Patent Publication Number: US-2007119975-A1

Title: Method and Apparatus for Reducing the Precipitation Rate of an Irrigation Sprinkler

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims priority of U.S. patent application Ser. No. 11/062,968, filed Feb. 22, 2005, which in turn claims priority from U.S. patent application Ser. No. 10/295,689, filed Nov. 14, 2002 and now issued as U.S. Pat. No. 6,921,029, which in turn claims priority from U.S. patent application Ser. Nos. 60/348,488, filed on Nov. 28, 2001, now expired; 60/344,398, filed on Jan. 3, 2002, now expired; 60/360,420, filed on Mar. 1, 2002, now expired; and 60/360,883, filed on Mar. 4, 2002, now expired. The disclosures of each of the aforementioned patent applications are incorporated by reference herein in their entireties. 
    
    
     FIELD  
      This disclosure relates to irrigation sprinklers, and more particularly to a new and improved method and apparatus for reducing the effective precipitation rate of a fixed sprinkler, particularly of the pop-up type.  
     BACKGROUND  
      Probably the most common method of irrigating landscape areas of vegetation is by the use of sprinklers. In a typical irrigation system various types of sprinklers are used to distribute water over a desired area. In general, sprinkler devices are divided into two types, namely rotating stream type and fixed spray pattern type. The stream type sprinkler, commonly referred to as a rotor, trajects a stream of water outwardly from a nozzle, which is rotating slowly over a predetermined arc or complete circle. The spray type sprinkler sprays water from a stationary nozzle, the pattern of coverage being determined by the geometric shape of the discharge passage of the nozzle.  
      For reasons well known to those involved in the design of irrigation systems, the precipitation rate of the rotor type sprinklers is much lower than the precipitation rate of the fixed nozzle type sprinkler. For proper irrigation of plant life and conservation of water it is extremely important to have a uniform or prescribed amount of water delivered by the irrigation system to a specific area. Because of the difference in precipitation rates of the two types of sprinklers, heretofore it has been necessary to operate the rotor type of sprinkler for a longer time than the spray type sprinkler. In order to accomplish this, it has been necessary to have the two types of sprinklers operated separately whereby each type could be operated for a suitable time to supply the desired total precipitation to the irrigated area. Prior to this invention many attempts have been made to reduce the precipitation rates of spray type sprinklers. Most, if not all of such attempts have been concentrated on the design of the nozzles in order to reduce the rate of flow of water.  
      The fixed nozzle type sprinklers disclosed in U.S. Pat. No. 6,921,029 addresses this deficiency in prior fixed nozzle type sprinklers by providing a fixed pattern type sprinkler with attainable precipitation rates equivalent to the precipitation rates of rotary stream sprinklers. This advantageously makes it possible to operate rotary and spray type sprinklers on the same supply circuit and for the same length of time, thereby reducing the cost and simplifying the operating of the irrigation system. This is accomplished by reducing the effective time of operation of the sprinkler while using conventional flow rate nozzles by interrupting the flow of water to the sprinkler nozzle.  
      More specifically, interruption of the flow of water to the sprinkler nozzle is accomplished by turning the water supply to the nozzle on and off in timed durations using a water flow interrupter or valve assembly. The water flow interrupter assembly may be disposed within the riser and be moveable therewith, and functions to periodically shut-off the supply of pressurized water to the nozzle for a predetermined period of time without interrupting the supply of water from the source to the sprinkler. The flow interrupter assembly operates in a highly effective and efficient manner to permit controlled reduction in the effective precipitation rate of the sprinkler, and allows the use of any size nozzle and nozzle pattern without effecting the overall lowered precipitation rate of the sprinkler.  
      The periodic shut-off of the supply of pressurized water to the nozzle can be accomplished by selectively blocking a fluid flow passage using a sealing member. However, the periodic shut-off of the supply of pressurized water to the nozzle can cause the sprinkler to experience a water hammer effect due to pressure fluctuations within the sprinkler. As the sealing member moves from an open position to a closed position, the flow area between the fluid flow passage and the sealing member continually decreases in correspondence with the position of the sealing member from the fluid flow passage until the sealing member is blocking flow through the opening of the fluid flow passage. When the sealing member is blocking flow through the fluid flow passage, the abrupt change in the flow area between the sealing and the fluid flow passage from greater than zero, immediately prior to blocking, and zero, at the time of blocking, can cause a sudden pressure spike greater than the upstream pressure. More specifically, the pressure spike in the upstream pressure can be caused as the motion energy in the flowing fluid is abruptly converted to pressure energy acting on the components of the sprinkler. This pressure spike can cause the sprinkler to experience a water hammer effect, which can undesirably result in increased stress on the components of the diaphragm valve, as well as other components of the irrigation system, and can lead to premature failure of the components.  
      The water hammer effect can be exacerbated due to increased pressure fluctuations within irrigation sprinklers having the flow interrupter assembly. Increased pressure fluctuations can be caused by the use of larger radius nozzle sizes, undersized pipe diameters that result in increased water velocities, and large pipe losses that require higher input pressures.  
      The water hammer effect can also be exacerbated due to increased pressure fluctuations within a circuit having multiple irrigation sprinklers each with a flow interrupter assembly. For example, multiple such irrigation sprinklers can have their flow interrupter assemblies randomly synchronize, which can increase the amount system water flow that is shut-off. Similarly, synchronization of irrigation sprinklers having the flow interrupter assemblies can cause the dynamic pressure to increase and approach the static input pressure as flow demand drops. Moreover, the use of a pressure regulator, such as a pressure regulator that is built into a valve, can cause a lag time in compensating for pressure fluctuations due to the flow interrupter assemblies, which can magnify the resulting pressure waves.  
      Accordingly, there is a need for a fixed nozzle type sprinkler having a flow interrupter assembly, such as disclosed in U.S. Pat. No. 6,921,029, that is configured to reduce the water hammer effect.  
     SUMMARY  
      A fixed nozzle type sprinkler having a flow interrupter assembly, such as disclosed in U.S. Pat. No. 6,921,029, that is configured to reduce the water hammer effect is disclosed. A pressure relief valve is disposed to vent fluid when the pressure within the sprinkler exceeds a generally predetermined amount. When the flow interrupter assembly is in its closed position, blocking fluid flow through the nozzle, the pressure in the sprinkler can build up. Once that pressure reaches the generally predetermined amount, the pressure relief valve opens to vent fluid and reduce the pressure within the sprinkler. Thus, when the flow interrupter assembly moves from its open position to its closed position, the reduced pressure in the sprinkler can result in a reduced water hammer effect.  
      The pressure relief valve may automatically open in response to the fluid pressure within the sprinkler exceeding the generally predetermined amount. The pressure relief valve can be incorporated into the case of a sprinkler, or may be incorporated into a riser of the sprinkler. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a front elevation view of a first embodiment of a pop-up irrigation sprinkler having a flow stop valve and a pressure relief valve showing a riser extending from a case;  
       FIG. 2  is a top plan view of the irrigation sprinkler of  FIG. 1 ;  
       FIG. 3  is a cross-sectional view of the irrigation sprinkler of  FIG. 1  taken along line III-III of  FIG. 2 ;  
       FIG. 4  is a detailed cross-sectional view of the irrigation sprinkler of  FIG. 3  showing a pressure relief valve in a closed position;  
       FIG. 5  is a detailed cross-sectional view of the irrigation sprinkler of  FIG. 3  showing a pressure relief valve in an open position;  
       FIG. 6  is an exploded view of the flow stop valve of  FIG. 1 ;  
       FIG. 7  is a cross-sectional view of the flow stop valve of  FIG. 1  in a closed position with inner and outer pistons in extended positions;  
       FIG. 8  is a cross-sectional view of the flow stop valve of  FIG. 1  with the inner piston in the extended position and the outer piston in a retracted position;  
       FIG. 9  is a cross-sectional view of the flow stop valve of  FIG. 1  in an open position with the inner and outer pistons in the retracted positions;  
       FIG. 10  is a cross-sectional view of a second embodiment of a pop-up irrigation sprinkler having a flow stop valve and a pressure relief valve showing a riser extending from a case and the pressure relief valve in a closed position;  
       FIG. 11  is a cross-sectional view of the sprinkler of  FIG. 10  showing the pressure relief valve in an open position;  
       FIG. 12  is a graph comparing the maximum upstream pressures for the sprinklers of  FIG. 1  having both the pressure relief valve and the flow stop valve and sprinklers having only the flow stop valve for a period of about 130 seconds;  
       FIG. 13  is a detailed view of the period from 20 to 50 seconds of the graph of  FIG. 12 ; and  
       FIG. 14  is a graph comparing the inlet pressure, pressure inside the flow stop valve, and pressure upstream of a nozzle of the sprinkler for the sprinkler of  FIG. 1 . 
    
    
     DETAILED DESCRIPTION THE DRAWINGS  
      Pop-up spray-type sprinklers  300  and  400  for watering a fixed area around the sprinkler is disclosed and illustrated in  FIGS. 1-5 ,  10  and  11 . The sprinklers  300  and  400  are configured for reduced precipitation, permitting it to be used on the same circuit as a rotor, and for reducing the water hammer effect by venting excess pressure.  
      In a first embodiment, the sprinkler  300  includes a cylindrical case  304  adapted to be buried in the ground. A bottom end of the case  304  has a water supply inlet  302  at the bottom for attachment to a source of pressurized water. A top end of the case  304  has an overlying cover  306 . A hollow tubular riser  308  is disposed for reciprocation between an extended upper operating position, as shown in  FIG. 1 , and a lower inoperative position retracted inside the case  304 . The riser  308  has an internal bore  356 , extending between a lower end  358  disposed within the case  304  and an upper end  310  adapted to project above the case  304  and cover  306  when in the operative position. The upper end  310  of the riser  308  is adapted for mounting of a removable spray nozzle.  
      A conventional retract spring  360 , herein a coil spring, is disposed around the riser  308  within the casing  304 , and has one end abutting the underside of the cover  306  and the other end abutting an enlarged upwardly facing radial surface surrounding the lower end  358  of the riser  308 . The retract spring  360  operates to bias the riser  308  to the inoperative, retracted position within the case  304  when no water pressure is supplied to the sprinkler  300 , and to compress to the position shown in  FIG. 1  when water pressure is admitted to the sprinkler  300  via the inlet  302 , and the riser  308  is extended to the upper, operative position.  
      When in normal use, pressurized water enters the inlet  302  and flows through the case  304  and the riser  304  to the upper end  310  where it is ejected outwardly away from the sprinkler  300  through the nozzle in a fan-shaped spray pattern and at a precipitation rate determined by the spray nozzle and water supply pressure utilized. Depending on the type of nozzle installed on the riser  308 , the spray pattern can be any shape, typically from a full circle to a small pie-shaped part circle, such as a quarter circle pattern. When the supply of pressurized water is shut off, the retract spring  360  moves the riser  308  downwardly to the retracted inoperative position inside the case  304 . It should be noted that each time the supply of pressurized water is admitted to the case  304 , the rise in internal pressure causes the riser  308  to extend upwardly to the operative position, and as water pressure builds within the riser, water ejected though the nozzle results in a spray pattern that initially extends radially outwardly from adjacent the sprinkler  300  to the maximum distance away from the sprinkler for the specific nozzle and supply pressure utilized. On shutting off the supply of pressure to the inlet  302  of the case  304 , the water pressure decreases so that as the riser  308  retracts to the inoperative position within the case  304 , the spray pattern decays from the maximum radial distance back to the area adjacent the sprinkler. Thus, with each cycle of sprinkler operation, the area around the sprinkler  300  is watered from adjacent the sprinkler out to the maximum radial distance of throw of the nozzle.  
      A water flow interrupter or flow stop valve, generally designated by reference numeral  350 , is disposed within the riser  308  and is moveable therewith, and which functions to periodically shut-off the supply of pressurized water between the lower end  352  and upper end  310  of the riser  308 , and thus to the nozzle, for a predetermined period of time without interrupting the supply of water from the source to the inlet  302  of the sprinkler  300 . The flow interrupter assembly  350  operates in a highly effective and efficient manner to permit controlled reduction in the effective precipitation rate of the sprinkler  300 , and allows the use of any size nozzle and nozzle pattern without effecting the overall lowered precipitation rate of the sprinkler. Moreover, the flow interrupter  350  is relatively simple in construction, reliable in use and economical to manufacture, yet can be utilized with virtually any spray type sprinkler where it is desirable to reduce and control the precipitation rate during an irrigation cycle without having to turn the supply of pressurized water from the source on and off. The operation of the flow interrupter assembly  350  may be of the types described in greater detail in U.S. Pat. No. 6,921,029, the disclosure of which is incorporated by reference in its entirety, or as discussed herein. An in-stem pressure regulator  352  may optionally be included, as well as a Seal-A-Matic™ check valve.  
      When the flow interrupter assembly  350  functions to shut off the fluid flow through the riser  308 , fluid pressure upstream of the flow interrupter assembly  350  can undesirably increase. This increased water pressure upstream of the flow interrupter assembly  350  can cause the fluid pressure in the sprinkler  300  to increase, and can increase the water hammer effect. However, the water hammer effect is reduced by virtue of a pressure relief valve  312  that reduces pressure in the sprinkler  300  when the flow interrupter assembly  350  is blocking fluid flow to the nozzle. The pressure relief valve  312  opens to vent fluid and reduce the fluid pressure in the sprinkler  300  when the fluid pressure in the sprinkler  300  exceeds a generally predetermined amount.  
      The pressure relief valve  312  is positioned to vent fluid from the case  304  when in its open position, and to block fluid from exiting the case  304 , other than through the riser  308 , when in its closed position. The pressure relief valve  312  includes an opening  316  that is selectively blocked by a seal  324  attached to a plunger  322 . The plunger  322  is biased by a spring  326  to urge the seal  324  into a position blocking the opening  316 , as illustrated in  FIG. 4 . When the fluid pressure in the sprinkler increases beyond a predetermined amount, the fluid pressure urges the plunger  322  and attached seal  324  away from the opening  316 , against the biasing force of the spring  326 , to permit fluid to vent from the sprinkler  300  through the opening  316 , as illustrated in  FIG. 5 .  
      The amount of pressure at which the pressure relief valve  312  will open can be generally predetermined. That is, the pressure relief valve can be selected to open when certain predetermined pressures within the sprinkler  300  are exceeded. Due to operating variations of the sprinkler  300 , the predetermined pressure at which the pressure relief valve  312  will open likely will be within a range of pressures, and thus is generally predetermined. Some variation from the predetermined pressure is expected during operation.  
      For a given predetermined amount of pressure, the pressure relief valve  312  can be configured to shift from its closed position to its open position to vent fluid. More specifically, the area of the opening  316  and the amount of force exerted by the spring  326  can be configured to result in the pressure relief valve  312  opening for a given predetermined pressure according the following formula: F=(P)(A), where P is the predetermined pressure, F is the spring force, and A is the area of the opening. By way of example, for an opening that is about 0.50 inches in diameter and a desired pressure of 130 psi, a spring force of about 25.5 pounds is necessary to open the pressure relief valve  312 .  
      Turning now to more of the details of the pressure relief valve  312 , the plunger  322  is disposed in a bore  314  having a diameter that is larger than the diameter of the opening  316 . At the intersection of the opening  316  and the bore  314 , a recessed annular groove  336  is positioned facing the bore  314 , as illustrated in  FIGS. 4 and 5 . Between the groove  336  and the opening  316  is a raised annular rim  338 . The raised annular rim  338  is positioned to be engaged by the seal  324  positioned on the plunger  322 . The plunger  322  has a head  323  and a shaft  330 , with the head  323  having a larger diameter than the shaft  330 . The head  323  of the plunger  322  has a recessed region  334  with a diameter sized to accommodate the seal  324  and to position the seal  324  for engagement with the rim  338  to block the opening  316 . Opposite the recessed region  334 , the plunger  322  has a shaft  330  with an inner blind bore  332 . A disk  320  is positioned in the bore  314 , opposite the opening  316 , to close the bore  314 . The disk  320  has an inwardly projecting guide pin  328  that is sized to fit at least partially within the blind bore  314 .  
      Surrounding the shaft  330  is a valve spring  326 , which is positioned between the disk  320  and the head  323  of the plunger  322  to bias the plunger  322  away from the disk  320  and toward the opening  316  to block the opening  316  by engagement between the seal  324  and the rim  338 . When the biasing force of the spring  326  is overcome by the pressure in the sprinkler  300  acting on the seal  324  via the opening  316 , the plunger  322  and attached seal  324  are moved away from the opening  316  to permit fluid flow from the interior of the case  304 , through the opening  316  and into the bore  314 .  
      A vent passage  318  intersects the bore  314  and provides a path for fluid exiting the case  304  to flow through to atmosphere. The vent passage  318  preferably, though not necessarily, directs the vented fluid above-ground when the case  304  is at least partially buried underground in order to reduce erosion of the ground adjacent the exit of the vent passage  318 . As can be seen in  FIG. 3 , the vent passage  318  extends upwardly to a level approximately even with an outwardly facing surface of the cap  306 . A diffuser or redirecting feature may be placed at the opening of the vent passage  318  to either diffuse or redirect the exiting fluid.  
      In the illustrated embodiment of  FIGS. 1-5 , the vent passage  318  and bore  314  are integrally molded with the case  304 , and are connected to the case  304  via an extension  340 . The extension  340  includes a cut-out  342  to accommodate a depending portion of the cap  306 . The disk  320  may be sonically welded into the bore  314 , press-fit into the bore  314 , threaded into the bore  314 , attached with adhesive, or other such ways of joining. Alternatively, the disk  320  may be integrally molded with the case  304  or other components of the pressure relief valve  312 .  
      Comparison tests were performed between a sprinkler having the pressure relief valve  312  and a control sprinkler lacking the pressure relief valve  312 . In both instances, the sprinkler was a Rain Bird Model No. 1812 having a Seal-A-Matic™ check valve and an in-stem pressure regulator. The sprinkler having the pressure relief valve  312  was tuned to vent at the generally predetermined pressure of 130 psi. This was accomplished by having a vent opening of about 0.50 inches in diameter and a spring force of about 25.5 pounds. In the tests, the piping was 0.50 inch diameter PVC.  
      In the first comparison test, a pair of the same type of sprinklers were on the same circuit with a pump and a valve, and the pressure at the pump was about 103 psi. The maximum pressures were measured at start-up were measured before the valve, after the valve and at the end of the line. As set forth in the below table, the maximum pressure upon start-up both after the valve and at the end of the line are significantly reduced in the sprinkler with the pressure relief valve as compared to the control. This correlates to a decreased water hammer effect during start-up.  
                                              Start-up Pressure Maximum                                 Before Valve   After Valve   End of Line                                         Control   143 psi   195 psi   292 psi       With Pressure Relief Valve   131 psi   141 psi   161 psi                  
 
      In the second comparison test, eight of the same type of sprinklers were on the same circuit with a pump and a valve, and the pressure at the pump was about 103 psi. The maximum pressures during operation were measured before the valve, after the valve and at the end of the line. As set forth in the below table, the maximum pressure upon start-up both after the valve and at the end of the line are significantly reduced in the sprinkler with the pressure relief valve as compared to the control. This correlates to a decreased water hammer effect during operation.  
                                              Operating Pressure Maximum                                 Before Valve   After Valve   End of Line                                         Control   140 psi   159 psi   243 psi       With Pressure Relief Valve   131 psi   137 psi   152 psi                  
 
      As shown in the graph of  FIG. 14 , the pressure at the inlet increases to its maximum in conjunction with the decrease in pressure downstream of the flow interrupter assembly  350  and upstream of the nozzle. That is, the maximum pressure upstream of the flow interrupter assembly  350  occurs when the flow interrupter assembly  350  closes. However, the sprinkler  300  having both the pressure relief valve  312  and the flow interrupter assembly  350  has consistently lower maximum operating pressures, as shown in the graphs of  FIGS. 12 and 13  comparing the maximum upstream pressures for the sprinklers  300  having both the pressure relief valve  312  and the flow interrupter assembly  350  and sprinklers having only the flow stop valve. The data used to generate this graph was measured at the end of the line of a circuit having eight sprinklers, a pump operating at 103 psi, 0.50 inch PVC piping and a nozzle size 15F. As is expected, the highest maximum pressures are experienced soon after start-up. Both soon after start-up and thereafter, the sprinkler  300  having the pressure relief valve  312  has reduced maximum operating pressures, which in turn, and as discussed above, can advantageously lead to a reduced water hammer effect.  
      Instead of incorporating a pressure relief valve  312  into the case  304 , a pressure relief valve may be incorporated into a modified riser  408 , as illustrated in the sprinkler  400  of  FIGS. 10 and 11 . The pressure relief valve of the sprinkler  400  is configured to vent excess fluid when the pressure in a case  404  of the sprinkler  400  exceeds a generally predetermined pressure in order to reduce pressure in the sprinkler  400 , and can be combined with the flow interrupter assembly  350 . Although not illustrated, it is understood that the flow interrupter assembly  350  can be positioned in the modified riser  408 .  
      The modified riser  408  has an upper span  422  of typical diameter, a reduced diameter span  410 , and an enlarged diameter span  420 . During normal operation, when the riser  408  is extended from the case  404  and when the operating pressure in the case  404  of the sprinkler  400  does not exceed the generally predetermined pressure, the upper span  422  of the riser  408  contacts a lip  355  of a seal  354  of the cap  406 , as illustrated in  FIG. 10 , to restrict fluid from exiting between the riser  408  and the lip  355 . However, when the pressure in the case  404  increases beyond the generally predetermined pressure, the riser  408  extends further from the case  404  to a vent position such that the lip  355  of the seal  354  is adjacent to but spaced from the reduced diameter span  410  of the riser  408 , as illustrated in  FIG. 11 , to permit fluid to exit between the riser  408  and the lip  355  and thereby vent fluid to reduce pressure in the case  404 .  
      A spring retention ring  412  is disposed around the riser  408  to provide a surface against which a retract spring  414  and a pressure relief spring  416  abut in order to bias the riser  408 . The retention ring  412  may be freely floating on the riser  408 , but restricted from passing from the end of the riser  408  by the enlarged diameter span  420 . More specifically, the retract spring  414  is positioned between an end of the riser  408  and the retention ring  412  to bias the riser  408  into a retracted position within the case  404 . During normal operating pressures within the case  404 , the riser  408  is biased against the force of the retract spring  414  into the extended position where the upper span  422  of the riser is engaged by the lip  355  of the seal  354 . The pressure relief spring  416  is positioned between the cap  406  and the retention ring  412  to bias the riser  408  against further extension from the extended position. As discussed above, when the pressure in the case  404  increases beyond the generally predetermined pressure, the riser  408  extends further from the case  404  to the vent position, against the biasing force of the pressure relief spring  416 , such that the lip  355  of the seal  354  is adjacent to but spaced from the reduced diameter span  410  of the riser  408 . The spring force of the retract spring  414  is less than the spring force of the pressure relief spring  416 , such that during normal operating conditions the riser  408  does not exceed its extended position prematurely. The amount of the spring force of the pressure relief spring  416  can be determined similarly to that for the pressure relief valve  312 . That is, by multiplying the desired venting pressure by the area being acted upon by the pressure, in this embodiment, the area of the riser  408  acted upon the operating pressure in the case  404  of the sprinkler  400 .  
      Turning now to details of a specific example of the flow interrupter assembly  350 , the assembly includes a cartridge cylinder  502  having an outer piston  504  reciprocal therein and an inner piston  506  being reciprocal in the outer piston  504 . A spring  508  biases the inner piston  506  from the outer piston  504 , and a spring  510  biases the outer piston  504  from the cartridge cylinder  502 . A valve head  512  of the inner piston  506  is configured to abut a valve seat  514  maintained in the riser  308  to block fluid from flowing past the assembly  350  when the biasing forces of the springs  508  and  510  are not exceeded.  
      The valve head  512  has a convex upper surface and, during pressurization of the sprinkler, the fluid flow is directed against the underside of the valve head  512  by an inclined surface of the valve seat  514 . The upper convex surface of the valve head  512  may be shaped liked a portion of a sphere, and the valve seat  514  may be shaped like the frustum portion of a cone. The use of this rounded valve head  512  and inclined valve seat  514  design facilitates the use of the low precipitation rate device at both relatively high and low source pressure conditions.  
      During operation, fluid flows upward between the case  304  and the flow interrupter assembly  350 . To facilitate this fluid flow, a plurality of internal ribs may be positioned inside the case  304 . The flow interrupter assembly  350  can be abutted by the ribs, leaving fluid flow paths between the case  304  and the fluid interrupter assembly  350 . The upwardly flowing fluid impinges on the inclined surface of the valve seat  514 , which redirects the fluid flow toward an upper surface  516  of the outer piston  504 . The impact of the redirected fluid on the upper surface  516  of the outer piston  504  urges outer piston  504  to retract against the biasing force of the spring  510 , while the fluid also impacts the underside of the valve head  512  to urge the same against the inclined surface of the valve seat  514 . When the outer piston  504  is retracted in the cartridge cylinder  502 , the spring  508  is compressed and urges the inner piston  506  into a retracted position relative to the outer piston  504 , and thereby moves the valve head  512  away from the valve seat  514  to permit fluid to flow therepast. However, as fluid flows past the valve seat  514 , the fluid pressure acting on the upper surface  516  of the outer piston  504  is reduced, and the spring  510  biases the outer piston  504  along with the inner piston  506  such that the valve head  512  abuts the valve seat  514  to block fluid flow.  
      The configuration of the valve head  512  and valve seat  514  helps prevent the assembly  350  from stalling, or ceasing to operate, at difference pressure conditions. When the valve head  512  moves slowly at either opening or closing, fluid turbulence may create force or pressure balance conditions that cause the valve head  512  to stop between the open and closed positions, leaving the assembly  350  partially open. In ordinary operation, the assembly  350  may stall at different pressure conditions depending on the stiffness of the springs  508  and  510  that are used to open the valve head  512 . When the springs  508  and  510  have a low spring coefficient, it will open the valve head  512  in low pressure conditions but may not be sufficiently stiff to quickly and completely pull the valve head  512  through turbulence in high pressure conditions. On the other hand, when the springs  508  and  510  have a high spring coefficient, they may not close the valve head  512  quickly enough through turbulent flow. To avoid stalling resulting from fluid turbulence, it can be desirable to move the valve head  514  quickly between open and closed positions.  
      The disclosed embodiment of the assembly  350  facilitates reduced stalling by using a spring  508  having a relatively high spring coefficient in conjunction with a valve head  512  and valve seat  514  configured to allow fluid flow to assist in closing the valve. The relatively stiff spring  508  facilitates the assembly  350  being opened quickly in both low and high pressure conditions. The use of an inclined valve seat  514  facilitates the assembly  350  being closed quickly by directing fluid flow against the underside of the valve head  512  to provide additional force on the valve head  512  at the time of closing. The valve head  512  is shaped to have an upper convex surface to provide a profile that facilitates the valve head  512  moving through turbulence more easily than a flat valve head design. Thus, the assembly  350  can move quickly through turbulence at both the point of opening and the point of closing.  
      In a flat valve design, the fluid approaching the valve as the valve head is approaching the closed position tends to impact the valve head in a horizontal manner, given the absence of an inclined valve seat. As a result, the momentum of the fluid does not provide much force to the valve head and does not contribute significantly to closing the valve. In the current embodiment, in contrast, the valve seat  514  is inclined to allow the fluid to impact the underside of the valve head  512  more directly, thereby urging it to close. This aspect is especially beneficial at high pressure conditions where the fluid flow provides greater force on the valve head  512  than at lower pressure conditions.  
      In this rounded valve head and inclined valve seat design, the valve head  512  also includes circular ribs and pockets, which are located on the convex upper surface of the valve head  512 . These circular ribs and pockets are positioned on the convex upper surface to change fluid pressure and velocity at the valve head  512 . Gradual change in pressure and velocity can tend to increase the likelihood that the assembly  350  will stall, i.e., become stuck in a partially open position. The use of circular ribs and pockets results in a more drastic change in pressure and velocity, which can decrease the occurrence of stalling.  
      The low precipitation rate devices described herein reduce the precipitation rate by causing intermittent operation of the sprinkler or precipitation device. The duty cycle, or ratio of precipitation time to non-precipitation time, describes how much the precipitation has been decreased. For example, a 15H nozzle has a precipitation rate of 1.58 inches/hour at 30 pounds per square inch (psi) input pressure. A device having a 38% duty cycle reduces the precipitation rate to about 0.6 inches per hour. The reduced precipitation rate more closely matches the typical soil intake rate of water, thereby reducing water run-off and wasted water.  
      If the duty cycle is constant regardless of source pressure, the precipitation rate will increase as pressure decreases. In other words, the precipitation rate can vary as the source pressure varies. The change in precipitation rate as a function of pressure for a 15H nozzle and a 38% duty cycle can vary between 0.60 and 0.80 inches per hour as the pressure increases from 15 to 30 psi.  
      The above embodiments of the invention preferably use a linear emitter design to compensate for this change in precipitation rate and to make the precipitation rate relatively constant, regardless of changes in pressure. As described in the above embodiments, the length of time during which the sprinkler or low precipitation rate device is in the “off,” or not precipitating, condition is controlled by an emitter. By designing the emitter to have a variable flow rate with a lower flow rate at lower pressure, the “off” time can be increased, which in turn reduces the precipitation rate of the device. Thus, by adjusting the flow rate at different pressures, the emitter can be linearized, i.e., provide a relatively constant precipitation rate regardless of pressure.  
      For example, the linear emitter duty cycle can be increased from approximately 0.30 to 0.40 as the pressure increases from 15 to 30 psi. By changing the duty cycle as a function of pressure, the precipitation rate can be maintained at a relatively constant level regardless of pressure. The precipitation rate of the 15H nozzle with linear emitter can be relatively constant at about 0.60 inches per hour.  
      The linear emitter design can facilitate bi-directional flow of fluid in and out of the emitter. Further, it can operate over a wider pressure range, such as 10 to 125 psi with surges up to 300 psi, than typical drip emitters, which are designed to operate in pressure ranges of 15 to 60 psi. The emitter is also designed to operate at faster operating cycles with frequent pressure changes than are typical emitters.  
      The linear emitter design uses a variable orifice to create a pressure compensating flow control. It uses a specially shaped restriction orifice that reacts with pressure differential displacement of an elastic diaphragm  520 , which may be held in place using a retainer  522 . The linear emitter uses a V-shaped groove  524  that has a constantly changing profile, and may extend radially outward.  
      The drawings and the foregoing descriptions are not intended to represent the only forms of the sprinkler incorporating a pressure relief valve in regard to the details of construction and manner of operation. Changes in form and in the proportion of parts, as well as the substitution of equivalents, are contemplated as circumstances may suggest or render expedient; and although specific terms have been employed, they are intended in a generic and descriptive sense only and not for the purposes of limitation. For example, although the foregoing benefits may be achieved in the presently-disclosed sprinkler having a pressure relief valve, other pressure relief valves may be configured differently and still result in these benefits.