Patent Publication Number: US-6981654-B2

Title: Apparatus for intermittent liquid dispersal

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation of and claims priority to U.S. patent application Ser. No. 09/885,378, entitled “Apparatus for Intermittent Liquid Dispersal” filed on Jun. 19, 2001 now U.S. Pat. No. 6,732,947, which claims priority from U.S. Provisional Patent Application No. 60/212,896, entitled “Apparatus for Periodic Liquid Dispersal” filed on Jun. 20, 2000, the disclosures of both applications are hereby incorporated by reference herein. 

   FIELD OF THE INVENTION 
   The present invention relates generally to intermittent liquid dispersal and more particularly to intermittent liquid dispersal for irrigation and pest control. 
   BACKGROUND OF THE INVENTION 
   A wide variety of irrigation systems are commercially available for use in watering crops, plants, and lawns. Sprinkler-based systems are generally the most popular, although systems that deposit water directly on the ground are also utilized, such as drip systems. In either case these systems are often automated so that they irrigate an associated area on a periodic basis without substantial human intervention. 
   Automated systems typically comprise an electronic controller and solenoid valve electrically coupled to the controller. The solenoid valve is typically located inline with a pressurized source of water. In operation, the valve opens to allow water to flow from the source, through a conduit, and out one or more sprinkler heads or drip emitters. When the cycle is complete, the controller signals the solenoid valve to close. Typically, these systems operate no more than a few times in day. A typical watering cycle may last anywhere from a few minutes to more than an hour. 
   After a watering cycle has been completed, it is not uncommon for the ground to be soaked and saturated. In the intervening period between cycles, the soil can become arid, especially in hot and dry climates. Both saturated and arid ground conditions can be damaging to certain types of plants. For instance, a seedling without a developed root system can be dislodged from the soil if enough water is added to the ground to cause puddling. Additionally, if the ground around a seedling is allowed to dry completely for even a short period of time the seedling can quickly dehydrate and die. Furthermore, there are types of plants that have root systems that are very intolerant of saturated soil conditions and can be damaged if exposed to saturated soil on a regular basis. 
   Ideally, it would be desirable to maintain soil at a predetermined and constant moisture level that is ideal for the plants growing therein. Increasing the frequency of irrigation cycles while reducing the time there between helps to maintain the soil at a more constant moisture level, but most electronic controllers are designed only to open an associated solenoid at most a few times every day. Even if controllers were available that allowed frequent watering cycles of short duration, the electronic solenoids generally available for use in sprinkler systems are not designed for continuous repetitive duty. 
   Another drawback of electronic systems is that they require coupling to an electrical power source that may not be conveniently available. Additionally, the conduits of electrical current, such as the wires between the solenoid and the controller, must be protected from moisture and other potential sources of damage. These requirements of traditional automatic systems make them complicated and consequently difficult and expensive to install. 
   Another problem that traditionally affects farmers and home gardeners alike is damage done to plants and crops by animals. It can be appreciated that animals in general will not bother plants or crops while a sprinkler is in operation because either they do not like the water or they are scared by sprinkler noise. Traditional sprinklers are relatively effective in deterring animals from entering an area being irrigated. Unfortunately, traditional sprinklers cannot be left on continuously for extended periods of time because of the amount of water used and the potential saturation of the underlying soil. Other objects, such as scarecrows, have very little effect on most animals. There are solutions that can be applied to the surfaces of plants that make them undesirable to animals, although the nature of the solutions often preclude there use on crops that are to be consumed by humans. 
   SUMMARY OF THE INVENTION 
   Various embodiments, presentations, and configurations of a valve for the intermittent distribution of a fluid according to the present invention is described herein. In one embodiment, the valve includes a valve housing defining an interior cavity and a valve member that is at least partially contained within the cavity. The fluid is in a reservoir capable of holding the fluid under pressure. An inlet port is provided to receive the fluid into the cavity of the valve housing and an outlet port is provided to permit the fluid to flow out of the cavity. The valve member is moveable within the cavity between a closed position and open position. In the closed position, the valve member blocks the flow of the liquid in the cavity between the inlet port and the outlet port. In the open position, the valve member permits the flow of the fluid between the inlet port and the outlet port. Additionally, a biasing mechanism is provided to control the movement of the valve member between the closed and open positions. In particular, the biasing mechanism provides (i) a retention force applied against the valve member to hold the valve member in the closed position when the valve member is in the closed position, and (ii) a biasing force encouraging the valve member into the closed position when the valve member is in the open position, the biasing force being less than the retention force, in one example. 
   In another embodiment, the valve housing defines a bore with the inlet port and the at least one outlet port extending through a wall of the housing from the bore. The inlet port is adapted for coupling to the source of liquid, and is in fluid communication with the at least one outlet port through the bore. The valve further includes a valve stem that is at least partially contained within the bore. The valve stem is movable between an open and closed position in the bore (the stroke), wherein the valve stem blocks the flow of liquid between the inlet and outlet ports in the closed position and permits the flow of liquid between the inlet and outlet ports in the open position. A biasing mechanism is provided that applies a retention force to hold the valve stem in the closed position within the bore when the valve stem is in the closed position. Additionally, the biasing mechanism provides a return force encouraging the valve stem into the closed position when the valve stem is in the open position, wherein the magnitude of the return force is less than the retention force. The stroke may be set by a retention strap or the like. In addition, the bore may define a second enlarged section of the bore, and the valve stem may include a stopper adapter to operate within the bore and thereby control the stroke. 
   In embodiments of the present invention, the biasing mechanism comprises two sets of one or more magnets, one coupled with the valve housing and the other with the valve stem. The retention force at least partially comprises the magnetic force between the magnets. Furthermore, the valve may include one or more O-rings that span the distance between the surface of the valve stem to the interior surface of the bore. 
   In other embodiments of the invention, a valve generally similar to those described above is utilized in conjunction with a reservoir designed to hold a fluid in a pressurized state. Operationally, the valve member moves into the open position from the closed position when the pressure in the reservoir exceeds a critical level, i.e., an activation force is equal to or exceeds the retention force. The valve member does not move back into the closed position from the open position until the pressure level in the reservoir drops to a second level that is less than the critical pressure level. Accordingly, a volume of liquid that is the difference between the volume of liquid contained within the reservoir at the critical pressure and the volume of liquid stored in the reservoir at the second pressure level is expelled from the reservoir through the periodic fluid release valve. 
   In another preferred embodiment, the reservoir is fluidly coupled to a pressurized source of liquid by way of a flow control regulator that controls the rate at which the reservoir is filled. A combination system including the regulator, the reservoir and the periodic control valve facilitates the repetitive cyclic release of liquid from the outlet port of the valve when a pressurized source of liquid is provided. 
   A fluid delivery system for periodic fluid dispersal and a method for the same are described herein. Embodiments of the present invention incorporate a pressure actuatable periodic fluid dispersion valve (periodic valve) as described below coupled with and located downstream from a reservoir containing a fluid. The reservoir is fluidly coupled with a fluid source with a flow regulator valve (regulator) intervening to control the flow rate of fluid from the fluid source. 
   In one embodiment, fluid flows into the reservoir from the source at a rate controlled by the regulator. The reservoir contains the fluid and expands as the volume of fluid in the reservoir increases. At a critical pressure, the periodic valve is triggered into an open position, wherein a portion of the fluid contained within the reservoir is expelled therefrom through the periodic valve. Once the pressure in the reservoir drops below a certain level that is generally lower than the critical pressure, the periodic valve closes. 
   In another method, the various embodiments of the valve are useful in forming a fire line to help prevent the movement of a fire passed a certain point. To form a fire line with the present invention, a plurality of sprinkler heads are distributed over an area of ground typically in a line. Each sprinkler head is fluidly coupled with the valve, and each valve is fluidly coupled with a reservoir. A source of fluid, such as water or other fire-line agents, is fluidly coupled with the reservoir so that the reservoir fills and the fluid therein is pressurized. The area of the ground in the broadest range of the sprinkler may then be kept at a level to reduce fire by the periodic dispersal of water from the periodic fluid dispersal valve assembly. 
   The operation of the periodic valve is in contrast to spring-loaded safety release valves that are commonly found on pressure vessels. Safety release valves open when a critical pressure is reached but close immediately once the pressure within an associated pressure vessel or reservoir decreases below the critical pressure. The volume of fluid released from a safety release valve is typically small, depending on the influx of fluid into the reservoir or pressure vessel from a pressurized source. 
   For purposes of illustration, the invention is described herein in terms of a periodic sprinkler for use in the irrigation of plants, lawns or crops and for use in forming a fire-line, as well as startling critters that may be after the crop. It is to be understood that other embodiments of the invention are contemplated for use wherever the periodic distribution of a fluid, liquid or gas, is required. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric view of a first embodiment of the present invention. 
       FIG. 2  is a section view of the first embodiment taken along line  2 — 2  of  FIG. 1 , and illustrating the valve in the closed position. 
       FIG. 3  is a section view of the first embodiment, and illustrating the valve in the open position. 
       FIG. 4  is an exploded isometric view of the first embodiment. 
       FIG. 5  is a partial section view of the first embodiment taken along line  5 — 5  of  FIG. 3 . 
       FIG. 6  is a partial section view of the first embodiment taken along line  6 — 6  of  FIG. 3 . 
       FIG. 7  is a block diagram illustrating the operation of the valve according to the present invention, in one example. 
       FIG. 8  is an isometric view of a second embodiment of the present invention. 
       FIG. 9  is a partial section view of the second embodiment taken along line  9 — 9  of  FIG. 8  showing the valve in its closed position. 
       FIG. 10  is a partial section view of the second embodiment taken along line  10 — 10  of  FIG. 9 , and illustrating an O-ring circumscribing the bottom portion of a valve stem, the O-ring being below an outlet port of the valve. 
       FIG. 11  is a section view similar to  FIG. 9  wherein the valve is in the open position. 
       FIG. 12  is a partial section view of the second embodiment taken along line  12 — 12  of  FIG. 11 , and illustrating the O-ring above the outlet port so that fluid pulses up into the valve and out of the outlet port. 
       FIG. 13  is an isometric view of an embodiment of the present invention adapted for high volume fluid dispersal. 
       FIG. 14  is a cross section view of the embodiment illustrated in  FIG. 13  taken along line  14 — 14  of  FIG. 13 , and showing the valve in the open position. 
       FIG. 15  is an isometric view of another embodiment of the present invention, having a flexible riser so that the sprinkler head may be adjusted to match the terrain that the valve is being used on. 
       FIG. 16  is a vertical cross section view of a valve according to another embodiment of the present invention, this embodiment includes a second O-ring and internal valve stem stroke control. 
       FIG. 17  illustrates a series of sprinklers arranged in a line to create a swath of land having high moisture content for use as a fire line according to one embodiment of the present invention, and one method for using the present invention. 
       FIG. 18  illustrates a series of sprinkler heads attached together via a supply hose to a single periodic valve for use in a fire line according to another embodiment and method of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates a perspective view of a sprinkler  5  including a intermittent liquid emitter valve (“valve”)  10  according to one embodiment of the present invention.  FIG. 2  illustrates a section view of the valve  10  taken along line  2 — 2  of  FIG. 1 , and illustrating the valve  10  in the closed position.  FIG. 4  is an exploded view of the valve  10  according to the first embodiment, and particularly illustrates each component of the valve  10 , in one example.  FIG. 3  also illustrates a section view of the valve  10 , but with the valve in the open position. Generally, the valve  10  periodically and cyclically or otherwise intermittently emits a fluid in response to a pressure increase in a reservoir  12 . As used herein, “fluid” includes a liquid, a gas, a combination thereof, and any material that exhibits fluid characteristics, i.e., the material is able to flow or move freely. 
   Referring to  FIGS. 1 and 2 , the valve  10  is supported by a base  14  adapted to hold the valve  10  in a generally vertical orientation. The base  14 , in one example, is conically shaped with an opening at the apex of the base, wherein a bottom end of the valve  10  is slid into the opening and secured therein by a clamp  16 . Within the base  14 , the bottom of the valve  10  is fluidly coupled with an at least partially elastic reservoir  12 . A partial portion of the reservoir  12  is shown exiting from the base  14  in  FIG. 1 . 
   Referring primarily to  FIGS. 1–4 , the components of the valve  10  of the first embodiment are described. The valve  10  includes a valve housing  20  defining a longitudinal bore  30  and at least one outlet port  22 . In one example, there are two outlet ports, with each outlet port  22  having a diameter of 11/64 inches. The valve housing  20  is an elongate cylinder with a threaded top portion  24 , a threaded bottom portion  26 , and a sidewall  28  extending between the top portion  24  and the bottom portion  26 , in one example. The valve housing  20  is typically fabricated from a polymeric material having a low coefficient of friction, such as Teflon™. The threaded top portion  24  and threaded bottom portion  26 , in one example, conform to the ¾-11.5 ANSI standard (external) hose coupling thread, with the threaded portion extending about 7/16 inches. 
   The longitudinal cylindrical bore  30  extends through the valve housing  20  between the top portion  24  and the bottom portion  26 , and has a diameter of about ¼ inch, in one example. The bore  30  is adapted to receive a valve stem  32 . The outlet ports  22  are located above the bottom threaded portion  26  of the valve housing  20 . Generally, the outlet ports  22  are perpendicular to the bore  30  and the sidewall  28 , and form an aperture therebetween. The embodiment illustrated in  FIGS. 1–6  includes two outlet ports  22 . It is envisioned, however, that the valve housing  20  includes at least one outlet port, and may include any number of outlet ports required to facilitate a particular fluid distribution pattern. 
   A liquid distribution channel  34  or conduit is fluidly coupled with the outlet port  22 . In the embodiment illustrated in  FIGS. 1–6 , there are two liquid distribution channels  34  fluidly coupled with the two outlet ports  22 , respectively. In one example, the channels  34  are defined by a polyethylene tubing having a 4 mm inside diameter (“I.D.”), which may be bent, such as through use of a heat gun, into a variety of distribution patterns according to the needs of a particular user. It is envisioned that other conduits, such as stainless steel, rubber hose, pre-formed tubing, adjustable tubing, ball-and-socket piping, and the like may be used. In the embodiment of  FIGS. 1–6 , the tubes  34  extend transversely from the valve  10  and then bend upwardly and substantially vertically, with the end of the tube  34  generally above the valve  10  so as to allow unimpeded liquid distribution from a sprinkler head  36  at the upper end of the channel. The tubing is available from United Green Mark, Inc., 15579 E. Hinsdale CR #200, Englewood, Colo. A fitting  38  that has a bore extending through it is press-fit into each of the outlet ports  22 . Each tube  34  has a threaded lower end that is received into a threaded section of the fitting bore. The sprinkler heads  36  illustrated in  FIG. 1  have a plurality of openings from which water pulses. The sprinkler heads  36  are press-fit in the upper end of the tube  34 . The sprinkler heads  36  are available from numerous sources, including Spotjet, 14452 Chestnut Street, Westminster, Calif. 92683. 
   An outlet portion  40  (shown best in  FIGS. 2–4 ) of the liquid reservoir  12  is fluidly connected with the bottom threaded portion  26  of the valve housing  10 . In one example, the liquid reservoir  12  is fluidly connected with an inlet portion  42  of the bore  30  by a section of conduit  41 , which is threadably connected with the threaded bottom portion  26  of the valve housing  10 . In the embodiment shown in  FIGS. 1–6 , a garden hose has been found to be effective as a reservoir  12 , wherein the length of the reservoir is directly related to the volume of water that will be released during each cycle of the valve  10 . It can be appreciated that in variations of the first embodiment, and other embodiments described herein, the same garden hose that is used to carry water from a source to the valve  10  may also double as a reservoir  12 . In one example, the reservoir  12  may be formed from any resilient elastic material that expands under pressure entering therein and stores energy, and releases the energy by expelling the fluid when an outlet for the fluid is provided. An inlet portion  43  of the reservoir  12  (shown in  FIG. 4 ) is fluidly connected with a liquid supply  47 . An inlet valve or regulator  45  is situated between the inlet  43  of the reservoir  12  and the fluid supply  47  to govern the flow rate of fluid into the reservoir. In one example, the inlet portion  43  of the reservoir defines a threaded male connection adapted to engage a threaded female connection of the fluid supply. The outlet portion  40  of the reservoir is in fluid communication with the inlet portion  42  of the bore  30 . 
   The inlet valve  45  allows fluid, such as water, to flow into the reservoir  12  from a source of fluid, such as a standard garden hose fluidly connected with a domestic water tap. In one example, the regulator  45  is a commonly available one-way emitter press-fit into the inlet  43  end of the reservoir  12  that permits the flow of water from the source at a given rate, and in other variations, the regulator is a standard bib or similar type adjustable valve that is only partially opened to restrict the flow of water therethrough. The emitter allows liquid to flow into the reservoir at a desired rate from the liquid, e.g., 1 gallon/hour, according to the emitter used, which, in one example, functions to control the cycling rate of the valve. Liquid flows into the reservoir  12  through the emitter and increases the pressure in the reservoir. As discussed below, the liquid is prevented from flowing out of the outlet portion  40  of the reservoir  12  until a retention force (R) is met or exceeded by the pressure in the reservoir, which acts as an activation force (A) on the bottom of the valve stem  32 . 
   In the first embodiment, the partially elastic reservoir  12  expands volumetrically when pressurized. When the valve  10  is opened, the partially elastic walls contract and force the water contained therein into the bore  30  as the walls contract into their nominal position. Accordingly, the reservoir  12  contains a greater volume of water at the pressure when the valve opens than it holds at the pressure level at which the periodic valve  10  closes. It is generally the difference in these volumes that is expelled from the reservoir  12  during each operational cycle of the valve  10 . If a substantially rigid reservoir were utilized, very little water, perhaps a negligible amount, would be expelled from the rigid reservoir before the pressure therein dropped below the level at which the periodic valve would close, since liquids are incompressible fluids. In embodiments of the invention adapted for use with compressible gaseous fluids, a rigid reservoir can be used since the expansion of the gas would act to maintain pressure therein. Furthermore, a rigid reservoir can be utilized with a liquid, if a portion of the reservoir contains a gas or other compressible medium, which expands as the liquid contained therein is expelled. 
   The valve stem  32  projects upwardly from the bore  30  adjacent the threaded top portion  24  of the valve housing  20 . The valve stem  32 , as best shown in  FIG. 4 , is fabricated from a rigid material that is resistant to corrosion from whatever fluid that is to be distributed from the valve  10 . In one example, the valve stem  32  is made of stainless steel with a 0.250+0/−0.001 inch diameter, and a length of 9 inches. Additionally, in one example, a small about 1/16 inch hole  50  is drilled through the valve stem  32  about 4.75 inches above the bottom of the valve stem to accept a hitch pin  52  (which is described in more detail below). The surface of the valve stem  32  is typically smooth to reduce its coefficient of friction, which provides smooth movement of the valve stem  32  within the bore  30 . In one example, the bore hole has a diameter that is slightly larger than the diameter of the valve stem  32 , which substantially reduces any contact between the valve stem  32  and the bore  30 , and which causes an O-ring  58  to be the only portion of the valve stem assembly that contacts the bore  30 . The O-ring  58 , discussed in further detail below, prevents fluid from flowing through the valve  10  until the retention force (R) is met or exceeded by the activation force (A). 
   A cap  44  is threadedly engaged to the top portion  24  of the valve housing  20 . The cap  44  defines an aperture for the valve stem  32  to pass through which, in one example, has a ⅜ inch diameter aligned with the longitudinal bore  30 . At least one magnet  46  (lower magnet) is attached to the cap  44 , such as by 3M Scotch Grip™ 1099 plastic adhesive. In this embodiment, three ceramic ring magnets  46  are connected with the cap  44 . Like the cap  44 , the magnets  46  define an aperture in alignment with the cap aperture that allows the valve stem  32  to pass therethrough. In another variation, a thin brass tube (not shown) having a diameter similar to the inside diameter of the lower ring magnets  46  is passed through the center apertures of the lower magnets and the cap  44 . The ends of the tube, which extend beyond the cap  44  and lower magnets  46  at either end, are then flared outwardly over the corresponding surfaces to hold the magnets  46  and cap  44  together. 
   In addition, at least one magnet (upper magnet)  48  is coupled with the portion of the valve stem  32  that projects upwardly from the bore  30 . The upper magnets  48  are adapted to magnetically engage the lower magnets  46  and thereby provide the retention force (R), which holds the valve stem  32  down in the closed position against the activation force (A). In one example, the upper and lower magnets are CR145 type 5 magnets. It is not necessary for the upper  48  and lower magnets  46  to contact. In this embodiment, an upper portion of the valve stem  32 , which is contiguous with the lower portion, extends above the top portion  24  of the valve housing  20 , through the aperture in the cap  44 , and through the apertures in the lower magnets  46 . The horizontal hole  50  extends through the upper portion of the valve stem  32  to receive the hitch pin  52  (such as a Western Wire Clip™ no. 237 from Western Wire Products Company, 770 Sun Park Drive, Fenton, Mo.), cotter pin, or the like, as best seen in  FIGS. 4–6 . As shown in  FIG. 4 , an upper bumper  54 , such as a section of latex tubing having 2¼ inch I.D. and 9⅜ inch outside diameter (“O.D.”), is supported on the hitch pin  52 . In this embodiment, the upper magnets  48  rest on the upper bumper  54 . 
   Generally, ceramic magnets are fairly brittle and therefore susceptible to chipping and cracking. To reduce the likelihood of damage to the upper magnets  48 , one or more tubular rubber or synthetic spacers (see  468 ,  470  of  FIG. 16 ) can be placed between the valve stem  32  and the inside diameter of the upper magnets  48 , thereby isolating the upper magnets from direct contact with the hard surface of the valve stem. Additionally, a collar  49  may protect the outside surfaces of the magnets  46 ,  48 . 
   The lower portion of the valve stem  32  that is located within the bore  30  of the valve housing  20  during operation defines a circumferential groove  56  adapted to secure the O-ring  58  (such as a size 006 50 durometer O-ring with a high lubricity coating and/or a high lubricity Buna formulation). In one example, the groove has a width of about 0.075″ and a diameter of about 0.132″ to about 0.135″ with a tolerance of +/−0.0005 inches. The O-ring  58  is positioned along the lower portion of the valve stem  32 , above a bottom face  59  of the valve stem  32 , so that the O-ring  58  is located below the outlet ports  22  when the valve stem  32  is in the closed position. The O-ring  58  spans the gap between the outside diameter of the valve stem  32  and the inside diameter of the bore  30  the portion of the bore above the O-ring  58  from fluid located below the O-ring, i.e., the fluid in the reservoir  12 , and to thereby prevent fluid from flowing through the valve  10  until the A≧R. Since the O-ring  58  slides in the bore  30  with the valve stem  32 , a lubricant such as SuperLube™ by Synco Chemical Corp., may be applied to the O-ring  58  to facilitate smooth movement in the bore  30  and to help break in the O-ring  58 . Generally, the sealed or closed position of the valve  10  (wherein water is not flowing through the outlet ports) is maintained while the retention force exceeds the pressure in the reservoir.  FIG. 2  shows the valve  10  in the closed position, and  FIG. 3  shows the valve in the open position. 
   In the closed position, the magnetic force between the upper magnets  48  and the lower magnets  46  provides the retention force (R) holding the valve stem  32  down in the closed position. The retention force may be adjusted by adding or removing magnets  46 ,  48 , and changing the separation between the upper and lower magnets  48  and  46 . The retention force may also be adjusted by utilizing magnets with various holding forces. In addition, the retention force may be adjusted by adding or removing counterweights  60  located above the upper magnets  48 . 
   In the first embodiment, the lower magnets  46  coupled to the valve housing  20  and the upper magnets  48  coupled to the valve stem  32  are separated in the closed position by the flexible, resilient, and water resistant separator or bumpers  54 , which are rubber or synthetic washer, in one example. Separation of the magnets  46 ,  48  allows much stronger magnets to be used than would be used in an embodiment without a separator. Hence, the magnetic force, or retention force, between the magnets is strong despite the separation of the magnets. By providing a strong magnetic coupling between the upper  48  and lower  46  magnets, the valve stem  32  accelerates, or “snaps” upwardly into the open position ( FIG. 3 ) and also accelerates downwardly into the closed position after fluid pulses from the reservoir  12  as discussed in more detail below. 
   Resting upon the upper magnets  48 , in one example, are one or more cylindrical counterweights  60 . The counterweights  60  define an aperture that is aligned with the apertures in the upper magnet  48 , and have the upper portion of the valve stem  32  passing therethrough. In one example, a rubber or synthetic washer (not shown) is placed between upper magnets  48  and the counterweight  60  to reduce the likelihood of damage to the upper magnets  48 . The counterweights  60  may be used to add our additional gravitational component to the retention force R and to the return force (discussed below). 
   As fluid is introduced into the reservoir  12  through the inlet valve  45 , pressure in the reservoir  12  increases. The pressure increases because the lower portion of the valve stem  32  along with the O-ring  58  prevents fluid from flowing into the valve  10 , i.e., through the outlet portion  40  of the reservoir, through the inlet portion  42  of the bore  30 , and out of the outlets  22 . The valve stem  32  is held, against the pressure in the reservoir  12 , in the sealed position by the retention force (R). As pressure increases in the reservoir  12  the upward activation force (A) on the bottom face of the valve stem  32  increases. When the pressure in the reservoir  12  causes the activation force (A) to meet or exceed the retention force (R), the valve stem  32  moves upwardly in the bore  30  in a snap action. The snap action is a function of the activation force (A) overcoming the retention force (R) and the valve stem  32  accelerating upwardly. As the valve stem  32  moves upward, the magnetic face component reduces significantly allowing the valve stem  32  to accelerate upwardly, which facilitates the fluid flow through the valve in a pulse. As the valve stem  32  moves upwardly in the barrel the O-ring  58  moves above the outlet ports  22 . After the O-ring  58  moves above the outlet ports  22 , the liquid pulses from the reservoir  12  and through the outlet ports  22 , into the liquid distribution channel  34 , and out a sprinkler head  36 . The liquid release through the outlet ports  22  reduces the pressure in the reservoir  12  and hence reduces the force on the bottom of the valve stem  32 . 
   The movement from the sealed position, with the O-ring  58  below the outlet ports  22 , to the release or open position, with the O-ring  58  above the outlet ports  22 , is the “stroke” of the valve stem  32 . In a lower portion of the stroke the O-ring  58  is below the outlet ports  22  and hence seals or closes the reservoir  12  from releasing liquid through the outlet ports  22 . In an upper portion of the stroke, the O-ring  58  is above the outlet ports  22  and hence unseals or opens the reservoir  12  and allows liquid to pulse through the outlet ports  22 . In the lower portion of the stroke, the valve stem  32  is held in place by the retention force (R). When the activation force (A) on the bottom of the valve stem  32  exceeds the retention force, the valve stem  32  snaps upwardly in the barrel  30 . As the valve stem  32  moves upwardly along the stroke toward the upper portion of the stroke, the force between the magnets  46 ,  48  decreases allowing the O-ring  58  to accelerate past the outlet ports  22 . The valve stem  32  is kept from exiting the barrel along the upward stroke, in one example, by a retention strap  62 , such as polypropylene webbing, nylon, or the like, which is configured to control the stroke of the valve stem  32  between about 1½ inches and about 2½ inches. In one example, the strap has a width of about ⅝ inches, and a section of vinyl tubing with a ⅝ inch O.D. and a ½ inch I.D, provides a protective sleeve  66  to protect the strap from the impart with the upper magnets. Note, the movement of the valve stem  32  upwardly in response to the force on the bottom of the valve stem  32  may not require the retention strap  62  if the return force is great enough to prevent the valve stem  32  from exiting the bore  30 . 
   When the valve stem  32  is in the upper portion of the stroke, the magnetic force between the upper  48  and lower  46  magnets, along with the gravitational force from the counterweight  60  provides a return force (RF), which is generally less than the retention force (R). After the pressure in the reservoir  12  decreases to a level equal to or less than the return force (RF) the valve stem  32  begins to moves downwardly in the bore  30 . The return force (RF) represents the force between the upper  48  and lower  46  magnets and the gravitational force in all positions of the valve stem  32  except in the closed position, which is the retention force (R). As the valve stem  32  moves downwardly and the separation between the upper  48  and lower  46  magnets decreases the return force (RF) increases causing the valve stem  32  to accelerate downwardly. This downward acceleration helps reseal and close valve  10  by accelerating the O-ring  58  downwardly past the outlet ports  22  into the closed position. After the reservoir  12  is closed off, the pressure in the reservoir  12  begins to increase again and the fluid distribution cycle is repeated. Accordingly, the valve  10  periodically pulses fluid out of the outlet ports  22  as long as fluid is supplied to the reservoir  12  in such a manner as to cause the activation force (A) on the bottom of the valve stem  22  to once again exceed the retention force (R). 
   In the uppermost portion of the stroke the return force (RF) is less than in the retention force (R) in the lower most portion of the stroke. The counterweight  60  is utilized in the upper most portion of the stroke to ensure that the force of the liquid on the bottom of the valve stem  32  is overcome to return the valve stem  32  to the closed position. The counterweight  60  providing downward force on the valve stem  32  through gravity alone, may be necessary when the magnetic force is insufficient to ensure that the valve stem  32  reseals the reservoir  12 . In addition, although not necessary to reseal the reservoir  12 , the counterweight  60  may also provide a quicker stroke than an embodiment without a counterweight by assisting the magnetic force in rapidly overcoming the force on the bottom of the valve stem  32  as liquid flows out of the outlet ports  22 . 
   As discussed above, the O-ring  58  may be lubricated using any commonly available water resistant lubricant to facilitate smooth movement of the valve stem  32  in the bore  30 . The use of a lubricant is especially important in low pressure applications wherein the pressure in the reservoir  12  and the retention force (R) are adjusted in such a manner as to allow low pressure liquid releases through the outlet ports  22 . 
   The retention (R) and return forces (RF) are primarily the sum of two individual force components: the magnetic force acting between the upper  48  and lower  46  magnets; and the gravitational force acting on the valve stem  32 , the upper magnets  48  and the one or more counterweights  60 . The strength of the attractive magnetic force is a function of the distance between the magnets  46 ,  48  with the force decreasing asymptotically as the distance is increased. In one example, the upper  48  and lower  46  ceramic magnets have a combined attractive force of about 48 pounds when in direct contact with each other, but have an attractive force of only about 4–5 ounces when separated by the ¼ inch bumper  54 . Increasing or decreasing the height of the bumper stack  54  can adjust this component of the retention force (R). As the distance increases between the upper  48  and lower  46  magnets, the magnetic force component of the retention force (R) quickly drops to negligible levels. 
   The gravitational force component of the retention (R) and return forces (RF) is directly related to the weight of the valve stem  32  and the weight of the components connected to the valve rod, such as the upper magnets  48  and the counterweight  60 . The gravitational force is constant and does not vary with the position of the valve stem  32  relative to the valve housing  20 . This component of the retention (R) and return forces (RF) encourages the valve stem  32  to move back into the closed position once the water pressure and consequently the activation (A) force drops. It can be appreciated that in order for the valve  10  to function properly, the valve stem  32  and the bore  30  must be at least partially vertically orientated with the top of the valve stem  32  disposed above the rest of the valve  10 . As the angle of the valve stem  32  deviates from vertical, the amount of gravitational force applied to push the valve stem  32  will be reduced accordingly, potentially effecting the operation of the valve  10 . Adjustments can be made, however, to facilitate operation on sloped surfaces by adding or removing counterweights  60  as necessary, changing the magnetic component force retention(R) and return forces (RF), and the like. 
   There are several additional components of the retention force (R) that may affect the level of the activation force (A) necessary to move the valve stem  32  into its open position. For instance, there is a static friction force associated with the static coefficient of friction of the O-ring  58  within the bore  30  that decreases to zero once the O-ring  58  begins to slide. There is also a dynamic friction force (typically much less than the static friction force) associated with a dynamic coefficient of friction that acts while the O-ring  58  slides along the surface of the bore  30 . In the first embodiment, these additional force components do not significantly affect the operation of the valve  10  but depending on the design of alternative valve assemblies, they can be significant. 
   Retention force components derived by other means may also be utilized in alternative embodiments of the present invention, so long as (i) the force as a whole is reduced significantly once the valve is opened to allow fluid to freely flow through the valve and (ii) only fraction of the full retention force (the return force) is necessary to close the valve  10 . For example, soft deformable O-ring may be utilized in place of the upper and lower magnets, wherein the soft O-ring deforms as the pressurized fluid from the reservoir  12  presses it against the wall of the bore  30 . Because of the deformation of the O-ring, a high activation force (A) is necessary to break the frictional engagement of the O-ring  58  with the wall. However, once this static friction has been overcome the amount of force necessary to push the valve stem  32  upwardly and hold it in its open position is reduced to a level primarily dependent on the gravitational force applied to the valve stem  32  based on the weight of the valve stem  32  and any counterweights  60  attached thereto. Once the pressure in the reservoir  12  has dropped to a low enough level, the gravitational force will cause the valve stem  32  and the then undeformed soft O-ring to move downwardly into its closed position. It is also contemplated that a spring member coupled between the valve housing  10  and the upper portion of the valve stem  32  may be used along with the magnets, and counterweights, or replace them in alternative embodiments of the present invention. 
     FIG. 7  is a block diagram illustrating the operation of the valve  10  according to one embodiment. The operations  710 – 730  are discussed with reference to the first embodiment ( FIGS. 1–6 ), however, the operations are generally applicable to the other embodiments of the invention illustrated and discussed herein. In the first operation  710 , fluid flows into the reservoir  12  to increase pressure in the reservoir. The pressure builds in the reservoir causing the force (A) on the bottom of the valve stem  32  to increase. In the second operation  720 , the activation force A on the bottom of the valve stem  32  exceeds the retention force (R) causing the valve stem  32  to begin moving upward in the bore  30 . In the third operation  730 , the O-ring  58  moves above the outlet port  22  allowing fluid to flow from the reservoir  12  into the valve  10  and through the outlet ports  22 . In the third operation  730 , the pressure in the reservoir decreases due to the release of fluid from the reservoir  12 . 
   It can be appreciated that in alternative embodiments either the upper  48  or lower  46  magnets may be replaced with a ferric attractor. For example, the counterweight  60  and the upper ceramic magnets  48  can be replaced with a single cylindrical shaped piece of ferritic stainless steel which will be attracted to the lower magnet  46  and will perform the counterweight function. Depending on the configuration of the valve  10 , the strength of the lower magnets  46  may need to be increased and/or the distance between the lower magnets and the stainless steel attractor decreased to provide an attractive magnetic force comparable to a valve  10  having both upper  48  and lower  46  magnets. 
     FIGS. 8–12  illustrate a second embodiment of the present invention. In general, the second embodiment is similar to the first embodiment. The valve  110  utilizes the same type of reservoir  112  and regulator  45  as described for the first embodiment. Like the first embodiment, the valve  110  incorporates a housing  120  defining a bore  130  extending longitudinally through it with the lower end of the bore  130  in fluid communication with the reservoir  112 , and at least one outlet port  122  extending generally transversely to the bore  130 . A liquid distribution channel  134  fluidly connects the outlet port  122  with a sprinkler head  136 . A T-fitting  168  having a pressure gauge  170  attached thereto is in fluid communication with the bore  130  between the outlet ports  122  and the reservoir  112 . The pressure gauge  170  allows a user to determine the pressure at which the activation force (A) exceeds the retention force (R) for use when adjusting the operational characteristics of the valve  110  by adding or removing counterweights  160  and/or adjusting the strength and/or number of the magnets  146 . The feedback from the pressure gauge  170  also helps the operator adjust the flow regulation rate into the reservoir  112 . For example, by observing how rapidly the pressure increases, you can easily adjust the flow of fluid into the reservoir  112 , such as by adjusting a tap, changing emitters, or modifying the volume of the reservoir  112 , to get the desired intermittent frequency rate for the valve  110 . The T-fitting  168  is connected with the valve housing  120 . The T-fitting  168  defines a second bore  131  in alignment with the bore  130  of the valve housing  120 . In one example, an intermediate fitting  133 , defining a third bore  135 , couples the valve housing  120  to the T-fitting  168 . It is envisioned that the gauge  170  could be used and fitted with the other embodiments discussed herein in substantially the same manner as discussed above. It is envisioned that the gauge  170  may be connected with the valve housing  120  in other configurations. For example, the gauge  170  can be directly connected with the valve housing  120  via a threaded aperture, and one or more of the intermediate fittings may be eliminated or arranged differently. The lower portion of the T-fitting  168  is directly connected with a second base  137 , or, in one example, via an additional fitting  148 . 
   As best illustrated in  FIG. 8 , the second base  137  includes a cross member  139  having a first end  141  and a second end  143 , and having a first support member  145  connected transversely to the first end  141  of the cross member and having a second support member  147  connected transversely to the second end  143  of the cross member. The second base  137  defines a generally H-shaped structure. The additional intermediate fitting  148  defines two threaded male ends and defines a fourth bore  150 , and is fluidly coupled at its lower end to a fluid channel  149  defined by the cross member  139 , the fluid coupling being about midway between the first support member  145  and the second support member  147 . In this embodiment, the fluid channel  149  is L-shaped (not shown) with the vertical section of the L defining and opening at the top of the support member  145 , and the transverse lower section of the L defining an opening in the front face of the support member  147 . A female fitting  151  adapted to receive and connect the reservoir  112  is connected to the front face of the cross member adjacent the L-shaped channel, and in fluid communication with the fluid channel  149 . Accordingly, in this embodiment, fluid flows between the reservoir  112  and the valve  110  through the fluid channel  149  defined by the second base  137 . This base  137  may be readily exchanged with the base  14  described with reference to the first embodiment, and the reservoir  112  fluidly connected with the inlet of the valve  110 . 
   The valve stem  132  is also similar to the valve stem  32  one described in the first embodiment incorporating a lower portion that is slidably contained within the bore  130  for movement between open and closed positions, and an upper portion that has counterweights  160  attached thereto, in one example. 
   The second embodiment, unlike the first embodiment, uses a weaker magnet  146  that engages one or more metallic components connected with the valve stem  132  to provide the retention force. The magnet  146  is attached to a magnet platform  172  that is threadably attached to the top of the valve housing  120 , in one example. As shown in  FIGS. 8 and 9 , the magnet  146  is held on the magnet platform  172  by nuts  174  and two retaining bolts  176 , in one example. An upper portion of the shaft of each retention bolt  176  extends upwardly from the magnet platform  172  until terminating at a bolt retaining head  177 . In this embodiment, the distance between the top nut  174  and the bolt head  177  deliver the stroke of the valve stem  132 . A corresponding counterweight platform  178  is attached to the valve stem  132  above the magnet platform  172  by way of a metallic collar  180  that is secured to the valve stem  132  with a setscrew  182 . As can be best seen in  FIGS. 9–11 , the counterweight platform  178  is rectangular and has a pair of holes through which the upper portions of the bolt shafts pass. Additionally, a center hole  185  permits a portion of the upper section of the valve stem  132  to pass through the counterweight platform  178 . The counterweight platform  178  provides a platform for holding the counterweights  160 , and serves as a stop for preventing the valve stem  132  from exiting the bore hole  130  by impacting the heads  177  of the aforementioned retaining bolts  176 . The metallic collar  180 , which is attached to the valve stem below the platform  178 , serves as an attractor for the magnet  146 , in one example. 
     FIG. 9  is a section view of the second embodiment taken along line  9 — 9  of  FIG. 8 , and illustrating the second embodiment in the sealed or closed position. The magnetic force used in the second embodiment is less than the magnetic force used in the first embodiment because the magnet  146  actually engages the collar  180 , which is connected with the valve stem  132 . With the magnet  146  and collar  180  in engagement, the magnetic force therebetween is maximized. Accordingly, a higher strength magnet would have a higher retention force than the magnet  146 , and accordingly require a higher activation force (A) to overcome the retention force (R); and accordingly, provide a higher pressure and higher volume burst of fluid from the valve  110 . 
     FIG. 10  is a section view taken along line  10 — 10  of  FIG. 9 . The bottom portion of the valve stem  132  as shown in  FIG. 10  has a substantially pointed bottom  153  having a flat tip  155 . The valve stem  32  illustrated in the first embodiment, in contrast, has a substantially flat bottom having rounded edges and a 0.020 inch width. The valve stem  132  in each embodiment operates similarly. The pointed bottom valve stem has a somewhat lower likelihood of binding with the edges of the bore  130 , when the valve stem  132  is moving between the open position and the closed position. In addition, the force distribution of pressurized fluid from the reservoir  112  along the pointed bottom is somewhat different than the force distribution on the flat bottom. However, the net effect of the different force distributions is substantially similar resulting in little net difference, if any, in the force acting on the bottom of the valve stem. 
     FIG. 11  is a section view taken along line  9 — 9  of  FIG. 8  illustrating the second embodiment of the valve in the open position. As with the first embodiment, as pressure increases in the reservoir  112  the force from the fluid on the bottom portion  153  of the valve stem  132  exceeds the retention force (R) and the valve stem  132  moves upwardly in the bore  130  until the O-ring  158  is above the outlet port  122 . When the O-ring  158  is above the outlet port  122  the fluid pulses from the reservoir  112  through the inlet portion  142  of the bore  130 , and through the outlet port  122 . When the fluid flows through the outlet port  122 , the pressure in the reservoir  112  decreases until the force on the bottom of the valve stem  132  is less than the return force (RF) allowing the valve stem to move back to the closed position as shown in  FIGS. 9 and 10 . 
   By using a lower power magnet  146  than the first embodiment, and using counterweights  160  (in some instances more than the first embodiment), the retention force (R) and the return force (RF) are somewhat modified, and hence the operation of the valve  110  is somewhat different. In this embodiment, the magnetic force field drops dramatically once contact is broken. Whereas, in the separated magnet embodiments, such as is illustrated in  FIGS. 1–6  and  13 – 16  have a much greater effective magnetic reach, and the force therebetween decreases more gradually as the valve stem moves from the closed to open position, and conversely increases more rapidly when the valve stem comes back down again to the closed position at the end of each cycle. For the retention force (R), the use of the low power magnet  146  serves to prevent the valve stem  132  from bobbing in response to the activation force (A) from the reservoir  112 , and accordingly facilitates a snap-action for the valve  110  generally. The majority of the retention force (R), however, is provided by the counterweights  160 . 
   When the valve activates and the valve stem  132  snaps upwardly, the movement tends to be more linear than with the first embodiment because the primarily force component is provided by the counterweights  160  rather than the magnet  146 . The initial uplifting impulse provided by the contact magnet  146 , however, is greater than with the non-contact configuration, e.g.,  FIG. 1 , because the retention force (R) of the contact magnet  146  effectively drops to zero almost immediately once the contact is broken. The valve stem  132  gets a more powerful initial boost this way. In the other embodiments, with the non-contact powerful magnets, the retention force (R) drops off more gradually. The initial boost is not as great. The advantage from the non-contact type magnets, however, is when the top magnets come back down again during the closing phase of the cycle. The magnetic attractive force progressively increases as the non-contacting magnets get closer. In the contact magnet configuration the magnet has little effect on the closure operation until full contact is made. The acceleration of this movement is fairly linear because the low power magnet provides little added acceleration back into the closed position, as with the first embodiment. It is envisioned that the various features of the valve of the second embodiment, such as the low power magnet, the counterweights, etc., are fully interchangeable with the various like features of the first embodiment and other embodiments disclosed herein. 
   Otherwise, the general principles discussed with reference to the first embodiment are applicable to the second embodiment. For example, the valve stem  132  moves upwardly into the open position allowing the fluid to pulse from the reservoir  112  into the valve when the activation force (A) from the fluid in the reservoir exceeds the retention force (R). And, the valve stem  132  moves from the open position back to the closed position when the return force (RF) exceeds the remaining activation force from the fluid in the reservoir  112 . 
   In a third embodiment, not illustrated herein, no magnets are used for the retention force. Instead, only counterweights are used to create the retention force. This embodiment simplifies the operation of the valve to some extent by removing the magnetic component of the retention and return forces. This embodiment is especially useful if it is desirable to have the valve bob up-and-down in small increments, and allow only small fairly low volume and low pressure bursts of fluid. This occurs primarily because the non-linear force component provided by the magnet, as found in the first two embodiments, is not present in the third embodiment. In addition, the use of soft O-rings can provide a high initial static friction, which facilitate the “snap action” effect to pop the valve stem up. 
   A fourth embodiment of the present invention is illustrated in  FIGS. 13 and 14 .  FIG. 13  shows an isometric view of the fourth embodiment of the sprinkler system  205  including an impulse-type sprinkler head  236 , such as those which are commonly available and well known to those skilled in the art.  FIG. 14  is a partial section view of the third embodiment illustrating the valve  210  is similar in configuration and operation to the valve  10  described in regard to the first embodiment. It is also envisioned that the second and third embodiments of the invention as described herein may also be used with a sprinkler head  236  and the related attachment structure described below. The valve  210  as best shown in  FIG. 14  has a single outlet port  222  as opposed to two outlet ports illustrated for the first and second embodiments. The fourth embodiment also differs from the first embodiment in the manner in which the valve  210  is incorporated into a sprinkler system  205 . The fourth embodiment is adapted for use with a standard size sprinkler head, such as the impulse-type head  236  shown, that are commonly available; whereas, the first and second embodiments are illustrated with smaller sprinkler heads ( 36 ,  136 ), such as the type used in low volume irrigation applications. The use of a larger sprinkler head ( 236 ) permits the sprinkler system  205  to provide coverage to a greater area and to expel a greater volume of fluid in a shorter amount of time. The fourth embodiment also incorporates a stake  284  in place of the base ( 14 ,  137 ) to secure it into the ground. These differences and others, along with the other components of the fourth embodiment are discussed in further detail hereafter. 
   Referring to  FIG. 14 , a female/female swivel coupling  286 , in one example, is threadably attached to the bottom end of the valve housing  220 ; thereby facilitating easy disconnection of a male connection associated with the reservoir  212  from the valve  210  without stripping the soft threads of the plastic valve housing  220 . In another example, a quick connect coupling is threadably attached to the bottom end of the valve housing  220 , which facilitates the connection and disconnection from the reservoir  212 , and reduces the likelihood of leaking. The convex side of the valve housing  220  is cradled in a vertical and concave side  288  of a support block  290 . The support block  290  has a hole  292  passing horizontally through it proximate the concave side, wherein a clamp  291 , such as a stainless steel hose clamp, passes through the hole  292  and around the valve housing  220  to secure the support block  290  thereto. In one example, the support block  290  is fabricated from a polymeric material such as polyethylene, although other types of materials can be used. The steel stake  284  extends downwardly from a hole  293  in the bottom surface of the support block  290 , and is staked into the ground to secure the sprinkler  205  in a generally vertical orientation. 
   The male end of a male/female fitting  295  is threadably received into an opening  297  along the top surface of the support block  290 . A vertical riser tube  294  is threaded into a threaded female portion  299  of the fitting  292  wherein the top of the riser  294  is configured to have any one of a number of standard sprinkler heads, such as sprinkler head  236 . Fitting  295  may be eliminated if the vertical riser tube  294  is threadably compatible with the threads of the opening  297  ( FIG. 14 ). The riser tube  294  is then directly received into opening  297 . 
   Referring to  FIG. 14 , a barbed threaded fitting  221  is received in the outlet port  222  of the valve  210 . The other end of the fitting  221  is press-fit into a horizontal bore  223  defining a liquid distribution channel  301  that extends horizontally and vertically through the support block  290  as best shown in  FIG. 14 . In one example, the channel  301  has a diameter of about ½ inch and the outlet port  222  has a diameter of about ⅜ inch. The fitting  221  is press-fit in the outlet port  222 , and the horizontal channel  223  is press-fit over the fitting  221 . Intersecting the horizontal bore  223  is a vertical bore  296  that defines an opening along the top surface of the support block  290 . Accordingly, when the valve  210  is in its open position, the reservoir  212  is in fluid communication with the sprinkler head  236  via the liquid distribution channel  301  as shown in  FIG. 14 . The liquid distribution channel has a larger diameter than other embodiments discussed herein, such as about ½ inch that acts to reduce the frictional energy losses from fluid flow through the channel  301  and increases intermittent flux and power, which facilitate the use of larger sprinkler heads, such as sprinkler  236 . 
   In this embodiment (similar to the first embodiment), the retention force (R) and the return force (RF) are provided primarily by three lower ceramic ring magnets  302  connected with the top of the valve housing  220 , three upper ceramic ring magnets  304  connected with the valve stem  232 , and a counterweight  260  coupled with the valve stem  232 . The upper magnets  304  are separated from the lower magnets  302  by one or more bumpers  352 . 
   A fifth embodiment of the present invention is illustrated in  FIG. 15 , which is similar to the fourth embodiment except that the riser tube  294  has been replaced with a section of ball-socket type tubing  394 , wherein the angle of the sprinkler head  236  can be varied to closely match the topography of the underlying ground, such as being oriented transversely to the ground, while maintaining the valve  210  in a generally vertical orientation. Other types of flexible tubing can be used in place of ball-socket tubing provided the tubing has sufficient rigidity to withstand the thrust incident on it from water exiting the sprinkler head  236 . For instance, a flexible interlocking metal hose with a plastic liner can be used in place of the ball-socket tubing. 
   A valve  410  incorporating an internal valve stem  432  stop in lieu of the retention strap  62  of the preceding embodiments is illustrated in  FIG. 16 . The longitudinal bore  430  of the valve housing  420  defines an enlarged section  431  proximate the upper portion of the valve housing  420 , the enlarged section  431  has a diameter greater than that of the valve stem  432  and greater than the diameter of the lower portion of the bore. The stop member  462  has a diameter less than that of the enlarged section  431  of the bore  430  and is placed over the valve stem  432  and secured thereto by any suitable means to prevent the stop  462  from moving from its attachment location relative to the valve stem  432 . For instance, the stop member  462  can be swaged in place, or held in place by a hitch pin. In one example, the stop member  462  is a flexible resilient bumper, such as a section of latex tubing, supported on a stainless steel cotter pin, and the cotter pin is connected with the valve stem via a 5/64 inch aperture in the valve stem  432 . A cap  444  defining an aperture  445  to allow the valve stem  432  to slide through it is threadably secured to the top end of the valve housing  420 , substantially enclosing the enlarged section  431  of the bore  430 . When the activation force (A) exceeds the retention force (R) of the valve  410  during a sprinkler cycle and the valve stem  432  is thrust upwardly, its upward movement is limited by the impact of the stop  462  with an inside surface  447  of the cap  444 , with a stroke of about one inch. 
   The valve stem  432  illustrated in  FIG. 16  includes a lower O-ring  450  circumscribing the valve stem  432  and an upper O-ring  452  circumscribing the valve stem  432 . The lower O-ring  450  and the valve stem  432  generally operate in substantially the same manner as described above with regard to other embodiments. In one example, the groove that the O-rings  450 ,  452  fit in has a width of about 0.075 inch and a diameter of about 0.136 inch. 
   The upper or second O-ring  452  is positioned so as to prevent backflow coming back through the outlet port  456  and up the bore  430  and out the top when the valve is in the closed position. Also, the second O-ring  452  is positioned low enough on the valve stem  432  so as to not enter the enlarged section  431  of the bore  430  at the top of the stroke and so as to provide an added barrier to any incidental leaking up through the bore  430  of the Teflon™ barrel when the valve stem  432  is in the open position. 
   As illustrated in  FIG. 16 , six ceramic ring magnets  458  (lower magnets) are connected with the cap  444 , and six ceramic ring magnets  460  (upper magnets) along with two counterweights  463  are connected with the upper portion of the valve stem  432  with a hitch pin  464 . A bumper  466 , such as a section of latex tubing, with a width of about ¼ inch to about 5/16 inch separates the upper magnets  460  and the lower magnets  458 . The force between the six upper and six lower magnets provides a component of the retention force (R) such as about 4 lb. of attractive force and the counterweight  463  add about 1 lb., providing a total (R) of about 5 lb., in one example. Alternatively, one large magnet having a similar magnetic force may be used. An upper latex bushing  470  and a lower latex bushing  468  between the valve stem  432  and the upper magnets  460  are used to reduce wear and tear on the magnets. Additionally, in one example, a rubber washer  470  is placed between the upper magnets  460  and the counterweights  463  to reduce wear and tear on the magnets. The additional upper  460  and lower  458  magnets are useful to provide a higher retention force as compared with, for example, three upper and lower magnets, which allows more pressure and fluid volume to build in the reservoir. 
   The inlet part  454  is fluidly connected with a reservoir (not shown) holding a fluid, such as water. The fluid flows into the reservoir through a valve, such as an emitter type valve, from a source, such as a tap, and gradually builds up pressure in the reservoir and on the bottom portion of the valve stem  432  and the lower O-ring  450 . When the pressure in the reservoir, i.e., the activation force (A), exceeds the retention force (R), the valve stem  432  snaps upwardly in the bore  430 . When the lower O-ring  450  moves upwardly past the outlet port  456 , the fluid from the reservoir pulses into the inlet port  454  and out the outlet port  456 . From the outlet port  456 , in one example, the fluid pulses into a distribution channel and into a sprinkler, such as is shown in  FIGS. 13 and 14 . 
   Applications Utilizing Embodiments of the Present Invention 
   Through the periodic release of water from a sprinkler incorporating the periodic valve as described herein, the ground and soil within the range of the sprinkler can be maintained at a generally constant moisture level. This is in contrast to the manner in which other types of automatic sprinkler systems are utilized, wherein the ground is drenched and then allowed to dry to a certain level before more water is applied. 
   The amount of water released by the sprinkler embodying the characteristics of the present invention depends on the size of the reservoir utilized, the rate at which the reservoir is refilled, the critical pressure necessary to open the periodic valve assembly (when the activation force overcomes the retention force), and the pressure level at which the valve closes (the return force). Each of these parameters is adjustable for specific applications. 
   Embodiments of the invention are particularly useful in maintaining a moisture level of seeds planted near the surface of the soil, thereby encouraging rapid germination. It can be appreciated that using traditional sprinkling methodology, seeds might dry-up between watering, thereby increasing the time necessary for them to germinate. Seedlings and other new emerging plants are typically very delicate and intolerant of significant variations of moisture levels. In particular, seedlings are prone to shrivel and die within a very short period of time if they cannot get the necessary fluids from the soil. Furthermore, if seedlings with undeveloped root systems are over watered they can wash away because they are not strongly anchored to the ground. The invention can ensure that the necessary amount of water is delivered to the soil at a constant rate, which can also be balanced with the rate of water evaporation to maintain the soil at a moisture level that is preferred by the seeds or seedlings. Furthermore, because the regulator controls the rate of consumption of water (i.e., the rate at which the reservoir fills), the sprinkler can be left on for extend periods of time without worry about excessive and wasteful water consumption, or the need for electrical timing components. Sprinklers incorporating embodiments of the invention can be utilized with adult plants as well, wherein an adult plant need not utilize its resources growing deep root systems to reach moist soil, thereby freeing resources to be utilized growing other portions of the plant, such as foliage and/or harvestable crops. As a further advantage, the moisture levels help promote the growth of aerobic bacteria and fungi which breakdown organic matter into nutrients that are usable by plants. 
   The frequent and potentially violent release of water a sprinkler incorporating embodiments of the invention is effective deterring various pests such as rats, mice, birds, and rabbits from the area in which the sprinkler is being utilized. In certain instances, the pests may be scared by the noise that accompanies the rapid release of water as the valve stem ( 32 ,  132 ) is thrust upwardly into the retention strap  62 , stop member  462 , or the like. In other instances, the pests may be deterred by the drenching from the cyclic bursts of water. 
   In another use unrelated to gardening, the present invention can be utilized in a series to create “wet lines” for use in fighting wild fires. Traditionally, firefighters create “fire lines” around a fire or along one front of a fire to contain it by preventing it from spreading to the other side of the fire line. Depending on the fire and the configuration of the associated land, “fire lines” may be dug into the ground or they may be created by burning or clearing the combustible materials from the ground that will constitute the “fire line.” Alternatively, the ground area that is to form a “fire line” may be doused with water to increase the moisture level of the ground and plants to an incombustible level (typically 15% moisture by weight). Although dousing an area with water can be effective, it is often impractical, because of the amount of water necessary (much of the water is absorbed into the ground) and the potential for breaks in the line if the water is not evenly applied along the entire length of the line. 
   A string of sprinklers incorporating the periodic valve assembly can be utilized to produce and maintain a “wet line” at a specified moisture level without excessive use of water. Sprinkler heads attached to the sprinklers can be utilized that broadcast water over relatively large areas that a wild fire could not easily jump. In one embodiment as illustrated in  FIG. 17 , a series of sprinklers  505 , each having its own reservoir  512  and regulator valve  513  are attached to a pressurized source of water, such as a tanker truck  515 , via a supply hose  517 , wherein each sprinkler  505  operates independently of the others. As shown the  FIG. 17 , each sprinkler broadcasts water over circular area  519 . An additional savings, such as about a 50% savings, in water for fire breaks can be realized by reducing the angular coverage of each sprinkler down to only 180 degrees. The semicircular sprinkling patterns obtained are sprinkler but then aligned along the axis of their diameters. This reduced area of coverage still results in a fire break over 30 feet wide, in one example, without changing the overlapping pattern of the sprinklers from the original full 360 degree arrangement. The coverage areas of neighboring sprinklers overlap to form a “fire line” of sufficient breadth. 
   In another embodiment as shown in  FIG. 18 , a series of sprinkler heads  636  are connected to a water supply hose  617  to form a line, and the supply hose  617  is connected to a single valve  610  of sufficient size to handle the volume requirements of all of the sprinkler heads  636  attached with the hose  617 . The periodic valve assembly  610  is attached to a large reservoir  612 , and the reservoir  612  is attached to a water supply  615  by way of a flow regulator  613 . 
   Other Alternative Embodiments 
   Although the present invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made by way of example, and changes in detail or structure may be made without departing from the spirit of the invention as defined in the appended claims. 
   Many of the specific components utilized in the described embodiments are merely exemplary and other components may be substituted for them without deviating from the scope of the invention. For instance, the O-ring seal can be replaced with any suitable type of sealing element that would prevent the fluid contained in the reservoir from flowing past it when the valve is in its closed position. Additionally, the materials that comprise the various components may vary. The valve housing which is made of Teflon™ in the embodiments described herein could be comprised of another polymeric material, such as ultra high-density polyethylene, or it could be comprised of a metallic material, such as brass. Likewise, the valve stem could be fabricated from a plastic or composite material instead of stainless steel. 
   The valve is described above primarily in terms of a sprinkler system for the irrigation of lawns, plants, and/or crops. In addition to serving this purpose, alternative embodiments of the sprinkler system may be utilized to scare away critters and varmints that might disturb plants and crops in the area surrounding the sprinkler. It can be appreciated that the noise emanating from the valve as it opens and closes may be relatively loud depending on how the valve is designed and that this noise can be used to startle animals. If additional noise is desired, other noisemakers, such as bells, may be affixed to the valve stem to create additional noise as the valve is actuated. In other embodiments, the valve may be used for purposes unrelated to sprinkler systems or irrigation. It is contemplated that the valve may be utilized in any number of applications where a periodic controlled release of fluid is required from a pressurized source. The fluid may be either liquid or gaseous or a combination thereof. 
   In one sense, the present invention is a valve for releasing a fluid from a pressurized source starting when the pressure in the reservoir reaches a first critical level and ending when the pressure of the fluid from its source drops below the critical level. The valve assemblies described above provide exemplary means for accomplishing the periodic release of a fluid from a pressurized source utilizing forces provided by weights and magnets. Other mechanisms, such as springs, electromagnetic, and the like, in lieu of magnets and weights are contemplated for providing a valve with similar functionality. 
   The present invention although described in an upright position wherein the valve stem moves up and down in the barrel may also be oriented in other positions. The principles described herein will work in a similar manner. The magnetic force, however, might require adjustment to account for differences in gravitational effect. The present invention is useful where any periodic liquid dispersal is desired.