WP4C uses the chilled-mirror dewpoint technique to measure
the water potential of a sample. In this type of instrument,
the sample is equilibrated with the headspace of a
sealed chamber that contains a mirror and a means of
detecting condensation on the mirror. At equilibrium, the
water potential of the air in the chamber is the same as the
water potential of the sample. In the WP4C, the mirror
temperature is precisely controlled by a thermoelectric
(Peltier) cooler. Detection of the exact point at which condensation
first appears on the mirror is observed with a
photoelectric cell. A beam of light is directed onto the mirror and reflected into a photodetector. The photodetector
senses the change in reflectance when condensation
occurs on the mirror. A thermocouple attached to the
mirror then records the temperature at which condensation
occurs. Values begin to be displayed indicating that
initial measurements are being taken. WP4C then signals
you by flashing a green LED and/or beeping when final
values are reached. The final water potential and temperature
of the sample is displayed.
In addition to the technique described above, WP4C uses an
internal fan that circulates the air within the sample chamber
to reduce time to equilibrium. Since both dewpoint and
sample surface temperatures are simultaneously measured,
the need for complete thermal equilibrium is eliminated.
The WP4C controls the sample temperature by means of
an internal thermo-electrical module that monitors and
stabilizes the sample block temperature according to how
it is set.