dpT calculation in waterBaseProp_dT

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Reported by m.graeber on 22 Oct 2009 10:15 UTC
I suppose there is a bug in the calculation of dpT in

Modelica.Media.Water.IF97_Utilities.waterBaseProp_dT

For Region 4 there are to different lines where dpT is calculated:

aux.dpT := BaseIF97.Basic.dptofT(T);

and some lines below

aux.dpT := (vap.s - liq.s)*(if (liq.d <> vap.d) 
	then (vap.s - liq.s)*liq.d*vap.d/(liq.d - vap.d) 
	else BaseIF97.Basic.dptofT(aux.T));

The first algorithm should be the right one.

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Migrated-From: https://trac.modelica.org/Modelica/ticket/224

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Modified by dietmarw on 22 Oct 2009 16:40 UTC

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Comment by hubertus on 23 Oct 2009 19:15 UTC
The second formula is the Clausius-Clapeyron equation: dp/dT = (s2-s1)/(v2-v1). In IF97, the analytical curve p = f(T) is an approximation, even though a good one. Using Clausius-Clapeyron instead makes dp/dT in my view more consistent with other parts of the fundamental equation. But you have a point in that the first one should be removed. It is no error, anyways. Check, e.g. http://theory.phy.umist.ac.uk/~judith/stat_therm/node44.html

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Comment by m.graeber on 25 Oct 2009 16:09 UTC
Isn't there still an error in the equation? Shouldn't it be

aux.dpT := if (liq.d <> vap.d) then 
    (vap.s - liq.s)*liq.d*vap.d/(liq.d - vap.d) else
    Modelica.Media.Water.IF97_Utilities.BaseIF97.Basic.dptofT(aux.T);

without the term (vap.s - liq.s) in front of the if equation?

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Comment by hubertus on 25 Oct 2009 20:16 UTC
No, this is the right formula. Compare it against the derivative of the approximation function dpofT.

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Modified by dietmarw on 26 Oct 2009 07:17 UTC

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Comment by hubertus on 28 Oct 2009 23:36 UTC
fixed in d7a0945 in trunk. Will be merged to maintenance later, with other media changes

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Comment by dietmarw on 25 Nov 2009 11:53 UTC
Milestone MSL3.1.1 deleted