A method for similitude testing of a turbomachine, the method comprising selecting a test gas having a molecular weight between 40 g/gmol and 150 g/gmol, a global warming potential (GWP) of less than 700, and a gas specific heat ratio of between 1 and 1.5; operating the turbomachine using the selected test gas; identifying at least one parameter while operating the turbomachine; establishing at least one test parameter associated with the at least one parameter; and comparing the at least one test parameter with a corresponding at least one specified parameter to determine whether the turbomachine has passed the similitude test.