Positive displacement meter

A screw flowmeter is composed of a set of screws (also called spindles) which form with the internal structure of the flowmeters' casing a measurement chamber.[1] The screw will get into rotation thanks to the medium passing through the device, which will then be transferred by the-said screws from one end to the other end of the measuring device. For this to be done, the pressure drop is essential and seen as a "necessary evil".[2] This rotation can then be recorded by a sensor which, combined with the processing unit (software and hardware), will be able to deliver a measurement according to the flowrate, viscosity and size of the measurement chamber.[3] ft

Screw flowmeters are well-acknowledged for their excellent linearity (±0.001%),[4][5] excellent repeatability (up to 0,006%)[6] and accuracy (±0.1%).[7][8] They have the propensity to be used as metrological international reference and/or standard by metrological institutes, due to their outstanding features and reliability. Thanks to screw meters, public and independent institutes of metrology worldwide can compare their respective work, facilities, or calibrate other flowmeters (e.g., master metering) or compare flowmeters' performance according to different measurement principles.[9][10][11][12]

The first positive displacement screw flowmeter. A KRAL flowmeter.

List of public and independent institutes of metrology using screw flow meters as international reference and/or standard:[13][14][15][16]

• Australia
• Austria
• Belgium
• Canada
• Czech Republic
• Denmark
• France
• Germany
• Japan
• Mexico
• Scotland
• Sweden
• Switzerland
• Taiwan R.O.C.
• The Netherlands
• The United Kingdom
• Vietnam

Each piston is mechanically or magnetically operated to fill a cylinder with the fluid and then discharge the fluid. Each stroke represents a finite measurement of the fluid (can be a single or multi-piston device).

Gear flow meters rely on internal gears rotating as fluid passes through them. There are various types of gear meters named mostly for the shape of the internal components

Oval gear

Two rotating oval gears with synchronized teeth “squeeze” a finite amount of fluid through the meter for each revolution.

With oval gear flow meters, two oval gears or rotors are mounted inside a cylinder. As the fluid flows through the cylinder, the pressure of the fluid causes the rotors to rotate. As flow rate increases, so does the rotational speed of the rotors.

Helical gear

Helical gear flow meters get their name from the shape of their gears or rotors. These rotors resemble the shape of a helix, which is a spiral-shaped structure. As the fluid flows through the meter, it enters the compartments in the rotors, causing the rotors to rotate. Flowrate is calculated from the speed of rotation.

A disk mounted on a sphere is “wobbled” about an axis by the fluid flow and each rotation represents a finite amount of fluid transferred. A nutating disc flow meter has a round disc mounted on a spindle in a cylindrical chamber. By tracking the movements of the spindle, the flow meter determines the number of times the chamber traps and empties fluid. This information is used to determine the flow rate.

A rotating impeller containing two or more vanes divides the spaces between the vanes into discrete volumes and each rotation (or vane passing) is counted.

Flow = volume of measuring chamber × RPM × 4

Fluid is drawn into the inlet side of an oscillating diaphragm and then dispelled to the outlet. The diaphragm oscillating cycles are counted to determine the flow rate.