In the tank, you have boiling liquid (not simply gas). Pretty much the same way as in your coolant system >100C. It self regulates itself;

* some liquid will be boiled

* pressure in the closed system raises

* until it reaches high enough pressure so more liquid will not boil

** this pressure depends on temperature - see measured curves

So the graphs are the boiling point at certain pressures. (shown rotated 90 degrees compared to traditional representation: pressure for certain temp, instead of temp for certain pressure)

If you have numeric representation of the tables, we can make an approximating formula, from a reasonable function

* using x, x*2 and 1/x and constants, for example.

** propertherm has some simulated annealing functions that we could use, or

** the simplest way is probably to use octave.org, see below

* Or we just make a table lookup.

Example for octave.org function to fit a parametric curves on numeric data stored in "transferfunc.dat" file columns 1:2 :

<code>

f(x) = a*x*x*x + b*x*x + c*x + d + e/x)

fit f(x) "transferfunc.dat" using 1:2 via a, b, c, d, e

plot "transferfunc.dat" using 1:2 with points 6, f(x)

</code>