WP
4C
PotentiaMeter
2. About the WP4C
5
limited by accuracy in the wet end of the water potential range. The
range of 0 to -5 MPa has an accuracy of 0.05 MPa. For example,
a measurement at -0.1MPa has an accuracy of ±50% of the read-
ing, and -1 MPa has an accuracy of ±5% of the reading. Function-
ally this means that the WP4C will not measure water potential
accurately near field capacity.
WP4C and Water Potential
Water potential is a measurement of the energy status of
the water in a system. It indicates how tightly water is
bound, structurally or chemically, within a substance.
Water potential can be computed from the vapor pressure
of air in equilibrium with a sample in a sealed measure-
ment chamber. For a more detailed description of water
potential, please refer to Chapter 9, titled “Theory: Water
Potential” of this manual.
How the WP4C works
WP4C uses the chilled-mirror dewpoint technique to mea-
sure the water potential of a sample. In this type of instru-
ment, 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 WP4C, 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
