- Frequency domain sensor
Frequency domain (FD) sensor is an instrument developed for measuring
soilmoisture content. The instrument has an oscillating circuit, the sensing part of the sensor is embedded in the soil, and the operating frequency will depend on the value of soil's dielectric constant.
There are two types of sensors:
* "Capacitance probe", or fringe capacitance sensor. Capacitance probes use capacitance to measure the dielectric permittivity of the soil. The volume of water in the total volume of soil most heavily influences the dielectric permittivity of the soil because the dielectric of water (80) is much greater than the other constituents of the soil (mineral soil: 4, organic matter: 4, air: 1). Thus, when the amount of water changes in the soil, the probe will measure a change in capacitance (from the change in dielectric permittivity) that can be directly correlated with a change in water content. Circuitry inside some commercial probes change the capacitance measurement into a proportional millivolt output. Other configuration are like the
neutron probewhere an access tube made of PVCis installed in the soil. The probe consists of sensing head at fixed depth. The sensing head consists of an oscillatorcircuit, the frequency is determined by an annular electrode, fringe-effect capacitor, and the dielectric constantof the soil.
* "Electrical impedance sensor", which consists of soil probes and using electrical impedance measurement. The most common configuration is based on the
standing waveprinciple (Gaskin & Miller, 1996). The device comprises a 100 MHz sinusoidal oscillator, a fixed impedance coaxial transmission line, and probe wires which is buried in the soil. The oscillator signal is propagated along the transmission line into the soil probe, and if the probe's impedance differs from that of the transmission line, a proportion of the incident signal is reflected back along the line towards the signal source.
time domain reflectometer(TDR), FD sensors are cheaper to built and have a faster response time. However because of the complex electrical field around the probe, the sensor needs to be calibrated for different soil types. Some commercial sensors have been able to remove the soil type sensitivity by using a high frequency.
* Gaskin G.J., Miller J.D. 1996. Measurement of soil water content using a simplified impedance measuring technique. Journal of Agricultural Engineering Research 63, 153-160.
*Campbell C.S., Campbell G.S., Cobos D.R.2004. Response of Low Cost Dielectric Moisture Sensor to Temperature Variation. Eos Trans. AGU, 85(17), Jt. Assem. Suppl. Abstract NS44A-05.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Frequency-shift keying — Passband modulation v · d · e Analog modulation AM · … Wikipedia
Time-domain reflectometer — A time domain reflectometer (TDR) is an electronic instrument used to characterize and locate faults in metallic cables (for example, twisted wire pairs, coaxial cables). The equivalent device for optical fiber is an optical time domain… … Wikipedia
Radio Frequency Identification — Die Neutralität dieses Artikels oder Abschnitts ist umstritten. Eine Begründung steht auf der Diskussionsseite. siehe: Abwahl Lesenswert Februar 2007 … Deutsch Wikipedia
Level sensor — Level sensors are used to detect liquid level. The liquid to be measured can be inside a container or can be in its natural form (e.g. a river or a lake). The level measurement can be either continuous or point values. Continuous level sensors… … Wikipedia
Radar sensor — Radio detection and ranging (radar) is used to sense angle, range and velocity of (moving) scatterers in the environment [Skolnik, M. I.: Introduction to Radar Systems, 3rd Ed. . McGraw Hill, 2005] [Stimson, G. W.: Introduction to Airborne Radar … Wikipedia
Fiber optic sensor — A fiber optic sensor is a sensor that uses optical fiber either as the sensing element ( intrinsic sensors ), or as a means of relaying signals from a remote sensor to the electronics that process the signals ( extrinsic sensors ). Fibers have… … Wikipedia
Ultrasonic sensor — Ultrasonic sensors (also known as transducers when they both send and receive) work on a principle similar to radar or sonar which evaluate attributes of a target by interpreting the echoes from radio or sound waves respectively. Ultrasonic… … Wikipedia
Irrigation scheduling — is the process used by irrigation system managers to determine the correct frequency and duration of watering. The following factors may be taken into consideration: * Precipitation rate of the irrigation equipment how quickly the water is… … Wikipedia
Outline of hydrology — The following outline is provided as an overview of and topical guide to hydrology: Hydrology – study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and… … Wikipedia
Soil moisture sensors — measure the water content in soil. A soil moisture probe is made up of multiple soil moisture sensors. One common type of soil moisture sensors in commercial use include the Frequency domain sensor such as the capacitance sensor. One of the major … Wikipedia