Click on an image to see the manufacturer's listing further down this page.

Netafim's Flori

SDEC's HMS9000

Delta-T's ThetaProbe

Troxler's Sentry 200 AP

PRISM - CMP with the Troxler Sentry 200AP Probe		“Gopher” system Sentek's Diviner	Sentek's "Cable Free" EnviroSCAN RT6	Adcon's C-probe
Aquaterr 200		ECHO probe by Decagon	Vitels Hydraprobe
Dynamax TH2O		Dynamax SM200	Dynamax ML2

  basic principles

For an excellent decscription of capacitance sensors see this description on Richard Mead's page.

All the sensors within this group use an oscillator to generate an AC field which is applied to the soil in order to detect changes in soil dielectric properties linked to variations in soil water content. Like TDR systems they are therefore also known as dielectric sensors.

Capacitance sensors consist essentially of a pair of electrodes (either an array of parallel spikes or circular metal rings) which form a capacitor with the soil acting as the dielectric in between. This capacitor works with the oscillator to form a tuned circuit, and changes in soil water content are detected by changes in the operating frequency.

Frequency Domain sensors work similarly, but use a swept frequency. The resonant frequency (at which the amplitude is greatest) is a measure of soil moisture, and the amplitude is a measure of soil electrical conductivity. Like capacitance sensors, their measurement is made at a single frequency, but the exact frequency depends on soil moisture content.

Delta-T's ThetaProbes also use an oscillator to generate an AC signal but, as in TDR systems, apply this to a coaxial transmission line that extends into the soil (via an array of parallel metal spikes). Soil moisture content is measured from the amplitude of the standing wave which is set up when the reflected AC signal interacts with the generated AC signal. They operate at a single fixed frequency.

Other models of sensor (example the SENTEK) operate from within access tubes and are not in contact with the soil. This allows multiple sensors to be lowered into an access tube and take measurements at all depths.
  advantages

Generally capacitance sensors permit more freedom of choice in the design of electrode geometry and operating frequency than is the case with TDR systems. Most of these sensors operate at lower frequencies (100MHz or less) and can therefore detect "bound" water in fine-particle soils. This bound water is water that is strongly attracted to the surface of soil particles and can constitute more than 10% soil moisture content. Much of it is available to plants, but is not detected effectively by TDR systems operating at frequencies > 250MHz.

Less critical measurements required compared to TDR, and no specific knowledge of analysing wave-forms is required.

Most sensors can be connected to conventional dataloggers.
  disadvantages

Readings are heavily influenced by moisture content and air gaps in the soil volume nearest the electrodes (this is also found with TDR systems). With access-tube models, particularly, it is extremely critical to have good sensor-tube-soil contact for reliable estimation of soil moisture.

Systems operating at lower frequencies (particularly at < 20MHz) are more susceptible to soil salinity errors.

Less "control" and detailed information is available compared to purist TDR wave analysis.
  general

Examples of use

Delta-T Thetaprobe ML2

7/7/99 The Macaulay Land Use Research Institute (MLURI) has produced a report (in Pdf format): ThetaProbe ML2x - Principles of Operation and Applications, 2nd edition, which contains information on ThetaProbe field applications in northern Scotland. www.mluri.sari.ac.uk/~jm0188/

4/4/99 The University of Florida Tropical Research and Education Center has a major irrigation-management related study underway using the Sentek Pty. Ltd EnviroSCAN system. The crops include avacados, limes, carambola, corn, potatoes, and beans. For info on the equipment http://www.sentek.com.au/ The center is within one hour of the Miami airport. The researcher in charge is Dr. Bruce Schaffer. (305)246-6397

Netafim's Flori more info on their sowacs Feature Page, past correspondance on sowacs, or their website.       sowacs FEATURE ARTICLE

Delta-T Devices Ltd. offer a range of soil moisture equipment . The sensors include the ThetaProbe ML2 for soil water content, tensiometers for soil water potential and the SigmaProbe for pore water conductivity - they are all described in a past Feature Article. Delta-T is represented by experienced distributors in many countries of the world." (see address details page).       sowacs FEATURE ARTICLE

Dynamax, Inc. manufactures and distributes a wide range of irrigation scheduling and soil moisture measurement equipment for private, commercial and research markets. Our most popular products include the IL2-MC irrigation system controller system, the IL2-MS soil moisture and irrigation scheduler, TH20 soil moisture meter and the industry standard Vadose TDR system for soil moisture research applications. In the USA Dynamax distributes Delta-T Devices’ GP1 data logger and irrigation monitor, DL6 soil moisture logger and the SM200 or ML2 soil moisture sensor, as well the PR2 soil moisture profile probes. They also supply TDR and other equipment

The C-Probe supplied through Adcon Telemetry also appears to be suited for use with access tubes. They also have a one page pdf data sheet. (see address details page) and their       sowacs FEATURE ARTICLE

Troxler Sentry 200 AP capacitance probe is distributed in the US by Irrigation Scheduling Methods Inc., as part of their irrigation scheduling/soil monitoring system complete with scheduling software (PRISM - ISS) (see address details page) and their       sowacs FEATURE ARTICLE

Decagon Devices Inc. for Biophysical research Instrumentation (see address details page) a sowacs SUPPORTER

Developmental Technologies, who are marketing the Automated Crop Irrigation System (ACIS) claim to have all the problems solved!

Sentek Pty Ltd May 02 release of EnviroSMART and EasyAG slimline based on their successful EnviroSCAN technology. These new sensors have multiple interfaces SDI, voltage, current, S232 (Modbus) and RS485 (Modbus). The original model RT6 was a fixed system dedicated to continuous monitoring of soil moisture The Diviner 2000 using the same sensor technology is a portable system allowing quick infield readings of the whole root zone. Sentek offers flexible telemetry solutions in its new Windows based software and its products are distributed worldwide.

SDEC's HMS9000. A French company (address details page) that also makes tensiometers. Some sowacs correspondance.

AquaPro~Sensors (was the Water Watcher instrument.) Contact Jack Hillen on sales@aquapro-sensors.com - see address details page.
Please note this is NOT the electrical resistance based WaterWatcher instrument.

The “Gopher” system, developed and manufactured in Australia, is a portable capacitance soil moisture profiler. The system comprises a 1.1m length staff, hand held microprocessor, a capacitance sensor head, software and upload cable. The access tubes (installed vertically) are standard Class 6 50mm PVC pipes, therefore the cost per site is minimal.

Vitel Hydra Soil Moisture Probe operates at a maximum of about 50 MHz, and has a purpose built datalogger available, or can be connected to other loggers. See the User guide.
Vitel has been taken over by Stevens Water Monitoring Systems, Inc. Contact to be confirmed - see address details page.

Didcot Instrument Company Ltd. UK NO LONGER IN BUSINESS (address details page)


  literature

literature
  details

See the article "TDR vs FDR vs Capacitance: What exactly are these?"
  other

 

Relevant Links

• A home made sensor complete with design schematic, connected to a Hobo logger, for use in Alpine research
  the 3 page pdf describes it in more detail, and is made by CYCLOBIOS, Dreiheiligenstr.1a, A-6020 Innsbruck, Austria, Austrian Patent AT403213B, © Cyclobios 1997

Campbell Scientific supports Sowacs

Data Logging Application and Measurement Systems Engineers


Soil Moisture Meter



Remote data acquisition and control systems


PRISM - CMP by Irrigation Scheduling Methods, Inc.

Research, Products, Consultancy on Real-Time Water Dynamics