I don't have CoolProp installed so I can't try this, but this could happen when using REFPROP, too. It's usually about the range of the pressure and internal energy we've asked the function to ask the Coolprop/REFPROP for. For some corner cases, Coolprop/REFPROP just can't generate a value, most likely due to the bound set for the fit function inside. Nothing you can do about it directly from MATLAB side. Change the range of pressure and internal energy to a narrower range and try again. The values at the location of failure provide a hint on which bound to change.
When I try this with REFPROP, I can't go as high as 3MPa with lower bound of internal energy to be -5.99. Setting the high pressure to be below 2.54MPa seems to work. I can get the following to work.
>> HeTables = twoPhaseFluidTables([-5.99,500],[0.2275,2.5],25,25,60,'HELIUM','C:\Program Files (x86)\REFPROP\')
Alternatively, changing the lower bound of the requested internal energy seems to help, too. This one also works in REFPROP.
>> HeTables = twoPhaseFluidTables([-1,500],[0.2275,3],25,25,60,'HELIUM','C:\Program Files (x86)\REFPROP\')
Hope this helps.
