WP4C 6 SAMPLE PREPARATION
Make sure that the rim and outside of the sample cup are clean.
Wipe any excess sample material from the rim of the cup with a clean
tissue. Material left on the rim or the outside of the cup will con-
taminate 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 material left on the cup rim will be transferred to the
block, preventing this seal and contaminating future samples.
If a sample will be read at some future time, put the sample cup
disposable lid on the cup to restrict water transfer. For short-term
storage (< 3 hours) the cup lid is acceptable. If it will be a long time
before the measurement is made, seal the lid with tape or Parafilm
R
completely around the cup and lid junction.
6.3 Dry Samples
Samples that have a water potential drier than about −300 MPa
cannot be accurately measured with the WP4C. However, samples
with such dry water potential values are rare. When a sample water
potential value is drier than about −300 MPa, WP4C will display
an error message indicating the lowest reading it could make on that
particular sample. For example, if you were measuring a dry sample
and the following screen appeared:
This screen indicates that the last water potential reading the WP4C
measured on this sample was −301.8 Megapascals. Therefore, the ac-
tual water potential of the sample is lower than the instrument can
measure.
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