WP
4
PotentiaMeter
Theory
46
fan is to speed equilibrium and to control the
boundary layer conductance of the dew point sen-
sor.
From these measurements, the vapor pressure of
the air in the headspace is computed as the satura-
tion vapor pressure at dewpoint temperature. When
the water potential of the sample and the headspace
air are in equilibrium, the measurement of the head-
space 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 inter-
nal equilibrium of the sample itself is important. If a
system 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 system.
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 tem-
perature. 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 0.1
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