A sub-plot shows the variation of isentropic (i.e. adiabatic) efficiency with flow, at constant speed. Some examples of theoretically isentropic thermodynamic devices are pumps, gas compressors, turbines, nozzles, and diffusers. This expression is based on the form of Bernoulli's equation applicable to isentropic compressible flow. For an isentropic flow, entropy density can vary between different streamlines. The isentropic upwelling solid mantle material exceeds the solidus temperature and melts. This equation applies to a steady, uniform, isentropic flow. Sometimes it is more convenient to use an isentropic velocity ratio. However, the actual stagnation pressure may be less than the isentropic stagnation pressure. See table of isentropic flow Mach number relationships. It is caused mainly from isentropic heating of the air molecules within the compression wave. Work is performed on the system during the lower isentropic compression process. Engineers classify this type of flow as an "isentropic" flow of fluids. General standard practice is to interpret these efficiencies as isentropic rather than polytropic. Next, a great deal can be computed for isentropic processes of an ideal gas. The volume ratio formula 2 is called the "isentropic expansion ratio".