BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present application relates to filtration systems which include replaceable filter cartridge housings that have a bypass feature associated therewith, more particularly, to filter cartridge housings which include a bypass channel that allows fluid to proceed from the inlet portion of the cartridge to the discharge portion of the cartridge without contacting the filer media disposed within the cartridge housing, and still more particularly, to a replaceable cartridge which provides economic advantages through improved filtration capacity while maintaining the hardness of the water output within a desirable range. 
     2. Background of the Related Art 
     Often in both consumer and industrial applications a fluid, such as water, is filtered prior to its use in an intended application. As a result, filtration systems are installed either internally or externally within the industrial system or consumer appliance. For example, in today&#39;s modern society, refrigerators which provide drinking water and produce ice are widespread. Commonly this type of refrigerator utilizes an internal filtration system for purifying the water used for drinking and ice production. Additionally, since water is the base for many beverages such as soda and coffee, it is also commonplace to find water filtration assemblies installed within or in conjunction with commercial vending machines that dispense coffee or soft drinks. 
     Like most filtration systems, frequent replacement of the filter media is required to sustain proper filtration and the quality of the output flow. Therefore, the aforementioned filtration assemblies are typically equipped with filter cartridges that can be easily disengaged from a filter head, discarded and replaced, without requiring the removal of the remaining components of the filter system. 
     Other than ease of use, filter cartridge manufacturers, as well as their buyers and end users, consider characteristics such as long-life, filtration ability, low cost design to be of primary importance. The filtration ability is probably the most variable quality since it depends highly on the characteristics of the input water and desired characteristics of the output flow. The life of the filter is largely related to the operative flow rate through the filter and type or degree of filtration desired. Customization of the filter cartridges to meet the desired filtration ability will result in increase costs. Lower costs may be achieved and maintained if one or a few standard filter cartridges can be mass-produced and utilized for a plurality of applications. 
     A good example of the relationship between filtration ability and filter life may be found in applications which incorporate such replaceable filters for water hardness reduction. Preliminary, it should be noted that hardness of water, as used herein, refers to the content of calcium and magnesium salts, which may be bicarbonates, carbonates, sulfates, chlorides, or nitrates. Furthermore, hardness is a measure of calcium and magnesium salts expressed as equivalent calcium carbonate content and is usually stated in parts per million (ppm) or grains per gallon (gpg) as follows: very soft water, less than 15 ppm; soft water, 15 to 50 ppm; slightly hard water, 50 to 100 ppm; hard water, 100 to 220 ppm; and very hard water, over 220 ppm. 
     In the case of water hardness reduction, most filter media in use today have the ability to remove a substantially high percentage, if not essentially all, of the calcium and magnesium from the input water. Most applications utilize the filter cartridges for total removal of calcium and magnesium, or at the very least, can endure such removal without harmful results. However, a high level of calcium and magnesium removal is not desirable for all applications, such as applications requiring “recipe quality water” (e.g., beverage mixing applications) that must maintain a level of hardness within a constant range to deliver an end product having a pleasing taste, among other things. 
     In addition, a high rate of removal reduces the life of the filter, and thus, filtration capability. Many end users utilize such filters for applications in which a lowered rate of calcium and magnesium removal may be tolerated. It is unnecessary for these end users to pay higher costs resulting from filter replacement and decreased filtration capability, especially in situations where the input water is hard or very hard. 
     The aforementioned issues, and manner in which they may be addressed, poses a problem for manufacturers and end users alike. Customization of the filter cartridge, filtration head, or filtration system apparatus to achieve the desired recipe quality water can satisfy specific needs, but such individualized changes can result in significantly increased costs and/or problems with equipment operability, among other things. 
     Thus, what is needed is a viable cost-effective solution to the aforementioned problems with respect to water hardness, as well as with respect to similar problems achieving desirable output water characteristics and/or decreasing the cost of operation by operating the filtration system with a decreased removal rate. 
     SUMMARY OF THE INVENTION 
     The present invention solves the problems of the prior art by, among other things, providing a connective end for a filter cartridge that provides fluid communication between a filter media chamber and a filter head having a receiving core, and first and second fluid conduits. The connective end in accordance with the present invention includes a substantially hollow neck defining a distal end portion, an elongated portion and a proximal end portion for mating with the filter media chamber. A tubular member is disposed axially in the neck which defines an inner axial passage and a radially outer axial passage bounded by the periphery of the tubular member and inner surface of the neck, so that the inner axial passage and the outer axial passage provide independent conduits for dual fluid flow through the neck. 
     The tubular member has an opening in the distal end portion of the neck and at least one opening to the outer axial passage is positioned in the elongated portion of the neck. In addition, a bypass opening to the outer axial passage is located in the distal end portion of the neck. 
     The neck includes a plurality of seals disposed along the outer periphery of the elongated portion for providing a substantially water-tight engagement between the neck and the receiving core. Preferably, a proximal radial seal is disposed adjacent to the proximal end and a distal radial seal is disposed adjacent the distal end portion. Together, the proximal and distal radial seals help maintain the integrity of fluid communication (i.e., limit contact between the influent and effluent streams, among other things) between the first conduit and the outer axial passage opening, or openings, and between the second conduit and the tubular opening and the bypass outer axial passage opening, respectively, when the connective end is engaged in the receiving core. 
     In a preferred embodiment of the aforementioned connective end, the bypass opening is substantially circular, and may have a diameter ranging from about 0.762 millimeters to about 1.78 millimeters. Preferably, the opening has an area ranging from about 0.45 mm 2  to about 2.5 mm 2 . 
     The present invention is also directed to a filter cartridge including a filter media chamber, a substantially hollow neck defining a distal end portion, an elongated portion and a proximal end portion mated with the filter media chamber, and a tubular member disposed axially in the neck and extending into the filter media chamber. The tubular member defines an inner axial passage and a radially outer axial passage bounded by the periphery of the tubular member and inner surface of the neck, the inner axial passage and the outer axial passage providing independent conduits for dual fluid flow through the neck and the filter media chamber. 
     The tubular member has an opening located in the distal end portion of the neck and at least one outer axial passage opening positioned in the elongated portion of the neck. Also, a bypass outer axial passage opening is positioned in the distal end portion of the neck. 
     A proximal radial seal disposed along the outer periphery of the elongated portion of the neck adjacent the proximal end portion provides a substantially water-tight engagement between the neck and the receiving core. A distal radial seal disposed along the outer periphery of the elongated portion of the neck adjacent the distal end portion positioned also provides a substantially water-tight engagement between the neck and the receiving core. Together, the proximal and distal radial seals maintain the integrity of fluid communication between the first conduit and the outer axial passage opening, or openings, and between the second conduit and the tubular opening and the bypass outer axial passage opening, respectively. 
     In a preferred embodiment of the aforementioned filter cartridge, the bypass opening is substantially circular, and may have a diameter ranging from about 0.762 millimeters to about 1.78 millimeters. Preferably, the opening has an area ranging from about 0.45 mm 2  to about 2.5 mm 2 . 
     The present invention is also directed to a filter cartridge and filter head assembly. The filter cartridge includes a filter media chamber, a substantially hollow neck, and a tubular member disposed axially in the neck and extending into the filter media chamber. The hollow neck includes a distal end portion, an elongated portion and a proximal end portion which is mated with the filter media chamber. The tubular member defines an inner axial passage and a radially outer axial passage within the neck. The outer axial passage is bounded by the periphery of the tubular member and inner surface of the neck. The inner axial passage and the outer axial passage provide independent conduits for dual fluid flow through the neck and the filter media chamber. 
     The tubular member has an opening located in the distal end portion of the neck and at least one outer axial passage opening positioned in the elongated portion of the neck. Also, a bypass outer axial passage opening is positioned in the distal end portion of the neck. 
     A proximal radial seal disposed along the outer periphery of the elongated portion of the neck, adjacent the proximal end portion, provides a substantially water-tight engagement between the neck and the receiving core. A distal radial seal disposed along the outer periphery of the elongated portion of the neck, adjacent the distal end portion, provides another substantially water-tight engagement between the neck and the receiving core. Together, the proximal and distal radial seals maintain the integrity of fluid communication between the first conduit and the outer axial passage opening, or openings, and between the second conduit and the tubular opening and the bypass outer axial passage opening, respectively. 
     The filter head of the present invention includes a receiving core configured to engage the neck of the filter cartridge, a first conduit, and a second conduit for fluid flow therein. The first conduit includes an opening to the receiving core which is positioned so that it is adjacent to the elongated portion of the neck when the neck of the filter cartridges is engaged within the receiving core. The second conduit includes an opening to the receiving core which is positioned so that it is adjacent to the distal portion of the neck when the neck of the filter cartridge is engaged within the receiving core. 
     The proximal and distal radial seals define a first reservoir bounded by the proximal and distal radial seals and the receiving core in fluid communication with the first conduit opening and the outer axial passage opening, or openings. A second fluid reservoir is also defined by the distal seal and the receiving core in fluid communication with the second conduit opening, the tubular opening and the bypass outer axial passage opening. 
     Other objects and advantages of the application will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that those having ordinary skill in the art to which the present application appertains will more readily understand how to make and use the same, reference may be had to the drawings wherein: 
         FIG. 1  is a perspective view of a disposable filter cartridge constructed in accordance with the present invention; 
         FIG. 2  is a side elevational view of the upper portion of the filter cartridge shown in  FIG. 1 , including the cover and neck; 
         FIG. 3  is a top plan view of the cover for a cartridge shown in  FIG. 1 ; 
         FIG. 4  is a side elevational view of the upper portion of the filter cartridge shown in  FIG. 1 , including the neck and fluid opening therein; 
         FIG. 5  is an elevational cross-sectional view of the cover of FIG.  3  through line  5 — 5 ; 
         FIG. 6  is a bottom plan view of the cover of FIG.  4  through line  6 — 6 ; 
         FIG. 7  is a perspective view of an exemplary filter head in which a filter cartridge constructed in accordance with the present invention may be installed; 
         FIG. 8  is a perspective view of another exemplary filter head in which a filter cartridge constructed in accordance with the present invention may be installed; 
         FIG. 9  is a perspective view of another exemplary filter head in which a filter cartridge constructed in accordance with the present invention may be installed; 
         FIG. 10  is a cross-sectional enlarged view of a disposable cartridge constructed in accordance with the present invention and mated with the filter head of  FIG. 9 ; 
         FIG. 11  is an elevational cross sectional view of a disposable cartridge which is mated with the filter head of  FIG. 9 ; and 
         FIG. 12  is a cross-sectional view of a disposable cartridge constructed in accordance with the present invention and mated with the filter head of FIG.  8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The advantages of a filtration assembly constructed or retrofitted in accordance with the present invention will become more readily apparent to those having ordinary skill in the art from the following detailed description of certain preferred embodiments taken in conjunction with the drawings which set forth representative embodiments thereof. Unless otherwise apparent, or stated, directional references, such as “lower” and “upper”, are intended to be relative to the orientation of a particular embodiment of the invention as shown in the first numbered view of that embodiment. Also, a given reference numeral indicates the same or similar structure when it appears in different figures. 
     Referring now to  FIG. 1 , in which there is illustrated a filter cartridge constructed in accordance with a preferred embodiment of the present disclosure and designated by reference numeral  10 . Filter cartridge  10  includes a main housing  12 , preferably cylindrically-shaped, which includes a closed lower portion  14  and a generally planar cover  16 . The cover  16  may be mated with lower portion  14  after a filtering material (not shown in  FIG. 1 ) is disposed in the housing. A preferred filter media for use in filtering water for soft drinks and in particular carbonated water, is activated carbon. However, proprietary materials are often employed in housing  12 . In addition, pre and/or post filtering media, such as screens or polishing agents, may be included in housing  12 . 
     In the following description, an axis extending longitudinally through the cylindrically-shaped housing  12  (i.e., from lower portion  14  through the center of cover  16 ) is used as a reference to describe features and components of the inventive filter cartridge as being “axial” and “radial” with respect to the aforementioned longitudinal axis. It should also be understood, however, that filter cartridges in various shapes and sizes may be constructed according to the present invention and still fully benefit from the advantages described herein. 
     With continuing reference to  FIG. 1  in conjunction with  FIGS. 2 through 6 , a neck  18  of substantially uniform outer diameter projects from the cover  16  of main housing  12 . Neck  18  is formed by a proximal end  20  connected with cover  16 , an elongated portion  22  extending axially from the proximal end  20  and a distal end  24 . Preferably, proximal end  20  is mated together or integrally formed with cover  16 . 
     A pair of seals, preferably O-rings, which include a distal O-ring  26  and a proximal O-ring  28  are mounted within annular recesses  29  along the outer periphery of neck  18  which are configured to accommodate the O-rings therein. O-rings  26  and  28  are preferably fabricated of an elastic, durable and inert material. Neck  18  is at least partially hollowed out to form an axial passage into housing  12 . The axial passage in neck  18  is divided into a radially inner axial passage  30  and radially outer axial passage  32 . In this embodiment, the radially inner axial passage  30  is formed by a tube  34  which extends axially into lower portion  14  and is supported in neck  18  by a pair of diametrically opposed indentations  36  which extend radially inward from the inner surface of neck  18 . Thus, the radially outer axial passage  32  is defined by the outer circumference of tube  34  and the inner circumference of neck portion  18 , as shown particularly in  FIG. 6. A  pair of diametrically opposed openings on elongated portion  22  including opening  38  provide access to the radially outer axial passage  32 . A tube opening  40  at distal end  24  provides a second conduit through neck  18  and into the lower portion  14  of the housing. A second opening  42  in distal end  24  provides access to outer axial passage  32 , referred to herein as bypass opening. Preferably, bypass  42  is bored, or otherwise created, adjacent tube opening  40 . 
     The neck  18  of filter cartridge  10  is adapted and configured for reception within a central core of a filter head, such as filter heads  44 ,  46  and  48  shown in  FIGS. 7 ,  8  and  9 , respectively. Each of the three exemplary filter heads include a core for receiving neck  18  of cartridge  10 , and conduits for providing fluid communication with openings  38 ,  40  and  42  when neck  19  is installed within in the core. The conduits are attached, threadably or otherwise, to a fluid supply and fluid receiving apparatus, which may include further filter heads or a device for end use, such as a fluid mixer. 
     As shown  FIGS. 10 and 11 , neck  18  is installed in core  50  of a filter head which is substantially similar to filter head  48  of FIG.  9 . Installing cartridge  10  in core  50 , defines a first annular fluid reservoir  54  bounded radially between the inner periphery of core  50  and the outer periphery of elongated portion  22 , and bounded axially between proximal and distal O-rings  26  and  28 . A second fluid reservoir  56  is also defined by installing cartridge  10  in core  50  between the interior distal end of core  50  and distal end  24  of neck  18 . 
     In operation, untreated fluid is fed through conduit  58  in filter head  48  to first reservoir  54 , which in turn flows into outer axial passage  32  of cartridge  10  via openings  38 . Fluid entering outer axial passage  32  primarily flows into lower portion  14  and contacts filter media  60  for treatment. A portion of the fluid flow entering outer axial passage  32  exits cartridge  10  untreated via bypass  42  which feeds to second reservoir  56 . The portion of fluid influent contacting filter media  60  thereafter enters tube  34  through a proximal opening  62 . The pressure of the fluid in cartridge  10  results in fluid flow through tube  34  and into the second reservoir  56  via opening  40  in distal end  24 . Thus, when in operation, treated fluid from opening  40  is mixed in second reservoir  56  with a portion of untreated fluid from bypass  42 . The combined effluent flow is supplied by reservoir  56  to a conduit  64  defined in filter head  48 , and may thereafter be directed to process equipment or additional filter heads and cartridge systems in accordance with the present invention, or otherwise. The flow of fluid through this embodiment is illustrated by arrows  66  in  FIGS. 10 and 11 . Alternatively, the flow of fluid may be reversed such that fluid influent derives from reservoir  56  and flows into openings  40  and  42 , exiting the cartridge via opening  38  into reservoir  54 . 
       FIG. 12  depicts another exemplary mating between a cartridge  110  and a filter head  146 , wherein reservoirs  154  and  156  differ in volume, when compared to the previous embodiment of  FIGS. 10 and 11 , due to the different internal design of filter head  146 . 
     The following test results illustrate some of the advantages of a filter cartridge constructed in accordance with the present invention. Filter cartridges having filter media for reducing water hardness were tested to determine the effect of bypass size (by diameter) on hardness reduction and removal efficiency. The hardness, that is, the presence of calcium (Ca) and magnesium (Mg), was tested in influent and effluent water samples for all cartridges. 
     The samples of water were removed under the same conditions (e.g., after 20 gallons at a flow rate of 0.6 gallons per minute and a line pressure of 50 pounds per square inch) each time the test was conducted and the presence of Ca and Mg in the samples was determined via Inductive Coupled Plasma Atomic Emission Spectroscopy. Hardness may be expressed in grains per gallon (gpg) or as below, in parts per million (ppm or mg/L) as calcium carbonate (CaCO 3 ) equivalent (note that 1 gpg hardness is equal to 17.1 ppm or mg/L). 
     The following formula is used to determine the hardness in ppm:
 
Hardness, ppm=([Ca, ppm]*2.497)+([Mg, ppm]*4.116)
 
     For example, the influent sample after 20 gallons had flowed into a cartridge  10 , wherein the bypass  42  possessed a diameter of 0.052 inches (1.32 millimeters), was found to have concentrations of 33 ppm of Ca and 7.0 ppm of Mg. Thus, in accordance with the above formula:
         Hardness, ppm=([33 ppm]*2.497)+([7.0 ppm]*4.116)   Hardness, ppm=82.4 ppm+28.8 ppm   Hardness, ppm=111 ppm       

     A sample taken from the effluent was determined to contain 6.7 ppm of Ca and 1.9 ppm of Mg, thus the hardness was calculated to be 25 ppm. 
     The hardness removal efficiency is determined by dividing the difference in hardness between the influent and effluent by the hardness of the influent. In the above example, the hardness removal efficiency is calculated to be 77.5%. 
     For comparative purposes, it should be understood that similar filter cartridges without a bypass  42 , or that otherwise do not mix influent with effluent, which are put under the same conditions would output an effluent flow containing negligible amounts, if any, of Ca and Mg. Thus, the aforementioned filter cartridges effectively output an effluent stream having a hardness of zero and hardness removal efficiency of 100%. The reduction in hardness removal efficiency is therefore the difference between total hardness removal (i.e., 100%) and actual hardness removal, which in the present example is 22.5%. 
     
       
         
               
               
             
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Reduction 
               
             
          
           
               
                   
                   
                 in 
               
             
          
           
               
                 Bypass 
                   
                 Influent 
                 Effluent 
                 Hardness 
               
             
          
           
               
                 Diameter 
                 Sample 
                   
                 hardness 
                   
                 hardness 
                 Removal 
               
               
                 (inches) 
                 (gallons) 
                 pH 
                 (ppm) 
                 pH 
                 (ppm) 
                 Efficiency 
               
               
                   
               
               
                 0.052 
                 20 
                 7.44 
                 111 
                 5.37 
                 25 
                 22.5% 
               
               
                 0.059 
                 20 
                 7.42 
                 121 
                 5.36 
                 30 
                 24.8% 
               
               
                   
               
             
          
         
       
     
     Preferably, the bypass is substantially circular in shape, and could be created on an already existing filter cartridge by using a circular drill or boring device. Thus, in the above examples, the cartridge with a bypass diameter of 0.052 inches has an opening area of 0.00212 in 2  (1.37 mm 2 ) and the cartridge with a bypass diameter of 0.059 inches (1.5 millimeters) has an opening area of 0.00273 in 2  (1.77 mm 2 ). 
     Table 2 illustrates the results of tests conducted with a cartridge  10  having a bypass  42  with a diameter of about 0.0465 inches (1.181 millimeters) and an opening area of 0.0017 in 2  (2.58 mm 2 ). The pressure in the feed water line was 70 psi and the flow rate to cartridge  10  was varied as shown below. 
     
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Reduction in 
               
             
          
           
               
                 Flow 
                   
                 Influent 
                 Effluent 
                 Hardness 
               
             
          
           
               
                 Rate 
                 ΔP 
                   
                 Alkalinity 
                 hardness 
                   
                 Alkalinity 
                 hardness 
                 Removal 
               
               
                 (gpm) 
                 (psia) 
                 pH 
                 (ppm) 
                 (gpg) 
                 pH 
                 (ppm) 
                 (gpg) 
                 Efficiency 
               
               
                   
               
             
          
           
               
                 0.25 
                 4.02 
                 7.58 
                 70 
                 6 
                 6.37 
                 20 
                 2 
                 33.3% 
               
               
                 0.5 
                 9.66 
                 7.71 
                 70 
                 6 
                 6.23 
                 30 
                 2 
                 33.3% 
               
               
                 0.75 
                 16.01 
                 7.72 
                 70 
                 6 
                 6.28 
                 25 
                 2 
                 33.3% 
               
               
                 1 
                 24.21 
                 7.71 
                 80 
                 6 
                 6.38 
                 25 
                 2 
                 33.3% 
               
               
                   
               
             
          
         
       
     
     As shown in Table 2, the hardness removal rate (66.7%) for a given bypass diameter stayed constant over various flow rates. Therefore, the reduction of the hardness removal rate also stayed constant. Table 3 below illustrates the results of the same test as shown in Table 2 applied to a filter cartridge much like cartridge  10 , but without having a bypass  42 . 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Flow 
                   
                 Influent 
                 Effluent 
                 Reduction in 
               
             
          
           
               
                 Rate 
                 ΔP 
                   
                 Alkalinity 
                 hardness 
                   
                 Alkalinity 
                 hardness 
                 Hardness Removal 
               
               
                 (gpm) 
                 (psia) 
                 pH 
                 (ppm) 
                 (gpg) 
                 pH 
                 (ppm) 
                 (gpg) 
                 Efficiency 
               
               
                   
               
             
          
           
               
                 0.25 
                 5.5 
                 7.58 
                 70 
                 6 
                 5.52 
                 5 
                 0 
                 0% 
               
               
                 0.5 
                 11.9 
                 7.48 
                 60 
                 6 
                 5.64 
                 10 
                 0 
                 0% 
               
               
                 0.75 
                 19.8 
                 7.6 
                 70 
                 6 
                 5.94 
                 10 
                 0 
                 0% 
               
               
                 1 
                 28.5 
                 7.61 
                 70 
                 6 
                 6.11 
                 15 
                 0 
                 0% 
               
               
                   
               
             
          
         
       
     
     As shown in Table 3, hardness removal is 100%. It should also be noted that the pressure drop is higher than that of the cartridge  310  under the same conditions. 
     It should be readily apparent that a filter cartridge constructed in accordance with the present invention is not suitable in a system requiring 100% hardness reduction. However, as shown herein, the rate of hardness reduction may be controlled by varying the diameter of the bypass in a filter cartridge constructed in accordance with the present invention. Furthermore, the filter cartridge of the present invention results in a lower pressure drop and longer life, by, among other things, allowing a portion of the influent stream to pass through untreated. The present invention is advantageously adapted to existing systems without the need for redesigning or reconfiguring existing equipment, or radically changing the design of the cartridge itself. 
     While the systems and methods for using the systems contained herein constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise systems and methods of use, and that changes may be made thereto without departing from the scope of the invention which is defined in the appended claims.
A filter cartridge 10 includes a connective end for mating with a filter head 48 and providing fluid communication between the filter media chamber and the filter head. The connective end includes a tubular member 34 disposed axially in a hollow neck 18 to define inner and radially outer axial passages ( 30, 32) providing independent conduits for dual flow through the neck. The outer axial passage includes an opening 38 in the elongated portion 22 of the neck and a bypass opening 42 in the distal end portion of the neck. The inner axial passage includes an opening 40 in the distal end of the neck. Proximal and distal radial seals ( 26, 28) are provided on the outer periphery of the elongated neck portion 22 to ensure water-tight engagement when the neck is inserted into the receiving core of the filter head.