WP
4
PotentiaMeter
Sample Preparation
35
cup lid on it to preserve the moisture. Try not to
cool the sample too much, since this will
lengthen the equilibrium time.
2. After cooling it for a minute or so, place the
sample back in and note the temperature differ-
ence. If it is close enough to the block tempera-
ture, turn the knob to the READ position to
begin reading.
There is a linear relationship between the sample’s
dewpoint temperature and its water potential. The
dewpoint decreases -0.12 °C per MPa. For example,
a very dry sample at -40MPa can be 4.8 °C (-.12 x -
40) above the chamber temperature without
condensing. A sample at -1 MPa (fairly dry for most
soils) can be 0.12°C above the chamber temperature
without condensing. Therefore, if you know the
general range of your sample’s water potential, you
can gage at which temperature it will condense
moisture.
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