PID temperature control / HERMS

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Jabo1428

Jabo1428

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I've been building up my brewing kit for a few years and last year decided to try a PID controlled HERMS brewery. After a lot of research, development and making I have a brewery but the PID does not seem to operate as expected. My understanding is that it reads the temperature via the sensor (which was supplied with the PID), turns on the output if the temp is lower than the set temp and turns off the output if the temp is higher than the set temp. The output of the PID turns on/off a SSR which controls heating elements. The problem is, when the set temp is reached the heating elements stay on, therefore not giving auto temp control. I've checked my wiring but can't find a problem. Should I be setting up the PID or doing anything with it that I'm not aware of in order for it to operate as expected?
Any guidance would be appreciated.
Cheers.
 
The PID modulates it's output according to the difference in temp between the set point and the measured value (that's the proportional bit, the integral and differential functions are a bit harder to explain).

What will happen in most setups is you (should) end up with almost zero I and D functions and a large P. This has the downside of resulting in offset where the controller gradually cuts the power as the temp approaches the set point, resulting in it never actually reaching it. Which tends to mean the power will remain 'on', just at a low level. It cuts the power by turning the SSR on/off for a variable amount of time, is the light on the SSR blinking?

Things to check:
1) the PID is definitely an SSR output and is wired up correctly, and the output is set to SSR in the menu. Some can drive anything from an internal relay, to SSR, or 4-24ma.
2) Has the SSR blown, the fotec ones that come with ebay PID's are often fakes (a real 40A SSR isn't 50p, more like £50), they fail 'on'.
3) The switching frequency of the PID might be too high, an SSR on a 50hz supply can't switch quicker than 1/100th of a second, some PID's switch in the khz or mhz range to power motors (unlikely for a temp controller unless it was designed to drive a pump rather than a heating element).
4) Some other setting in the PID.
 
Thanks for the advice. When in use the PID output indicator is on, then flashes as it approaches the set temperature and then goes off when the temperature is reached, as I would expect, suggesting that the PID is ok and it is, as suggested, the SSR that is at fault. I'll replace the SSR with one from somewhere like Rapid and hopefully it will work as expected.

"Everyone needs something to believe in; I believe I'll have another beer"
 

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