8 SAMPLE PREPARATION AquaLab
condense on the mirror during the reading process, but do not evap-
orate 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 most effective method to determine if volatiles are a problem
is to compare dew point readings to capacitance readings. If the
dew point readings are more than 0.018 higher than the capacitance
readings, volatiles are likely a problem.
Decagon designed the Series 4TEV 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, as described in Section
5. The only difference in operation is a lower accuracy of ±0.015
a
w
for the capacitance sensor. After measuring volatiles with the
Capacitance 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, the AquaLab displays an error message indicating the
lowest reading it attained on that particular sample. See the trou-
bleshooting problem number five in Section 12 for possible solutions.
If your sample is not below 0.03 a
w
, but is still getting the error
message, refer to Section 12 for other possible explanations.
8.6 Samples Not at Room Temperature
Samples that are 4
◦
C colder or warmer than the instrument chamber
temperature need to equilibrate to instrument temperature before
you can make a fast and accurate reading. Rapid changes in temper-
ature over short periods of time causes the water activity readings
to rise or fall until the temperature stabilizes. When the tempera-
ture stabilizes within an optimal one or two degrees of the chamber
temperature, you can proceed with normal measurements.
52
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