@Samir Khan 

First of all I would like to thank you for the many instructive video you made to help newbies like myself. Thank you for providing the correct syntax. Is there somewhere a good instruction manual present that explains the coolprop thermodynamics package?  

Dear Preben, thank you for your rapid reply! That works fine 

@acer 

Dear Acer, thank you so much this solved my issue. I just have one remaining question why do you have to use combine in this situation? I would expect that Maple would automatically do this conversion.