If a gas is compressed adiabatically, i.e. without outflow of heat from the system, the temperature rises (due to the pressure increase) at a higher rate with respect to isothermal compression, where the performed work is dissipated as heat. The exponent, , with which the expansion of the gas can be calculated by the application of heat is called the isentropic &ndash; or adiabatic coefficient. Its value is determined by the Rüchardt experiment.
An adiabatic and reversible running state change is isentropic (entropy S remains the same as temperature T changes). The technique is usually an adiabatic change of state. For example, a steam turbine is not isentropic, as friction, choke and shock processes produce entropy.