A discussion of the contingencies leading to the invention of the Irristat (e.g., extensive experience from my work, with B.J. Davis, during our development of polyacrylamide gel electrophoresis, and problems with my bonsai hobby) quoted from my downloadable:
"Tenuous but contingent connections" (264K) (abstracted on, my Home Page) follows:
(if you don't already have it !)
When first working out gel formulations for electrophoresis, BJ and I learned how readily slightly-cross-linked polyacrylamide gels would swell in water. An osmometer filled with a solution of non-ionic, long-chain, linear, hydrophilic polymer behind a water-permeable, polymer-impermeable membrane will, at equilibrium, develop a head-pressure equal to the chemical potential of the water in any medium in contact with the other side of membrane. As the concentration of such polymer inside the osmometer is increased, the osmotic potential rises faster than the increase in molarity  . In fact, a very-slightly-cross-linked gel will behave similarly, even though it can be considered to be a single molecule which, "ideally" should develop no osmotic pressure at all. Only when substantial levels of cross-linking are achieved, will a neutral gel behave "ideally" and not swell in distilled water.
Soil moisture availability can be measured by the chemical potential of water, which is equal to the osmotic potential. Therefore, I realized I could design a device, consisting of a small cylindrical chamber, a porous membrane or fine mesh across one end, a piece of slightly-cross-linked polyacrylamide gel (e. g., about 0.1 % cross-linking monomer) behind the membrane, a cylindrical piston behind the gel, and a flexible water conduit between the piston and a valve-seat closing the bottom ofthe cylinder. When this device is placed in soil in the root-zone of a plant, one end of the conduit attached to a pressurized water reservoir e. g., 0.5 to 15 Ibs./in.2) by narrow-bore (e. g., 0.03" I.D.) plastic capillary tubing, and the other, attached to a short length of tubing which leads to, or near to, the soil surface, this "Irristat" valve will open and close in negative feedback (Fig. 6). As the plant's demand for water varies (e. g., between night and day), much like a thermostat, the valve will hold the soil moisture at a pre-set level, determined by the composition and size of the gel and the size of the cylindrical chamber . The 2 cm3 Irristat valve can support potted plants, agricultural crops and trees, and has been successfully and fairly extensively tested [115, 116]. It provides the most efficient possible irrigation. It is just now going into production for home and agricultural markets. Irristat valves now support all of my Bonsais, all of my houseplants and some of my foundation planting and landscape.
When musing on its possibilities for agriculture in arid lands, I tried to imagine the consequences of covering the Sahara Desert with a forest irrigated with the extensive fossil aquifers that lie beneath it. Unless the forest induced sufficient increase in natural rainfall by positive feedback , such a plan would eventually fail as the aquifers dried out. This has involved me with global weather modeling at the Goddard Institute for Space Studies in New York City . Even this tangential excursion was contingent on connections to people and experiences leading to and from Disc Electrophores." This study is covered on CureForGlobalWarming.html
The detailed technical background describing the Irristat, U.S. Patent 4,182,357, can be found in my unpublished downloadable:
The Irristat : "A Key to a New Irrigation Technology" (77K), which is abstracted below:
"With existing irrigation technology, it would be too expensive and wasteful of water even to try to support highly productive agriculture on the vast tracts of hilly, erosion-prone, semi-arid lands which have soils that vary in depth and permeability almost from one plant location to the next. Inexpensive, self-regulating, moisture-sensitive irrigation-valves used one-to-a-plant in drip-irrigation systems, might compensate perfectly for such variability with little waste of water and at a reasonable cost for the device and its installation. Relevant plant physiology, soil physics and physical chemistry are reviewed to define the sensitivity required for such a device. Analysis reveals that previously described designs lack the required sensitivity. The characteristics of osmotic tensiometers are examined for their suitability as moisture sensors for such valves, and the non-ideal osmotic behavior of polymers and hydrogels (e.g., very slightly cross-linked polyacrylamide) is shown to provide the requisite sensitivity for a workable design. The 2-cubic centimeter Irristat, and its operating characteristics, are then described. Its performance at a number of test-installations is briefly reviewed and its advantages in agricultural applications are outlined."
You can also download,
"The Irristat: A Moisture-Sensitive Self-Regulating Water Valve for Drip Irrigation Systems" (132K), Drip/Trickle Irrigation in Action Vol. 2 ASAE Pub. 10-85, St. Joseph, Mich., pp.623-629; (Proc. of the Third Intern. Drip/Trickle Irrigation Congress, Nov. 18-21, 1985, Fresno, CA),
which contains a description of the Irristat, a general discussion of its applications in agriculture, including a description of a system I installed for R. G. Evans and E. Proebsting for 60 mature sweet cherry trees at the Irrigated Agriculture Research and Extension Center of Washington State University, Prosser, WA. An excerpt of the introductory portion follows:
The Irristat (Figs. l & 2) uses a uniquely formulated, synthetic polyacrylamide gel as its moisture-sensing element. It is buried in the soil near a plant's roots (e.g., Fig. 3).
Water is conducted by capillary tubing from a water supply, through a thin-walled rubber tube within the Irristat (See Fig. 1), and then through an attached length of capillary tubing to, or near to the soil surface. The water percolates down through the soil, passes through the Irristat's porous polyester fiber membrane, and reaches the moisture-sensing element,the gel.
As the gel becomes more moist, it swells, pushing the Irristat's piston against the rubber tube. When the moisture in the soil surrounding the Irristat reaches a predetermined set-point, the swollen gel causes the piston to pinch the rubber tube closed, cutting off the supply of water.
As the plant draws moisture from the soil, the gel shrinks, reversing the cycle. When the moisture level falls below the Irristat's set-point, the piston moves back, relieving the pressure on the rubber tube, and water begins to flow (Fig. 4).
Buried in the soil, Irristats will function reliably for many years. The body parts and piston are molded of polypropylene; the water conduit is made of silicone rubber; the semi-permeable membrane, of Dacron-like polyester fibers; and the moisture-sensitive gel, of slightly-cross-linked polyacrylamide. All are chemically, biologically and physically durable.
You can also download:
Evans, R.G., Proebsting, E.L., and Kroeger, M.W. " Water-Sensitive Valves for Monitoring Crop Water Use Patterns" (528K) Presented at the Metting of ASAE, Chicago IL, December 16-19, 1986,
which describes the performance of the Irristat on the 60 cherry trees referred to above, and is abstracted below:
"Water application and pan evaporation data were collected for two and one-half years on mature sweet cherry trees. They show that it is possible to inexpensively monitor daily and seasonal crop water use patterns for perennial, woody plants. Such data have been previously unavailable. Linear regression analyses showed high correlation between pan evaporation and the water applied by a soil water-sensitive, self-regulating valve/sensor located in the root zone. Water applications were affected by the location of the valve/sensor with respect to the application point. Daily analyses of the data provide an indication of the crop coefficients throughout the season. The Irristat valve/sensor can be used in crop water use investigations for many crops, but it is particularly advantageous with perennial, woody horticultural crops."
You can also download:
"The Irristat Instruction Manual: Irristat for Potted Plants" (176K) ;
"Irrigation of New Plantings in Competition with Established Major Plantings" (44K) by S. Bogyo, Pres. Irristat International, Inc.; a discussion of the use of Irristats in Landscaping Applications.
For access to information on Drip, Trickle and Microirigation technology in general, go to TRICKLE_L and the WWW Virtual Irrigation Library, and for specific access to a listing of of Irrigation Industries clich here.
For access to Soil Water Content Sensor technology, go to SOWACS.
For information on the use of Polyacylamide in Agriculture to control erosion, go to USDA at Kimberley.
For more information about IRRISTAT INTERNATIONAL INC., and the Irristat contact<email@example.com>