3.4.3 Displacer

In general, provers use one or more spheres, or a piston as the displacer. Spheres should be made of a resilient material that is compatible with the test liquids, and they shall meet the specified minimum and maximum sizes. Piston displacers shall be made of rigid material with resilient seals that are in contact with the pipe prover wall. The diameter of the sphere(s) shall be such that a seal is provided without excessive friction.