Various types of drip irrigation apparatus are known in the patent literature and in the marketplace. A particularly successful type of drip irrigation apparatus is described and claimed in Israel Pat. No. 45211 which discloses a drip irrigator comprising a continuous uninterrupted drip irrigation hose which is extruded around drip irrigation elements.
U.S. Pat. No. 4,519,546, which is assigned to the present assignee, describes a differential pressure responsive, volume-controlled drip irrigation apparatus which also employs a continuous uninterrupted drip irrigation hose.
U.S. Pat. No. 4,210,287 also described differential pressure responsive, volume-controlled drip irrigation apparatus employing a continuous outer hose. The apparatus described in the Mehoudar patent has the disadvantage that it is susceptible to blockage when water of at least a predetermined pressure is supplied thereto. In the apparatus described in the Mehoudar patent, the predetermined pressure at which blockage occurs decreases as the size of the outlet aperture is increased. However, a decrease in the size of the outlet aperture increases its susceptibility to clogging.
U.S. Pat. No. 4,095,745 described drip irrigation apparatus including flexible tubular outlet means in the form of flaps which protrude into the water flow stream and cause a pressure drop thereacross. The structure described therein is intended to provide flushing upon initial flow through the drip line and volume control thereafter, as the result of bending of the tubular outlet means.
U.S. Pat. No. 4,502,631 to Rainbird proposes drip irrigation apparatus having a narrowed outlet region underlying a flexible membrane, which is intended to prevent the membrane from completely blocking the outlet at high pressure. Other than this feature, the apparatus described in U.S. Pat. No. 4,502,631 is similar to that shown in U.S. Pat. No. 3,954,223 to the same assignee as U.S. Pat. No. 4,502,631.