The sensor mote consists of two main elements as shown in the
photos above. The sensor probe is installed horizontally at around nine or
ten inches depth. The enclosure houses a Tmote Sky control/wireless
module, custom-built probe interface circuitry, and batteries. The enclosure
is elevated around twenty inches above ground.
The probe is an ECH2O-TE from
Decagon Devices . We collect dielectric permittivity
(moisture), electrical conductivity, and temperature readings every
15 minutes. In addition we collect
temperature and humidity readings for the ambient air within the
mote enclosure, which is vented. For this prototype we store
readings in the control module and periodically download them
manually to a laptop. However once operational the control
module will tranmit readings wirelessly to the base station at
the Development building.
Data collection failed initially during the cold nighttime
period. We isolated the problem to faulty synchronization in the
communication between the probe and control module. Since Nov. 28 data
collection has worked flawlessly. In addition power usage thus far
indicates we can expect several months of use from the four AA batteries.
Next we describe how the raw readings from the sensor are converted to
useful information, and the checks we performed to verify the accuracy of
Moisture is described as volumetric water content (VWC). We calculate
VWC using a linear equation for generic mineral soils:
θ = 1.087 * 10-3 * P - 0.629
where P is the dielectric permittivity reading from the sensor.
We confirmed that the raw counts in air and water match the
manufacturer's specs, which indicates accuracy of approximately
±3% for volumetric water content (VWC). We can perform more
precise calibration for this soil in the lab by wetting a known amount
with increasing quantities of water.
Electrical conductivity (EC) is described as pore water
EC, which measures the conductivity of the water in the pore space of the
soil. This value is not the same as solution EC, which commonly is returned
by a soil test. Solution EC is calculated by wetting the soil to saturation
with distilled water, and then measuring the conductivity of that solution.
According to Decagon, it is difficult to compare pore water EC with solution
Pore EC does not measure salinity per se. However, we are
most interested in changes to pore EC. In this case the main agent of change
is the water applied to the soil, and we expect the saline irrigation water
to increase pore EC and distilled rain water to decrease pore EC.
Pore EC is calculated using all three of the probe's readings:
σp = εp σb /
(εb - εb=0)
For more background, see the ECH2O-TE manual  and
Hilhorst (2000) .
σp is Pore EC measured in dS/m
εp = 80.3 - 0.37 * (Tsoil - 20), where
Tsoil is the soil temperature in °C
σb is the sensor bulk electrical conductivity
εb = 7.64 * 10-8 * P3 - 8.85 *
10-5 * P2 + 4.85*10-2 * P - 10,
where P is the sensor dielectric permittivity
εb=0 is assumed to be a generic value of 6
We performed some rough verification of the sensor's electrical
conductivity readings. We found the conductivity for distilled water,
tap water, and potting soil within expected values. Decagon claims
accuracy of ±20% for Pore EC for moist soils.
The probe measures temperature using a thermistor and reports the
reading directly. We confirmed the soil sensor readings at room
temperature are with ±1°C. For production we plan to
verify over a greater range of temperatures.
We also verified the accuracy of the control module's
temperature sensor. We found it to be ±1°C at room
temperature, and no more than ±2°C at 0°C. The temperature
sensor's specs specify ±1°C at 0°C; however we are limited
by the accuracy of our reference thermometer at this temperature.
 M.A. Hilhorst. A Pore Water Conductivity Sensor.
Soil Sci. Soc. Am. J. 64:1922-1925 (2000).