Abstract: An apparatus and method for calibrating a flow meter are provided which utilize a tank for maintaining the apparatus liquid full. The tank has a freely movable barrier therein which divides the interior of the tank into a first section and a second section wherein the first section is in communication with a system which comprises at least one prover and a meter in series. A biasing means, such as a compressed gas in the second section of the tank, is also provided for urging the barrier toward the first section of the tank. Employing the tank according to the invention maintains a liquid filled system.