WP
4
PotentiaMeter
Sample Preparation
32
3. Make sure that the rim and outside of the
sample cup are clean. Wipe any excess sam-
ple material from the rim of the cup with a clean
tissue. Material left on the rim or the outside of
the cup will contaminate the sensor chamber
and will be transferred to subsequent samples.
The rim of the cup forms a vapor seal with the
sensor block when the drawer knob is turned to
the READ position. Therefore, any sample mate-
rial left on the cup rim will be transferred to the
block, preventing this seal and contaminating
future samples.
4. If a sample will be read at some future
time, put the sample cup’s disposable lid on
the cup to restrict water transfer. For short-
term storage (< 3 hours) the cup lid is accept-
able. If it will be a long time before the measure-
ment is made, seal the lid with tape or Parafilm
completely around the cup/lid junction.
Dry Samples
Samples that have a water potential drier than
about -300 MPa cannot be accurately measured with
the WP4. However, samples with such dry water
potential values are rare. When a sample’s water
potential value is higher than about -300 MPa, WP4
will display an error message indicating the last
reading it could make on that particular sample. For
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