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WP4C
WP4C computes water potential from the vapor pressure of air in equilibrium with the
sample in the sealed sample chamber. Large temperature differences between the sample
and sample block will cause longer reading times, since a complete and accurate reading
will not be made until the difference between the sample temperature and the sample
block temperature is <1.0 °C.
To monitor the temperature difference between the sample and the block, access the
Sample Temperature screen (Section3.2.4.4).
3.3.2 MEASURING WATER POTENTIAL
The water potential of a solid or liquid sample can be found by relating the sample water
potential reading to the saturation vapor pressure of air in equilibrium with the sample. The
relationship between the sample water potential (
Ψ) and the saturation vapor pressure of
the air is determined using Equation 1:
Equation 1
Ψ=
RT
M
eT
eT
ln
()
()
sd
where
• e
s
(T
d
) is the saturation vapor pressure of the air at dew point temperature,
• e
s
(T
s
) 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.
In the WP4C, the sample cup is sealed against a sensor block, while a fan speeds
equilibration of the sample with the headspace vapor and controls the boundary layer
conductance of the dew point mirror. The dew point mirror measures the dew point
temperature of the air, and an infrared (IR) thermometer measures the sample temperature.
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 vapor 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 measurement)
which is not the true water potential of the sample.
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