does seem the most reasonable for the resources I have at my disposal.

So, provided what I wrote above is right (and please, tell me if it's not), is this sensible? Or is there something I'm missing?

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    \$\begingroup\$ I'm guessing that about 9 of 10 questions that start with "is it wise?" on stack exchange get an answer "no". Furthermore, for this EE site, it's probably more like 95 out of 100 \$\endgroup\$ Jan 10 at 16:42
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    \$\begingroup\$ @JosephDoggie Is that such a bad thing? The way I see it, means people are at least aware of their own limitations. Everything seemed reasonable to me here, but I knew there was likely some factors I couldn't see on my own. It makes for a good question/answer pair. On stackoverflow the 95 out of 100 is "how do I launch a rocket to the moon with no knowledge or effort", I don't think you'd prefer it. \$\endgroup\$
    – TrisT
    Jan 11 at 2:48
  • \$\begingroup\$ my comment was meant to be humorous (-: with estimated percentages, but there's some truth in it. \$\endgroup\$ Jan 11 at 14:03
  • \$\begingroup\$ FYI, Fresh article about a related problem in the range of tens of watts: edn.com/… \$\endgroup\$
    – kimstik
    Feb 15 at 10:36

1 Answer 1

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Measuring the temperature of the heater is not really measuring the temperature of whatever it is you are trying to control. I'm also not sure the Kanthal elements will last all that long if you're operating that close to the maximum but it really depends on the atmosphere they will be exposed to. You also need to find a way to calibrate it (a thermocouple or an optical pyrometer) and 50ppm/°C means you will have trouble with tight control. I think it can be made to work at least for a short time (without my doing a lot of evaluation of your proposed method) but I don't think it will be comparable to a thermocouple controller unless you have a very special need to control the temperature of the heating elements themselves.

A not insignificant problem is that the resistance and the elements themselves will change significantly during use so that your actual controlled temperature will drop continuously as the elements erode away and the resistance increases.

A standard platinum-rhodium thermocouple can easily handle those temperatures (type R or S are most common, and work up to 1450°C). They don't have to be fat like AWG 8 thermocouples, even a fine wire will last a long time, because Pt is very non-reactive ('noble'). The output is quite a bit less than common base-metal thermocouples such as K or N type, but still not a problem for modern electronics.

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    \$\begingroup\$ Damn what an amazing answer. The furnace was gonna be closed which is why controlling the temperature of the element was roughly enough, and I had an idea in mind for getting rid of the oxygen - but the eroding part throws a massive wrench in the works. That is what I was missing. But hey I just learned about the existence of the optical pyrometer, maybe eventually I can make sapphire windows and measure with light. Any advice on where to find these platinum-rhodium thermocouples? Ebay search yields weird ones with a pink 'handle' but can't find much anywhere else, especially in the EU. \$\endgroup\$
    – TrisT
    Jan 10 at 8:06
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    \$\begingroup\$ There are many. For example, this one: thermagmbh.de/en/thermocouples as well as US (?) companies such as Cole-Palmer and Omega. Material suppliers are fairly thick on the ground too, but I suspect you want a complete product. \$\endgroup\$ Jan 10 at 8:48
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    \$\begingroup\$ Should be Cole-Parmer. \$\endgroup\$ Jan 10 at 15:04
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    \$\begingroup\$ Therma doesn't seem to have anything above K/J (perhaps on request), but they apparently only focus on selling to companies either way. Cole-Parmer seems to have some (very expensive) R and S types but they're still only rated for 1150C(2100F). I found omega before you mentioned, so I looked a little harder - availability changes with country but looking at germany, france and spain it seems at least the tungsten-rhenium wire pairs are consistently available at a reasonable price. I'll also check local places but I imagine they'll tell me to take a walk. \$\endgroup\$
    – TrisT
    Jan 11 at 3:47
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    \$\begingroup\$ You may have some luck with pottery supply places. R and S is rare in North America hobby work, because of cost (though long term it is not more expensive) but not so much in other places, as I understand it. \$\endgroup\$ Jan 11 at 3:49

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