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WP
PotentiaMeter
4C
9. Theory: Water Potential
sure at dewpoint 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 cal-
culated) 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 equilib-
rium 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.
Effect of Temperature on Water
Potential
Temperature plays a critical role in water potential deter-
minations. Most critical is the measurement of the differ-
ence 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 measure-
ments to be accurate to 0.05 MPa, temperature difference
measurements need to be accurate to 0.006°C.
WP4C's infrared thermometer measures the difference in
temperature between the sample and the block. It is care-
fully 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.
37
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