22-09-2017, 10:49 AM
Soil moisture sensors measure the volumetric content of water in the soil. Since direct gravimetric measurement of soil free moisture requires the removal, drying and weighting of a sample, soil moisture sensors indirectly measure the volumetric water content using some other soil property, such as electrical resistance, constant dielectric or neutron interaction as an indicator of moisture content. The relationship between measured property and soil moisture should be calibrated and may vary depending on environmental factors such as soil type, temperature or electrical conductivity. Reflected microwave radiation is affected by soil moisture and is used for remote sensing in hydrology and agriculture. Portable probe instruments can be used by farmers or gardeners.
Soil moisture sensors usually refer to sensors that estimate the volumetric water content. Another class of sensors measures another property of moisture in soils called water potential; these sensors are usually called soil water potential sensors and include tensiometers and gypsum blocks.
The technologies commonly used to indirectly measure the volumetric content of water (soil moisture) include:
• Frequency Domain Reflectometry (FDR): The dielectric constant of a certain volume element around the sensor is obtained by measuring the operating frequency of an oscillating circuit.
• Time Domain Transmission (TDT) and Time Domain Reflectometry (TDR): The dielectric constant of a given volume element around the sensor is obtained by measuring the propagation velocity along a buried transmission line.
• Neutron Moisture Meters: The moderating properties of water for neutrons are used to estimate soil moisture content between a source and a detection probe.
• Soil resistivity: To determine the moisture content of the soil, it is possible to measure how strongly the soil resists the flow of electricity between two electrodes.
• Galvanic cell: The amount of water present can be determined by the voltage produced by the soil because the water acts as an electrolyte and produces electricity. The technology behind this concept is the galvanic cell.
Soil moisture sensors usually refer to sensors that estimate the volumetric water content. Another class of sensors measures another property of moisture in soils called water potential; these sensors are usually called soil water potential sensors and include tensiometers and gypsum blocks.
The technologies commonly used to indirectly measure the volumetric content of water (soil moisture) include:
• Frequency Domain Reflectometry (FDR): The dielectric constant of a certain volume element around the sensor is obtained by measuring the operating frequency of an oscillating circuit.
• Time Domain Transmission (TDT) and Time Domain Reflectometry (TDR): The dielectric constant of a given volume element around the sensor is obtained by measuring the propagation velocity along a buried transmission line.
• Neutron Moisture Meters: The moderating properties of water for neutrons are used to estimate soil moisture content between a source and a detection probe.
• Soil resistivity: To determine the moisture content of the soil, it is possible to measure how strongly the soil resists the flow of electricity between two electrodes.
• Galvanic cell: The amount of water present can be determined by the voltage produced by the soil because the water acts as an electrolyte and produces electricity. The technology behind this concept is the galvanic cell.