METER GROUP AquaLab 4TE - Page 9

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AQUALAB
2.2.1.2. LOW WATER-EMITTING SAMPLES
Some extremely dry, dehydrated, highly viscous, water-in-oil emulsions (e.g., butter), high
fat, or glassy compositions may require multiple readings because of their slow water-
emitting properties. This is because the slow emission of water decreases the change in
water activity sufficiently that the instrument determines the test to be complete, even
though changes in water activity are still occurring.
The most effective way to test these types of samples is to run them in the AQUALAB using
Continuous or Custom mode and wait for the water activity readings to stabilize.
For faster reading, it is important to have the relative humidity of the chamber at or below
the water activity of these types of samples. This causes the sample to release water to the
vapor phase and equilibrate with the chamber. If the relative humidity of the headspace is
greater than the water activity of the sample, the sample will require a long period of time to
reach equilibrium.
To reduce test time when testing low water-emitting samples, see Section3.2.4.2 to set
Low-Emitting mode.
2.2.1.3. VOLATILE SAMPLES
Samples with certain volatiles in high enough concentrations may give inaccurate water
activity values in the AQUALAB 4TE and AQUALAB 4TE DUO models. Volatiles condense on
the mirror during the reading process but do not evaporate from the mirror as water does.
As a result, the reading on samples with volatiles may not be accurate. The concentration
of volatiles that causes interference is variable and matrix dependent.
The AQUALAB 4TEV measures volatiles such as propylene glycol and ethanol. The AQUALAB
4TEV contains both a chilled-mirror dew point and a capacitance sensor. The capacitance
sensor has a lower accuracy of ±0.015 a
w
than the dew point sensor, which has an accuracy
of ±0.003 a
w
.
To determine if volatiles are a problem, compare dew point readings to capacitance readings.
If the dew point readings are >0.018 a
w
higher than the capacitance readings, volatiles are
likely a problem and the AQUALAB 4TEV, AQUALAB 4TEV DUO, or AQUALAB TDL will need to be
used for the sample.
After measuring volatiles with the capacitance sensor, clean the chamber (Section4.2),
place charcoal in the chamber, and seal for a minimum of 5 min before switching to the dew
point sensor.
2.2.1.4. LOW WATER ACTIVITY
A sample’s water activity value may be below the cooling capacity of the AQUALAB. The
AQUALAB will display an error message indicating the lowest reading it attained on that
particular sample. The AQUALAB may also give the error message even if the sample is not
below 0.03 a
w
. Section4.5 provides possible solutions to the error message.