Leak detector with two stage piston chamber

Abstract

A pressure sensing piston connected to a valve of a leak detector senses leakage of gasoline from underground piping and restricts the flow of gasoline through the piping whenever any leakage exceeds a certain, predetermined rate of flow. The piston is shiftably received in a piston chamber and carries an inner cylindrical wall that is sealingly engageable with a mating cylindrical seal of structure connected to the piston chamber whenever the valve of the detector is in a leak sensing position to enhance movement of the valve in accordance with relatively small changes in the volume of fluid within the region. Once pressure in the piping has built to a value sufficient to raise the piston and disengage the walls from the seal, fluid from the piping is presented to the entire cross-sectional area of the piston chamber including the entire area of the piston face so that a greater volume in the piston chamber is presented to accumulate gasoline and to substantially preclude movement of the valve back to a reduced flow, metering position due to thermal contraction of gasoline within the piping. A venting device mounted in a passageway of the piston releases gasoline that would otherwise be trapped below the piston in areas outside of the wall as the piston is moved toward the bottom of the piston chamber.