WP4C 8 THEORY: WATER POTENTIAL
of a closed chamber, then measuring the vapor pressure of that
headspace. In the WP4C, 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 temperature sensor, and an infrared thermome-
ter. The dew point sensor measures the dew point temperature of
the air, and the infrared thermometer measures the sample temper-
ature. The purpose of the fan is to speed equilibrium and to control
the boundary layer conductance of the dew point sensor.
From these measurements, the WP4C computes the vapor pressure
of the air in the headspace as the saturation vapor pressure at dew
point temperature. When the water potential of the sample and the
headspace air are in equilibrium, the measurement of the headspace
vapor pressure and sample temperature (from which saturation va-
por pressure is calculated) gives the water potential of the sample.
In addition to equilibrium between the liquid phase water in the
sample and the vapor phase, the internal equilibrium of the sample
itself is important. If the sample is not at internal equilibrium, one
might measure a steady vapor pressure (over the period of measure-
ment) which is not the true water potential of the sample.
8.3 Effect of Temperature on Water Potential
Temperature plays a critical role in water potential determinations.
Most critical is the measurement of the difference between sample
and dew point temperature. If this temperature difference were in
error by 1
◦
C, an error of 8 MPa would result. In order for water
potential measurements to be accurate to 0.05 MPa, temperature
difference measurements need to be accurate to 0.006
◦
C.
The WP4C infrared thermometer measures the difference in tem-
perature between the sample and the block. It is carefully calibrated
to minimize temperature errors, but achieving 0.006
◦
C accuracy is
difficult when temperature differences are large. Best accuracy is
therefore obtained when the sample is near chamber temperature.
Another effect of temperature on water potential occurs with samples
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