A positive displacement (PD) flowmeter is a device that measures fluid flow by mechanically trapping fixed volumes of liquid and counting how many times those volumes are displaced from inlet to outlet. The measuring chamber is precisely machined so that each cycle of the measuring elements corresponds to an exact volume of fluid. This counting mechanism makes PD meters inherently accurate and well-suited for applications where absolute volumetric totalization is required. Positive displacement meters are one of the oldest and most trusted technologies in the oil measurement industry.
Positive displacement flowmeters are used extensively in oil applications including fuel dispensing, chemical dosing, tank truck loading, oil transfer, and custody transfer at low to medium flow rates. They are available in several design variations, including oval gear, bi-rotor, rotary piston, and nutating disc types, each optimized for specific fluid viscosity ranges, pressure ratings, and flow capacities. The oval gear design is the most popular for oil applications, offering good accuracy across a wide viscosity range and robust construction in materials suitable for hydrocarbons. Rotary piston meters are common in lubrication oil systems and fuel oil burner systems.
One of the primary advantages of PD meters in oil applications is their ability to accurately measure flow independently of fluid viscosity and density, provided the viscosity is within the meter's rated range. They do not require a minimum fluid velocity to operate, making them reliable at very low flow rates. However, PD meters are sensitive to overspeed—exceeding the maximum flow rate damages the measuring elements—and must be protected by a suitable valve to prevent surge flows. Regular maintenance including inspection of gears, seals, bearings, and the register mechanism ensures long-term accuracy. Despite the advent of more sophisticated meter technologies, positive displacement meters remain the workhorses of oil measurement in many industries.