WO 2007/022322 PCT/US2006/032067 COVER FOR A SWIMMING POOL MAIN DRAIN BACKGROUND OF TIlE INVENTION [0001] The present invention relates to covers for swimming pool main drains. 5 [0002] A swimming pool has one or more main drains that remove water continuously from the bottom of the pool. The removed water is subsequently filtered of both suspended particulate and sediment to keep it both clear and clean before it is returned to the pool. From time to time, larger debris, such as water-soaked leaves, will descend to the bottom of the pool and can be drawn into the drain through its cover for removal. 10 [0003] However, main drains, because of the relatively powerful suction required for normal operation, pose hazards to swimmers, particularly small children. The suction can trap swimmers and sometimes hold them to the drain, which is commonly located in the deepest part of the pool. At other times, the configuration of the drain, in combination with the suction, will entangle the swimmer's hair or clothing. Either of these circumstances can 15 be fatal to the swimmer. [0004] Drain covers are frequently designed with many small openings to avoid the aforementioned problems. However, they will then not be suitable for draining larger debris and may still cause entrapment and entanglement. [0005] A drain cover needs to have a total opening area sufficient to allow the 20 pool to drain and to admit leaves and other larger debris, but avoid or at least reduce the possibility of the entrapment and entanglement of swimmers. SUMMARY OF THE INVENTION [0006] Briefly recited and according to its major aspects, the present invention is a main drain cover for a swimming pool. The particular feature of the present invention is its 25 utilitarian shape, which is characterized by a gently domed overall shape and a single, extended opening with rounded edges. The present invention may also include an adaptor collar, to fit the cover to the main drain housing, and the housing itself. [0007] In two of the embodiments described herein, the pool is drained through a single extended opening characterized by smooth, curved edges., The first of these is a 30 straight, elongated opening; the second is shaped roughly like the letter "H." The size and extent of the openings is an important feature of the present invention. Because of their size and extent, the openings are relatively more difficult for a swimmer to block than more compact openings would be. Because of their smoothly rounded edges, free of corners and 1 WO 2007/022322 PCT/US2006/032067 projections, they are much less likely to entangle the hair or clothing of swimmers. Furthermore, larger debris will be more easily drawn in through the larger, rounded openings. [0008] An advantage of the second embodiment of the present invention is that the cover can be used on both new and previously existing drain housings and on housings 5 with two and with three ports, such as those with entrapment relief ports in addition to suction ports and hydrostatic relief ports. [0009] Other features and their advantages will be apparent to those skilled in the art of swimming pool water circulation system design from a careful reading of the Detailed Description of Preferred Embodiments accompanied by the following drawings. 10 BRIEF DESCRIPTION OF THE DRAWINGS [0010] In the drawings, [0011] Fig. 1 is top view of a cover for a main drain of a swimming pool, according to a first preferred embodiment of the present invention; [0012] Fig. 2 is a side, cross sectional view of the main drain of Fig. 1, taken 15 along lines 2-2; [0013] Fig. 3 is a top view of a cover for a main drain of a swimming pool, according to a second preferred embodiment of the present invention [0014] Fig. 4 is a side, cross sectional view of the main drain of Fig. 3, taken along lines 4-4; and 20 [0015] Fig. 5 is a detailed, cross sectional view of the main drain of Fig. 3, taken along lines 5-5. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS [0016] The present invention is a device for covering a swimming pool main drain; a main drain with a cover; and a cover and adaptor collar for covering a main drain. 25 While specifically intended for use in swimming pools, the present drains could be used for a drain of a body of water in a tank or reservoir where entrapment, entanglement, or draining of larger debris is a concern. Two embodiments of the present main drain cover are presented in the figures and will be described below. [0017] Referring now to the Figs. 1 and 2, there is illustrated a main drain 10 30 according to a first preferred embodiment of the present invention, and generally indicated by reference number 10. Main drain 10 is made of a material such as polyvinyl chloride that is substantially resistant to the environment of a swimming pool and to the typical chemicals used for maintaining the pool water free of bacteria and mold. 2 WO 2007/022322 PCT/US2006/032067 [0018] Main drain 10 comprises a cover 12 and a housing 14 that are joined together in a way that allows cover to be removed periodically for cleaning the interior of housing 14, such as by a friction fit between the walls 16 of housing 14 and depending flanges 18 of cover 12. Alternatively, cover 12 and housing 14 can be formed with threads. 5 [0019] Housing 14 has a major dimension and a minor dimension with the former being longer than the latter by typically a factor of two. Housing 14 also has a hydrostatic relief port 26 and a bottom suction port 28. Bottom suction port 28 allows water and heavier debris to be sucked from the interior of a swimming pool from a region adjacent main drain 10 through main drain 10 and thence to a canister filter (not shown). Hydrostatic relief port 10 26 allows water that may have accumulated under a pool to be removed. Typically bottom suction port 28 has a 2 2-inch diameter opening; hydrostatic relief port 26 has a 2-inch opening. Some drain housings have an additional port, as will be described below in connection with the embodiment shown in Figs. 3-5 but which may be incorporated into first embodiment if desired. Thus, the embodiment of cover 12 shown in Figs 1 and 2 can be used 15 in connection with a two-port housing as shown or with a three-port housing as described below in connection with the second embodiment of the present invention. In the Embodiment with a two-port housing, the drains should always be installed in pairs connected to the suction source by a tee or a X to further defeat entrapment. [0020] Running parallel to the major dimension is a narrow, linear opening 40. 20 Opening 40 has an internally curved flange 42 that runs the length of opening 40. While flange 42 adds stiffness, its primary purpose is to avoid sharp edges that may injure those whose fingers are inserted into opening 40 and to avoid entanglement. Opening 40 is preferably the width of a large human finger so that children's fingers are unlikely to become stuck fast. Furthermore, larger debris such as leaves can enter opening 40 when sucked into 25 main drain 10 more easily than they can enter many smaller openings. [0021] In an alternate preferred embodiment, shown in Figs. 3, 4 and 5, another main drain 50 is shown. Main drain 50 has a cover 52 and a housing 54. Cover 52 is not joined directly to housing 54 but is coupled to it using a collar 56 that allows cover 52 to be adaptably connected to various pre-existing housings 54 having different dimensions. 30 Rather than one long, linear opening as illustrated in Figs. 1 and 2, an H-shaped opening 60 permits the entrance of pool water and debris. Opening 60 is an internally curved surface 64 (best seen in Figs. 4 and 5) to provide both structural stiffening while reducing the likelihood of entanglement and entrapment. 3 WO 2007/022322 PCT/US2006/032067 [0022] As with main drain 10, main drain 50 has a hydrostatic relief port 66 and a bottom suction port 68. In addition, as best seen in Figs. 4 and 5, main drain 50 has an entrapment relief port 70 that typically is connected to a line that runs to a different part of the pool such as a wall. In the event opening 60 is blocked, suction from bottom suction port 5 68 would be relieved through entrapment relief port 70. As stated above, cover 52 may be used on a two-port housing of the type shown in Figs. 1 and 2, or the three-port housing 54 as shown in Figs. 3-5. While the three-port housing if Figs. 3-5 is preferred over the two port housing of Figs. 1 and 2, a cover either of the type shown in Figs. 1 and 2 or of that shown in Figs. 3-5 is even more important in a two-port housing where there is no additional relief port 10 if the opening of the cover is blocked. The extensive shapes of the openings of covers 12, 52, disclosed are more difficult to completely block than less extensive openings. [0023] Collar 56 has a lateral flange 74 is secured to pool surface 76 by friction fit as shown and perhaps also with screws 58 while supporting an upstanding flange 78 and a depending flange 80. Cover 52 has a depending flange 82 that is friction fitted to upstanding 15 flange 78 of collar 56, while depending flange 80 of collar 56 is friction fitted to the interior of housing 54. Depending flange 80 can be formed on collar 56 a little more radially inward or outward depending on the circumference of the particular size of housing 54 so that depending flange 80 and housing 54 properly engage. [0024] Both embodiments shown are characterized by a cover 12, 52 having a 20 single opening 40, 60 that is extended, covering either much of cover 12, 52, so that it is less likely that a swimmer will block, either accidentally or intentionally, every part of opening 40, 60, and thus cut off the flow into drain 10, 50. Opening 40, 60 of cover 12, 52 is curved, rounded, and smooth, free of corners or sharp angles that would tend to trap and entangle hair along the opening. 25 [0025] It will be readily apparent to those skilled in the art of swimming pool circulation system design that many changes and substitutions can be made to the foregoing preferred embodiments without departing from the spirit and scope of the present invention, defined by the appended claims. 4