Decagon Devices AquaLab 4TE Operator's Manual page 17

AquaLab
3. Water Activity Th eory
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 equilibrium, 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 important to
remember the restriction of the defi nition of water activity to equi-
librium.
Temperature Eff ects
Temperature plays a critical role in water activity determination.
Most critical is the measurement of the diff erence between sample
and dewpoint temperature. If this temperature diff erence were in er-
ror by 1°C, an error of up to 0.06 a could result. In order for water
w
activity measurements to be accurate to 0.001, temperature diff er-
ence measurements need to be accurate to 0.017°C. AquaLab's in-
frared thermometer measures the diff erence in temperature between
the sample and the block. It is carefully calibrated to minimize tem-
perature errors, but achieving 0.017°C accuracy is diffi cult when
temperature diff erences are large. Best accuracy is therefore obtained
when the sample is near chamber temperature.
Another eff ect of temperature on water activity occurs when sam-
ples are near saturation. A sample that is close to 1.0 a and is only
w
slightly warmer than the sensor block will condense water within the
block. Th is will cause errors in the measurement, and in subsequent
measurements until the condensation disappears. A sample at 0.75 a
w
needs to be approximately 4°C above the chamber temperature to
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