Mechanical flowmeters and Piston meter/rotary piston

A positive displacement flow meter can be compared to a bucket and a stopwatch.The stopwatch starts when traffic starts and stops when the bucket reaches its limit.Volume divided by time gives flow rate.For continuous measurement, we need a system that constantly fills and empties the bucket to divert the flow without letting it flow out of the pipe.These continuously forming and contracting volumetric displacements can take the form of pistons reciprocating in cylinders, gear teeth meshing with the inner wall of a gauge, or progressive chambers formed by rotating oval gears or helical screws.

Piston meter/rotary piston: 

Due to its use in domestic water measurement, piston meters (also known as rotary piston or semi-displacement meters) are the most common flow measuring devices in the UK and are used in almost all ).Piston flow meters work on the principle that a piston rotates within a chamber of known volume.For each revolution, a certain amount of water passes through the piston chamber.Analog dials and odometer displays improved by gear mechanisms and sometimes magnetic drives.

Oval gear meter:

An oval gear flow meter is a positive displacement flow meter that uses two or more oval gears configured to rotate at right angles to each other, forming a T-shape.Such a meter has two sides, which may be called A and B.With no fluid passing through the center of the meter, the teeth of the two gears are always meshing.On one side of the meter.

(A).The teeth of the gear close off fluid flow as the elongated gear on side A protrudes into the measuring chamber, while on the other side of the meter.

(B).The cavity houses a measuring chamber A fixed volume of fluid.When fluid pushes against the gear, the gear rotates, releasing fluid in the B-side measurement chamber to the outlet port.Simultaneously, fluid entering the inlet will be driven to the now open A-side measuring chamber.The teeth on the B side will now block fluid from entering the B side.The cycle continues as the gears rotate and fluid is metered through alternating measuring chambers.A permanent magnet in the rotating gear can transmit a signal to an electric reed switch or current sensor for flow measurement.Despite claims of high performance, they are often not as precise as sliding vane designs.

Gear meter

Gear meters differ from oval gear meters in that the measurement chambers are made up of the gaps between the teeth of the gears. These openings divide up the fluid stream and as the gears rotate away from the inlet port, the meter's inner wall closes off the chamber to hold the fixed amount of fluid.The outlet port is located in the area where the gears are coming back together.The fluid is forced out of the meter as the gear teeth mesh and reduce the available pockets to nearly zero volume.

Helical gear

Helical gear flowmeters 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.The length of the rotor is sufficient that the inlet and outlet are always separated from each other thus blocking a free flow of liquid.The mating helical rotors create a progressive cavity which opens to admit fluid, seals itself off and then opens up to the downstream side to release the fluid.This happens in a continuous fashion and the flowrate is calculated from the speed of rotation.

Nutating disk meter

This is the most commonly used measurement system for measuring water supply to a home.Fluid (most commonly water) enters from one side of the gauge and strikes an eccentrically mounted nutating disc.Because the bottom and top of the disk remain in contact with the mounting chamber, the disk must "wobble," or nutate, about a vertical axis.A partition separates the inlet and outlet chambers.As the disc oscillates, it is a direct indication of the volume of liquid that has passed through the meter, as the volumetric flow is indicated by a gear and register arrangement attached to the disc. Flow measurement within 1% is reliable.

Turbine flowmeter

Turbine meters (better described as axial turbines) convert the mechanical action of a turbine rotating about its axis in a liquid flow into a user readable flow rate (gpm, lpm, etc.).A turbine tends to have all airflow go around it.A turbine wheel is disposed in the path of the fluid flow.The flowing fluid impacts the turbine blades, applying force to the blade surfaces and moving the rotor.When a steady rotational speed is reached, the speed is directly proportional to the fluid velocity.Turbine flow meters are used to measure natural gas and liquid flow.At low flow rates, turbine meters are not as accurate as displacement and jet meters, but the measuring element does not occupy or severely restrict the entire flow path.The flow direction is usually straight through the flowmeter, resulting in higher flow rates and lower pressure loss than displacement flowmeters.They are the meters of choice for major commercial users, fire protection and water distribution system master meters.A filter is often required in front of the water meter to protect the measuring element from gravel or other debris that could enter the water distribution system.Turbine meters are typically suitable for pipe sizes from 4 to 30 cm (1+1⁄2–12 in.) or larger.Turbine flowmeter bodies are usually made of bronze, cast iron, or ductile iron.Internal turbine elements can be plastic or non-corrosive metal alloys.They are accurate under normal operating conditions but are highly affected by flow profile and fluid conditions.

Firefighting flowmeters are a special type of turbine flowmeter approved for the high flow requirements required by firefighting systems.They are usually approved for fire protection by Underwriters Laboratories (UL) or Factory Mutual (FM) or similar.Portable turbine flow meters can be temporarily installed to measure the water consumption of fire hydrants.The gauges are usually made of aluminum to reduce weight and usually have a capacity of 7.5 cm (3 in).They are often required by water utilities to measure building water, pool fills, or where a permanent water meter has not yet been installed.