Grace Instrument Industries, LLC created a device to measure the viscosity of oil used in oil drilling wells. To create the pressure used to make an accurate viscosity measurement, it used a pressurizing fluid to compress the drilling oil to be measured. To prevent the pressurizing fluid from mixing with the drilling oil in a measurement chamber, Grace included an “enlarged chamber” above the measurement chamber where the two fluids could mix.

The district court found that the phrase “enlarged chamber” rendered the claims indefinite because the patent provided no baseline for what the size of the chamber was larger than, rejecting Grace’s argument that “enlarged chamber” could be defined by its purpose.

The Federal Circuit reversed, holding that the recitation of “enlarged chamber” did not render the claims indefinite, and remanded the case back to the district court while leaving open whether other aspects of the claim could render claims indefinite.

The Federal Circuit concluded that, in the context of Grace’s patent, “enlarged chamber” does not require that chamber to be larger than some baseline object; rather it must be large enough to accomplish a particular function. The chamber is “enlarged” if any mixing of the pressurization fluid and compressed drilling oil occurs within the enlarged chamber and not in a lower measurement chamber. That is, the enlarged chamber has to be able to contain enough drilling oil at the pre-pressurization stage such that, during pressurization, the drilling oil level does not fall below the bottom of the enlarged chamber and into the viscometer’s lower measurement chamber.

This case illustrates the risks with patent ambiguity and difficultly of proving claim indefiniteness. Inventors should view this decision with caution, as without this forgiving view of the patent specification and claims, Grace’s patent could have remained invalid.