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I am trying to control the addition of caustic or acid based to a tank based on the pH value of the influent entering the tank. I have tried the PID block in the AB PLC 5 but since it is nonlinear the proportional, integral and derivative terms must have values that change with the measured process value. I am contemplating using a third degree polynominal function to control the addition. Has anyone found an accurate way to control pH in an Allen-Bradley PLC 5?