Pawkit
6
VERIFICATION AND CALIBRATION
6
Verification and Calibration
The Pawkit takes water activity measurements by measuring the
change in electrical properties of a special polymer held between two
electrodes. Due to the nature of the capacitance humidity sensor, it is
important to verify the AquaLab water activity calibration against
known standards to guarantee optimal performance and accuracy.
METER recommends verification daily, once per shift or before each
use. METER also recommends annual factory calibration to main-
tain optimal performance.
6.1
Verification Standards
The Pawkit uses four calibration standards: 6.00 mol/kg NaCl (0.760
a
w
), 13.41 mol/kg LiCl (0.250
a
w
), and 2.33 mol/kg NaCl (0.920
a
w
).
You received a small supply of these standards with your instrument.
These standards are specially prepared salt solutions at specific con-
centrations for constant and accurate water activity measurements.
They have been produced under a strict quality assurance regime,
and their accuracy is verified by an independent third party instru-
ment. They are very accurate, easy to use, and readily available from
METER. Most importantly, they greatly reduce preparation errors.
Because of these reasons, we recommend using these standards for
the most accurate calibration of your Pawkit. The verification stan-
dards are shelf-stable for one year.
If these standards are not available you can make a saturated Sodium
Chloride (NaCl) slurry with a water activity value of 0.75
a
w
. To
make a salt slurry of NaCl add water until the salt can absorb no
more water, as evidenced by the presence of free liquid. The slurry
should take the shape of the cup and flow when tipped with the
amount of free liquid at a minimum.
Note: To avoid inaccurate water activity readings, verification stan-
dards should be used once immediately after opening and not stored
in sample cups for repeated use.
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