PIDAT Instruction to control concentration in two directions

[TigerLily]

Hello,

I am trying to use a PID loop to correct a concentration reading that can be too high or too low.

I want to add or subtract my MV from my current device set point and that will correct the concentration.

I have never used a PID loop in this way and it is not working like I had hoped, currently I am flipping the PID direction between forward and reverse depending on the direction I need it to go and then subtracting or adding the MV from the current set point.

Has anyone achieved similar PID control and if so could you please offer me some tips on the best way to do this?

Thank you!!