What are departure functions and how are they used?

Departure functions express the difference between real fluid properties and ideal gas properties at the same T and P: H_real - H_ideal, S_real - S_ideal. Calculated from equation of state via integrals involving compressibility factor or volume. Usage: Real property = Ideal gas property + Departure + Reference state contribution. This approach separates ideal gas calculations (known analytically) from non-ideal corrections (from EOS). Enables consistent property calculations using EOS. Important for accurate enthalpy and entropy in high-pressure gas systems.