WP4C 5 CALIBRATION AND VERIFICATION
5 Calibration and Verification
5.1 Verification
The WP4C uses the chilled mirror dew point technique for mea-
suring water potential. This is the primary measurement method,
though instrument cleanliness can affect the calibration. We fix the
calibration slope during factory calibration. The user can adjust the
zero offset and calibrate successfully with any solution of known wa-
ter potential. We recommend using the 0.5 mol/kg KCl verification
standard available from METER.
5.2 Verification Standards
Verification standards are specially prepared salt solutions that have
a specific molality and water potential. The potassium chloride (KCl)
verification standards are accurate, easy to use, and readily available
from METER. Most importantly, they greatly reduce preparation
errors.
The standards are produced under a strict quality assurance regime
and are shelf stable for one year. If for some reason you cannot obtain
METER’s verification standards and need to make a salt solution for
verification, refer to Appendix A.
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.
5.3 When to Verify Calibration
The calibration of your WP4C should be checked with the KCl stan-
dard before each use. It can also be checked by measuring distilled
water, but this is often not a good choice for checking calibration.
When using distilled water, the humidity of the chamber approaches
100% which can cause condensation to occur if the sample is warmer
than the chamber. For batch processing, you should regularly check
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