AquaLab 4TEV Series User Manual

AquaLab 3 WATER ACTIVITY THEORY
Temperature Effects
Temperature plays a critical role in water activity determination.
Most critical is the measurement of the difference between sample
and dew point temperature. If this temperature difference were in
error by 1
◦
C, an error of up to 0.06 a
w
could result. In order
for water activity measurements to be accurate to 0.001, tempera-
ture difference measurements need to be accurate to 0.017
◦
C. The
AquaLab infrared thermometer measures the difference in temper-
ature between the sample and the block. It is carefully calibrated
to minimize temperature errors, but achieving 0.017
◦
C accuracy is
difficult when temperature differences are large. Best accuracy is
therefore obtained when the sample is near chamber temperature.
Another effect of temperature on water activity occurs when sam-
ples are near saturation. A sample that is close to 1.0 a
w
and is
only slightly warmer than the sensor block condenses water within
the block. This causes 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 cause condensation. The AquaLab warns the user if a sample is
more than 4
◦
C above the chamber temperature, but for high water
activity samples the operator needs to be aware that condensation
can occur if a sample that is warmer than the block is put in the
AquaLab.
3.3 Water Potential
Some additional information may be useful for understanding what
water activity is and why it is such a useful measure of moisture
status in products. Water activity is closely related to a thermody-
namic property called the water potential, or chemical potential (µ)
of water, which is the change in Gibbs free energy (∆G) when water
concentration changes. Equilibrium occurs in a system when (µ) is
the same everywhere in the system. Equilibrium between the liquid
and the vapor phases implies that (µ) is the same in both phases. It
is this fact that allows us to measure the water potential of the va-
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AquaLab 4TEV Series Specifications

General

Brand	AquaLab
Model	4TEV Series
Category	Measuring Instruments
Language	English