This paper will verify the history, requirements and operation of all Provers accepted for liquid pipeline meter uncertainty verification in the Liquid Oil/Gas Industry. It will continue with an explanation and the industries wide acceptance of the Uni-directional Captive Displacement Prover (UDCDP). This document will supply the reader with information regarding meter types and the flow volumes that can be used with the UDCDP and will look at the opportunities for the use of a UDCDP as a mass prover. It will also provide the information for field verification of provers known as a water draw.