Decagon Devices Dielectric Leaf Wetness Sensor Besitzer-/Bedienerhandbuch - Seite 15

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5 LWS THEORY

LWS Theory

5.1

How the LWS Works

The LWS 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 dependends on the presence of

moisture or frost on the sensor surfaces. The sensor outputs a mV

signal proportional to the dielectric of the measurement zone, and

therefore proportional to the amount of water or ice on the senor

surface.

5.2

How the LWS Mimics a Real Leaf

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

, and the thickness of a typical leaf is 0.4 mm, then the heat

capacity of the leaf is 1,425 J

by the thin (0.65 mm) ﬁberglass construction of the LWS, which has

a heat capacity of 1,480 J

namic properties of a real leaf, the LWS 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 radi-

ation 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 LWS absorbs well in the

near-infrared region, but the white color reﬂects most of the visible

radiation. Spectrophotometer measurements indicate that the over-

all 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 LWS. This is expected and does not aﬀect the

probes function.

The surface coating of the LWS is hydrophobic – similar to a leaf

, the density is estimated to be 0.95

This is closely approximated

. By mimicking the thermody-

LWS