Pawkit
Theory
40
Figure 1. Water Activity Diagram—adapted from
Labuza
Water activity of a system is measured by equilibrat-
ing the liquid phase water in the sample with the
vapor phase water in the headspace and measuring
the relative humidity of the headspace. In the Paw-
kit, a sample is placed in a sample cup which is
sealed inside a chamber. Inside the sensor block is a
dielectric humidity sensor. Changes in the electrical
conductance of the dielectric sensor occur as the
relative humidity of the chamber changes. By moni-
toring the change in electrical conductance, the rel-
ative humidity of the headspace is computed. When
the water activity of the sample and the relative
humidity of the air are in equilibrium, the measure-
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