Instruction Manual

IN-E-HyLab-V4_11
Rotronic AG
Bassersdorf, Switzerland
Document code Unit
Instruction Manual
Document Type
HygroLab bench top indicator version 4:
instruction manual
Document title
Page 40 of 51
© 2006; Rotronic AG IN-E-HyLab-V4_11
data request |{M00RDD} +CR note the | character (ASCII 124) preceding the command
answer: {M00RDD 0020.41;0019.87;----.--;----.--;#E
note: 0020.41: humidity probe 1 (%RH)
0019.87: temperature probe 1 (°C as per instrument configuration), etc.
17 Water activity measurement
17.1 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.