Decagon and METER Phytos 31 Leaf Wetness Sensors

How do the Decagon and the METER Phytos 31 Leaf Wetness sensors work?

The Davis EnviroMonitor system and The Yield’s microclimate sensing system (MCSS) are compatible with both the Phytos 31 sensor and the Decagon Leaf Wetness sensor - which are manufactured by the METER group. The sensor was first produced by Decagon but was later manufactured by METER group. So whilst the names are different, the sensors are the same.

The sensors work by measuring the electrical resistance between two electrodes on the sensor. When the leaf is dry, the resistance is high. When the leaf is wet, the resistance decreases as the water conducts electricity. The sensor uses this change in resistance to determine the wetness of the leaf and outputs a signal that can be interpreted by other equipment to indicate the presence of moisture.

How the METER Phytos 31 mimics a real leaf

This sensor has been specially designed to closely approximate the thermodynamic properties of a leaf. 

If the specific heat of a leaf is estimated at 3750 J kg-1 K-1, the density is estimated to be 0.95 g/cm3 , and the thickness of a typical leaf is 0.4 mm, then the heat capacity of the leaf is 1425 J m-2 K -1. This is closely approximated by the thin (0.65mm) fiberglass construction of the sensor, which has a heat capacity of 1480 J m-2 K-1. By mimicking the thermodynamic properties of a real leaf, the sensor is able to more closely match 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 sensor 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 sensor. This is expected and will not affect the sensor’s function.

The surface coating of the sensor 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 sensor 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. Painting and individual sensor calibration is not necessary with the Phytos 31.

What are the technical specifications of the Phytos 31?

Specifications: PHYTOS 31 Tech Specs

I want to learn more, can you share more information on my leaf wetness sensor?

Check out these useful resources for more information:

• METER Phytos 31 Leaf Wetness Sensor

• Instructional Guide for the Phytos 31 Leaf Wetness Sensor

• Decagon Leaf Wetness Sensor Overview