How is the size chosen?

By the flow range and the viscosity together, not by the pipe size. A meter that is too small for the flow turns too fast and wears; with a viscous liquid the pressure loss across the meter rises, and the same flow may need a larger meter. Tell us the liquid, its viscosity at the operating temperature and the minimum and maximum flow, and we will propose the size.

FLOW MEASUREMENT TECHNOLOGIES

Positive displacement flowmeters: volume measured directly, turn by turn

A positive displacement flowmeter divides the flow into small portions of known volume and counts them. In the oval gear design, two oval gears in a measuring chamber are turned by the liquid itself; with every turn they carry a fixed volume from the inlet to the outlet, so counting the turns gives the volume directly. It needs no straight run, it measures oils, fuels and other non-conductive liquids that an electromagnetic flowmeter cannot, and it becomes more accurate, not less, as the liquid gets thicker. It is for clean liquids: gases, and liquids with solid particles, call for other meters. Positive displacement flowmeters are not yet in our online catalogue; we supply oval gear flowmeters from Flomec.

The manufacturer we work with

How an oval gear flowmeter works

The measuring chamber is shaped like two overlapping circles, and in it sit two oval gears that mesh with each other, each turning on its own shaft. The gears fit the chamber walls with very small clearances, so the liquid cannot simply flow past them. The pressure difference between the inlet and the outlet pushes on the gears and turns them — the liquid itself drives the meter, with no external power to the measuring element.

As they turn, each gear in turn closes off a pocket of liquid between itself and the chamber wall, carries it from the inlet side to the outlet side and releases it there. Each gear does this twice per revolution, so one revolution of the pair moves four pockets — a fixed volume, set by the geometry of the chamber and the gears. Counting revolutions therefore measures volume directly: it does not depend on the velocity or on the flow profile, which is why the meter needs no straight run.

The turns are counted from outside the chamber. Magnets in one of the gears pass a sensor in the cover — usually a reed switch or a Hall-effect sensor — and every pass gives an electrical pulse that stands for a known volume of liquid; the pulses per litre are the meter’s K-factor. An electronic register adds them up to a total and works out the flow rate, or passes them on to a PLC as pulses or as a 4–20 mA signal. Simpler meters drive a mechanical counter through a magnetic coupling and need no power at all.

A small amount of liquid always slips through the clearances without being counted. With viscous liquids this slip is very small, so the accuracy improves as the viscosity rises; with thin liquids such as water or solvents it grows, mainly at low flows. And what the meter measures is volume at the operating temperature: fuels and oils expand as they warm up — by roughly 0.1% per °C — so where volume is billed, it is often converted to a reference temperature, usually 15 °C.

The flow of the liquid
The two oval gears, turned by the liquid itself
The pocket of liquid of known volume, closed off between a gear and the chamber wall — four per revolution
The sensor in the cover, which detects the magnet in the gear as it passes
One pulse each time the magnet passes — each pulse is a known volume
The volume is proportional to the number of pulses

Where a positive displacement flowmeter fits — and where it does not

A strong choice for

Oils, fuels, resins, adhesives and other viscous liquids: the thicker the liquid, the smaller the slip through the clearances, so the accuracy holds even at low flows
Non-conductive liquids — oils, fuels, solvents — which an electromagnetic flowmeter cannot measure
Installations with no room for a straight run: the meter does not depend on the flow profile, so it can go right after a bend, a valve or a pump
Fuel and oil transfer, dosing and batching: it counts the volume that has passed directly, and with a batch controller the transfer stops at the quantity you set
Fuel consumption of engines, boilers and burners, and the oil delivered to each machine
Simple installations: many meters have a mechanical register or a battery-powered display and need no external power — from portable transfer to fixed installations

Think twice when

The liquid carries solid particles or abrasives: they wear the gears and the chamber, or jam them — a strainer before the meter is essential, and slurries need another kind of meter, such as an electromagnetic one
The fluid is a gas or steam: oval gear meters are for liquids. For gases there are other positive displacement meters — rotary and diaphragm gas meters — which this page does not cover
The liquid is thin, like water, and the flow is low: the slip through the clearances grows and the accuracy at the low end falls — for water, an electromagnetic flowmeter is usually the better choice
Air or gas can get into the line, for example when a tank runs empty: the meter counts it as liquid and reads high, unless an air eliminator removes it first
The pipe is large or the flow very high: size, weight, cost and pressure loss rise quickly, and the moving parts wear faster — electromagnetic or ultrasonic flowmeters suit large lines better
The flow must never stop: if the gears jam, the meter blocks the line — so there a positive displacement flowmeter needs a bypass

How it compares with the other technologies

Typical accuracy of each flow measurement technology, ± % of the reading, on a logarithmic scale. The further left, the more accurate. The technology of this article is highlighted.

Coriolis±0.1–0.2%
Electromagnetic±0.2–0.5%
Ultrasonic±0.5–2%
Differential pressure±0.5–2%
Variable area±1.6–4% *
Vortex±0.75–2%
Thermal mass±1–3%
Positive displacement (the technology of this article)±0.2–0.5%
Turbine±0.5–1%

Typical ranges for liquids (thermal mass: gases), ± % of the reading; for variable area, the accuracy class (*). The accuracy of a specific instrument depends on the model, the size and the installation — we confirm it for your application.

All technologies

Installing a positive displacement flowmeter

A positive displacement flowmeter needs no straight run before or after it, because it does not depend on the flow profile. What it needs is clean liquid, free of air: a strainer before the meter, with the mesh the manufacturer specifies for the meter and the liquid, and — wherever air or vapour can get into the line, as when a tank runs empty or a pump draws air on its suction side — an air eliminator before the meter.

The meter must stay full of liquid, so put it where it cannot drain — not at a high point of the line — and fit control valves after it, not before, so that it works under back pressure. Install it with the gear shafts horizontal — the usual requirement; the manufacturer’s instructions decide — and with isolation valves before and after it and a bypass, so that the meter and the strainer can be taken out for cleaning or service without stopping the line. Then:

A strainer before the meterSolid particles wear or jam the gears, so a strainer — here a Y-strainer — goes before the meter, with the mesh the manufacturer gives. The particles collect in its leg, and it has to be cleaned at intervals.

Remove the air firstAir or gas in the line passes through the meter and is counted as liquid. An air eliminator before the meter lets the bubbles rise and vents them through a valve at the top, so that only liquid reaches the meter.

Gear shafts horizontalInstall the meter so that the gears turn in a vertical plane, with their shafts horizontal — the usual requirement, though the manufacturer’s instructions decide. The same meter turned so that the gears lie flat, with their shafts vertical, is installed the wrong way.

Isolation valves and a bypassWith an isolation valve before and after the meter and a bypass around it, the meter and its strainer can be taken out for cleaning or service while the liquid keeps flowing through the bypass. In normal operation the bypass valve stays closed — and it must not leak, or liquid passes uncounted.

Flush new pipework before the meter goes in — with a spool piece in its place or through the bypass — so that welding slag and other debris do not reach the gears.
At start-up, fill the line slowly and vent the air: a rush of air or liquid into an empty line can spin the gears far beyond their maximum speed and damage them.
Size the meter for the flow range and the viscosity together — the pressure loss rises with both — and keep the flow within the meter’s range: running above the maximum flow wears the gears and bearings quickly.
Clean the strainer at intervals — a rising pressure drop across it shows that it is clogging — and, where the volume is billed, check the meter against a known volume, for example a certified measuring can, at regular intervals.

Positive displacement flowmeters by industry

Oil & GasFuels and lubricants at terminals, depots and refuelling points, on tank trucks and in transfers between tanks, and the additives dosed into fuels — where the volume is often what is billed.

ChemicalSolvents, resins, adhesives and other non-conductive or viscous liquids, dosed into batches and mixers — with the materials of the gears and seals chosen for the liquid.

Food & BeverageVegetable oils, syrups, glucose and other viscous ingredients, dosed into batches — with a design and materials suitable for food.

Power & EnergyDiesel and heavy fuel oil to generators, boilers and burners, measured on the supply line — and on the return line, where there is one — for the actual fuel consumption.

MarineFuel consumption of main and auxiliary engines, and lubricating oil — measured on the supply and return lines of each engine.

Oval gear flowmeters from Flomec

Positive displacement flowmeters are not yet in our online catalogue. We supply them from Flomec, a Great Plains Industries brand, which makes oval-gear flow meters — as well as turbine and electromagnetic meters — for fuels, oils, chemicals and water, from portable transfer to fixed installations. Tell us the application: the liquid and its viscosity, the flow range, the pressure and temperature, and whether you need a local display, pulses or a 4–20 mA signal — and our engineers will suggest the right meter, with price and delivery time.

FlomecFlow meters for fuels, chemicals and water Learn more

Positive displacement flowmeters: frequently asked questions

Our comparison table gives ±0.2 to ±0.5% of reading as a typical range — deliberately conservative: some meters do better. The accuracy depends above all on the viscosity and on where in the range you measure: with viscous liquids it holds down to low flows, while with thin liquids it falls at the low end, where the slip through the clearances matters most. Wear and dirt in the gears lower it over time. We confirm the expected accuracy for your liquid and flow range before you order.

Volume — the actual volume of the liquid at the temperature it flows at. Fuels and oils expand as they warm up, by roughly 0.1% per °C, so 1,000 litres of fuel at 35 °C correspond to about 980 litres at 15 °C. Where volume is billed, it is often converted to 15 °C using the temperature of the liquid; some registers do this themselves. If you need mass, multiply by the density — or consider a Coriolis flowmeter, which measures mass directly.

The clearances between the gears and the chamber are very small, so solid particles wear the gears or jam them — the strainer keeps them out. And the meter counts whatever passes through its chamber: a pocket of air is counted as if it were liquid, so the reading comes out too high. The air eliminator removes air and gas before the meter — necessary wherever the line can draw air, as when tanks are emptied.

Positive displacement flowmeters are not yet in our online catalogue. We supply oval gear flowmeters from Flomec, which makes oval-gear, turbine and electromagnetic meters for fuels, oils, chemicals and water, from portable transfer to fixed installations. Send us the liquid and its viscosity, the flow range, the pressure and temperature, and the output you need, and we will propose the right meter, with price and delivery time.

Ask about positive displacement flow measurement

Tell us the liquid, its viscosity at the operating temperature, the flow range, the pressure and temperature, and whether you need a local display, pulses or a 4–20 mA signal — and our engineers will suggest the right meter, with price and delivery time.