Are water content and absolute humidity of airlayer the same?

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WEI LI
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Are water content and absolute humidity of airlayer the same?

Post by WEI LI » Sun Mar 21, 2021 11:37 am -1100

Hi there,
Are the water content and absolute humidity of the air layer the same?
According to the definition, it looks like the same.
However, I have calculated the absolute humidity using the formula in WUFI help, the values are not the same as the water content in the WUFI graph.

Thanks for your reply in advance.

Thomas
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Re: Are water content and absolute humidity of airlayer the same?

Post by Thomas » Mon Mar 22, 2021 2:30 am -1100

WEI LI wrote:
Sun Mar 21, 2021 11:37 am -1100
Are the water content and absolute humidity of the air layer the same?
Hi Wei Li,

these are not the same. WUFI has been developed to investigate the moisture transfer in porous materials. These always contain some liquid water, and the moisture storage function which describes how that amount of liquid water depends on the relative humidity of the pore air is independent of the temperature.

On the other hand, air layers contain no liquid moisture, and the amount of moisture they contain is strongly influenced by temperature. Originally, they were introduced in WUFI only because many components contained air layers which therefore needed to be included in the simulation. The air layers were only intended to model the air gaps in the component in so far as these act as resistances for heat and moisture flows. The material database contains air layers "without additional moisture capacity" whose moisture storage function has been defined in such a way that their "liquid water content" (as reported by WUFI) corresponds roughly to the absolute humidity of the air. However, it is not strictly possible to describe the temperature-dependent absolute humidity of the air by means of the temperature-independent moisture storage function, so the water contents reported by WUFI can only be a rough approximation. They are not intended to be analysed. (The relative humidity of the air layer, on the other hand, should be much more realistic, relative humidity being WUFI's primary variable for describing moisture processes.)

For a more detailed description of the air layers, see the topic Reference | Material Data | Special Materials | Air Layers in WUFI's help file. We are working on a more realistic handling of the moisture content of air layers, but this is a complex task and it will take a while.

Kind regards,
Thomas

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