LWS Dielectric Leaf Wetness Sensor
4
(≈5) are much higher than that of air (≈1), so the measured dielectric constant
is strongly dependent 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 sensor surface.
5.2 Leaf Mimicry
The LWS is designed to approximate the thermodynamic properties of most
leaves. If the specific heat of a typical leaf is estimated at 3750 J kg
–1
K
–1
,
density estimated at 0.95 g/cm
3
, and thickness estimated at 0.4 mm, then the
heat capacity of the leaf is
≈1425 J m
–2
K
–1
. This heat capacity is closely
approximated by the thin (0.65 mm) fiberglass construction of the LWS, which
has a heat capacity of 1480 J m
–2
K
–1
. By mimicking the thermodynamic
properties of a leaf, the LWS closely matches the wetness state of the canopy.
The sensor closely matches the radiative properties of real leaves. Healthy
leaves generally absorb solar radiation in much of the visible portion of the
spectrum, but selectively reject much of the energy in the near-infrared. The
surface coating of the LWS 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 coating, which does not affect the
function of the sensor. The surface coating is hydrophobic — similar to a leaf
with a hydrophobic cuticle. The sensor matches the wetness state of these types
of leaves, but may not match the wetness duration of pubescent leaves or
leaves with less waxy cuticles.
6. Specifications
Features:
•
Imitates characteristics of a leaf
•
Does not require painting or calibration of individual sensors
•
Detects trace amounts of water or ice on the leaf surface
•
Compatible with Campbell Scientific CRBasic data loggers:
CR200(X) series, CR300 series, CR6 series, CR800 series, CR1000,
CR1000X, CR3000, CR5000, and CR9000(X)
Settling Time:
10 ms
Excitation:
2.5 Vdc (2 mA) to 5.0 Vdc (7 mA)
Minimum Excitation Time:
10 ms
Output:
300 to 1250 mV (depends on excitation
voltage)
Operating Temperature:
–40 to 60 °C
Length:
12.0 cm (4.7 in)
Width:
5.8 cm (2.3 in)
