AquaLab 8 SAMPLE PREPARATION
accurate water activity values. This is because the 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 will not be accurate. The concentration of volatiles that
will cause interference is variable and matrix dependent. The most
effective method to determine if volatiles are a problem is to com-
pare dew point readings to capacitance readings. If the dew point
readings are more than 0.0153 higher than the capacitance readings,
volatiles are likely a problem.
Decagon designed the Series 4TEV is designed for measuring volatiles
such as propylene glycol and ethanol. The Series 4TEV contains both
a chilled mirror dew point and a capacitance sensor. Simply choose
the sensor you want to use from the menu in the instrument. The
only difference in operation is a lower accuracy of ±0.015 a
w
for the
capacitance sensor. All other operations and features will be the
same, including measurement times and adjusting for linear offset.
After measuring volatiles with the volatiles sensor, it is a good idea
to clean the chamber and run charcoal before switching to the dew
point sensor.
8.5 Low Water Activity
When a sample water activity value is below the cooling capacity of
the Series 4, your AquaLab will display an error message indicating
the lowest reading it attained on that particular sample. See the
troubleshooting problem number five in Chapter 12 for possible so-
lutions. If your sample is not below 0.03 a
w
, but is still getting the
error message, refer to Chapter 12 for other possible explanations.
8.6 Samples Not at Room Temperature
Samples that are 4
◦
C colder or warmer than the instrument (cham-
ber) temperature will need to equilibrate to instrument tempera-
ture before you can make a fast and accurate reading can be made.
Rapid changes in temperature over short periods of time will cause
the water activity readings to rise or fall until the temperature stabi-
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