Dear Yuan,
        Here is a physical chemists answer, which may or may not be what you
want!
The water absorption rate of a substance may be defined as the amount
of moisture taken up (often expressed as a percentage the 'dry' weight
of the material) as a function of time (e.g. 0.01%per hour), for a
specified temperature and humidity condition (e.g. 25C 90%RH). Water
aborption may also refer to immersion in the liquid rather than uptake
from atmospheric moisture.
Absorption rates are complex properties, as they are often time
dependent, as well as being dependent upon the geometry of the sample.
ie. a thin film will absorb moisture faster than a large block of
material.
In the context of your question , the absorption rate should be
distinguished from the equilibrium moisture content of a material,
which is usually expressed as a percentage of the dry mass of the
material. Hence a substrate such as polyimide (used in flexi circuits
for example) will have an equilibrium moisture content of about 2% if
stored at 25C and 95% RH for some time. This may be the quanitity you
are referring to in your posting rather than the absorption rate.

If all this sounds like 'double dutch' then I apologise!




Dr. Carl Levoguer
CMMT
National Physical Laboratory
Queens Rd, Teddington, Middlesex.
TW11 0LW
Direct Line : +44 181 943 6263
Fax: +44 181 943 6453

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