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Departure function
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<p>In <a href="/facts/Thermodynamics/q4hIx3yP">thermodynamics</a>, a departure function is defined for any thermodynamic property as the difference between the property as computed for an <a href="/facts/Ideal_gas/80EfYGFj">ideal gas</a> and the property of the species as it exists in the real world, for a specified temperature <i>T</i> and pressure <i>P</i>. Common departure functions include those for <a href="/facts/Enthalpy/bBou8lCE">enthalpy</a>, <a href="/facts/Entropy/s52IXeFz">entropy</a>, and <a href="/facts/Internal_energy/IYdsoxhR">internal energy</a>. </p><p>Departure functions are used to calculate real fluid extensive properties (i.e. properties which are computed as a difference between two states). A departure function gives the difference between the real state, at a finite volume or non-zero pressure and temperature, and the ideal state, usually at zero pressure or infinite volume and temperature. </p><p>For example, to evaluate enthalpy change between two points <i>h</i>(<i>v</i>1,<i>T</i>1) and <i>h</i>(<i>v</i>2,<i>T</i>2) we first compute the enthalpy departure function between volume <i>v</i>1 and infinite volume at <i>T</i> = <i>T</i>1, then add to that the ideal gas enthalpy change due to the temperature change from <i>T</i>1 to <i>T</i>2, then subtract the departure function value between <i>v</i>2 and infinite volume. </p><p>Departure functions are computed by integrating a function which depends on an <a href="/facts/Equation_of_state/tLILkepz">equation of state</a> and its derivative. </p>