Most flow meters infer flow indirectly — they measure pressure drop, rotor speed, or float position and back-calculate volume. Positive displacement (PD) meters do something fundamentally different: they trap a known, fixed volume of fluid in a chamber, move it from inlet to outlet, and count the cycles. Total flow is literally chamber volume × number of cycles. This makes them the most accurate volumetric meters available, with no need for a flow profile, straight pipe runs, or Reynolds number assumptions.

Common PD meter types:

• Oval gear: Two intermeshing oval rotors trap crescent-shaped pockets of fluid between the gear teeth and housing. Excellent for viscous fluids like oil, ink, and resin.
• Nutating disc: A wobbling disc divides the chamber into known volumes that fill and discharge with each nutation. The classic residential water meter design.
• Rotary piston: A cylindrical piston rotates eccentrically inside a chamber, sweeping a fixed volume per revolution. Used in water utility metering for billing.
• Helical gear / lobed rotor: Two helical or trilobed rotors mesh without contacting, trapping fluid pockets. Handles high flow rates and is common in fuel dispensing.
• Diaphragm: Flexing diaphragms displace measured gas volumes — the basis of every residential gas meter.

Why engineers reach for PD meters: Accuracy of ±0.1% to ±0.5% of reading, repeatability under 0.05%, and performance independent of viscosity changes. Higher viscosity actually improves sealing between rotors and housing, reducing slip. That's why they dominate custody transfer of oil, lubricants, and chemicals — when money changes hands per gallon, you want a meter that physically counts gallons.

Rule of thumb: For each rotor revolution, an oval gear meter passes 2 × (crescent volume between gear and housing). If the displacement is 50 mL per revolution and you count 1,200 revolutions per minute, flow rate is 50 × 1,200 = 60,000 mL/min = 60 L/min. Pulse output from a reed switch or Hall sensor gives you direct totalization in your PLC or flow computer.

Tradeoffs: PD meters have tight clearances — typically 5 to 50 microns between rotor and housing — so they're vulnerable to particulate damage. Always specify upstream strainers (often 100 mesh or finer). They also create pressure drop (5–15 psi typical) and pulsation, which can be a problem for downstream sensitive equipment. Don't use them on dirty water, slurries, or fluids with abrasive particles. And because they have moving parts in the flow path, mean time between failures is shorter than for magnetic or ultrasonic meters.