Mr Ron,
I have a question and I hope you have the time to help me finding the answer.
I really understood this thread, and I got the concept that why “Proportional” control only will not eliminate the error.
In your example “the oven”, the temperature won’t just “keep right on climbing” because at some definite point  the amount of heat leaving the oven eventually BALANCES the amount of heat entering the oven ... it EXACTLY balances ... and that’s what causes the steady state condition.
I quote some words from your post, and they are perfectly clear to me.
Now, the problem is that my mind can’t apply this fact on other “Process variables” e.g. level, flow, pressure and so on.
To be more clear: I understand that the oven will continue to loose HIGER amount of heat due to the continuous INCREASE of temperature difference until the system achieves a balance or steady state condition, so PV (i.e. temperature) will stop changing which in turn will cause a constant error, but what would prevent other “PVs” from just climbing and climbing until they reach SP [using “Proportional” only]? Can you give me other example?
And finally, I have to thank you gain for this great post.
