Each budget, of mass, forces and energy, is written per unit of mass of the fluid. Then, continuity introduces a new parameter Theta = ln(u/u), called the parameter of volume variation, u is the mass volume, u(r) a constant value of reference. When \Theta\ < In 2, this parameter, taken as a new variable, led to a budget separated into two distinct equations. The first equation is invariable in writing compared to an incompressible situation; in particular the order of correlation in these equations remains unchanged. The second equation is directly related to the volume variation. If the law of state of perfect gas is applied, the pressure is not any more a variable for the system. A flows classification gives the different cases, which permits the use of this separation.