The fluid may be compressible or incompressible. A hydraulic system uses an incompressible liquid as its fluid, rather than a compressible gas. In fact, thermodynamics is typical of compressible flows and degenerates in incompressible flows. The hydraulic fluid itself is sensitive to temperature and pressure along with being somewhat compressible. This expression is based on the form of Bernoulli's equation applicable to isentropic compressible flow. Compressors operate on compressible fluids, typically gases. These changes occur because the fluid is compressible. The equation is also usable as a PVT equation for compressible fluids (e.g. polymers). Compressible flow accounts for varying density within the flow. The term "gas" refers to a compressible fluid phase. Choked flow is a compressible flow effect. It works because liquids are not appreciably compressible. Above Mach 0.3, the problem flow should be described using compressible aerodynamics. As a result, pressure coefficients can be greater than one in compressible flow. Otherwise the more general compressible flow equations must be used.