Positive Displacement (PD) Flow Meters: The Complete Guide

Whether it’s the gasoline pump filling your vehicle, the water meter at your residence, or the precise blending of chemicals in a factory, you've likely relied on a technology that has been proven for more than 100 years.

Positive Displacement (PD) flow meters represent a foundational and highly reliable method for fluid measurement.

This guide provides a comprehensive overview of their operation, benefits, types, and practical application considerations.

What is a Positive Displacement Flow Meter?

A Positive Displacement flow meter is a volumetric instrument that directly measures the volume of fluid passing through it. 

The core principle involves mechanically separating the fluid into fixed, known volumes within a high-precision chamber and then counting them. 

Think of it like repeatedly filling a beaker to a set level and counting how many times the container is filled to determine the total volume.

Internal rotating components, moved by the fluid's energy, trap these pockets of liquid. Because this mechanism directly measures discrete volumes, the meter’s output is proportional to the volumetric flow rate. This direct measurement method makes PD meters exceptionally accurate and dependable.

Contact our experts today for a personalized recommendation on the best PD meter for your application.

The Top 5 Reasons to Use a PD Flow Meter

PD meters offer significant advantages in many industrial and commercial settings. Their enduring popularity is a testament to their performance and reliability.

1. Unmatched Accuracy and Repeatability

PD meters are renowned for their high accuracy, with standard models typically achieving +/-0.5% of reading. Some designs, such as the Helical Screw and Slide Vane types, can attain even greater precision, with linearity of +/-0.1% or better. Their outstanding repeatability and reliability have led to their use as international metrological standards for custody transfer applications.

2. The Perfect Choice for Viscous Fluids

These instruments excel at measuring high-viscosity fluids like oils, fuels, and greases. As viscosity increases, the internal leakage or "slippage" between moving parts decreases, which can actually improve the meter's accuracy. This capability makes them ideal for hydraulic fluids, chemicals, and even food products like chocolate and honey.

3. Installation Flexibility (No Straight Pipe Needed)

Unlike inferential meters such as orifice plates or turbines, PD meters are not sensitive to flow profile distortions. This attribute eliminates the requirement for long, straight sections of upstream and downstream pipe, allowing for more compact and flexible installations. They can be connected directly to elbows or valves without compromising measurement integrity.

4. High Turndown Ratio and Low-Flow Capability

A meter's turndown ratio indicates its effective measurement range. PD meters can maintain their accuracy over a wide span of flow rates, with turndowns of 100:1 being common, especially with viscous liquids. This allows for precise measurement of both very low flows and normal process flows with the same device.

5. Durability and Independence

The mechanical design of many PD meters means they can operate without an external power supply, making them perfect for remote locations. Built with precision-machined parts, they are known for their long service life. It's not unusual for these meters to perform reliably for 10 or even 20 years with minimal maintenance, particularly when measuring clean, lubricating fluids.

Different Types of Positive Displacement (PD) Flow Meters

Various designs of PD meters exist, each suited for specific applications and fluid types. Below is a summary of the most common configurations.

1. Oval Gear

Two interlocking, oval-shaped gears rotate within a chamber, trapping and transferring a fixed fluid volume with each revolution.

• Best For: General industrial use, high-viscosity fluids, chemical processing, and fuel measurement.

2. Helical Screw (or Gear)

Two interlocking helical rotors spin to move fluid axially through the meter. This design virtually eliminates pulsation.

• Best For: High-accuracy custody transfer, fuel and oil measurement, and applications demanding exceptional linearity and repeatability. They are often used as master meters by metrology institutes.

3. Nutating Disc

A disc mounted on a central sphere wobbles (nutates) in a circular path, displacing a known volume of fluid with each cycle.

• Best For: Residential water metering and measuring both low and high viscosity fluids. They are less susceptible to damage from particulates than other designs.

4. Reciprocating/Oscillating Piston

A piston oscillates within a cylinder of a known volume, filling and discharging fluid with each stroke.

• Best For: Low-flow rate applications, measuring viscous fluids on engine test stands, and some residential water services.

5. Rotary Vane

A rotor with multiple sliding vanes creates discrete volumetric compartments against the chamber wall, with each rotation representing a measured quantity.

• Best For: The petroleum industry, including aviation fuels, oils, and LPG.

6. Diaphragm

An oscillating diaphragm or bellows alternately fills and empties with fluid (typically gas), driving a crank mechanism that records the volume.

• Best For: Residential natural gas metering, where wide-ranging gas totalization is required.

7. Lobed Impeller / Roots Meter

Two figure-eight or peanut-shaped impellers rotate in opposite directions, trapping and transferring fluid around the outside of the chamber.

• Best For: Handling viscous fluids in chemical processing and food and beverage industries.

Need help choosing between an oval gear and a piston meter? Schedule a free consultation with our flow measurement specialists.

Positive Displacement (PD) Flow Meters Selection, Installation, and Maintenance Checklist

Choosing and maintaining the right PD meter will lead to years of reliable service. Consider these practical points.

PD Flow Meters Key Selection Criteria

• Fluid Characteristics: Go beyond just viscosity. Consider the fluid's lubricity, as some designs rely on the process fluid to lubricate moving parts. For non-lubricating liquids like water or solvents, choose meters with self-lubricating materials. Also, evaluate chemical compatibility and solids content.
• Filtration is Non-Negotiable: PD meters have tight internal clearances and require clean fluid to prevent damage and premature wear. A filtration level of 100 microns is a standard best practice.
• Pressure Drop: All PD meters introduce a pressure drop as they obstruct the flow path. Designs with wiping seals typically cause a higher pressure drop than those relying on a liquid seal. This factor must be included in system design calculations.
• Output and Integration: Meters can have a simple mechanical register or electronic sensors (like Hall effect or Reed switches) that produce a pulse output. For batching applications, the pulse resolution (pulses per volume unit) is a critical specification.

PD Flow Meters Installation Best Practices

While straight pipe runs are not necessary, proper installation is still important. Orient the meter according to the manufacturer's specifications to keep it full of liquid. For systems with reciprocating pumps, consider installing pulsation dampeners upstream to protect the meter's components from hydraulic shock, even though the meter will accurately measure pulsating flow.

Troubleshooting and Longevity

• Common Issues: If a meter stops registering, the most common cause is a blockage from a large particle. An inaccurate reading may indicate internal wear from abrasive particles or a lack of lubricity.
• Maximizing Lifespan: The key to a long service life is feeding the meter clean, compatible fluids. By following proper filtration and selection guidelines, routine maintenance can often be eliminated.

Conclusion

For over a century, Positive Displacement meters have provided accurate and direct volumetric flow measurement. Despite the advent of newer electronic technologies, they remain the preferred choice for many applications due to their high accuracy, reliability with viscous fluids, and simple, durable construction.

By understanding the different types and applying practical selection criteria, you can leverage this proven technology to achieve precise and dependable fluid control in your operations.

Ready to achieve precise flow measurement? Request a quote from us for industry-leading PD flow meters today.