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What will furnace pid values actually look like?
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coppercone2:
So i have a decent mechanical and thermal design of a furnace ready along with a high power supply that accepts simple analog control.

The idea is to get to like the limits of kanthal a1 temperature in a box thats like 8x8 inches and possibly poorly isolated by design, with optional removable insulation.

The amount of power and heating element is pretty heavy overkill for tge size of the furnace.

Practically do you think i can get away with analog pid control? Will i get ridiculous values for a d term or something. The furnace is like 4kw where a typical one would be 1.5 for performing with poor insulation so it can do cycling. Its ramp would obviously be limited so it does not self destruct.
ajb:
Depends on what your time constants look like.  How much thermal mass vs how much thermal power, rate of heat loss, how much input and sensor lag you have, etc.  Small systems with reasonable power to mass ratios and minimal lag between control input and feedback should be pretty simple.  If you don't need really tight control you don't even need PID.  A simple on/off control could do the job, either hysteretic or constant on-time once the system is characterized.
coppercone2:
so not enough information right now to ball park it with 8x8x8 inch chamber, up to 4000W of heating from coils evenly distributed on the walls and firebrick?

Reason I am asking is because I don't have the capability to finish that job at the moment, mainly due to vacuum system design problems, but I can play around with a PID on a bread board or even make a PCB, but I would like to know if the control problem is unrealistic for analog components.

I don't need numbers, just a sanity test. If it turns out I need some garbage can sized capacitor or w/e for small signals, then I could investigate digital control or using a off the shelf PID (I would like to make everrything myself though).

Say a big load would be 2kg of copper for a vacuum brazing heat exchanger/heat sink job or a couple of hundred grams of ferrite for things like sintering inductors or w/e. I figure copper is the worst case.

Theoretically since the chamber is 8x8x8 or so, you could fit like 18kg of copper in there (5x5x5 inch cube).

I thought since those controllers are all over the place someone might have a feel for what will happen.
floobydust:
How is the thermocouple sampling load temperature verses chamber temperature? It's in a vacuum too?

Do you have proportional control for the 4kW heating element, like an SSR that can do x# powerline cycles?

I would consider a REX-C100 PID controller for under $10 (chinese clone of RCK Instruments Japan).
It may be less time and hassle.
coppercone2:
i have a high power DC supply that will be used. Maybe two so the heaters are not all in series to make it more uniform to maintain a more even temperature. It will segway into building multizone tube furnaces eventually. dont want to deal with weird ass ac shit like triac, ssr

not sure on thermocouple placement thats a good point

wanna do it myself, if analog, if digital then I have some kinda mid-range shit I can mess with but I don't want to use someone else's thing for this job.

I would say I am not impressed with the quality of (electronic) stuff from Omega (at ALL, but I have no issues that I know of from their sensors) and I defiantly won't  be using some 10$ Chinese shit. For simple system I would go with a PIC that just has a setpoint based on a 10 turn pot value so you don't need to mess with interface controls etc. And it would probably be more reliable then omega lol. But I would like to avoid any digital code.

For two thermomemters and a PID I thought that maybe one thermometer can dampen the pid response based on gradient but it seems complicated to put a buncha variable resistors or whatever to some how slave it.

Whats the algorithm/relationship between having two therometers in a kiln?
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