try the computer programs available from NASA, distributed by Cosmic, such as :<http://www.openchannelfoundation.org/projects/CET93_PC/>The ability to calcualte the thremodynamic propoerties of any gas is fundamental to the process uses by the types of objects that Nasa designs, and the referenced program ( or its background references ) may provide a means to obtain a value for Cp/ Cv. RE: Sonic