Umwelt-Geräte-Technik GmbH | Eberswalder Str. 58 | D-15374 Müncheberg | Tel.: +49 (0) 33 43 2 - 89 575 | www.ugt-online.de Seite 8 von 12
3. Mode of operation of the UMP - 2
3.1. Water content measurement
The water content measurement procedure of the UMP-2 sensor is can neither be fully
described as FDR nor TDR method.
The basic mode of operation however corresponds to the FDR process, because the sensor
operates with a continuous signal at constant frequency (120 MHz) and not with a pulse.
However phase shift instead of frequency change is used to calculate the measurement
reading.
The measuring line consists of two stainless steel rods (antennas) of 50 mm length and with
a diameter of 3 mm. These dimensions and the measurement frequency produce a
measurement volume of approximately 500 ml. Faults within the measurement volume such
as vessel walls, large air pockets or cables can influence the measurement result. A
continuous signal runs along the antennas in the ground. The waves of the signal running
back and forth are compared here. The dielectric constant εof the surrounding soil/water
mix is determined from the phase difference of the signals running back and forth. The
dielectric constant for pure water at 20°C is ε=80. Dry soil has an εof 2 to 5. The dielectric
constant of the soil/water mix lies between these two levels, and is higher the more water
there is in the soil.
Using the calculated dielectric constant, the volumetric soil water content is then
determined according to the Topp/Davis equation.
The εvalue can also be called up when making independent water content calculations using
other mathematical formulas.
3.2. Electrical conductivity measurement
The UMP-2 sensor records the electrical conductivity of the soil by means of an additional
circuit (IC) independently of the soil moisture measurement. This system offers two main
advantages. On the one hand an extremely precise measurement of conductivity is possible
using the components and measurement principle. On the other hand the independently
measured conductivity can be used to correct the calculated water content.
Electrical conductivity effects the measurement of the dielectric constant of the soil. With an
increased electrical conductivity, the dielectric constant increases, which leads to falsely
elevated water content readings. Using a formula to adjust to a higher conductivity prevents
an offset in soil water content.
In order to measure conductivity, the soil moisture measurement is interrupted for a short
period of time, and a signal in the kHz range transmitted over the antennae. The
corresponding IC measures the impedance of several different frequencies and averages the
results.
