<< Click to Display Table of Contents >> Navigation:  Software > HW5 (obsolete) > General HW5 feature > Water Activity > Definition

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, 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.