“Filtering” Appropriate Claim Construction Reasoning

| Emily K. McCorryDouglas B. Wentzel


Before Reyna, Bryson, and Cunningham. Appeal from the United States District Court for the Western District of Wisconsin.

Summary: The district court correctly construed the claim of one patent in view of the prosecution history but erred in construing a second patent by limiting the claimed “filter” to specification examples.

SSI filed suit against DZEM, alleging infringement of two patents. The patents describe sensors for determining the characteristics of fluid in a fuel tank or other container. DZEM asserted invalidity and tortious interference counterclaims. The district court granted DZEM’s motion for summary judgment of noninfringement and dismissed its invalidity counterclaims. The district court also granted summary judgment to SSI on DZEM’s tortious interference counterclaim.

For one patent, the ’153 patent, the district court relied on the prosecution history in construing “a dilution of the fluid is detected while the measured volume of the fluid decreases” to require that the measured fluid volume be considered in the claimed contaminant determination. For the other patent, the ’038 patent, the district court construed “filter” to mean “a porous structure defining openings, and configured to remove impurities larger than said openings from a liquid or gas passing through the structure.” The district court also found that SSI forfeited an argument that DZEM infringed the ’038 patent under the doctrine of equivalents. SSI appealed these issues. DZEM cross-appealed on other issues.

Regarding the ’153 patent, the Federal Circuit affirmed the district court’s construction. The Federal Circuit agreed that the prosecution history showed an intent to capture an error-detection capability that required considering the measured fluid volume. The Court further found that the claim language supported the district court’s construction because another construction would have rendered the word “measured” superfluous. Regarding the ’038 patent, the Federal Circuit rejected the construction of “filter” adopted by the district court because that construction improperly limited the claim to examples in the specification. The Federal Circuit adopted SSI’s construction and thus vacated and remanded all summary judgment rulings on the ’038 patent. The Federal Circuit also held that SSI had not forfeited its doctrine of equivalents argument for the ’038 patent because the parties addressed that issue in their summary judgment briefs, including with testimony from SSI’s expert.

Further, the Federal Circuit affirmed the district court’s dismissal of invalidity counterclaims for the ’153 patent because there was no apparent risk of future actions against DZEM. The Federal Circuit also affirmed the grant of summary judgment on the tortious interference counterclaim because it agreed that SSI’s infringement claims were not “objectively baseless.”

Editor: Paul Stewart