For an ideal gas, this function is independent of temperature (although temperature remains as a valid input).
function beta
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.PressureReciprocal beta "Isothermal compressibility";
end beta;