Thanks Michael and David...
Your apportations and feedback is very important to me, and i want to thank you for your time and will to help me.
About the Michael's answer, indeed I've tried to increase a little the Derivative gain expecting that the loop anticipates the response of the system, but I note that the controller's output has a maximum only until the PV cross the SP with a positive slope. In other words, to make this works, I need to decrease the Proportional gain to guarantee a smooth response and increase the derivative gain to decrease the slope of the CV, but doing this I'm observing that the control action takes a lot of time to be observed in the output of the system.
Describing with a little detail the system I have:
1) A buffer tank with a total capacity of 4,000.00 litres.
2) The injection method is a 2" venturi using a recirculating bomb with a constant speed that takes water from the bottom of the tank, pass it through the venturi and feeding again the water to the tank by a lateral pipe.
3) I'm using an ozone analyzer to measure the concentration of the dissolved ozone into the tank. I'm using the signal of this analyzer as an analog input to my PLC using this information to control the system.
4) I have an additional bomb that drains ozonated water to the process, when the level runs down certain minimum level the system begins to restore the level by opening a valve on the top of the tank wich fills the tank with fresh water.
5) The ozonated water gets into a closed loop into the process, some part of this is rejected to drain and the rest enters again to my tank.
6) The valve that restores the level in my tank only act when the initial level is below the level setpoint and if the drained water flow is excesive and provoke a low level in my tank. Remember that I'm re-circulating water so in practice, the level in the tank is "constant".
About if the system is controllable, well, I knew about some other companies that can achieve control about +/- 10% of deviation of the proposed setpoint usign feedforward loops, one of these is Pacific Ozone with its DirectDrive technology.
I cannot pretend to achieve similar results, I really don't need it, as far as i know this strict control is only for human consumption products, when the setpoint of dissolved ozone rounds the 0.13 PPM where a difference of 10% compromise the quality of the product, in my case the setpoints rounds 2.0 PPM and my only goal is mantain the ozone in a "closer" neighboor near this SP.
I don't know if I can add images to my replies, maybe I can add some drawings or graphs that I've obtained during my tests.
Thanks for your quick answers, I'll be looking forward for your posts.
An apologize for the poor english, isn't in good shape yet...
Regards...