When it comes to measuring the flow of liquids—especially those with high viscosity or at low flow rates—positive displacement (PD) flow meters offer unmatched precision and reliability. From chemical batching to oil metering, these robust instruments provide volumetric accuracy where other flow technologies may fail.
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What Is a Positive Displacement Flow Meter?
A positive displacement flow meter is a volumetric flow measurement device that directly counts the volume of liquid passing through it. Unlike other flow meters that infer flow based on velocity, pressure drop, or thermal properties, PD flow meters physically trap and release precise volumes of fluid, ensuring high-accuracy measurement.
They are particularly well-suited for:
- High-viscosity liquids (like oil, honey, or glue)
- Low flow rate conditions
- Applications requiring totalization (batching, dispensing, blending)
Positive displacement meters are often regarded as mechanical counters—each rotation, oscillation, or movement corresponds to a fixed volume of fluid, enabling accurate totalization without the need for complex calculations.
How Does a Positive Displacement Flow Meter Work?
At the core of every PD meter is a chamber and mechanical components (e.g., gears, pistons, rotors) that rotate or move to trap a fixed volume of fluid. Each cycle of movement corresponds to a known, consistent volume. By counting these cycles, the meter calculates total volume and flow rate.
Here’s a simplified process of how it works:
- Fluid Enters the Meter The incoming fluid is directed into the measuring chamber.
- Mechanical Displacement Internal rotating or oscillating components (e.g., gears, disks, pistons) trap a fixed volume of fluid.
- Fluid Discharge As the chamber moves, the fluid is pushed out and exits the meter.
- Volume Counting Each movement is detected by sensors or magnets, and the cumulative count represents the total volume passed.
Because the meter measures flow volumetrically, its accuracy is largely unaffected by changes in fluid properties such as viscosity, density, or turbulence—making it ideal for precise, repeatable measurement in challenging conditions.
Types of Positive Displacement Flow Meters
There are several different types of PD flow meters, each suited for different flow rates, fluid types, and applications. Here are the most commonly used varieties:
1. Gear Flow Meters
How They Work:
Gear flow meters use two intermeshing gears—typically oval or helical—to form pockets of fluid that are moved from the inlet to the outlet.
Best for:
- High-viscosity liquids (oil, glue, syrup)
- Low flow conditions
- Precise dispensing
- Designed for low flow liquids from 0.6 L/H
- 150:1 wide turndown ratio
- Capable of measuring highly corrosive medium including sulfuric acid, hydrofluoric acid, and other aggressive chemicals
- Designed for high-viscosity fluid measurement, handling challenging media such as syrup, molten asphalt, and honey with reliable accuracy
- High-precision measurement with up to ±0.5% accuracy
2. Rotary Vane Flow Meters
How They Work:
A rotor with multiple vanes turns inside a cylindrical housing. Each rotation traps a known volume of liquid and transfers it through the meter.
Best for:
- Water-based fluids
- Heating oil and fuel
- Industrial process flows
3. Piston Flow Meters
How They Work:
Fluid is drawn into and pushed out of chambers by oscillating or rotating pistons. Each stroke displaces a fixed volume.
Best for:
- Clean, low-viscosity fluids
- Chemical injection
- Pharmaceutical and medical dosing
4. Nutating Disk Flow Meters
How They Work:
A disk mounted on a central ball “wobbles” (nutates) inside a chamber, displacing fluid in fixed volumes per cycle.
Best for:
- Residential water meters
- Beverage dispensing
- Custody transfer systems
5. Bi-Rotor Flow Meters
How They Work:
Two rotors rotate inside a cavity, creating fluid pockets that are displaced through the meter. These meters have minimal internal friction, making them ideal for sanitary applications.
Best for:
- Food-grade fluids
- Pharmaceuticals
- Viscous fluids like oil, resin, or syrup
- Custody transfer systems
- High accuracy up to ±0.1 %, high process pressure up to 110 bar (1595 psi)
- Extremely long service life, economical low maintenance
- No oscillating, reciprocating or sliding parts or cranks to wear or disturb the balanced rotary
- Stable in running with low voice
- Good anti-interference performance and long service life
- Strong applicability to viscosity changes
6. Oval Gear Flow Meters
How They Work:
Oval gear meters use two interlocking oval-shaped gears that rotate with the flow of fluid. Each rotation displaces a known volume, and the number of rotations is proportional to the total volume passed through.
Best for:
- Viscous fluids like oil, resin, or syrup
- Smooth spiral rotor operation with minimal vibration for consistent performance
- Wide measurement range with good repeatability
- High-precision measurement with up to ±0.2% accuracy
- Not sensitive to the viscosity change
- Optimized for higher viscosity liquid measurement
- Simple installation without requiring straight pipe runs
Advantages of Positive Displacement Flow Meters
Positive displacement flow meters offer many performance and operational advantages, making them the go-to solution in applications where accuracy and repeatability are critical.
- Direct Measurement of Volume: They don’t estimate or infer—PD meters measure the actual fluid volume passing through, which eliminates many common sources of error.
- High Accuracy at Low Flow Rates: They are among the most accurate flow meters at very low flow levels, which makes them ideal for dosing and micro-batching applications.
- Insensitive to Fluid Properties:PD meters are not affected by fluid density, viscosity, or temperature to the extent that other technologies are.
- No Need for Straight Pipe Runs: Unlike turbine or ultrasonic meters, PD meters don’t require long upstream or downstream pipe lengths, which is a major benefit in compact installations.
- Excellent Repeatability: Their mechanical design ensures highly consistent performance over time.
- Totalizing and Batch-Friendly: Perfect for batching, filling, or blending systems where cumulative volume matters.
Disadvantages of Positive Displacement Flow Meters
While PD flow meters offer numerous benefits, there are some limitations to consider when choosing this technology.
- Not Ideal for Dirty Fluids: Solids, debris, or particulates can damage internal moving parts or cause clogging.
- Mechanical Wear and Tear: Over time, mechanical components can wear down, especially if proper filtration or maintenance is not observed.
- Size and Cost at High Flow Rates: For high-flow applications, PD meters become bulky and expensive compared to other technologies.
Recommended Fluids and Mediums for PD Flow Meters
Positive displacement flow meters work best with clean, viscous, and incompressible liquids. Here are common media types that are well-suited for PD technology:
Oils
- Hydraulic oil
- Lubricating oil
- Engine oil
- Crude oil
Chemicals
- Resins and adhesives
- Solvents
- Additives and catalysts
Food and Beverage
- Honey
- Syrup
- Flavor concentrates
- Milk and dairy products
Pharmaceutical and Cosmetic
- Glycerin
- Alcohol-based fluids
- Emulsions
- Liquid medication
Applications of Positive Displacement Flow Meters
Thanks to their accuracy and reliability, PD flow meters are used in a variety of industries and processes. Here are some of the most common use cases:
1. Chemical Injection and Dosing
In water treatment, oil refining, and specialty chemical manufacturing, PD meters ensure precise injection of expensive or reactive chemicals.
2. Oil and Fuel Measurement
They are widely used in lube oil blending plants, diesel fuel transfer systems, and engine testing labs to measure and totalize fuel flow.
3. Food and Beverage Processing
In high-viscosity food products such as honey or syrups, PD meters ensure consistent dosing during filling and packaging.
4. Cosmetics and Personal Care
PD meters accurately dispense lotions, creams, and gels in controlled environments.
5. Pharmaceutical Manufacturing
For high-value drugs and bio-fluids, PD meters are used in ultra-precise filling, mixing, and formulation applications.
6. Paints and Coatings
In pigment and resin applications, PD meters help maintain proper ratios and minimize waste.
Explore Metlan’s PD Flow Meter Solutions
At Metlan Instruments, we offer industry-grade positive displacement flow meters tailored for low flow, high-viscosity, and precision-critical applications.
Need help finding the right meter for your system? Contact us today—we’ll help you select the perfect solution for your application.
