How does the WP4 measure water potential?
WP4 uses the chilled-mirror dewpoint technique to measure the water potential of a sample. In a dewpoint 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 relative humidity of the air in the chamber is the same as the water potential of the sample. In the WP4, 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 cell. 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. WP4 then signals you by flashing a green LED and/or beeping. The final water potential and temperature of the sample is then displayed.
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