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Comment on the relative advantages and disadvantages of

Venturimeter, orifice plate meter and rotameter as flow measuring devices.

The orifice plate meter is fairly accurate in its measurement

It is also the easiest to construct.

Orifice meter occupies less space thus it could be useful to measure fluid flows within space

constraints

The orifice plate meter has a significant head loss.

Its level of accuracy falls as the flow rate rises. The sudden contraction of the

diameter causes high loss factor due to turbulent flow.

A tube with diameter larger then the orifice meter plate is required to reduce this

energy loss.

Furthermore, as the diameter of the tube is reduced, the angle between the surface of t

he tube with the horizontal should not be too large, so as to reduce energy loss and improve

accuracy, but that would demand even more horizontal space.

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Comment on the head losses associated with all the flow metersstudied in this experiment, emphasising the relationship between the

mechanism of loss generation and its magnitude

For the venturimeter, its long section enables gradual constriction and epansion

of diameter.

The head loss is thus caused by the dissipation of energy via the friction between

the fluid and the inner surface of the pipe.

It has been observed that the head loss in the venturimeter is relatively

small relative to the orifice plate.

In the !enturi meter, the gradual reduction and epansion of the diameter

reduces the separation of flow as well as reduces the separation in the

deceleration portion of the meter.

"ence, the energy loss is mainly due to friction with the wall of the tube rather toinefficient miing and separate flow

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The orifice plate has a significant head loss when compared to the other parts of

the apparatus. !Ois observed to be very high too.

As fluid velocity across the plate is relatively high, the sudden contraction

and epansion of the diameter before and after the orifice place may result in a

sharp and significant change in momentum about the orifice, causing

turbulence to form on both ends.

The turbulence increases as the flow increases.

#onsequently, this turbulence dissipates energy, thus resulting in the head loss

"ence the energy loss may be due to the high energy flow across the small

orifice plate, resulting in a significant loss in energy.

$ith the orifice plate eperiencing as much as % times more head loss

it can be implied that turbulence contributes to significantly higher energy loss than friction

with the internal walls.

Explain with the aid of simple sketches what is the

vena contractor of an

orifice meter.

!ena contracta is the point in a fluid stream where the

diameter of the stream is

the least. The contraction ta&es place at a section slightl

y downstream from theorifice, where the fluid flow is horizontal. 'treaml

ines will converge (ust

downstream of the diameter change, and a region of sep

arated flow occurs from

the sharp corner of the diameter change and etends past

the vena contracta.

!ena contracta is the narrowest central flow region

of a (et that occurs (ust downstream to the

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orifice plate. It is characterized by high velocity

, laminar flow. )aminar flow, sometimes

&nown as streamline flow, occurs when a fluid flows

in parallel layers, with no disruption

between the layers.

The vena contracta refers to the point in the fluid stre

am where the diameter of

the streamlines is the smallest, and it occurs (ust slightly d

ownstream of the

orifice, where the flow is nearly horizontal and is

concentric with the orifice and

flow channel. After the vena contracta, the streamlines

diverge and a region of

separated flow occurs and etends past it.

A cross section of a circular orifice of diameter *o

is shown. The thic&ness of the wall is

assumed small compared to the diameter of the orifice.

*ue to the convergence of the

streamlines approaching the orifice, the cross section o

f the (et decreases slightly until thepressure is equalized over the cross+section, and the ve

locity profile is nearly rectangular.

This point of minimum area is called the

vena contracta

. eyond the vena contracta, friction

with the fluid outside the (et -air slows it down, an

d the cross section increases perforce.

This divergence is usually quite small, and the (et is ne

arly cylindrical with a constant/0

velocity. The (et is held together by surface tension, o

f course, which has a stronger effect the

smaller the diameter of the (et

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. #omment on the limitations and ma(or sources of err

or in this eperiment

)imitations of the eperiment1

+ The eperiment enables only a small range of flow m

easured. The

apparatus does not allow measurement over larger range.

+ This eperiment is conducted using water. Apparatus w

ill not wor& for fluid

with higher viscosity as very large energy will be requi

red.

+ The weighing tan& is of limited size, not large enou

gh to measure

rotameter reading of value lesser than 23. The water i

n tan& will overflow.

4a(or sources of errors1

5

The reading of manometer. The readings may not be acc

urate as it tends

to (ump, even after allowing some time before the rea

ding is ta&en. Thus

reading ta&en might not be the actual result.

5

The manometer is not totally transparent, due to prese

nce of water vapour

and bubbles in the tube, thus readings may not be accurat

e.

5

4easuring of a few meters using the same apparatus may

result in more

error in the measurements and calculation.

5

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6sage of stopwatch and weight to measure the mass flow

rate may not be

accurate due to human errors.

5

7aralla error reading from the apparatus may occur due to the colour of

the fluid used.

#onclusion

From the eperiment, we are able to recognize the wor

&ing mechanism of the

venturimeter, orifice meter and rotameter used to measu

re flow.

$e have also compared the advantages and disadvantages of

the three meters,as well as calculated the head loss from each type of meter

. From these, we are

able to better understand the principles behind the w

or&ing mechanism of each

meter, the efficiency of each meter and thus determine

wisely which meter is

better suited for various applications.

In this eperiement, the 8 devices + the venture me

ter, orifice meter and the rotameter has been close

ly

eamined.

A

lso, the coefficient of discharge , #d

, for the!

enturi meter and orifice meter is determined9 where

#d

for

the

!

enturi meter is 3.:;

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>A

? -3.3/::@ 3.88// /3%,

Futhermore, the energy losses in the

!

enturi meter, orifice meter, rotameter as well as t

he wide angle

diffuser and a :3BFelbow is determined.

A

ll in all, the ob(ectives of this eperiment are ac

hieved

)imitations of the eperiment1

+ The eperiment enables only a small range of flow m

easured as thepump=s power is limited and some of the apparatus do no

t allow

measurement over a larger range of flow rates.

+ This eperiment cannot be conducted with fluids of gre

ater viscosity as a

lot of energy will be required to pump the fluid aro

und the eperiment set

up. Also corrosive fluids or fluids at higher temperatur

es cannot beeperimented with as some of the connections and tubing

s are less

resistant to corrosion andC or high temperatures.

+ The weighing tan& is of limited size, and is not suitab

le to be used to time

greater mass flow rates of fluids and inaccuracies will ari

se if it fills up too

fast. Also, the water in tan& will overflow easily if the

flow is too high.

+ Also, the eperiment may not be suitable for dar& colo

ured or opaque

fluids as the rotameter float has to be visible to ta&e

that reading.

4a(or sources of errors1

5

The pressure of the A# pump is not constant and fluctuat

es quite a bit.

This will result in estimation errors while trying to r

ead the measurement

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values off the instruments. The readings will fluctuate,

even after allowing

the set up some time to settle. This problem may be solved

by using a

stabiliser for the pump pressure or a *# pump.5

#ondensation and dirt build up occurs inside the manome

ter tubes,

ma&ing them not totally transparent. This may cause inaccura

cies in the

readings, even if efforts are ta&en to reduce paralla e

rrors. #onducting

the eperiment in a dryer environment may reduce the r

is& ofcondensation.

5

6sing a hand stopwatch and manually loading weights to

measure the

mass flow rate may not be accurate due to human errors an

d bouncing of

the lever armature. It may be better to use an electron

ic timer attached to

the contact point between the arm and the frame and using mechanically

loaded weights or a calibrated spring to counter the

weight of the water.

Implementing this will ma&e the eperiment more accura

te but will also

increase the overall cost.

5

7aralla error reading from the apparatus may occur as

the manometerdiameter is small. Also, at the thic&ness, surface tension

effects may

become significant, resulting in inaccurate readings.

5

The tubings and connections may not be totally airtight

or watertight and

any lea&age of either fluid will result in inaccuracies i

n the reading ta&en.

#onclusionFrom this eperiment, we have become more familiar wit

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h the wor&ing

mechanisms of the various types of flow measuring devices such

as the !enturi

meter, orifice meter and rotameter. $e have also dete

rmined the coefficient ofdischarge of the !enturi meter and calibrated the rota

meter.

$e have also compared the advantages and disadvantages of

the three meters,

and calculated the head loss from each measuring apparat

us. From these, we

are able to better understand the principles behind t

he wor&ings of each meter,

and determine which meter is better suited for various applications. Thus, I feel

that this eperiment has been a satisfactory one.

"uman reaction time and paralla is one possible error.

b

$ater is used in this eperiment. Any other substances th

at have higher viscosity may not

be used. There still eist some air bubbles inside the tub

e that could have led to

inaccurate readings and affect subsequent calculations.

c

The flow rate in the eperiment is control by the pum

p which may not pump water at a

constant rate

.

d

Friction at the (oints of the lever system may cause

inaccuracies when weighing the

water.

e

The readings on the manometer are always fluctuating and

consistent results cannot be

obtained.

#OD#)6'IOD 1

The usage of the venture meter, orifice meter and the rotameter has been eamined. From the

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three, the venture meter gives us the most accurate re

ading and the rotameter involves the

most heat loss. $e can also see that the main reaso

n for any energy loss is due to turbulent

flow, friction and drag loss. In order to obtain an accurate reading, the pressure and velocity

difference had to be significant across the meters.

#oefficient of discharge for venturimeter and orifice

4eter,

#d

? /.3/;A

against Eotameter reading, we obtain the calibration curv

e equation,

>A

? -3.3/