Vaisala Hydromet Data Collection Platform - Manual (Page 88)

User's Guide ______________________________________________________________________
86 __________________________________________________________________ M210933EN-C
Soil Moisture Sensor EC-5
Soil Moisture Sensor EC-5 uses capacitance to measure the dielectric
permittivity of the surrounding medium. The volume of water in the
total volume of soil influences most heavily the dielectric permittivity
of the soil because the dielectric permittivity of water is much greater
than that of the other constituents of the soil. Thus, when the amount of
water changes in the soil, the EC-5 probe measures the change in
capacitance (from the change in dielectric permittivity) that can be
directly correlated with a change in water content. Circuitry inside the
EC-5 probe changes the capacitance measurement into a proportional
millivolt output.
MAWS software calculates the volumetric water content (VWC, m
3
/
m
3
) values based on the sensor millivolt output. MAWS software uses
calibration coefficients provided by the sensor manufacturer.
Calibration coefficients are provided for mineral soil, potting soil, and
rock wool. VWC of saturated soils is generally 40–60%, which is why
MAWS uses the manufacturer's calibration coefficients, purposed for
that range. VWC from 0 to 100% needs sensor-specified soil type
calibration. MAWS does not support this feature as factory default.
EC-5 factory calibration equations (VWC, m
3
/m
3
) are as follows:
Mineral soil: 11.9 * 10-4 * mV - 0.401
Potting soil: 0.3 * 10-4 * mV - 0.334
Rock wool: 2.63 * 10-6 * mV2 + 5.07 * 10-4 * mV - 0.0394
The result of the equation is (VWC, m
3
/m
3
). Millivolt (mV) values of
the equations are the sensor raw output values, which are used in the
calculation. These coefficients are valid only when the data logger
provides 2500 mV excitation voltage for the sensor. These equations are
based on the information that VWC of saturated soils is generally 40–
60%.