I am doing a lab report and it is to do with compressible flow through rotameters. An upstream and downstream rotameter to measure flow rate with a needle valve between the two. The needle valve acts as a fixed orifice in one part of the experiment. Then the inlet pressure is altered. Plotting pressure against mass flow rate yields a graph where above 13psig the flow is sonic and it follows a straight line. However at subsonic speeds it no longer follows a curve. I am struggling to explain why this is the case? Any ideas?