ROTRONIC HygroLab - Manual (Page 43)

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 43 of 51
© 2006; Rotronic AG IN-E-HyLab-V4_11
17.3 Principle of measurement
The standard (or conventional) method of measuring water activity consists in placing a sample of
the product to be measured in a sealed container. The product sample slowly exchanges moisture
with the air inside the sealed container until equilibrium is reached. Experience shows that even
with a relatively small amount of product, it is the moisture contained in the product that ends up
determining the humidity of the air, and not vice versa.
The equilibration process is monitored by measuring the humidity of the air above the product with
a relative humidity sensor (%RH = 100 x Aw). Because temperature is an important factor when
measuring water activity, the temperature of the air above the product, is also monitored.
By definition, water activity is equal to %RH / 100 when equilibrium has been reached. At that time,
the product no longer interchanges moisture with the surrounding air.
The following below uses the Rotronic AW-DIO and AwVC-DIO water activity probe as an example.