Temperature Effects - Decagon Devices pawkit Operator's Manual

Pawkit User Manual
3. Water Activity Theory
in the headspace and measuring the relative humidity of the
headspace. In the Pawkit, a sample is placed in a sample cup
which is sealed inside a chamber. Inside the sensor block is a
capacitive humidity sensor. Changes in the electrical capaci-
tance of the polymide layer of the sensor occur as the relative
humidity of the chamber changes. By monitoring the
change in electrical capacitance, the relative humidity of the
headspace is computed. When the water activity of the sam-
ple and the relative humidity of the air are in equilibrium,
the measurement of the headspace humidity gives the water
activity of the sample.
In addition to equilibrium between the liquid phase water in
the sample and the vapor phase, the internal equilibrium of
the sample is important. If a system is not at internal equilib-
rium, one might measure a steady vapor pressure (over the
period of measurement) which is not the true water activity
of the system. An example of this might be a baked good or a
multi-component food. Initially out of the oven, a baked
good is not at internal equilibrium; the outer surface is at a
lower water activity than the center of the baked good. One
must wait a period of time in order for the water to migrate
and the system to come to internal equilibrium. It is import-
ant to remember the restriction of the definition of water
activity to equilibrium.

Temperature Effects

Temperature plays a critical role in water activity determina-
tions. Most critical is the measurement of the difference
between sample and dew point temperature. Best accuracy is
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