GENERAL DESCRIPTION

A gas meter is tested for accuracy of registration by passing through it a volume of air from a prover and comparing the registration of the index of the meter with the accurate volume which is indicated by the prover.

As usually constructed, the bell prover (Figure 12.1.1) consists of a cylindrical metal tank open at the top and nearly filled with a light oil of low vapor pressure (see page 12.1.18 for specifications), in which a smaller cylindrical tank called the bell, open at the bottom and having a dome-shaped top, can be raised or lowered. The liquid acting as a seal prevents air from entering or leaving the bell except through a pipe called the dry well which extends from the outside of the prover tank down underneath it and up on the inside to a point above the level of the liquid. The prover tank is generally made of two concentric tanks with a distance of about two inches between them, the inner tank having an air tight top through which the air supply pipe passes. This method of construction decreases the quantity of oil required to seal the bell and permits more rapid temperature adjustment of the oil. The prover tank is mounted on three short legs which raise it off the floor, so that better air circulation can be obtained around the tank.