Gas meter prover

A Gas Meter Prover is a piece of equipment used to verify the accuracy of a gas meter. Presently, gas meter provers (or more simply “provers”) refer to manually or electronic and/or computer controlled equipment used to calculate accuracy of a gas meter. Typically used in many gas meter repair facilities, municipal gas meter shops, and public works shops, provers typically work by comparing a known volume of air across a meter with the gas meter’s index or register. A series of mathematical computations are utilized to produce what is called a proof. A proof is a numeric value given to represent the accuracy of the meter as compared to the known volume of air used during the test. Many present day provers are capable of computing a meter’s proof result and displaying it on a computer screen.

Proving Equipment

Manual Bell Provers

Around since the early 1900’s, Bell provers are to date the most common reference standard used in gas meter proving, and has provided standards for the gas industry that is unfortunately susceptible to a myriad of immeasurable uncertainties.

A bell prover (commonly referred to in the industry as a “bell”) consists of a vertical inner tank surrounded by an outer shell. A space between the inner tank and outer shell is filled with a sealing liquid (usually oil). An inverted tank, called the bell, is placed over the inner tank. The liquid provides an air-tight seal. Bell provers are typically counter-weighted to provide positive pressure through a hose and valve connected to a meter. Sometimes rollers or guides are installed on the moving part of the bell which allows for smooth linear movement without the potential for immeasurable pressure differentials caused by the bell rocking back or forth.

For purposes of meter proving, bells provide a volume of air that may be predetermined by calculated temperature, pressure and the effective diameter of the bell. Bell scales are unique to each bell and are usually attached vertically with a needle-like pointer. When proving a meter using a manually controlled bell, an operator must first fill the bell with a controlled air supply or raise it manually by opening a valve and pulling a chained mechanism, seal the bell and meter and check the sealed system for leaks, determine the flow rate needed for the meter, install a special flow-rate cap on the meter outlet, note the starting points of both the bell scale and meter index, release the bell valve to pass air through the meter, observe the meter index and calculate the time it takes to pass the predetermined amount of air, then manually calculate the meter’s proof accounting for bell air and meter temperature and in some cases other environmental factors.

Uncertainties commonly experienced (and possibly unaccounted for within a test) when using bell provers can lead to incorrect proofs, by which an operator may adjust a gas meter incorrectly. Temperature inconsistencies between the bell air, meter and connecting hoses can account for most meter proof inaccuracies. Other factors may be mechanical such as stuck or sticky bell rollers or guides, loose piping connections or valves, a dent in the test area of the bell, incorrect counterweights, and human errors in the operation or calculations.

Automated Bell Provers

The invention of PLCs (Programmable Logic Controls) allowed gas meter repair facilities to automate most of the manual bell prover’s process and calculations. Instead of manually raising and lowering the bell prover, solenoid valves connected to a PLC control air flows through the meter. Temperature, pressure, and humidity sensors can be used to feed data into an automated bell PLC, and calculations for meter proofs can be handled by a computer or electronic device programmed for such a purpose. In the early 1990’s, the PLC was replaced by PACs (Programmable Automated Controls) and modern computer systems. Sensors to read the index of a meter were added to further automate the process, removing much of the human error associated with manual bell provers.

Sonic Nozzle Provers

The natural evolution of the automated bell and PAC controls led itself to the use of vacuum driven provers with arrays of sonic nozzles (also known as a venturi) to provide precise flow rates. Such a use eliminated the need for a bell, as the flow rate is provided through the nozzles. When sufficient vacuum is applied to a sonic nozzle it creates a constant flow rate. Bernoulli's principle is applied to calculate the chosen flow rates chosen by the user or automated by a computer. Computers and PAC systems automate the process, and most sonic nozzle provers are capable of displaying not only meter proofs to a user, but are also capable of transmitting proofs as well as other important data to database systems across a computer network.