WP
4
PotentiaMeter
About the WP4
6
water is bound, structurally or chemically, within a
substance. Water potential is the vapor pressure of
air in equilibrium with a sample in a sealed mea-
surement chamber. For a more detailed description
of water potential, please refer to Chapter 9, titled
“Theory: Water Potential” of this manual.
How the WP4 works
WP4 uses the chilled-mirror dewpoint technique to
measure the water potential of a sample. In this
type of instrument, the sample is equilibrated with
the headspace of a sealed chamber that contains a
mirror and a means of detecting condensation on
the mirror. At equilibrium, the water potential of the
air in the chamber is the same as the water potential
of the sample. In the WP4, the mirror temperature is
precisely controlled by a thermoelectric (Peltier)
cooler. Detection of the exact point at which con-
densation first appears on the mirror is observed
with a photoelectric cell. A beam of light is directed
onto the mirror and reflected into a photodetector
cell. The photodetector senses the change in reflec-
tance when condensation occurs on the mirror. A
thermocouple attached to the mirror then records
the temperature at which condensation occurs. WP4
then signals you by flashing a green LED and/or
beeping. The final water potential and temperature
of the sample is then displayed.
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