Campbell Scientific LWS Decagon Leaf Wetness Sensor - Manual (Page 9)

LWS-L Dielectric Leaf Wetness Sensor
7.2 Leaf Mimicry
The LWS-L 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 (0.65mm) fiberglass construction of the LWS-L,
which has a heat capacity of 1480 J m
-2 K -1 . By mimicking the thermodynamic
properties of a real leaf, the LWS-L 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-L absorbs well in the near-infra red 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
probe’s function. The surface coating 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 pubescent
leaves or leaves with less waxy cuticles.
8. Example Programs
8.1 CR10X Datalogger Program
Color Description CR10X
White Excitation EX1
Red Analog Out SE1
Bare Analog Ground AG
;{CR10X}
;
*Table 1 Program
01: 60
Execution Interval (seconds)
1: Excite-Delay (SE) (P4)
1: 1
Reps
2: 5 2500 mV Slow Range
3: 1 SE Channel
4: 1 Excite all reps w/Exchan 1
5: 1 Delay (0.01 sec units)
6: 2500 mV Excitation
7: 1 Loc [ LWS_mV ]
8: 1 Multiplier
9: 0 Offset
5