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in_hypalm_aw1_v3.doc
Water Activity Measurement
Water Activity: definition and applications
Definitions
The moisture content of a product can be defined as the percentage weight of water in relation to
the dry weight of the product.
Products in which moisture can be present can be classified in two categories: hygroscopic and
non hygroscopic. Examples of hygroscopic materials are salts, vegetal fibers, most metal oxides,
many polymers, etc. Examples of non hygroscopic products are metal powders, glass granules,
etc.
Regarding the moisture content of a product, we define static equilibrium as a set of conditions
under which the product does not exchange any moisture with its environment. Under conditions
of static equilibrium, the moisture content of a hygroscopic product depends on the nature of the
product and also on the two following factors:
(a) the partial pressure of water vapor in the immediate environment of the product
(b) the temperature of the product
If the moisture content of a product is not dependent on both these factors, then the product is not
hygroscopic.
Hygroscopic products may absorb water in different ways: sorption with formation of a hydrate,
binding by surface energy, diffusion of water molecules in the material structure, capillary
condensation, formation of a solution, etc. Depending on the absorption process, water is bound
to the product with more or less strength. Moisture content can include both an immobilized part
(e.g. water of hydration) and an active part.
Water activity Aw (or equilibrium relative humidity %ERH) measures the vapor pressure
generated by the moisture present in a hygroscopic product.
Aw = p / ps and %ERH = 100 x Aw, where:
p : partial pressure of water vapor at the surface of the product
ps : saturation pressure, or the partial pressure of water vapor above pure water at the product
temperature
Water activity reflects the active part of moisture content or the part which, under normal
circumstances, can be exchanged between the product and its environment.
Water activity is usually defined under static conditions of equilibrium. Under such conditions, the
partial pressure of water vapor (p) at the surface of the product is equal to the partial pressure of
water vapor in the immediate environment of the product. Any exchange of moisture between the
product and its environment is driven by a difference between these two partial pressures.
Finally, water vapor can also be present in a gas or gas mixture. The relative humidity of a gas is
defined as %RH = 100 x p/ps, where (p) is the partial pressure of the water vapor present in the
gas mixture and (ps) is the saturation pressure, or the partial pressure of water vapor above pure
water at the temperature of the gas.
