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The claim phrase “preventing passage” of air was correctly construed as keeping out all air, even though experts from both sides agreed that persons skilled in the art would understand that the claimed device was not air-tight, because that was the way the invention was described and the claims were written, the U.S. Court of Appeals for the Federal Circuit held May 3 ( Aqua-Aerobic Systems Inc. v. Aerators Inc., Fed. Cir., No. 98-1465, 5/3/00). Downflow mixers are used in water treatment ponds and tanks. They consist of pumps mounted on flotation devices with propellers that are suspended below the surface and enclosed in stabilizer tubes. The liquid to be mixed is pumped above the propeller and flows down onto it, where it is recirculated into the pond or tank. Prior art downflow mixers suffered from vibration and propeller damage due to atmospheric air entering near the top of the propeller shaft and passing down into the stabilizer tube. Aqua-Aerobic Systems Inc. owns U.S. Patent No. 4,422,771, which claims a downflow mixer that prevents vibration and damage by sealing out air. Aqua filed suit against Aerators Inc. in the U.S. District Court for the Northern District of Illinois, claiming Aerators’ “Aqua-Lator” downflow mixer infringed the ’771 patent. Aerators argued that two claim elements were not present in the Aqua-Lator. Specifically, paragraph 12 of Claim 1 requires a “wall means … for preventing passage of atmospheric air” and paragraph 13 of Claim 1 requires a “seal means … for preventing the flow of atmospheric air.” The district court construed the two phrases differently. Reasoning that “flow” means a “movement that is smooth, continuous and interrupted,” it held that the shaft seal had to eliminate a continuous flow of air but allow entry of a miniscule amount of air. “Passage,” it reasoned, means “to pass from one place to another.” Consequently, the wall means had to prevent the entry of all air. Since the Aqua-Lator did not prevent the entry of all air, the district court granted Aerators’ motion for summary judgment of noninfringement. Aqua appealed, and the Federal Circuit affirmed. EXPERT TESTIMONY Aqua argued on appeal that both claim clauses must be interpreted as permitting the passage of more than a miniscule amount of air because experts from both sides agreed that persons of ordinary skill in the art would understand that the claimed structure was not air-tight. Judge Pauline Newman, writing for the Federal Circuit, disagreed with Aqua. The specification explicitly excludes all air, the court said. The claims must be construed as they are written and as the invention is described, not as extrinsic evidence dictates. “Expert testimony is often useful to clarify the patented technology and to explain its meaning through the eyes of experience, but it may not correct errors or erase limitations or otherwise diverge from the description of the invention.”

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