WP
4
PotentiaMeter
Theory
45
Measuring Water Potential
The water potential of a solid or liquid sample can
be found by relating the sample water potential
reading to the vapor pressure of air in equilibrium
with the sample. The relationship between the sam-
ple’s water potential (Ψ) and the vapor pressure of
the air is:
where p is the vapor pressure of the air, p
o
is the
saturation vapor pressure at sample temperature, R
is the gas constant (8.31 J/mol K), T is the Kelvin
temperature of the sample, and M is the molecular
mass of water. The vapor pressure of the air can be
measured using a chilled mirror, and p
o
is computed
from sample temperature.
The WP4 measures water potential by equilibrating
the liquid phase water of the sample with the vapor
phase water in the headspace of a closed chamber,
then measuring the vapor pressure of that head-
space. In the WP4, a sample is placed in a sample
cup, which is sealed against a sensor block. Inside
the sensor block is a fan, a dew point sensor, a tem-
perature sensor, and an infrared thermometer. The
dew point sensor measures the dew point tempera-
ture of the air, and the infrared thermometer mea-
sures the sample temperature. The purpose of the
Ψ
RT
M
-------
p
p
o
-----ln⋅=
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