Potential inconsistency in component enthalpies of ideal gases
For ideal gases, enthalpy is independent of pressure (see last paragraph of the introduction here or here). We end up with
dH = c_p dT
for enthalpy changes as function of temperature.
Some components implement
static const Scalar gasEnthalpy(Scalar temperature, Scalar pressure)
{ return gasHeatCapacity(temperature, pressure) * temperature; }
However, with the above equation we should actually from a reference temperature (or state) to the given temperature.
Some Observations:

CH4
implements this:
// method of Joback
const Scalar cpVapA = 19.25;
const Scalar cpVapB = 0.05213;
const Scalar cpVapC = 1.197e5;
const Scalar cpVapD = 1.132e8;
return
1/molarMass()* // conversion from [J/(mol*K)] to [J/(kg*K)]
(cpVapA + T*
(cpVapB/2 + T*
(cpVapC/3 + T*
(cpVapD/4))));
it says this is according to the method of Joback but I am having trouble finding out where the magic numbers come from. I cannot relate them to the Joback tables (e.g. Appendix C of [1]). Also, the equation looks as if this actually returns the average cp (integration of cp from 0 to T and then division by T). Sampling enthalpy values at specific temperatures also verifies that, as one gets closer to reported values when one removes the divisors (/2
, /3,
/4`)...
If we cannot find out where exactly these values come from, we could use Appendix A, Table Section C, row 26 of [1] for enthalpy computations. Seems to match reported values.
 The current implementation (
cp*T
) vs integration led to differences between 15% and > 30% in for150 < T < 600
I suggest the following for all ideal gases that have this kind of implementation:
 (1) Use Appendix A from [1] or properly implement & cite the Joback method (i.e. Appendix C from [1]) for the heat capacities
 (2) integrate the enthalpy from a reference state to the given temperature using eq 31.5 of [1]
 (3) (maybe) reevaluate the validity bounds (temperature, but also species) of the Joback method, and maybe emit a warning when one leaves the validity range? (Tricky, because it could happen in some Newton iterations but be fine at the end of the solve...)
 [1] also shows other methods than Joback, we could have a look if they provide advantages...
[1] B. Poling et al. (2001, fifth edition, pp A.35)  https://www.eng.uc.edu/~beaucag/Classes/ChEThermoBeaucage/Books/Bruce%20E.%20Poling,%20John%20M.%20Prausnitz,%20John%20P.%20O%27Connell%20%20The%20properties%20of%20gases%20and%20liquidsMcGrawHill%20Professional%20(2000).pdf