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Positive displacement (PD) flowmeters are precision instruments that measure flow by dividing the fluid into fixed volumes and counting these volumes as they pass through the meter. Because they operate on a volumetric principle, they provide excellent accuracy at low flow rates and with highly viscous fluids. They are widely used in oil, chemical, pharmaceutical, food, and automotive industries.

WORKING PRINCIPLE

PD flowmeters measure flow by separating the fluid into known volumes within a mechanical chamber and counting these cycles. Each cycle corresponds to a fixed volume, and the number of cycles is proportional to total flow.

Equation:
Qv = n · Vc
Qv: volumetric flow rate (m³/s), n: cycle frequency (Hz), Vc: chamber volume (m³).

Common types include:

Oval gear meters
Piston meters
Helical rotor meters

STRUCTURAL FEATURES

Body materials: stainless steel, cast iron, aluminum
Measuring elements: oval gears, pistons, helical rotors
Output: mechanical register, magnetic pickup, digital pulse
Excellent performance with viscous fluids
Capable of measuring very low flow rates

ADVANTAGES AND LIMITATIONS

Advantages:

Very high accuracy (±0.1% – ±0.5%)
Suitable for low flow measurement
Excellent performance with viscous fluids
No external power required (mechanical designs)

Limitations:

Moving parts require periodic maintenance
Wear from abrasive or particulate fluids
Limited use in very large pipe diameters

APPLICATION AREAS

Fuel and oil measurement
Solvent and polymer measurement in chemical industries
Syrup, honey, and oils in the food industry
Precise dosing in pharmaceuticals
Hydraulic systems for oil flow monitoring

STANDARDS AND CALIBRATION

OIML R 117: International standard for liquid measuring devices
ISO/IEC 17025: Calibration laboratory accreditation
ATEX-certified models for explosive environments
Regular calibration is essential to maintain accuracy

CONCLUSION

Positive displacement flowmeters provide unmatched accuracy and reliability, especially under low flow and high viscosity conditions. They are indispensable for applications requiring precise dosing and consumption measurement across various industries.

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