For an ideal gas, this function is independent of pressure (although pressure remains as a valid input).
function c_p import FCSys.Utilities.Polynomial; extends Modelica.Icons.Function; input Q.TemperatureAbsolute T = 298.15*U.K "Temperature" annotation( Dialog(__Dymola_label = "<html><i>T</i></html>")); input Q.PressureAbsolute p = p0 "Pressure" annotation( Dialog(__Dymola_label = "<html><i>p</i></html>")); output Q.CapacityThermalSpecific c_p "Isobaric specific heat capacity" annotation( Dialog(__Dymola_label = "<html><i>c<sub>p</sub></i></html>")); end c_p;