Traditionally, meters used in petroleum marketing operations have been analog devices. Liquid flows into one port of the mechanical meter and out another. The movement of the liquid, through the meter, causes pistons to rotate a connected shaft. The rotation of the shaft, in turn, creates movement of wheels on which numbers are embossed. By observing the numbers displayed on these wheels, the system operator can determine how many gallons of product have flowed through the system.
In the petroleum marketing equipment industry, the measuring device through which liquid flows in a fuel dispenser is referred to as the meter. The connected but separate device, which houses the gallonage wheels, is referred to as the register. If this separate device also includes wheels that display price figures, it is referred to as a computer.
Today, electronic meters predominate in petroleum marketing. These meters are digital. Rotation of pistons in the meter produces an electronic pulsing. The pulses are calibrated to reflect gallonage figures, which are displayed in discrete digital numbers.