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Help with the Wagner Equation

  1. Jun 25, 2008 #1
    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.

  2. jcsd
  3. Jun 25, 2008 #2
    Don't think there is an analytical solution for this equation.
  4. Jun 25, 2008 #3


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    I also don't think there is an analytical solution.

    If you must, you can probably solve it approximately in Excel (without scripting!) with bisection. If you want 9 decimals of precision you'll need ~30 rows for the calculation.

    You could solve it with VBA in Excel, though you'll get warnings about the script being potentially unsafe whenever you open it. (Kinda makes you wish Excel could tell when no file access was being done by the script...)

    You could download any number of programs, like Pari/GP, that would make solving it simple. Of course then you'd have to enter the data into Excel manually.
  5. Jun 27, 2008 #4
    I used linear regression in Excel..my boss is too stingy to give me access to VBA.
    Thanks for the help guys !
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