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What they are Tensiometers are simple, reliable instruments which provide a measure of the moisture status of the soil. They have been in use for many years, and due to their simple design and rugged construction, continue to be the standard instrument for tension measurement.
What they do Tensiometers help answer the irrigators' questions of when and how much irrigation water to apply. Maintaining proper moisture conditions is necessary for achieving optimal plant growth and quality. Monitoring the moisture status of the soil allows for timely and efficient irrigations, and for avoiding unnecessary irrigations.
How they are used At a typical tensiometer station, two tensiometers are installed. One, located in the upper root zone, monitors the active root area and is used to determine when an irrigation is needed. A second tensiometer, installed near the bottom of the root zone, is used to adjust the irrigation amount or system run-time in order to ensure that sufficient water is being applied, and to avoid over-irrigation and loss of water and chemical amendments due to drainage beyond the root zone. The moisture status of the soil is monitored by reading the tensiometers periodically. When the tension readings in the upper tensiometer reach a certain level, an irrigation is needed. This tension level is determined by the irrigator, and depends upon such factors as crop type, soil condition, and root depth. The lower tensiometer is monitored to ensure that enough water has been applied to refill the root zone.
What the readings mean Tensiometers measure the tension with which water is held in the soil. A plant root must overcome this tension in order to extract water held onto the soil particles, and to take in water and nutrients. Low tension readings indicate moist conditions, with water readily available and easy for the plants to extract. As the soil dries, the remaining water is held more tightly to the soil. Tension readings increase, and plant roots have more difficulty extracting water and maintaining optimal growth. At some point, irrigation becomes necessary to maintain growth and quality, and to avoid plant damage.
When to irrigate Factors such as plant variety, soil texture, and growth stage determine the tension at which irrigation should commence. Some examples of recommended tension readings at which to begin irrigation are shown in the following table, with information for other crop types found in the Tensiometer User's Guide.
Installation and maintenance Tensiometers are simple to install and use. A soil auger is used to make the proper-sized hole in the ground. The tensiometer is then inserted into the hole and left permanently. Periodically, the tensiometer may need to be refilled with water. Under irrigated conditions, however, the tensiometer will usually refill itself when irrigation water is applied to the field.
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Model Ten Dial-Gauge TensiometersThe Model Ten tensiometer offers versatility as well as reliability. The dial gauge and ceramic cup are easily replaceable, and threaded connections allow easy attachment of accessories. The construction of the Model Ten make it a rugged and versatile instrument. The porous ceramic cup is attached to the body tube with a threaded connection, and is easily replaced in case of damage. The large inner-diameter body tube forms a large water reservoir, resulting in longer intervals between refilling. The attachment point for the dial gauge is angled, making it easier for the user to read the gauge. The Bourdon-tube dial gauge is designed for prolonged outdoor use, and is recalibratable. The user can easily adjust the gauge to account for the effect of the water-column and any slight gauge offset, and obtain more accurate readings. A plastic endcap screws into a threaded fitting at the top and seals the tensiometer. The 1/4-NPT threaded fitting can also be used to attach accessories, such as an electronic pressure transducer or a vacuum switch.
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Mini-TensiometersMini-Tensiometers are used under conditions in which a tensiometer with a full-size ceramic cup cannot be used. These conditions might include: in a greenhouse environment to monitor plants growing in small containers; in laboratory column studies; or outside under turf, for measurements very near the surface. The mini-tensiometers are constructed with a length of flexible tubing which allows the ceramic sensing cup to be placed exactly where it is needed. Several different ceramic cup sizes and shapes are available, ranging in size from 2 mm to 13 mm in diameter. The larger water reservoir tube is then placed in a more convenient location. The silicon-rubber endcap allows periodic measurements to be made with a septum-and-needle-type portable electronic transducer, or a dial gauge can be attached. Continuous measurements can be made by attaching an electronic transducer and wiring the transducer to an electronic datalogger.
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Tensiometer TubesThe Tensiometer Tubes provide the basis for a custom tensiometer measurement system. The tensiometer tubes are simple and inexpensive, and can be used in a variety of ways. The tensiometer tubes consist of a porous ceramic cup, a clear PVC body tube, and a silicon-rubber endcap. Measurements can be made directly with a septum-and-needle-type portable electronic transducer. Periodic manual measurements can be made by attaching a dial gauge in place of the rubber endcap. Continuous measurements can be made by attaching an electronic transducer, and wiring the transducer to an electronic datalogger.
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See also the Tensiometer Accessories page for equipment related to tensiometer installation and routine maintenance.
More information about tensiometer installation and use
can be found in the
Tensiometer
User's Guide.
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| Santa Barbara, CA 93111 USA Phone:1+805-967-2726
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