Werner;
Thank you once again. I think you would agree that this is an interesting problem as it seems to tax Mathcad solvers, but do believe you are correct and that any values above 5.976 L/s will fail as this flowrate value is a real physical constraint of the system. The water in the small chamber (given the unique input values) length width and so on, never gets above 442 mm. So I can only assume at 6.4 L/s discharge the actual height of water that must be achieved to drive the water from the small chamber must be in excess of 422 mm which is physically impossible. What I do not understand however is why this problem is so numerically unstable. For example if you turn on multithreading for example Mathcad will fail to converge to an answer. I don't know if this is a Mathcad bug or something in the way the problem has be cast. If it is a bug it probably should be reported to PTC. Also as you have remarked previously if one decreases the length of the tank even just for from 12 m in length to 10 m in length Mathcad fails to produce an answer. All these things don't make sense to me especially if Mathcad is using the state of the art numerical solvers.
Kind Regards, Mark
