Rain Bird XFS-CV Subsurface Check Valve Dripline 0.6 GPH 18 in. Spacing 500 ft. (Sold per Roll). Rain Bird XFS-CV Dripline with an improved 4.3 PSI check valve delivers 10 feet of hold-back - the highest in the industry. With pure copper chips in every emitter to protect against emitter root intrusion, XFS-CV dripline is an all-in-one dripline suitable for any application - on-surface, sub-surface, sloped or level-grade. When used in applications where elevation changes exist, the patent-pending check valve keeps the dripline charged with water, delivering better irrigation uniformity while preventing over-watering and puddling at the low-point in the zone.