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SERVICE
4. SERVICE
This section describes the calibration and maintenance of the PAWKIT. Troubleshooting
solutions and customer service information are also provided.
4.1 VERIFICATION AND CALIBRATION
The PAWKIT takes water activity measurements by measuring the change in electrical
properties of a special polymer held between two electrodes. Because of the nature of the
capacitance humidity sensor, it is important to verify the PAWKIT 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 maintain optimal performance.
4.1.1 VERIFICATION STANDARDS
The PAWKIT uses three calibration standards:
• 6.00 mol/kg NaCl (0.760 a
w
)
• 13.41 mol/kg LiCl (0.250 a
w
)
• 2.33 mol/kg NaCl (0.920 a
w
)
Verification standards are specially prepared, unsaturated salt solutions having a specific
molality and water activity value that are accurately measurable. The verification standards
that were sent with the initial shipment are very accurate and readily available from METER.
The verification standards have been produced under a strict quality assurance regime, and
their accuracy is verified by an independent third party instrument. They are very accurate,
easy to use, and readily available from METER. Using verification standards to verify
accuracy can greatly reduce preparation errors. For these reasons, METER recommends
using these standards for the most accurate PAWKIT calibration. The verification standards
are shelf-stable for 1 year.
To use a verification standard, remove the twist top and pour the contents into an AQUALAB
sample cup. Information about the standard value and molality can be found printed on
the outside of the plastic vial. If for some reason METER verification standards are not
available, contact Customer Support for instructions on how to create a verification standard
salt solution.
The capacitance sensor can exhibit hysteresis. When a high water activity standard or
sample is measured, this may influence future readings. After measuring a high water
activity sample, allow 1 h for the sensor to dry. Additionally, desiccant or activated carbon
can be used to accelerate the drying process.
NOTE: To avoid inaccurate water activity readings, verification standards should be used once immediately after
opening and not stored in sample cups for repeated use.
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