Saturation vapor pressure formula

[DAPA]

Hello,
I would like to know the formula for calculating the saturation vapor pressure used in WUFI.
I don't see the vapor pressure in indoor and outdoor atmospheres as output from WUFI.
Thanks in advance for the reply.

[Thomas]

Hi DAPA,

please see here:

WUFI: Sättigungsdampfdruck - Saturation vapour pressure
(scroll down for the English version).

Kind regards,
Thomas

[DAPA]

Thank you Thomas.
I need saturation vapor pressure formula because I try to simulate the EN ISO 12572 test method (Determination of water vapour transmission properties — Cup method)using WUFI Pro.
I try it on several materials but I found a difference between the vapor diffusion resistance factor of the material choosen from the data base (dry cup) and the vapor diffusion resistance factor calculated by WUFI Pro.
Temperature 23°C
HR 0% / 50% (delta p = 1406,81 Pa)
Heat resistance = 0 for exterior and interior surfaces
Material : ISOVER GW Integra ZKF - 035
Thickness : 14 cm
Water vapor diffusion factor from the data base : 1
Vapor diffusion flux calculated by WUFI: 1.96086E-0006 kg/(m.s)
Vapor diffusion resistance factor calculated : 1,0158
Could you explain this différence of 1,6 % ?
Best regards
Daniel

[Thomas]

Dear Daniel,

this is because WUFI uses a slightly different version of the formula for the vapour permeability of air than EN ISO 12572. The formula used by WUFI is

$\delta_{air} = \frac{1.968e-7 \cdot (\vartheta+273.15)^{0.81}}{P}$

$\delta_{air}$: vapour permeability of air, kg/(m s Pa)
$\vartheta$: temperature, °C
$P$: barometric pressure, Pa

For
$\vartheta$ = 23 °C and
$P$ = 101300 Pa (the default pressure used by WUFI if no other pressure is specified via a climate file),
we have
$\delta_{air} = 1.95137 \cdot 10^{-10} \ \mathrm{kg/(m \ s \ Pa)}$.
The saturation vapour pressure formula gives us $p_{sat} = 2813.62 \ \mathrm{Pa}$.

These data result in the $\mu$-value

$\mu = \frac{\Delta RH \ \cdot \ p_{sat} \ \cdot \ \delta_{air}}{g \ \cdot \ d} = \frac{0.5 \ \cdot \ 2813.62 \ \cdot \ 1.95137 \ \cdot \ 10^{-10}}{1.96086\cdot10^{-6} \ \cdot \ 0.14} = 1.000001$

Kind regards,
Thomas

[DAPA]

Dear Thomas,
Thank you a lot for your answer.
It's perfect now.
Best regards,
Daniel