# EnggPedia - The Engineering Encyclopedia

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# Flow Meter - Flow Mesurement, Definition, Uses of Flow Meter & Types

## Definition:

A device that measures the movement of liquid in a pipe. Flow meters are coupled into the pipe and derive the flow rate from the rotation of a turbine or paddle wheel that is moved by the liquid.

## Flow Measurement:

Is the quantification of bulk fluid movement. It can be measured in a variety of ways.

## Types of Flow Measurement

### Venturi Meter:

A Venturi meter constricts the flow in some fashion, and pressure sensors measure the differential pressure before and within the constriction. This method is widely used to measure flow rate in the transmission of gas through pipelines, and has been used since Roman Empire times. The coefficient of discharge of Venturi meter ranges from 0.93 to 0.97.

### Orifice Plate:

An orifice plate is a plate with a hole through it, placed in the flow; it constricts the flow, and measuring the pressure differential across the constriction gives the flow rate. It is basically a crude form of Venturi meter, but with higher energy losses. There are three type of orifice: concentric, eccentric, and segmental.

### Pitot Tube:

A Pitot tube is a pressure measuring instrument used to measure fluid flow velocity by determining the stagnation pressure. Bernoulli's equation is used to calculate the dynamic pressure and hence fluid

### Optical flow meters:

Optical flow meters use light to determine flow rate. Small particles which accompany natural and industrial gases pass through two laser beams focused in a pipe by illuminating optics. Laser light is scattered when a particle crosses the first beam.

The detecting optics collects scattered light on a photo detector, which then generates a pulse signal. If the same particle crosses the second beam, the detecting optics collect scattered light on a second photo detector, which converts the incoming light into a second electrical pulse. By measuring the time interval between these pulses, the gas velocity is calculated as V = D / T where D is the distance between the laser beams and T is the time interval.

### Magnetic flow meters:

The most common flow meter apart from mechanical flow meters is the magnetic flow meter, commonly referred to as a "mag meter" or an "electromag". A magnetic field is applied to the metering tube, which results in a potential difference proportional to the flow velocity perpendicular to the flux lines. The physical principle at work is Faraday's law of electromagnetic induction. The magnetic flow meter requires a conducting fluid, e. g. water, and an electrical insulating pipe surface, e.g. a rubber lined nonmagnetic steel tube.

### Flow Nozzles:

Flow nozzles are often used as measuring elements for air and gas flow in industrial applications. The flow nozzle is relative simple and cheap, and available for many applications in many materials. The Turn-down Rate and accuracy can be compared with the orifice plate.

### Velocity Flow meters:

In a velocity flow meter the flow is calculated by measuring the speed in one or more points in the flow, and integrating the flow speed over the flow area.

### Turbine Flow meter:

There is much different manufacturing design of turbine flow meters, but in general they are all based on the same simple principle:
If a fluid moves through a pipe and acts on the vanes of a turbine, the turbine will start to spin and rotate. The rate of spin is measured to calculate the flow.
The turndown ratios may be more than 100:1 if the turbine meter is calibrated for a single fluid and used at constant conditions. Accuracy may be better than +/-0, 1%.

### Mass Flow meters:

Mass meters measure the mass flow rate directly.