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You don't need a PID controller for temperature control.
The temperature is not a function of the changes in the heat transfer rate, but only of the total amount of heat accumulated so far.
That's why unlike electrical or mechanical systems, heat transfer systems are not affected by the "D" coefficient.

(Professionally speaking,this is a first order system, and not a second order system).

That's why a PI controller is all you need when controlling a thermal system.

Adjustable PID controllers can be set to run in P, PI, or PD modes depending on their settings. Why try to make or find a PI-only controller when PID controllers are commonplace?

Any PID controller may be used as PI, simply by setting D=0.
But, if you build (or program) the controller by yourself, it's easier to make a PI than a PID.
If you buy the controller, temperature controllers are much cheaper, not only because they lack the "D", but because they may be much slower then the ones intended for use with electro-mechanical processes.
Using a PID controller for temperature control, is a major overkill, and not a good engineering practice.

Another good engineering practice is using parts you have around to get the job done...:) Temperature and PID controllers go for about the same prices on eBay, and I prefer more features to fewer, given that I plan to use this to control different systems. Curious why PID controllers are commonly sold for temperature control when you insist that they are overkill?

I agree that it is a good engineering practice, to use parts that you already have.
In a project I managed in the past I had to control 3 motion axis and a heater, so I purchased a 4 axis motion controller, and used 1 channel to control the temperature.

Anyway, I didn't mean to criticize the project.
I noticed the link to the theoretical explanation on PID, and I wanted only to make a point that from the theoretic point of view, heat system do not require full PID controller.

Regarding your question, I don't know much about prices of small PID controllers, as I've experienced only accurate expensive (about 1-2 k$)programmable multi axis controllers, or small self built analog controllers.
I assume that with the prices of electronics compared to all the overhead, the difference between Temp. & motion controllers may be neglectable, as long as the circuitry is in the 10-12 bits area, and expensive noise reduction is not required.

Another issue, is that temperature controllers usually include feature not found in most PID controllers - controlled Hysteresis.
This feature is very important mainly if an electro-mechanical relay is used, but can save switching noise when other switching components are used as well.

Interesting to learn about off-the-shelf controllers-- the box idea was to make a simple "black box" enclosure for what is normally an industrial black box controller built into a larger system. I was careful cutting the square hole-- it's a 1/16 DIN form factor, so it's trivial to swap out the PID for a temperature controller... or for any controller, really! "1/16 DIN controller" yields all sorts of wonderful little black boxes with simple interfaces...:)

I recently launched a product doing exactly that:
http://freshmealssolutions.com/index.php?option=com_frontpage&Itemid=1
Fresh Meals Solutions - Home

That is very cool. I foolishly hadn't connected tight temperature control with areas of cooking outside coffee.

I just setup a PID control loop for espresso machines using an arduino. The espresso machine modding world is all about "PID" control. Coming from there, I never realized that the D was never used. Well, maybe I did, but in a different way... does it relate to the fact that you cant put the brakes on the heater? If a there was a heating element and cooling element, could you take advantage of the D then?

If anyone is curious, the code for my setup is here:
http://www.arduino.cc/playground/Main/BarebonesPIDForEspresso

there is a processing ap that goes with it to to aid tuning.

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