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WP4C
4. SERVICE
This section describes the calibration and maintenance of the WP4C. Troubleshooting
solutions and customer service information are also provided.
4.1 CALIBRATION
The WP4C uses the chilled mirror dew point technique for measuring water potential.
METER adjusts the calibration slope during factory calibration. The user can adjust the zero
offset and calibrate successfully with any solution of known water potential (Section4.1.1).
Calibration can also be checked by measuring distilled water, but this is often not a good
choice. 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. METER
recommends using the 0.5 mol/kg KCl verification standard available from METER.
The WP4C calibration should be checked before each use and at regular intervals. This will
alert the user to the possibility of contamination of the unit or shifts in the calibration from
other causes. The WP4C calibration should also be checked after cleaning to identify any
change that may have occurred during cleaning procedures.
If a change has occurred, refer to Section4.1.2 for steps to correct the calibration. If the
instrument continues to read incorrectly, contact Customer Support.
4.1.1 VERIFICATION STANDARDS
Verification standards are specially prepared salt solutions that have a specific molality
and water potential. METER recommends using potassium chloride (KCl) verification
standards. The KCl standards are accurate, easy to use, and readily available from METER,
which greatly reduces preparation errors.
The standards are produced under a strict quality assurance regime and are shelf stable for
1 year. If for some reason verification standards are not available from METER, instructions
for making a salt solution for verification are located in Appendix A.
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.
4.1.2 CALIBRATION PROCESS
Since errors in the calibration values result in errors in all values subsequently measured,
carefully follow the calibration instructions to get accurate values.
NOTE: The WP4C will automatically shift to Precise mode for calibration.
1. Check to be sure the sample temperature is below sample block temperature (lower
right button).
2. Press the lower left button to access the System Setup screen.
3. Press the upper right button to begin calibration.
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