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Comment on the relative advantages and disadvantages of

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

From table 3, it is observed that loss factor Kv is the smallest.

This suggests that the venturimeter may give more accurate measurement as

compared to that of orifice plate.

its experimental flow rates that are quite close to the theoretical value (d

= 1.0178 1).

This implies that it is also able to measure a greater range of values of flow rate

as the will be less energy lost at high flow velocities.

The meter has a smooth internal surface that helps to reduce friction as the water

flows through it.

!oreover, the gradual reduction and expansion in the diameter of the tube

reduces this loss.

"owever, to cater for the gradual reduction and expansion, a longer tube is

required and the #enturimeter occupies quite a lot of space($%& ratio ofapproximate ') and is an expensive device as it is more difficult to construct

compared to the other device.

*t also cannot be altered for measuring pressure beyond a maximum velocity.

The orifice plate meter is fairly accurate in its measurement

*t is also the easiest to construct.

+rifice 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.

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

diameter causes high loss factor due to turbulent flow.

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 hori-ontal should not be too large, so as to reduce energy loss and improve

accuracy, but that would demand even more hori-ontal space.

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Comment on the head losses associated with all the flow meters

studied 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 expansion

of diameter.

The head loss is thus caused by the dissipation of energy via the friction betweenthe fluid and the inner surface of the pipe.

*t has been observed that the head loss in the venturimeter is relatively

small relative to the orifice plate.

*n the #enturi meter, the gradual reduction and expansion 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 to

inefficient mixing and separate flow

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

the apparatus. #+is observed to be very high too.

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

and expansion 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

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"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 experiencing 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 isthe least. The contraction ta/es place at a section slightl

y downstream from the

orifice, where the fluid flow is hori-ontal. 0treaml

ines will converge 1ust

downstream of the diameter change, and a region of sep

arated flow occurs from

the sharp corner of the diameter change and extends past

the vena contracta.

#ena contracta is the narrowest central flow region

of a 1et that occurs 1ust downstream to the

orifice plate. *t is characteri-ed 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 1ust slightly d

ownstream of the

orifice, where the flow is nearly hori-ontal and is

concentric with the orifice and

flow channel. fter the vena contracta, the streamlines

diverge and a region of

separated flow occurs and extends past it.

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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 1et decreases slightly until the

pressure is equali-ed over the cross2section, 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 1et (air) slows it down, an

d the cross section increases perforce.

This divergence is usually quite small, and the 1et is ne

arly cylindrical with a constant

45

velocity. The 1et is held together by surface tension, o

f course, which has a stronger effect the

smaller the diameter of the 1et

. omment on the limitations and ma1or sources of err

or in this experiment

$imitations of the experiment6

2 The experiment enables only a small range of flow measured. The

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apparatus does not allow measurement over larger range.

2 This experiment is conducted using water. pparatus w

ill not wor/ for fluid

with higher viscosity as very large energy will be requi

red.2 The weighing tan/ is of limited si-e, not large enou

gh to measure

rotameter reading of value lesser than 7. The water i

n tan/ will overflow.

!a1or sources of errors6

8

The reading of manometer. The readings may not be acc

urate as it tends

to 1ump, even after allowing some time before the reading is ta/en. Thus

reading ta/en might not be the actual result.

8

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.

8

!easuring of a few meters using the same apparatus mayresult in more

error in the measurements and calculation.

8

9sage of stopwatch and weight to measure the mass flow

rate may not be

accurate due to human errors.

8

:arallax error reading from the apparatus may occur du

e to the colour ofthe fluid used.

onclusion

From the experiment, we are able to recogni-e 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

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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.

*n this experiement, the 3 devices 2 the venture me

ter, orifice meter and the rotameter has been close

ly

examined.

lso, the coefficient of discharge , d

, for the

#

enturi meter and orifice meter is determined; where

d

for

the

#

enturi meter is .4, while ?d

for the orifice meter is .3

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measurement over a larger range of flow rates.

2 This experiment cannot be conducted with fluids of gre

ater viscosity as a

lot of energy will be required to pump the fluid aro

und the experiment setup. lso corrosive fluids or fluids at higher temperatur

es cannot be

experimented with as some of the connections and tubing

s are less

resistant to corrosion and% or high temperatures.

2 The weighing tan/ is of limited si-e, 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 toofast. lso, the water in tan/ will overflow easily if the

flow is too high.

2 lso, the experiment may not be suitable for dar/ colo

ured or opaque

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

that reading.

!a1or sources of errors6

8

The pressure of the pump is not constant and fluctuates quite a bit.

This will result in estimation errors while trying to r

ead the measurement

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.

8 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 parallax e

rrors. onducting

the experiment in a dryer environment may reduce the r

is/ of

condensation.8

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9sing 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. *t may be better to use an electronic timer attached to

the contact point between the arm and the frame and u

sing mechanically

loaded weights or a calibrated spring to counter the

weight of the water.

*mplementing this will ma/e the experiment more accura

te but will also

increase the overall cost.

8 :arallax error reading from the apparatus may occur as

the manometer

diameter is small. lso, at the thic/ness, surface tension

effects may

become significant, resulting in inaccurate readings.

8

The tubings and connections may not be totally airtight

or watertight and

any lea/age of either fluid will result in inaccuracies in the reading ta/en.

onclusion

From this experiment, we have become more familiar wit

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 of

discharge of the #enturi meter and calibrated the rotameter.

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 a

pplications. Thus, * feelthat this experiment has been a satisfactory one.

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)

"uman reaction time and parallax is one possible error.

b)

ater is used in this experiment. ny other substances th

at have higher viscosity may not

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

e that could have led to

inaccurate readings and affect subsequent calculations.

c)

The flow rate in the experiment is control by the pum

p which may not pump water at a

constant rate

.

d)

Friction at the 1oints 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.

+E$90*+E 6The usage of the venture meter, orifice meter and the r

otameter has been examined. From the

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. *n order to obtain an acc

urate reading, the pressure and velocitydifference had to be significant across the meters.

oefficient of discharge for venturimeter and orifice

!eter,

d

A 4.4=>

A .'

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