Re: pressure units
Evgenii Rudnyi
> Firstly, in what units of pressure does tdlib compute? I know
> this may sound absurd, but I haven't found any information regarding the
units of calculation in the guide. I assumed them to be in SI units,
meaning the unit for pressure in tdlib would be in Pa, am I right?
Actually not, the pressure is not in Pascals. Thermodynamics is a bit
special. There is a standard state, now it is 1 bar (10e5 Pa) and earlier
it used to be 1 atm (101325 Pa, if I remember it correctly).
So usually one defines thermodanymics function for the standard state, say
G(T, p0), where p0 is the standard pressure. Then one computes the
difference between the standard function and the given pressure
G(T, p) = G(T, p0) + f(T, p/p0)
and usually the function f is defined in terms of ratio p/p0. This way
pressure is dimensionless and this is how I have programmed TDLIB. This
means that if you use the standard state 1 bar (this is the case for
NIST), then you should use pressure in bar. If you use a standard state in
atmospheres, then in atmospheres.
Best wishes,
Evgenii
Evgenii Rudnyi
> pressure is dimensionless and this is how I have programmed TDLIB.
I have just checked the code and I see a problem in the case of the ideal
gas. Oh-oh. I should have checked the code before I have written this.
It happens that V and dV/dp for the ideal gas are not computed correctly
as they assume p as the absolute and not relative pressure. I have to
change it. I hope that you do not need this properties right now. Well, it
will be just necessary to figure out in what units you recieve them.
For other properties everything what I have written is correct.
By the way, by default TDLIB uses R = 8.31441. One can changed it with the
global object, for example
<globals
R=8.31441>
</globals>
This should be placed at the very beginning, if necessary to change it.
Evgenii