Flow Measurement(basics)

- Ashvani Shukla
- CI
- Reliance

INTRODUCTION

- In the physical world, mechanical engineers are

frequently required to monitor or control the

flow of various fluids through pipes, ducts and

assorted vessels. This fluid can range from thick

oils to light gasses. While some techniques work

better with some groups of fluids, and less well

with others, some are not at all suitable for

some applications. In this primer on fluid flow

instrumentation we will look at a wide variety of

flow transducers and their application in the

physical world.

Fluid flow measurement

- Fluid flow measurement can encompass a wide

variety of fluids and applications. To meet this

wide variety of applications the instrumentation

industry has, over many years, developed a wide

variety of instruments. The earliest known uses

for flow come as early as the first recorded

history. The ancient Sumerian cities of UR and

Kish, near the Tigris and Euphrates rivers

(around 5000 B.C.) used water flow measurement to

manage the flow of water through the aqueducts

feeding their cities. In this age the a simple

obstruction was placed in the water flow, and by

measuring the height of the water flowing over

the top of the obstruction, these early engineers

could determine how much water was flowing. In

1450 the Italian art architect Battista Alberti

invented the first mechanical anemometer. It

consisted of a disk placed perpendicular to the

wind, and the force of the wind caused it to

rotate. The angle of inclination of the disk

would then indicate the wind velocity. This was

the first recorded instrument to measure wind

speed. An English inventor, Robert Hooke

reinvented this device in 1709, along with the

Mayan Indians around that same period of time.

Today we would look down our noses at these crude

methods of flow measurement, but as you will see,

these crude methods are still in use today.

TYPE OF FLOW

- There are in general three types of fluid flow in

pipes - laminar
- turbulent
- transient
- Laminar flow
- Laminar flow generally happens when dealing with

small pipes and low flow velocities. Laminar flow

can be regarded as a series of liquid cylinders

in the pipe, where the innermost parts flow the

fastest, and the cylinder touching the pipe isn't

moving at all. - Shear stress in a laminar flow depends almost

only on viscosity - µ - and is independent

of density - ?. - Turbulent flow
- In turbulent flow vortices, eddies and wakes make

the flow unpredictable. Turbulent flow happens in

general at high flow rates and with larger pipes. - Shear stress in a turbulent flow is a function

of density - ?. - Transitional flow

CONTINUE..

- Transitional flow is a mixture of laminar and

turbulent flow, with turbulence in the center of

the pipe, and laminar flow near the edges. Each

of these flows behave in different manners in

terms of their frictional energy loss while

flowing and have different equations that predict

their behavior. - Turbulent or laminar flow is determined by the

dimensionless Reynolds Number. - Reynolds Number
- The Reynolds number is important in analyzing any

type of flow when there is substantial velocity

gradient (i.e. shear.) It indicates the relative

significance of the viscous effect compared to

the inertia effect. The Reynolds number is

proportional to inertial force divided by viscous

force. - The flow is
- laminar when Re lt 2300
- transient when 2300 lt Re lt 4000
- turbulent when 4000 lt Re

TYPE OF FLOW

- Uniform Flow, Steady Flow
- It is possible - and useful - to classify the

type of flow which is being examined into small

number of groups. If we look at a fluid flowing

under normal circumstances - a river for example

- the conditions at one point will vary from

those at another point (e.g. different velocity)

we have non-uniform flow. If the conditions at

one point vary as time passes then we have

unsteady flow. Under some circumstances the flow

will not be as changeable as this. He following

terms describe the states which are used to

classify fluid flow - uniform flow If the flow velocity is the same

magnitude and direction at every point in the

fluid it is said to be uniform. - non-uniform If at a given instant, the velocity

is not the same at every point the flow is

non-uniform. (In practice, by this definition,

every fluid that flows near a solid boundary will

be non-uniform - as the fluid at the boundary

must take the speed of the boundary, usually

zero. However if the size and shape of the of the

cross-section of the stream of fluid is constant

the flow is considered uniform.) - steady A steady flow is one in which the

conditions (velocity, pressure and cross-section)

may differ from point to point but DO NOT change

with time. unsteady If at any point in the

fluid, the conditions change with time, the flow

is described as unsteady. (In practice there is

always slight variations in velocity and

pressure, but if the average values are constant,

the flow is considered steady.

CONTINUOUS

- Combining the above we can classify any flow in

to one of four type - 1. Steady uniform flow. Conditions do not change

with position in the stream or with time. An

example is the flow of water in a pipe of

constant diameter at constant velocity. Fluid

Mechanics Fluid Dynamics The Momentum and

Bernoulli Equations. - 2. Steady non-uniform flow. Conditions change

from point to point in the stream but do not

change with time. An example is flow in a

tapering pipe with constant velocity at the inlet

- velocity will change as you move along the

length of the pipe toward the exit. - 3. Unsteady uniform flow. At a given instant in

time the conditions at every point are the same,

but will change with time. An example is a pipe

of constant diameter connected to a pump pumping

at a constant rate which is then switched off. - 4. Unsteady non-uniform flow. Every condition of

the flow may change from point to point and with

time at every point. For example waves in a

channel.

CONTINUOUS

- Compressible or Incompressible All fluids are

compressible - even water - their density will

change as pressure changes. Under steady

conditions, and provided that the changes in

pressure are small, it is usually possible to

simplify analysis of the flow by assuming it is

incompressible and has constant density. As you

will appreciate, liquids are quite difficult to

compress - so under most steady conditions they

are treated as incompressible. In some unsteady

conditions very high pressure differences can

occur and it is necessary to take these into

account - even for liquids. Gasses, on the

contrary, are very easily compressed, it is

essential in most cases to treat these as

compressible, taking changes in pressure into

account.

continuous

- Three-dimensional flow Although in general all

fluids flow three-dimensionally, with pressures

and velocities and other flow properties varying

in all directions, in many cases the greatest

changes only occur in two directions or even only

in one. In these cases changes in the other

direction can be effectively ignored making

analysis much more simple. Flow is one

dimensional if the flow parameters (such as

velocity, pressure, depth etc.) at a given

instant in time only vary in the direction of

flow and not across the cross-section. The flow

may be unsteady, in this case the parameter vary

in time but still not across the cross-section.

An example of one-dimensional flow is the flow in

a pipe. Note that since flow must be zero at the

pipe wall - yet non-zero in the Centre - there is

a difference of parameters across the

cross-section. Should this be treated as

two-dimensional flow? Possibly - but it is only

necessary if very high accuracy is required. A

correction factor is then usually applied.

continuous

- Flow is two-dimensional if it can be assumed that

the flow parameters vary in the direction of flow

and in one direction at right angles to this

direction. Streamlines in two-dimensional flow

are curved lines on a plane and are the same on

all parallel planes. An example is flow over a

weir foe which typical streamlines can be seen in

the figure below. Over the majority of the length

of the weir the flow is the same - only at the

two ends does it change slightly. Here correction

factors may be applied.

One dimensional flow

Two dimensional flow

Flow rate.

- Mass flow rate If we want to measure the rate at

which water is flowing along a pipe. A very

simple way of doing this is to catch all the

water coming out of the pipe in a bucket over a

fixed time period. Measuring the weight of the

water in the bucket and dividing this by the time

taken to collect this water gives a rate of

accumulation of mass. This is know as the mass

flow rate. - Volume flow rate - Discharge. More commonly we

need to know the volume flow rate - this is more

commonly know as discharge. (It is also commonly,

but inaccurately, simply called flow rate). The

symbol normally used for discharge is Q. The

discharge is the volume of fluid flowing per unit

time. Multiplying this by the density of the

fluid gives us the mass flow rate.

Type of flow measurement

- The most common principals for fluid flow

metering are - Differential Pressure Flow meters
- Velocity Flow meters
- Positive Displacement Flow meters
- Mass Flow meters
- Open Channel Flow meters

1.Differential Pressure Flow meters

- In a differential pressure drop device the flow

is calculated by measuring the pressure drop over

an obstructions inserted in the flow. The

differential pressure flow meter is based on

the Bernoulli's Equation, where the pressure drop

and the further measured signal is a function of

the square flow speed.

- Common types of differential pressure flow meters

are - Orifice Plates
- Flow Nozzles
- Venturi Tubes
- Variable Area - Rota meters
- Orifice Plate
- An orifice plate is a device used for measuring

flow rate, for reducing pressure or for

restricting flow (in the latter two cases it is

often called a restriction plate). Either a

volumetric or mass flow rate may be determined,

depending on the calculation associated with the

orifice plate.With an orifice plate, the fluid

flow is measured through the difference in

pressure from the upstream side to the downstream

side of a partially obstructed pipe. The plate

obstructing the flow offers a precisely measured

obstruction that narrows the pipe and forces the

flowing fluid to constrict.

continuous

- Orifice Plate is the heart of the Orifice Meter.

It restricts the flow and develops the

Differential Pressure which is proportional to

the square of the flow rate. The flow measuring

accuracy entirely depends upon the quality of

Orifice plate, its installation and maintains. - When measuring wet gas or saturated steam a weep

hole is drilled in a concentrically bored orifice

plate. This is a small hole drilled on the

orifice plate such that its location is exactly

on ID of the main pipe.

- The Orifice plates are manufactured as per ISA /

AGA/ API / ANSI standards and in various

materials - such as SS304 /SS316 / SS316L /Hestoly C / Monel

/ PTFE coated. - Various bores are used for various applications.
- Orifice Plate is categories in two types -

Paddle Type Universal Palate. - Paddle Type Orifice Plate
- This plate is sandwiched between two Orifice

Flanges. Tag Plate of orifice plate projects out

from Orifice flanges and it indicates the

existence of Orifice plate. Details such as Tag

NO /Orifice ID / Pipe ID / Plate Material are

stamped on one side of the tag plate which faces

upstream side of the pipe line. Outside diameter

of the orifice plate equals to PCD-1 Bolt Dia.

This ensures the concentricity with the main pipe

line. The other method to maintain the

concentricity is by using sleeves on the bolts or

by providing dowel pins on the Orifice Flanges. - Universal Orifice Plate
- This is a circular plate designed to fit in the

Orifice fittings / Plate holders / carrier rings

/ Ring Type Joints(RTJ).

Technical Specification

1.Size for Integral Design 15, 20, 25, 40 mm

2.Size for Flanged Design 25, 40, 50, 65, 80,

100, 150 ...250 mm 3.Material- Flanges

Carrier Ring A105 / SS304/ SS316 / SS316L / CS

Other materials on request. 4.Orifice

Plate SS304, SS316, SS316L, Hast C, Monel, PP,

PVC,PTFE, Coated or Clad with PP / HDPE / PTFE.

5.Gasket CAF / SS Spiral Wound CAF / PTFE /

PVC / Rubber, Other materials as per special

request. 6.Stud / Nut ASTM A193 Gr B/ASTM

A194 CI 2H A193 B16/A194 C14 7.Standards

Applicable Design - ISA RP 3.2 / DN 1952 / BS

1042 - 1981-84 8.Bore Calculation ISO 5167 /

BS 1042 / RW Miller / L. K. SPIN / AGE - 3.7

9.Flanges ANSI B-16-36 / or Equivalent

10.Types Square edge concentric, Quadrant

edges, Conical entrance, Eccentric.

11.Pressure Toppings For 1" to 16" - Flange

Taps / Corner Taps. Above 16" - D x D/2

Type Orifice Plate

- Paddle Type Orifice Plate
- Concentric Beveled Bore
- Application This Most Common Bore Used In The

Industries. This Is The Only Type Generally

Accepted For Use In Custody Transfer Measurement,

Since Adequate Data Is Not Available For Other

Bores. Used Primarily For Clean Homogeneous

Liquids, Gases, Non Viscous Fluids. The Bevel Is

Matched At 45 Angle To The Desired Throat

Thickness.

2) Restriction BoreApplication This Type Is

Not Used For Flow Measurement But For Dropping

The Pressure Considerably And Reducing The Flow

Accordingly. The Bore Is Not Beveled But Kept

Straight. The Beta Ratio Has No Limit As Accuracy

Is Not The Goal

Eccentric Bore

- Application Used For Measurement Of Flow For

Fluids Containing Solids And Slurries. It Is Also

Used For Vapors And Gases Where Condensation Is

Present. The Eccentric Bore Is Offset To Where

The Bore Edge Is Inscribed In A Circle That Is

98 The Line Id.

4) Segmenta Bore

- Application
- The Segmental Bore Is Located In The Same Way

That The Eccentric Bore Is. This Type Is Used

Primarily For Slurries Or Extremely Dirty Gases

Where The Flow May Contain Impurities Heavier

Than The Fluid.

Quadrant Bore

- Application Used For High Viscous Fluids Such

As Heavy Crude, Syrups And Slurries. It Is

Always Recommended For Flow Where Reynolds Number

Is Less Than 10,000.The Inlet Is Quarter Of A

Circle And The Plate Thickness Must Be At Least

Radius Of The Inlet.

6) Ring Type Joint Integral

- Application
- These Are Available In Oval Or Octal Shapes.

Orifice Plate Is A Part Of RTJ Gasket.

Ring Type Joint- Separate

- Application These Are Available In Oval Or

Octal Shapes. The Orifice Plate Is Universal

Type And Snap Fitted On The RTJ Gasket By Screws.

Universal Orifice Plates

Application This Is A Circular Plate Designed

To Fit In The Orifice Fittings / Plate Holders /

Carrier Rings / Ring Type Joints(RTJ).

Various Orifice Assemblies

WNRF - Flange Taps

WNRF - Corner Taps

Orifice working principle

- Working
- The orifice plate, being fixed at a section of

the pipe, creates an obstruction to the flow by

providing an opening in the form of an orifice to

the flow passage.

When an orifice plate is placed in a pipe

carrying the fluid whose rate of flow is to be

measured, the orifice plate causes a pressure

drop which varies with the flow rate. This

pressure drop is measured using a differential

pressure sensor and when calibrated this pressure

drop becomes a measure flow rate. The flow rate

is given by.

Where, Qa flow rateCd Discharge

coefficientA1 Cross sectional area of pipeA2

Cross sectional area of orificeP1, P2 Static

Pressures

- The main parts of an orifice flow meter are as

followsA stainless steel orifice plate which

is held between flanges of a pipe carrying the

fluid whose flow rate is being measured. - It should be noted that for a certain distance

before and after the orifice plate fitted between

the flanges, the pipe carrying the fliud should

be straight in order to maintain laminar flow

conditions. - Openings are provided at two places 1 and 2 for

attaching a differential pressure sensor (U-tube

manometer, differential pressure gauge etc) as

shown in the diagram.

Operation of Orifice Meter

- The detail of the fluid movement inside the pipe

and orifice plate has to be understood. - The fluid having uniform cross section of flow

converges into the orifice plates opening in its

upstream. When the fluid comes out of the orifice

plates opening, its cross section is minimum and

uniform for a particular distance and then the

cross section of the fluid starts diverging in

the down stream. - At the upstream of the orifice, before the

converging of the fluid takes place, the pressure

of he fluid (P1) is maximum. As the fluid starts

converging, to enter the orifice opening its

pressure drops. When the fluid comes out of the

orifice opening, its pressure is minimum (p2) and

this minimum pressure remains constant in the

minimum cross section area of fluid flow at the

downstream. - This minimum cross sectional area of the fluid

obtained at downstream from the orifice edge is

called VENA-CONTRACTA. - The differential pressure sensor attached between

points 1 and 2 records the pressure difference

(P1 P2) between these two points which becomes

an indication of the flow rate of the fluid

through the pipe when calibrated.

- Applications of Orifice Meter
- The concentric orifice plate is used to measure

flow rates of pure fluids and has a wide

applicability as it has been standardized. - The eccentric and segmental orifice plates are

used to measure flow rates of fluids containing

suspended materials such as solids, oil mixed

with water and wet steam. - Advantages of Orifice Meter
- It is very cheap and easy method to measure flow

rate. - It has predictable characteristics and occupies

less space. - Can be use to measure flow rates in large pipes.
- Limitations of Orifice Meter
- The vena-contracta length depends on the

roughness of the inner wall of the pipe and

sharpness of the orifice plate. In certain cases

it becomes difficult to tap the minimum pressure

(P2) due to the above factor. - Pressure recovery at downstream is poor, that is,

overall loss varies from 40 to 90 of the

differential pressure. - In the upstream straightening vanes are a must to

obtain laminar flow conditions. - Gets clogged when the suspended fluids flow.
- The orifice plate gets corroded and due to this

after sometime, inaccuracy occurs. Moreover the

orifice plate has low physical strength. - The coefficient of discharge is low.

Venturi Tube

- Due to simplicity and dependability, the Venturi

tube flowmeter is often used in applications

where it's necessary with higher TurnDown Rates,

or lower pressure drops, than the orifice plate

can provide. - In the Venturi Tube the fluid flow rate is

measured by reducing the cross sectional flow

area in the flow path, generating a pressure

difference. After the constricted area, the fluid

is passes through a pressure recovery exit

section, where up to 80 of the differential

pressure generated at the constricted area, is

recovered. With proper instrumentation and flow

calibrating, the Venturi Tube flowrate can be

reduced to about 10 of its full scale range with

proper accuracy. This provides a TurnDown

Rate 101.

Venturi 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 Turndown Rate and accuracy can be compared

with the orifice plate. - The Sonic Nozzle - Critical (Choked) Flow Nozzle
- When a gas accelerates through a nozzle, the

velocity increase and the pressure and the gas

density decrease. The maximum velocity is

achieved at the throat, the minimum area, where

it breaks Mach 1 or sonic. At this point it's not

possible to increase the flow by lowering the

downstream pressure. The flow is choked. - This situation is used in many control systems to

maintain fixed, accurate, repeatable gas flow

rates unaffected by the downstream pressure.

Recovery of Pressure Drop in Orifices, Nozzles

and Venturi Meters

- After the pressure difference has been generated

in the differential pressure flow meter, the

fluid pass through the pressure recovery exit

section, where the differential pressure

generated at the constricted area is partly

recovered. As we can see, the pressure drop in

orifice plates are significant higher than in the

venturi tubes.

Variable Area Flow meter or Rota meter

- The Rota meter consists of a vertically oriented

glass (or plastic) tube with a larger end at the

top, and a metering float which is free to move

within the tube. Fluid flow causes the float to

rise in the tube as the upward pressure

differential and buoyancy of the fluid overcome

the effect of gravity.

continuous

- The float rises until the annular area between

the float and tube increases sufficiently to

allow a state of dynamic equilibrium between the

upward differential pressure and buoyancy

factors, and downward gravity factors. - The height of the float is an indication of the

flow rate. The tube can be calibrated and

graduated in appropriate flow units. - The rotameter meter typically have a TurnDown

Ratio up to 121. The accuracy may be as good as

1 of full scale rating. - Magnetic floats can be used for alarm and signal

transmission functions.

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

Pitot Tubes

- The pitot tube are one the most used (and

cheapest) ways to measure fluid flow, especially

in air applications like ventilation and HVAC

systems, even used in airplanes for speed

measurent. The pitot tube measures the fluid flow

velocity by converting the kinetic energy of the

flow into potential energy. - The use of the pitot tube is restricted to point

measuring. With the "annubar", or multi-orifice

pitot probe, the dynamic pressure can be measured

across the velocity profile, and the annubar

obtains an averaging effect.

Calorimetric Flow meter

- The calorimetric principle for fluid flow

measurement is based on two temperature sensors

in close contact with the fluid but thermal

insulated from each other.

One of the two sensors is constantly heated and

the cooling effect of the flowing fluid is used

to monitor the flow rate. In a stationary (no

flow) fluid condition there is a constant

temperature difference between the two

temperature sensors. When the fluid flow

increases, heat energy is drawn from the heated

sensor and the temperature difference between the

sensors are reduced. The reduction is

proportional to the flow rate of the

fluid. Response times will vary due the thermal

conductivity of the fluid. In general lower

thermal conductivity require higher velocity for

proper measurement. The calorimetric flow meter

can achieve relatively high accuracy at low flow

rates

Vortex Flow Meter An obstruction in a fluid flow

creates vortices in a downstream flow. Every

obstruction has a critical fluid flow speed at

which vortex shedding occurs. Vortex shedding is

the instance where alternating low pressure zones

are generated in the downstream.

- Electromagnetic Flowmeter
- An electromagnetic flowmeter operate on Faraday's

law of electromagnetic induction that states that

a voltage will be induced when a conductor moves

through a magnetic field. The liquid serves as

the conductor and the magnetic field is created

by energized coils outside the flow tube. - The voltage produced is directly proportional to

the flow rate. Two electrodes mounted in the pipe

wall detect the voltage which is measured by a

secondary element. - Electromagnetic flowmeters can measure difficult

and corrosive liquids and slurries, and they can

measure flow in both directions with equal

accuracy. - Electromagnetic flowmeters have a relatively high

power consumption and can only be used for

electrical conductive fluids as water. - The Electromagnetic Flowmeter Principle - An

introduction to the electromagnetic flowmeter

principle

Ultrasonic Doppler Flow meter

- The effect of motion of a sound source and its

effect on the frequency of the sound was observed

and described by Christian Johann Doppler. - The frequency of the reflected signal is modified

by the velocity and direction of the fluid flow - If a fluid is moving towards a transducer, the

frequency of the returning signal will increase.

As fluid moves away from a transducer, the

frequency of the returning signal decrease. - The frequency difference is equal to the

reflected frequency minus the originating

frequency and can be use to calculate the fluid

flow speed. - The Ultrasonic Doppler and Time of Flight Flow

meter

Positive Displacement Flowmeter

- The positive displacement flow meter measures

process fluid flow by precision-fitted rotors as

flow measuring elements. Known and fixed volumes

are displaced between the rotors. The rotation of

the rotors are proportional to the volume of the

fluid being displaced. - The number of rotations of the rotor is counted

by an integral electronic pulse transmitter and

converted to volume and flow rate. - The positive displacement rotor construction can

be done in several ways - Reciprocating piston meters are of single and

multiple-piston types. - 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. Shaft rotation can be

monitored to obtain specific flow rates. - Notating disk meters have movable disks mounted

on a concentric sphere located in spherical

side-walled chambers. The pressure of the liquid

passing through the measuring chamber causes the

disk to rock in a circulating path without

rotating about its own axis. It is the only

moving part in the measuring chamber. - Rotary vane meters consists of equally divided,

rotating impellers, two or more compartments,

inside the meter's housings. 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.

The revolutions of the impeller are counted and

registered in volumetric units. - The positive displacement flowmeter may be used

for all relatively nonabrasive fluids such as

heating oils, lubrication oils, polymer

additives, animal and vegetable fat, printing

ink, Dichlorodifluoromethane R-12, and many more. - Accuracy may be up to 0.1 of full rate with a

TurnDown of 701 or more.

Mass Flow meters

- Mass meters measure the mass flow rate directly.
- Thermal Flow meter
- The thermal mass flowmeter operates independent

of density, pressure, and viscosity. Thermal

meters use a heated sensing element isolated from

the fluid flow path where the flow stream

conducts heat from the sensing element. The

conducted heat is directly proportional to the

mass flow rate and the temperature difference is

calculated to mass flow. - The accuracy of the thermal mass flow device

depends on the calibrations reliability of the

actual process and variations in the temperature,

pressure, flow rate, heat capacity and viscosity

of the fluid. - Coriolis Flow meter
- Direct mass measurement sets Coriolis flowmeters

apart from other technologies. Mass measurement

is not sensitive to changes in pressure,

temperature, viscosity and density. With the

ability to measure liquids, slurries and gases,

Coriolis flowmeters are universal meters. - Coriolis Mass Flowmeter uses the Coriolis effect

to measure the amount of mass moving through the

element. The fluid to be measured runs through a

U-shaped tube that is caused to vibrate in an

angular harmonic oscillation. Due to the Coriolis

forces, the tubes will deform and an additional

vibration component will be added to the

oscillation. This additional component causes a

phase shift on some places of the tubes which can

be measured with sensors. - The Coriolis flow meters are in general very

accurate, better than /-0,1 with an turndown

rate more than 1001. The Coriolis meter can also

be used to measure the fluids density.

Open Channel Flow meters

- A common method of measuring flow through an open

channel is to measure the height of the liquid as

it passes over an obstruction as a flume or weir

in the channel. - Common used is the Sharp-Crested Weir, the

V-Notch Weir, the Cipolletti weir, the

Rectangular-Notch Weir, the Parshall Flume or

Venturi Flume.