SMT100

The SMT100, designed and manufactured by TRUEBNER, combines the advantages of the low cost FDR sensor systems with the accuracy of a TDR system. Like a TDR, it measures the travel  time of a signal to determine the dielectric constant of the soil. And like a FDR, it converts this dielectric constant into an easy to measure frequency. But unlike an FDR it isn’t based on a capacitor, but utilizes a ring oscillator to transform the signal’s travel time into the measure frequency. The resulting frequency (>100 MHz) is high enough to operate well in clayey soils.

Compact, functional and robust: the PCB board based design allows for an economic design, and the blade shape facilitates installation. The casing and cable is water sealed. Different communication protocols are available, as well as analogue output.

The SMT100 has a broad measurement range. It is maintenance free and frost resistant. With short response times and low disturbance by salinity it is designed for a long term observation.

Specification
Accuracy Soil volumetric water content (VWC):
Using factory calibration up to ± 3% (VWC) in mineral soils
with moderate salinity from 0 to 50% VWC
Using medium specific calibration up to ± 1% (VWC)

Temperature:
Typical ± 0.2°C, max. ± 0.4°C over full range
Analog version ± 0.8°C

Additional output (digital versions only):
Raw measurement data
Dielectric permittivity
Resolution 0.1% volumetric water content or better
0.01°C or better (analog version 0.2°C)
Range 0 to 60% volumetric water content
(up to 100% volumetric water content with limited accuracy)
Temperature: -40 to +80°C (analog version -40 to +60°C)
Output options RS485 with TBUS
RS485 with Modbus
RS485 with ASCII
SDI-12
Analog: 0 - 10 V (other voltage ranges on request)
Power 4-24 V DC, up to 40 mA during measurement
(Analog version 12 - 24 V DC for  0 - 10 V output)
Measurement time digital versions: less than 50 ms
Measurement time analog versions: less than 200 ms
Cable length 10 m
Sensor dimensions 18.2 cm x 3 cm x 1.2 cm
Data logger compatibility logger capable of appropriate power excitation and RS485
(TBUS, Modbus, ASCII), SDI-12 or analog input
Free PC logger software available on request