Decagon Devices AquaLab 4TE Operator's Manual page 16

AquaLab
3. Water Activity Th eory
Fig. 1: Water Activity Diagram—adapted from Labuza
Water activity of a system is measured by equilibrating the liquid
phase water in the sample with the vapor phase water in the head-
space and measuring the relative humidity of the head-space. In
the AquaLab, a sample is placed in a sample cup which is sealed
inside a sample chamber. Inside the sample chamber is a fan, a dew
point sensor, a temperature sensor, and an infrared thermometer.
Th e dewpoint sensor measures the dewpoint temperature of the air
in the chamber, and the infrared thermometer measures the sample
temperature. From these measurements, the relative humidity of
the head-space is computed as the ratio of dewpoint temperature
saturation vapor pressure to saturation vapor pressure at the sample
temperature. When the water activity of the sample and the rela-
tive humidity of the air are in equilibrium, the measurement of the
head-space humidity gives the water activity of the sample. Th e pur-
pose of the fan is to speed equilibrium and to control the boundary
layer conductance of the dewpoint sensor.
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