I am modeling a pipe that can be either full, empty or partially full.  I am trying to use the following code segment, but I keep getting errors.

if level <= 0 then

  //empty pipe

      phi = 0.0;

  fitting_in.P = fitting_out.P;

    //Full pipe 

elseif level >= 2 * R then

  //full pipe

      phi = 2 * Modelica.Constants.pi;

  fitting_in.q = -fitting_out.q;

else

  //partially full pipe

  phi = 2 * acos((R - level) / R);

  fitting_in.P = fitting_out.P;

end if;

    A = 0.5 * R ^ 2 * (phi - m.sin(phi));

And when I run my simulation, I get the following errors:
assert | warning | The following assertion has been violated at time 35.327627
assert | debug | Model error: Argument of acos(DIVISION(pipeArray1.pipeSegment[2].R - pipeArray1.levels[2]) / pipeArray1.pipeSegment[2].R) outside the domain -1.0 <= -1.0017 <= 1.0

In this simulation, the level should be going up, until the pipe is full. 
level represents the height from the bottom of the pipe and R is a parameter.  What I am trying to do is if the level ever rises to, or above, the top of the pipe, the phi, the angle fro the center of the pipe to the fill level, should be fixed at 2pi.  But, during my simulation, the above error shows that the //full pipe calculation is not being used, and instead it is still using the acos calculation. 
From above, the acos should only be used when level is < 2*R, so (R-level)/R) should be >-1. 

Essentially, I am trying to have three different modes of operation, and using one to protect against acos of an out of range number.  But, for some reason the simulation is calculating into this "illegal" range and giving me an error. 
Any guidance about how to handle this kind of situation would be very helpful.