Positive Displacement Flow Meters (Volumetric Flow Meters)

A positive displacement (PD) flow meter is a volumetric flow instrument that measures flow by passing a precise, known volume of fluid with each revolution of its internal components. It is the only type of flow meter that measures actual volume directly. Every other technology infers the flow rate from some other measurement.

PD flow meters are precision instruments whose internal moving components are hydraulically locked in tandem with the volume of fluid moving through the flow meter. The meters can measure intermittent flows, very low flow rates, and liquids of almost any viscosity. When fluid flows, PD meters instantly move. And when the fluid stops, the meter stops.

How does a positive displacement flow meter work?

A positive displacement flow meter works by dividing the fluid into fixed volumes and counting them. Fluid pressure moves the meter’s internal components, whether pistons, gears, vanes or a disc. Each cycle traps and releases the same known volume, and a sensor counts the cycles. Flow rate is the number of cycles per unit of time multiplied by the volume of each cycle.

Moving in lock step with the fluid also means that measurements are not affected by the liquid’s viscosity, density or turbulence in the pipe. Each measured volume of fluid is uniquely separated from the next by a frictionless liquid seal. The sealing effectiveness is a function of manufacturing precision and the ease of rotation of the meter’s components. The same close clearances that make the precision parts work also mean that relatively clean fluids must be measured; a 10 micron filter upstream is the usual recommendation. Materials of construction vary widely and allow the family of PD meters to measure a wide variety of process liquids. Their high precision has allowed PD flow meters to be almost universally accepted for billing and transfer standard applications, such as dispensing gasoline, fuel oil, and natural gas or water measurement.

Output signals and K-factors

There are a variety of methods to provide output signals and readouts. PD meters have the ability to offer simple once per revolution counting to high resolution encoder equivalent signals which can take more than 4,000 measurements per revolution. Because these measurements are not sensitive to changes in pressure, temperature, or viscosity, the meter’s K-factor typically does not need to be compensated for during use.

The K-factor is the number of pulses the meter produces per unit of volume. A Max P002 piston flow meter produces 1,000 pulses per cc, so a single pulse represents one thousandth of a cubic centimeter. A P215 piston flow meter produces 20,000 pulses per liter. Dividing the pulse count by the K-factor gives the total volume passed; the pulse frequency gives the flow rate. Quadrature outputs add direction, so one meter reads forward and reverse flow and can track a null crossing. Analog 4-20 mA and other transmitter options are available where a pulse input is not.

How accurate are positive displacement flow meters?

Accuracy and operating range, or turndown ratio, vary widely by type and manufacturer, but many positive displacement meters fall in the range of 0.1% to 1.0% of reading, with turndown ranging from 10:1 to 200:1 or more. Due to their high accuracy, PD meters are the most widely used meters for custody transfer applications.

PD meter accuracy is stated as a percentage of reading, not of full scale, because the measurement is a physical volume that does not change with flow rate. A Max piston meter rated 0.2% of reading over a 200:1 range holds that 0.2% at 2,000 cc/min and at 10 cc/min. A meter rated 1% of full scale would be allowed twenty times the error at the low end of the same range. Read more on accuracy of reading vs. accuracy of full scale.

Advantages of positive displacement flow meters

• Direct volume measurement, with no inference from velocity or mass.
• Accuracy of reading over wide turndown ranges, up to 200:1 or more.
• Very low flow capability, down to 0.005 cc/min in reciprocating piston designs.
• Instant response to intermittent, pulsing and bi-directional flow.
• Unaffected by viscosity over most application ranges, and accuracy improves as viscosity rises.
• No straight pipe runs or flow conditioners required, so the meter can be installed close to valves, pumps and elbows.
• Mechanical or electronic output, from a simple register to high resolution pulse and quadrature signals.

Limitations of positive displacement flow meters

• The fluid must be relatively clean. Particles can score the measuring chamber or jam the rotors, so filtration is required.
• Moving parts wear over time, though a well-specified meter with clean fluid runs for decades.
• Pressure drop is higher than in open-tube meters, and rises with viscosity and flow rate.
• Not economical for large pipe diameters. Above a few inches, velocity or mass technologies are usually the better fit.
• Gas measurement is limited to specific designs; most precision PD meters are liquid meters.

Positive displacement vs. other flow meter technologies

	Positive displacement	Turbine	Coriolis	Electromagnetic	Ultrasonic
What it measures	Volume, directly	Velocity	Mass	Velocity	Velocity
Typical accuracy	0.1% to 1.0% of reading	0.5% to 1.5% of reading	0.1% to 0.5% of reading	0.3% to 1% of reading	About 1% of rate
Turndown	10:1 to 200:1 or more	Up to 100:1 on one fluid	Up to 100:1	Up to 100:1	Wide
Lowest practical flow	0.005 cc/min	Limited by rotor friction, typically tenths of a liter per minute	Limited by zero stability, typically grams per minute	Needs velocity above roughly 0.3 m/s	Needs velocity above roughly 0.3 m/s
Viscosity	Any; accuracy improves with viscosity	Up to about 50 cSt	Any	Any conductive liquid	Clean liquids
Straight pipe required	None	About 10 diameters upstream, 5 downstream	None	About 5 upstream, 3 downstream	About 10 upstream
Intermittent or pulsing flow	Responds instantly	Poor	Slow to respond	Fair	Fair
Fluid cleanliness	Filtered, 10 micron recommended	Clean	Any	Slurries acceptable	Clean, or with particles for Doppler types
Relative cost	Low to high, by precision	Low to moderate	Highest	Moderate	Moderate

Types of positive displacement flow meters

There are many types of positive displacement flow meters, including reciprocating piston, oscillating or rotary piston, bi-rotor types (spur gear, oval gear, helical gear, rotary vane), and nutating disc (wobble plate). PD meters are an established technology that covers the spectrum of measurement devices from some of the most widespread, easiest to use, and most affordable, like rotary piston or nutating disc meters, to some of the highest precision, highest accuracy, highest resolution meters available today, like reciprocating piston meters. New materials, like composites, new manufacturing techniques, like additive manufacturing, and new electronic sensing technology, like high resolution hall effect sensing, continue to keep the basic idea of measuring a known volume of fluid at the forefront of the fluid measurement industry.

Piston flow meters

Piston flow meters are of single and multiple-piston types. They function by rotating, oscillating, or reciprocating. For any motion, the pistons displace fluid in the same way that a syringe operates. Each piston displacement captures the same amount of fluid.

Oscillating piston meters. Typically seen in residential water service and to measure relatively viscous fuels, these meters are a single piston design that oscillates between the inlet and outlet ports as fluid flows. Measurement output is usually achieved by having a magnet installed on the piston and a transmitter picks up the change in the magnetic field as the piston moves.

Reciprocating piston meters. Typically linked via a crankshaft and opening and closing ports in the cylinder, reciprocating piston meters act similarly to piston engines that power most automobiles and trucks. The sensor output is usually driven from the rotating crankshaft and can be mechanical, electrical, optical, or magnetic. These meters are mainly used to measure very low flow rates, down to 0.005 cc/min. Max piston flow meters are radial multi-piston designs of this type.

Gear flow meters

Gear flow meters use two round gears that are mounted in overlapping compartments. The measured fluid is trapped in the voids of the gear teeth and transported from the inlet port to the outlet port as the fluid flow causes the gears to rotate. Spur gear meters handle a wide range of viscosities, from thin fuels to greases, and are the workhorse of hydraulic, lubrication and adhesive dispensing measurement. Max gear flow meters are spur gear designs.

Oval gear meters have two rotating, oval-shaped gears with synchronized, close fitting teeth. A fixed quantity of liquid passes through the meter for each revolution.

Helical gear flow meters use two screw-shaped rotors to slice the fluid stream into fixed displacement volumes. The rotors’ orientation is in-line with the fluid flow path. These meters rotate with very low pressure drop, and can turn at high rpm’s, making them accurate over wide flow ranges and compatible with very high viscosity fluid applications. Max helical flow meters measure fluids up to 1,000,000 centistokes.

Other PD flow meters

Rotary vane meters consist of equally divided, rotating impellers, in two or more compartments, inside the meter’s housing. The impellers are in continuous contact with the casing. A fixed volume of liquid is swept to the meter’s outlet from each compartment as the impeller rotates.

Nutating disc meters have a moveable disc mounted on a concentric sphere located in spherical side-walled chambers. The pressure of the liquid passing through the measuring chamber causes the disc to rock (wobble) in a circulating path without rotating on its axis. The disc and sphere are the only moving part in the measuring chamber.

What is a positive displacement water meter?

A positive displacement water meter is the meter on most homes and small commercial buildings. Oscillating piston and nutating disc designs dominate because they are inexpensive, accurate at the low flows of a dripping tap or a single fixture, and need no power. The utility reads the register, which totals the volume that has passed.

Industrial water and aqueous chemical measurement uses the same principle with stainless steel construction. Max water flow meters are stainless positive displacement meters for aqueous solutions such as reverse osmosis antiscalants, defoamers, anti-corrosives and sterilization chemicals, measuring 1 to 2,000 cc/min at 0.4% accuracy.

Where positive displacement flow meters are used

PD meters are chosen wherever a small or intermittent volume has to be known exactly.

• Custody transfer and dispensing of gasoline, diesel, fuel oil, water and gas.
• Fuel consumption testing, from a single injector to a running engine, in automotive and aerospace development.
• Injector, servo valve and hydraulic component testing, where flow starts, stops and reverses in milliseconds.
• Chemical and odorant injection in oil and gas production.
• Adhesive, sealant, paint and coating dispensing on production lines.
• Lubrication and hydraulic monitoring on machine tools and industrial equipment.
• Laboratory and calibration work needing NIST traceable volume.

What do positive displacement flow meters cost?

With such a large variety and diverse operating range, prices of positive displacement meters can vary widely. A simple water meter for your home may cost less than $100, while prices for the most sophisticated and largest meters can run up to several thousand dollars. Precision test-stand meters with high resolution transmitters, hazardous area certification and NIST traceable calibration sit at the upper end of that range.

Max positive displacement flow meters

Max Machinery has built positive displacement flow meters in Healdsburg, California since 1967. Every meter holds its accuracy as a percentage of reading, needs no straight pipe, responds in 1.6 milliseconds, and is calibrated NIST traceable across its full flow range.

Technology	Flow range	Accuracy	Viscosity	Typical use
Piston flow meters	0.005 cc/min to 35 L/min	0.2% of reading over 200:1	0.5 to 50 cSt	Fuel testing, injector and valve testing, chemical injection, laboratory
Gear flow meters	0.5 cc/min to 240 L/min	0.3% of reading over 100:1	1 to 100,000 cSt	Hydraulics, adhesive dispensing, lubricants, fuels, chemicals
Helical flow meters	100 cc/min to 500 L/min	0.2% of reading	1 to 1,000,000 cSt	Heavy fuels, coatings, polymers, resins
Water flow meters	1 to 2,000 cc/min	0.4% of reading	Aqueous solutions	Treatment chemical dosing, process water

For the lowest flows and fastest response, the P002 piston flow meter measures 0.5 to 2,000 cc/min at pressures to 7,250 psi with a 1,000 pulse per cc output. For higher flows, the P215 piston flow meter covers 25 cc/min to 35 liters per minute at 0.2% of reading. Call +1 (707) 433-2662 or request a quote and an application engineer will size the meter to your fluid, flow range and pressure.

Frequently asked questions