You are making life tougher than it needs to be. No PID or control is required....
Tom is exactly right, of course; my suggestion over-complicates the issue.
OT (but related, it's been that kind of Friday ...):
When testing large steam turbines (half a gigawatt and up, IIRC), my Dad (metioned twice on one day, and still another half a dozen threads to go, how about that?) said GE would run what they call a "valves wide open" test - isolate the process (turbine), open the inlets steam, and measure everything ASAP after commissioning to get a baseline (also to prove the as-built turbine meets the contract heat-rate, Btu/kWh, to ensure the final payment from the customer is made). Basically they treat the turbine as a flow element, analogous to an orifice meter or other pressure-drop flow measurement element.
Later, when the same valves-wide-open test is run and
if the results are changed from the baseline,
then something in the process changed (erosion, scale built up on the buckets, whatever).
With enough measurement data they could actually pinpoint where the changes occurred, at which point you can plan activities for the next maintenance an actually estimate an ROI. They could measure turbine performance to an absolute accuracy of around a quarter of a percent, so this metric was fairly useful.