# Help with the Wagner Equation

1. Jun 25, 2008

### CrazyDiamond

I'm using the Wagner Equation for Vapour Pressure at work.This equation is :

lnp=((-7.7069(1-T)) + (2.4932(1-T)^1.5158) -(2.9212(1-T)^1.9907) -(3.8684(1-T)^4.3798) + (0.46898(1-T)^1.7461) - (0.00008336))/x

[I haven't been able to find an online version of this equation]

Here, T and p are variables. Its obviously easy to find p if I know T, but now I need to find T given that p is known. This is getting very complicated, and I don't have access to MatLAB either.Could someone who has MatLAB please help me out - I simply need an equation for T in terms of p.

Online Equation solvers give me an answer for T if I feed in the equation, but they do not give me an equation for T, which is what I need so I can put it into an Excel Spreadsheet.

Thanks.

2. Jun 26, 2008

### caslav.ilic

It is not possible to get equation of the form T(p), since p(T) is a complicated enough rational polynomial. Wherever you need to use T(p), you will have to have a numerical solver for it in the background (what those online solvers do). Perphaps Excel has a function for that too?

If not, you may try another thing to patch up for the moment. In the range of T that you expect, plot in Excel the graph of p(T); if it doesn't look to non-linear, make a regression fit to it, using one of the expressions that can be easily inverted to obtain an approximate T(p).

--
Chusslove Illich (Часлав Илић)

3. Jun 27, 2008

### CrazyDiamond

And that worked perfectly.Thanks !

4. Jun 29, 2008

### DuncanM

I just happened to browse this forum and this post piqued my curiosity.

What is the Wagner Equation? Can you refer me to any online references where I can read more about it?