6 SAMPLE PREPARATION WP4C
6.4 Samples and Temperature
If samples are warmer than the chamber when they are placed in it
(Ts-Tb is a positive number), condensation may occur and moisture
may condense inside the block. In order to prevent this, follow steps
1 and 2.
1. Place your sample in the chamber, slide the drawer closed and
press the lower right button to access the sample temperature
screen and look at the temperature difference. If the sample
temperature is shown to be a positive number, take the sample
out immediately and let it cool on a cold surface with the cup
lid on it to preserve the moisture. Do not cool the sample too
much, or the equilibrium time will be lengthened (ideally the
Ts-Tb will be between −0.5 and 0).
2. After cooling it for a minute or so, place the sample back in
and note the temperature difference. If it is close enough to
the block temperature, turn the knob to the READ position to
begin reading.
There is a linear relationship between the sample dew point temper-
ature and its water potential. The dew point decreases −0.12
◦
C
per MPa. For example, a very dry sample at −40 MPa can be 4.8
◦
C (−.12 ∗ −40) above the chamber temperature without condens-
ing. 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 water potential, you can
gauge at which temperature it will condense moisture. For samples
that are more than 1
◦
C below chamber temperature, the WP4C
waits until their temperature increases to 1
◦
C below chamber tem-
perature to start a reading. Readings are therefore sped up if sample
temperature is just a little below chamber temperature.
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