Thermodynamic Expressions
Values based on approximations (particularly ) are in many cases not sufficiently accurate for practical engineering calculations such as flow rates through pipes and valves. An experimental value should be used rather than one based on this approximation, where possible. A rigorous value for the ratio can also be calculated by determining from the residual properties expressed as:
Values for are readily available and recorded, but values for need to be determined via relations such as these. See here for the derivation of the thermodynamic relations between the heat capacities.
The above definition is the approach used to develop rigorous expressions from equations of state (such as Peng-Robinson), which match experimental values so closely that there is little need to develop a database of ratios or values. Values can also be determined through finite difference approximation.
Read more about this topic: Heat Capacity Ratio
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