Enthalpy of orthohydrogen does not match the values from Leachman at al.

[Rich780]

Hi,

it appears that the enthalpy data of normal hydrogen and parahydrogen are very well in line with the data stated in Leachman et al. (https://doi.org/10.1063/1.3160306). However, for orthohydrogen I am getting for example at 7560 Pa a value of h = 391.101 kJ/kg from Refprop compared to -53.820 kJ/kg for saturated liquid (table 15) and 845.692 kJ/kg vs. 400.77 kJ/kg for saturated vapor (table 15).

Refprop 10.0
Windows 11, Spyder, Python

[nist-aharvey]

As you probably know, enthalpy is only meaningful relative to an arbitrary "reference state". Only enthalpy differences between state points are meaningful, and I think/hope you would find identical differences between points in REFPROP and in that table 15.

It appears that the default reference state for ortho-H2 in REFPROP has been set differently than it was for the Leachman paper (unfortunately, I don't find any information on reference states in that paper). If you look at comments near the top of the Fluid files ORTHOHYD.FLD and PARAHYD.FLD, you will see some text that explains the reason for this change, along with some advice for getting consistent results if you mix ortho and para.
I believe you would reproduce the values in Table 15 for ortho-H2 if you changed REFPROP's Reference State to "NBP" -- in the Windows GUI this is done in the Reference State dialog under Options.

But I will reiterate that the reference state issue will not matter if you are only calculating pure ortho-H2, or if you are calculating a mixture with a fixed composition. IF, however, you want to compare enthalpies between different ortho/para ratios, that gets tricky and you will need to also change the para-H2 reference state as discussed in the comment in the PARAHYD.FLD fluid file. And I think such a change would produce an offset between n-H2 and the 75/25 mixture of ortho and para -- but again it would be a constant offset and calculation of changes between state points should match pretty well.