Positive Displacement Flow
About Positive Displacement Flow Meters
Positive Displacement (PD) flow meters are volumetric flow measurement instruments ideal for low flow rates, highly viscous fluids, and measurement of flows involving starts and stops or pulsing. Positive displacement flow meters are similar in design to the gear pump. The principle of operation is reversed however; instead of the gears driving the fluid, the fluid drives the gears. A non-intrusive hall-effect sensor detects the movement of the gear and as each gear tooth passes the sensor, a square wave pulse is produced and a discrete volume of liquid is measured. The resulting pulse train is proportional to the actual flow rate and provides a highly accurate representation of the fluid flow. Positive displacement flow meters come in a variety of styles, and we carry three of them:
In Spur gear positive displacement flow meters, fluid travels around two round gears and is pushed through the meter’s internal chamber from inlet to outlet. The discrete volume of liquid trapped between each gear tooth is measured, as opposed to the Oval gear meter where a much larger volume between gears is swept through the chamber.
Possibly the most accurate PD meter in which two intersecting cylindrical bores are fitted with two interlocking helical screw-shaped gears. Because gear orientation is inline with flow, as the fluid passes through, the gears rotate with very little pressure drop. These flow meters are highly accurate over wide flow ranges and are compatible with very high viscosity fluids.
In this design a rotary piston oscillates in the circular internal chamber displacing a fixed amount of fluid with each rotation. This style accommodates ultra low flows and high flows, as well as high pressures.
Benefits of Positive Displacement Flow Meters
- High resolution flow measurement: high pulses per gallon for greater flow assurance
- High accuracy with high turndown, even at very low flow rates
- Handles dirty, viscous and corrosive fluids
- Unaffected by flow stops: Since the fluid drives the gears, accuracy in pulsing flows is still very accurate, which is ideal for batching and fast-acting processes.
- Flexible Installation: No need for straight run piping and can be installed in any orientation.