From your model description, the valves for the outflow of both tanks are controlled with hysteresis. The outlet valve should be opened as soon as the maximum level in a tank is reached and only close again when the level has fallen to the minimum.

You have used an if-clause for the controller, but that doesn't work. Starting at time t=0 with an empty tank 1 with a continuous inflow and closed outlet valve, the water level raises until the if expression h1>=max1 becomes true for the first time and the outlet valve will be opened. As the if-clause will be evaluated whenever any of the conditional expressions changes, it will switch very fast between the 'if h1>=max1'-branch and the else-branch and cause the chattering. The equation 'o1=o1' in the else-branch might also cause problems.

To implement the hysteresis, the last state of the valve must be taken into account. For this, a when statement is needed, that only activates equations when the defined condition becomes true. For your model a solution could be:

model TankSystem 

    parameter Real s=1.0; 

    Real h1(start=0.0); 

    Real o1; 

    parameter Real of1=2.0; 

    parameter Real min1=20.0; 

    parameter Real max1=80.0; 

    Real h2(start=0.0); 

    Real o2; 

    parameter Real of2=3.0; 

    parameter Real min2=20.0; 

    parameter Real max2=80.0;

                

equation

    der(h1) = s - o1;

    der(h2) = o1 - o2;

    

    // control valve 1 for outflow of tank 1 with hysteresis

    when h1>max1 then

      o1 = of1;

    elsewhen h1<min1 then

      o1 = 0;      

    end when;

       

    // control valve 2 for outflow of tank 2 with hysteresis

    when h2>max2 then

      o2 = of2;

    elsewhen h2<min2 then

      o2 = 0;     

    end when;

      

end TankSystem;

Well the result looks pretty much what I needed, thanks.
If I understand your words, the changes made by the "when" statement are preserved, whereas those made by the "if" are not meant to last.

hi, am facing the same error in openModelica and am usinf when and not if,can some one help me

Follow the guidelines given above. The error message tells you that your system is switching very fast between two states. This forces the integrator (dassl, etc) to restart frequently, killing performance.

Using some hysteresis is probably best. And sometimes noEvent() can help you in if-statements.

hi am having the same problem and am using when condition ,i tried the "noEvant" and i still have the same error msg and even with a hysterisis