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OpERATION
2.3 INTERpRETING DATA
Most leaf wetness applications (disease forecasting, etc.) only require knowledge if there
is any water on the surface of the leaf, not knowledge of the amount of water. To make this
determination, a sensor output threshold corresponding to the minimum wet state must be
identified.
A dry PHYTOS 31 outputs approximately 435 raw counts when read with METER data
loggers. When the sensor is totally wet, as in a heavy rain, the signal can range up to around
1,100 counts. Varying amounts of water on the surface of the sensor cause a sensor output
proportional to the amount of water on the sensor's surface. Ice has a much lower dielectric
constant than that of liquid water, so the sensor output from frost is much lower than that
from a similar amount of rain or dew (
Figure 5 Example raw pHYTOS 31 output
2.3.1 UNDERSTANDING DATA FROM METER DATA LOGGERS
With METER data loggers, users have two options for interpreting data: using number of
minutes wet (processed data) or raw probe output (raw data). The sensors are factory
calibrated to read 435 raw counts when dry. When configuring a METER logger port to read
the PHYTOS 31, the logger reads the sensor every minute and compares the data to two
predefined wetness thresholds of 450 and 460 raw counts to determine the number of
minutes wet during a measurement interval.