TENSIOMETERS : a type of Soil Water Content Sensors on the sowacs web pages


Sensors \| Dataloggers \| Suppliers	Systems \| Discussion - Archives Comparisons \| Theory \| Irrigation

				home
			Tensiometers how they work, some literature on them, where to get them and how much they cost:

photographs

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

Delta-T's three models
Soilmoisture's JetFill &
Quickdraw Portable Tensiometers

Earth
systems
Solutions
SDEC France
Irrometer
tensiometer

Soilmoisture's 2100F
Adcon's
Electrotensiometer
Adcon's electrotensiometer

basic principles

Tensiometers operate by allowing the soil solution to come into equlibrium with a reference pressure indicator through a permeable ceramic cup placed in contact with the soil.

advantages

They are not affected by the osmotic potential of the soil solution (the amount of salts dissolved in the soil water), as the salts can move into and out of the ceramic cup unhindered. This is not to say that the plant does not feel the effect of the osmotic potential (see the October 1996 sowacs archives for more dicsussion on this).

They measure soil matric potential with good accuracy in the wet range. Suited to applications where water stress and irrigation needs have to be monitored. Less subject to localised spatial variability than volumetric water content measurements (and therefore less sensitive to soil disturbance during installation).

disadvantages

Point measurement (inasmuch as there is no model capable of integrating a larger soil volume such as is possible with some sensors).

Slow reaction time.

Source of water during soil drying.

Usually only operate between saturation and about -70kPa (They are thus not much use for measurements in the dry end of the spectrum.) Air bubbles may enter at this point (termed the Air entry potential). If the soil is very coarse, this occurs even sooner (wetter).

Require maintenance to refill after dry periods.

If the ceramic cup looses contact with the soil (in an air pocket created by manuring for example), then this could cause an apparent "lack of response" of the instrument. If the tip were in an area of limited root activity, the readings could also "stall".
Air will enter a standard tensiometer certainly near the limit of its measurement--75-85 kPa. I have not kept up on this discussion, but I do recall a good bit of amendment with manure which may have served to create air pockets in the soil. If the tip of the instrument were to be located in one of these air pockets then contact between soil and tip would be lost.

general

See also the page on Watermark sensors (Granular Matrix Sensors) as these can replace tensiometers and get around some of the problems associated with tensiometers when absolute accuracy is not a primary concern.

Mini-Tensiometers
In relation to measuring water content very close to the surface of soils, cliff.hignett@soilwater.com.au sells 5mm diameter ceramic tubing which can be used for making micro tensiometers. (see my meb site www.soilwater.com.au under 'products')

manufacturers

Delta-T Devices Ltd. offer a range of electronic, pressure transducer tensiometers, including miniature and rugged-use models. Typical usage is in multiple arrays, automatically recorded by a field data logger. They measure soil water potential to an accuracy of ą0.2 kPa over the range +100 to -85 kPa. These sensors can also monitor water table height when submerged (and the overburden, if present). Suppliers index.

SoilMoisture Equipment Corporation . supply tensiometers as well as many other devices for monitoring soil and plant water portential Suppliers index.

Irrometer The Original American Suppliers since 1951. They also produce the Watermark sensor. Suppliers index.

SDEC's TENSIONICS. A French company that also makes a capacitance sensor. They also have a "Tensimeter" (electronic readout unit) which is designed for use as a portable gauge for use with tensiometers. Some sowacs correspondance. Suppliers index.

Earth Systems Solutions are the American distributors for various SDEC (French) tensiometers including accessories and electronic transducers for continuous logging. Suppliers index.

Adcon's Electrotensiometer has a standard ceramic cup, but the signal from the vacuum sensor above the water-filled tube is conditioned to be compatable with Adcon's data aquistion units. Adcon also supply telemtry solutions and have other sensors. Suppliers index.

literature

"1998 Article on use of moisture tension to irrigate greenhouse crops in highly amended media." To read about how this precision can be used in the nursery and greenhouse industry, where non-soil planting media is typically used, see the article by Dr. J. Heiner Lieth of University of California at Davis.

details

Please submit your suggestions...

other

The Advanced Tensiometer developed at the Idaho National Engineering and Environmental Laboratory (INEEL) has overcome the depth limitation of conventional tensiometers and is largely unaffected by changes in the ambient temperature. Advanced Tensiometers have been installed to depths exceeding 100 ft below land surface and have been continuously operated for about 3 years.

Relevant Links

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

Tensiometers
Time domain reflectometry (TDR)
Capacitance (or FDR)
Gypsum Blocks / Electrical Resistance
Neutron Probes
Granular Matrix
Heat Dissipation
Conductivity
Voltammetric
Psychrometry
Phase Transmission
Hydrotek's VDD
SH3
Non-Contact
About sowacs
Site map
Other sensors
Telemetry
Related links
See also
www. FindaSensor .com
12 Oct 2002