Measuring soil moisture is an essential thing in water management and agricultural research(Plant phenomics e.g. drought resistance) works. Although there are several methods but a few of them accepted for research purpose.
The Low-cost methods all depend on the electrical characteristics, the simpler ones are just Capacitive sensors which measures the capacitive reactance formed by Soil as a dielectric medium.
What are the existing sensor techniques ?
Most of them use the change in dielectric property of soil except the tensiometer and neutron sensor.
The tensiometer reads the pressure change by water evaporation from surrounding soil, and a gauge shows the pressure change interpreted or scaled as water potential.
The neutron gauge uses the neutron scattering power of water, the sensors contains some radioactive substance and radiates neutron particles if there is enough water in surrounding soil then it will be moderated and scattered by the hydrogen in water molecule. Although this sensors is most accurate for radioactive nature of the sensor makes it outofmarket, buyer have to get special permission from the government for this.
Rest of the sensors use the dielectric property of water so the overall dielectric property of soil, but the technique to read the dielectric property is different for each type of sensors.
Resistive Sensor
Capacitive Sensor
Time Domain reflectometry
Frequency Domain Reflectometry
Terawave sensing
Again the capactive sensors have different classes
Low Frequency i.e. 100-200KHz
High Frequency Sensors 60-80MHz
PCB based Capacitive Probe
Annular Ring type Probe (Gopher/MicroGopher/Redlight Green light)
Fringe Capacitance sensor
and the method of capacitance measurement
The Low-cost methods all depend on the electrical characteristics, the simpler ones are just Capacitive sensors which measures the capacitive reactance formed by Soil as a dielectric medium.
What are the existing sensor techniques ?
Most of them use the change in dielectric property of soil except the tensiometer and neutron sensor.
The tensiometer reads the pressure change by water evaporation from surrounding soil, and a gauge shows the pressure change interpreted or scaled as water potential.
The neutron gauge uses the neutron scattering power of water, the sensors contains some radioactive substance and radiates neutron particles if there is enough water in surrounding soil then it will be moderated and scattered by the hydrogen in water molecule. Although this sensors is most accurate for radioactive nature of the sensor makes it outofmarket, buyer have to get special permission from the government for this.
Rest of the sensors use the dielectric property of water so the overall dielectric property of soil, but the technique to read the dielectric property is different for each type of sensors.
Resistive Sensor
Capacitive Sensor
Time Domain reflectometry
Frequency Domain Reflectometry
Terawave sensing
Again the capactive sensors have different classes
Low Frequency i.e. 100-200KHz
High Frequency Sensors 60-80MHz
PCB based Capacitive Probe
Annular Ring type Probe (Gopher/MicroGopher/Redlight Green light)
Fringe Capacitance sensor
and the method of capacitance measurement
- Making a capacitance controlled oscillator and measure the frequency
- Using the Capacitor for Rectification of a HF wave and the output signal voltage is measured.
- Using the fringe effect on capacitance
the soil salinity affects a lot in low frequency capacitance measurement and high frequency measurement is least affected. and this makes the low frequency sensors obsolete now the most popular sensors are the 80MHz PCB Probe type sensors(Example Decagon SH100)
TDR Probes from different manufaturers
TDR150, TDR35 from specmeters
TDR200 from campbellscientific
Delta-T Theta Probe, ML3 Probe
HydraSense CampbellSci
TDR Probes from different manufaturers
TDR150, TDR35 from specmeters
TDR200 from campbellscientific
Delta-T Theta Probe, ML3 Probe
HydraSense CampbellSci
