In conventional system of measurement, the differential pressure generated by orifice plate is measured by D.P. transmitter. The output from D.P. transmitter after square rooting is accepted as proportional to flow – rate. This assumption is true only when density is constant. Unfortunately, density of compressible fluid is never constant. The density of compressible fluid changes with line pressure and line temperature. Thus, by measuring line pressure and temperature and usingthe relevant algorithms, instantaneous density can be found out and also the state of steam (i.e. saturated / superheated) .By knowing the correct density one can compute the accurate mass-flow rate. The further operation of integration, square rooting is similar to ordinary flow Totaliser.