Appeal No. 2002-0998
Application 09/296,139
In such a system, a vacuum pump generates a continuous negative
pressure in the range of 10-25 inches of Hg in the collection
tank and collection drain pipe to thereby pull liquid through the
drain pipe and into the tank. In the past, such vacuum
collection tanks where formed of steel, however, appellants note
that those tanks were overly costly to fabricate, overly heavy
for certain applications, required expensive treatment to resist
corrosion, and quickly conducted the outside temperature to the
stored liquid. While acknowledging that certain plastic
materials are known for having better weight, insulative, and
corrosion-resistance characteristics than steel, appellants urge
that such materials have not heretofore been used in vacuum waste
tanks. In forming a vacuum waste tank of thermoplastic material
(e.g., polypropylene) having particular characteristics
appellants have overcome the problems of the prior art by
recognizing the need for a thermoplastic material having a
flexural modulus of at least 175,000 psi and a wall thickness of
at least 0.4 inches capable of resisting a continuous external
pressure loading resulting from a negative pressure in the
collection tank in the range of 10-25 inches of Hg. Independent
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