pH control with solenoid valve

[KHartley]

Juventom,

PID control theory can be quite complicated. We do have several resources and examples that may help you with your project. 

Build a PID controller with your computer video

http://zone.ni.com/wv/app/doc/p/id/wv-1945

PID Control Theory

http://zone.ni.com/devzone/cda/tut/p/id/3782

Getting Started PID and Compact RIO

http://zone.ni.com/devzone/cda/tut/p/id/11197

The final example has to do with a compact rio but you could swap out the io variables for your daq task reads and writes and use it the same way. Please read through these and let me know if these are helpful to you.