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SAMPLE PREPARATION
METER recommends running the test as a DVS rather than DDI to ensure equilibrium is
reached. DDI tests on these sample types do not reach full equilibrium and can show increased
hysteresis. Sampling techniques are especially important for these sample types. The more
surface area to volume the better the test runs. Glass beads can be helpful in some instances.
8.3 VOLATILE SAMPLES
Samples with certain volatiles in high enough concentrations may give inaccurate water
activity values when using the chilled mirror. 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.
To determine if volatiles are a problem, compare the dew point readings to capacitance
readings. If the dew point readings are >0.02 a
w
higher than the capacitance readings,
volatiles are likely a problem. The sample should be read with volatiles turned to on.
After measuring volatiles with the capacitance sensor, clean the chamber (Section5.2),
place charcoal in the chamber, and seal for a minimum of 5 min before switching to the dew
point sensor.
8.4 LOW WATER ACTIVITY
When a sample water activity is below the cooling capacity of the chilled mirror, the VSA
displays a less-than symbol or gives a low water activity message in the software. If the error
persists, see Section9.1.
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