11
PHYTOS 31
3.3 THEORY
The PHYTOS 31 measures the dielectric constant of a zone approximately 1 cm from the
upper surface of the sensor. The dielectric constant of water (80) and ice (5) are much higher
than that of air (1), so the measured dielectric constant strongly depends on the presence of
moisture or frost on the sensor surfaces. The sensor outputs a millivolt signal proportional to
the dielectric of the measurement zone, and therefore proportional to the amount of water or
ice on the senor surface.
The sensor has been specially designed to closely approximate the thermodynamic
properties of a leaf. If the specific heat of a leaf is estimated at 3,750 J/kg
K, the density
is estimated to be 0.95 g/cm
3
, and the thickness of a typical leaf is 0.4 mm, then the heat
capacity of the leaf is 1,425 J/m
2
K. This is closely approximated by the thin (0.65-mm)
fiberglass construction of the PHYTOS 31, which has a heat capacity of 1,480 J/m
2
K. By
mimicking the thermodynamic properties of a real leaf, the PHYTOS 31 more closely matches
the wetness state of the canopy.
The sensor has also been engineered to closely match the radiative properties of real leaves.
Healthy leaves generally absorb solar radiation effectively in much of the visible portion of
the spectrum, but selectively reject much of the energy in the near-infrared portion of the
spectrum. The surface coating of the PHYTOS 31 absorbs well in the near-infrared region,
but the white color reflects most of the visible radiation. Spectroradiometer measurements
indicate that the overall radiation balance of the sensor closely matches that of a healthy
leaf. During normal use, prolonged exposure to sunlight can cause some yellowing of the
PHYTOS 31. This is expected and does not affect the sensor function.
The surface coating of the PHYTOS 31 is hydrophobic, similar to a leaf with a hydrophobic
cuticle. The sensor should match the wetness state of these types of leaves well, but may
not match the wetness duration of leaves with plentiful leaf hairs or less waxy cuticles. It is
impossible for any sensor to accurately mimic the properties of all leaves.
The PHYTOS 31 is engineered to be repeatable among units, so that relationships can be
determined between the wetness state of the sensor and the wetness state of various
agricultural or natural plant canopies.