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Flow Sensors

Flow - mass flow rate

- volume flow rate

- velocity

Types of flow - stream line parabolic velocity profile

- turbulent vortices

Methods of

measurement

- direct: positive displacement (batch sensors,

metering pumps)

- indirect: measurement of velocity

or kinetic energy

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Flow Sensors

Classification:

a) volume flow rate

b) mass flow rate

c) velocity

tVQ

tVQ mv == ,

tmQm =

SvQSvQ mv == ,

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Flow Sensors

Volume flow rate sensors:- rotameters (float)

- batch/oval gear

- velocity:- turbine, paddle wheel

- vortices

- electromagnetic- ultrasonic

- with moving marks

- obstruction devicesMass flow rate sensors:

- thermal

- Coriolis force

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Flowmeters with float

Rotameter

Float acts as force balance

indicator

viscosity insensitive =>sharp edges on float

Float(buoy)

Glass tube

Gravity

Flow

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Velocity based flowmeters

Turbine

flowmeter

linearity 0 1%

threshold 2 3% range

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Turbine flowmeters

frequency of pulses fproportional

to velocity:

KQvf =

Kconstant of flowmeterQV volume flow

Lower limit of accuracy: 3 5%

linearity: 0,1%

=

2

3 DfDQv rr

Criterion ofnonlinearity:

Ddiameter of turbine pipe

viskoziy of fluid

lin.dependence of ang, velocity of rotor

ron velocityof flowv

drop of pulse amplitude for low v (not in Hall sensor)

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Paddle-wheel sensor

+ cheaper

- less precise

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Vortex Shedding flowmeter

Detection of vortices:

thermoanemometersultrasonic detectors

pressure detectors

Accuracy ~ 1%

vaSrf =

ffrequency of vortices

A characteristic dimension of obstacleSr Strouhal number (char. for certain shape of obstacles)

Karman vortices

Bluff body

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Marking flowmeters

Mark - conductive (injection of electrolyte to liquid)

- optical (injection of colouring agent)

- thermal

- ionisation (admixture of radioisotope)

Principle: measurement of time interval of mark transit

between two points in direction of liquid flow

Correlation based velocity measurement

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Ultrasonic

flowmetersPulse ultrasonic flowmeter is based on

addition of vectors velocity of liquid

flow and velocity of ultrasoundpropagation.

The measured value is time of

propagation of pulse from transmmite

to receiver.

21

12

cos2 ttttLv

=

t1time interval of propagation

from (V2,P2) to (V1,P1 )

t2time interval of propagation

from (V1,P1)tok (V2,P2 )

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Ultrasonic flowmeters

Doppler type of ultrasonic flowmeter (non wetted type)Works in continuos wave mode CW (not in pulse mode)

Similarly to radar trafic speed measurement measure Doppler shift of frequency

Principle: reflection of ultrasonic wave from bubbles or dispersed particles

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Induction (electro-magnetic) flowmeters

Usual accuracy: through flow type 0,2%, immersion type 2%

analogy to Hall effect

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Induction (magnetic) flowmetersConstruction of flowmeter with saddlebacked coils

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Induction (magnetic) flowmetersImmersion type

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Sensors of flow with obstruction devices

Pressure drop (pressure difference

on obstructions devices(orifice,flow nozzle, Venturi tube)

21

2

24

ppdQv

=

Qvvolume,

expansion coefficientdiameter

Accuracy 2%

(0,5%)

2

2

1vpd =

Sensors with conversion of flow to

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Sensors with conversion of flow todeformation (drag-force flowmeters)

On the target immersed in the

flowing medium acts

drag - forceFd

2

2vSCF dd

=

Cdconstant of the target

Scrossection area

density of liquid

vvelocity

good dynamic response

- resonant frequency up to 200 HzAccuracy several %

O l fl i

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Oval gear flowmeters, metering pumps

1 dm3/h 103 dm3/h

Used for balance (audit) flowmetering

Badgermeter co.

Thermal mass flowrate sensors (flowmeters)

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Thermal mass flowrate sensors (flowmeters)

Thermoanemometer 2 ways of operation:-constant temperature of wire (feedback- excellent dynamic response

-constant current

Exchange of heat between source and surroundings (fluid)

- measurement of cooling of heat source (thermoanemometer)- measurement of warming up of fluid

Th l f fl t

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Thermal sensor of mass flowrate

Mode of operation of thermoanemometers

-const. current (change of flow velocity => change of temperature =>

=> change of resistance => bridge not balanced)

-const.temperature of wire (bridge balanced for maximum flow,

drop ofv => less cooling effect =>

=> drop of heating current)

Output current

m

Qbai +=2

a

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Response to the step change of velocity 1constant current2constant temperature

Differential thermoanemometer

Th l f fl t

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Thermal sensor of mass flowrateDifferential thermoanemometer

At v = 0 ...R1=R2at v > 0 cooling R1 and warming up R2 (

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Bridge evaluates the

temperature difference

1-2measured by S1 and S2

q

mPAQ

)( 12

=

Aconstant

Cpspecific heat capacity of fluid

PQthermal flux from heatingwindings T

sensor of small

mass flowrate

(shunt)

Differential sensor Thomas principle

Coriolis flowmeter

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Coriolis flowmeter

Coriolis forceFcis perpendicular to the axis of rotation and direction of

movement

FCdepends on - angular velocity f (ot/s)

- mass of the body m (kg)

- velocity of body w (m/s)

FC

= 2 m (w

) = 4 m w f

Coriolis flowmeter

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Coriolis flowmeter

Tube filled by liquid flowing with velocity w:

when rotating around axis z Coriolis forceFc acts on liquid

Fc is perpendicular to the axis of rotation and direction of flow and has a tendencyto bow the tube

lQmt

l

mvF mC =

== 222

Coriolis flowmeter

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Coriolis flowmeter

Magnetic forceFm causes vibrations of tube around the axis.Fcproduces

twisting of tube.

Coriolis flowmeter = type with U tube

m

mC

QtQdbdFM

== 42

One of rarely occuring principles of direct mass flowrate measurement

!sediments in U tube -> linear tube

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Level sensors

Level sensors

Analogue output

Binary output

(level switch)

Liquids only

Powders,

granules

Slurry

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Level sensors Many principles, but only a few really

massively used

90% of applications just 4 types:

Pressure / differential pressure

Float

Ultrasound

Radar

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Pressure

Open vessel Closed vessel =>

pressurized vapours

above level

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Pressure bubbler

Pressure sensor not in contact with liquid

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Float + reed switch (~analog)

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Float + reed switch (binary)

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Float + flag

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Float + weight (Archimedes)

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Float + Magnetostrictive

Balluff

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Ultrasonic (time of flight)

Beam width can be critical in narrow vessels

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Ultrasonic level switch

Transmission from source to detector

changes when water fills the space

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Radar TDR

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Radar FMCW

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Radar antennae - cone

Tanker (crude oil) radar sensors - SAAB

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Radar guided wave

Special sensors

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Special sensors

less frequently used

Tuning fork

Yo-yo

Paddle wheel

Optical

Conductive

Dip stick

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Tuning fork

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Yo-yo

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Paddle wheel

Motor must survive static

(jammed) operation

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Optical switch

Braking the condition of total reflection

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Conductive

Dip stick

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Dip stickNormal dip stick in every car (oil level checking)

Here: a high-tech dip stick

Documents

Flow Level