Device for measuring a pre-determined volume of liquid flowing therethrough

A gauge for measuring a pre-determined volume of liquid flowing therethrough comprising a conduit for said liquid and a measurement slug positioned within said conduit, the slug having substantially neutral buoyancy in the liquid to be measured and being constructed and arranged such that it will move along the conduit at substantially the same rate as liquid flowing therein and being capable of being moved through the conduit with substantially no net flow of liquid, the conduit comprising a constraint system defining a length of the conduit within which the slug is free to move, the constraint system comprising an upstream stop capable of preventing further movement of the slug, the measurement slug and the upstream stop being configured to block the conduit so as to substantially prevent the flow of liquid therethrough when the slug contacts the upstream stop, the gauge comprising detection means capable of detecting when the slug has moved a predetermined distance within the conduit.