Dear IBP,
typically moisture moisture storage function is based on few measured values (0.65, 0.80, 0.93, 0.97, free saturation) while the rest of the points are interpolated (0.99, 0.995, 0.999, 0.9995, 0.9999).
According to the "Material Information" I see that a 3 point formula is used.
I searched the web for a 3 point interpolation method but I failed to identify the one that reproduces the WUFI sorption isotherms.
Could you be more specific on the interpolation methodology that is applied (is there an analytical formula that is used for interpolation?) or indicate literature where to collect more information?
Thanks in advance
Best Regards
Marco
Moisture Storage Function  3 point formula

 WUFI User
 Posts: 3
 Joined: Thu May 13, 2021 2:52 am 1100
Re: Moisture Storage Function  3 point formula
Hi Marco,
the 3point formula has been introduced by Dr. A. Holm in his dissertation
Holm A.:
Ermittlung der Genauigkeit von instationären hygrothermischen Bauteilberechnungen mittels eines stochastischen Konzeptes,
Dissertation Uni Stuttgart, 2001 (in German)
https://publica.fraunhofer.de/entities/ ... cd/details
(See Formula (10) and the red dotted lines in Figures 19, 20, 21)
and
Holm A., Krus M., Künzel H.M.:
Approximation der Feuchtespeicherfunktion aus einfach bestimmbaren Kennwerten
IBPMitteilung 406, 2002
https://publica.fraunhofer.de/entities/ ... 90/details
\(u(p_c) = \frac{u_f}{1 + \left(\frac{p_c}{pk1}\right)^{pk2}}\)
\(u(p_c)\) water content as function of the capillary pressure \(p_c\), kg/m³
\(u_f\): water content at free saturation, kg/m³
\(p_c\): capillary pressure, Pa
\(pk1, pk2\): fit parameters
See also an alternative form and a short description in
Holm A., Kuenzel H.M.:
Uncertainty Approaches for Hygrothermal Building Simulations  Drying of an AAC Flat Roof in Different Climates
Seventh International IBPSA Conference
Rio de Janeiro, Brazil
August 1315, 2001
https://wufi.de/literatur/Holm,%20K%C3% ... ilding.pdf
The function has the shape of a "rounded step" function and thus reproduces the typical shape which a moisture storage curve has when the water content is plotted against the logarithm of the capillary pressure.
It's called "3point formula" in WUFI to distinguish it from the "twopoint" formula which WUFI uses for approximating the moisture storage function if two points (usually u80 and uf) are given. The 3point formula is more flexible, but it needs three points as input since there are three free parameters (\(u_f\), \(pk1\), \(pk2\)). We usually choose the points u(RH=80%), u(RH=95 %) and u_f, but other choices are possible as well. If more than three points of the moisture storage function are available we usually fit the curve to all of them in a leastsquares fit:
Best regards,
Thomas
Keywords: 3point formula, 3point function, 3PunkteFormel, 3PunkteFunktion, dreiPunkteFunktion, dreiPunkteFormel
the 3point formula has been introduced by Dr. A. Holm in his dissertation
Holm A.:
Ermittlung der Genauigkeit von instationären hygrothermischen Bauteilberechnungen mittels eines stochastischen Konzeptes,
Dissertation Uni Stuttgart, 2001 (in German)
https://publica.fraunhofer.de/entities/ ... cd/details
(See Formula (10) and the red dotted lines in Figures 19, 20, 21)
and
Holm A., Krus M., Künzel H.M.:
Approximation der Feuchtespeicherfunktion aus einfach bestimmbaren Kennwerten
IBPMitteilung 406, 2002
https://publica.fraunhofer.de/entities/ ... 90/details
\(u(p_c) = \frac{u_f}{1 + \left(\frac{p_c}{pk1}\right)^{pk2}}\)
\(u(p_c)\) water content as function of the capillary pressure \(p_c\), kg/m³
\(u_f\): water content at free saturation, kg/m³
\(p_c\): capillary pressure, Pa
\(pk1, pk2\): fit parameters
See also an alternative form and a short description in
Holm A., Kuenzel H.M.:
Uncertainty Approaches for Hygrothermal Building Simulations  Drying of an AAC Flat Roof in Different Climates
Seventh International IBPSA Conference
Rio de Janeiro, Brazil
August 1315, 2001
https://wufi.de/literatur/Holm,%20K%C3% ... ilding.pdf
The function has the shape of a "rounded step" function and thus reproduces the typical shape which a moisture storage curve has when the water content is plotted against the logarithm of the capillary pressure.
It's called "3point formula" in WUFI to distinguish it from the "twopoint" formula which WUFI uses for approximating the moisture storage function if two points (usually u80 and uf) are given. The 3point formula is more flexible, but it needs three points as input since there are three free parameters (\(u_f\), \(pk1\), \(pk2\)). We usually choose the points u(RH=80%), u(RH=95 %) and u_f, but other choices are possible as well. If more than three points of the moisture storage function are available we usually fit the curve to all of them in a leastsquares fit:
Best regards,
Thomas
Keywords: 3point formula, 3point function, 3PunkteFormel, 3PunkteFunktion, dreiPunkteFunktion, dreiPunkteFormel