Proving with open prover tanks consists of a comparison of the meters indicated volume of liquid and the known volume in the tank prover. The liquid should be passed through the meter under actual or simulated operating conditions of temperature, pressure, rate of flow, density and viscosity, then into the prover where its volume is determined from the gauge scales. The meter factor is the ratio between the known volume of the prover (corrected to standard conditions) as the numerator and the indicated volume of the meter (also corrected to standard conditions) as the denominator.

A volumetric proving tank is a reference standard used for proving meters under test. The tank is initially calibrated by an approved testing authority using a primary reference measuring instrument traceable to national standards, to an accuracy of ±0,05% or better. Calibration should be re-certified every five years, or sooner if it is suspected that the tank volume has been affected by damage. The tank is normally mounted on a wheeled trolley and has built-in spirit levels and adjustable/lockable legs for leveling.