Lecture 6 Manometers.pptx

FLUID
MECHANICS
LECTURENO. 6
PRESSURE
MEASUREMENT
(MANOMETERS)
PREP
AREDBY
:JAF
ARAALI
Jafar.dalo@koyauniversity.org
Oct. 2019

MEASU
REMEN
TOFPRESSU
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The pressure on a fluid is measured in different systems:
I. Absolute pressure: defined as the pressure which is
measured with reference to absolute vacuum
pressure.
II. Gaugepressure:defined asthepressurewhichis
measured with the help of a pressure measuring
instrument, in which the atmospheric pressure is taken
as datum. The atmospheric pressure on the scale is
marked as zero.

MEASU
REMEN
TOFPRESSU
RE
III. Vacuum pressure: is defined as the pressurebelow the
atmosphericpressure.
Absolute Pressure
A
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B
Gauge Pressure
Vacuum Pressure
Absolute Pressure
Absolute Zero Pressure
Pressure
Atmospheric pressure

MEASU
REMEN
TOFPRESSU
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Absolute pressure= Atmospheric pressure+ Gauge
pressure
Vacuum pressure= Atmospheric pressure – Absolute
pressure
The atmospheric pressure at sea level , 15 C is 101.3 kP
a.
The atmospheric pressure head is 760 mmof mercury or
10.33 mof water.

MEASU
RIN
GDEV
IC
ES
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The pressure of a fluid is measured by:
Manometers:defined as thedevices used for measuring
the pressure at a point in a fluid by balancing the column of
fluid by the same or another column of the fluid.
Mechanical Gauges:defined asthedevices usedfor
measuring the pressure by balancing the fluid column by the
spring or dead weight.

MANOMETERS
Manometers are classified as:
Simple Manometers
Differential Manometers
Piezometer U- tube manometer Single column manometer
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SIMPLEMAN
OMET
ER
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It consistsof a glasstube having oneof itsendsconnected
to a point where pressure is to be measured and other
end remains open to atmosphere.
The followings are types of simple manometer:
1. Piezometer
2. U- tube manometer
3. Single column manometer

PIEZOMETER
Piezometer tube is perhaps the simplest types of the pressure
measuring devices and consists of a vertical tube.
In its application one end is connected to the pressure to be
measured while the other end is open to the atmosphere.
The gauge pressure at A: PgA = pxgxh
The absolute pressure at A: PabA = pxgxh + Patm
This method can only be used for liquids (i.e. not for gases).
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U– T
U
BEMAN
OMET
ER
One of the earliest pressure measuring instruments is still in
wide use today because of its inherent accuracy and
simplicity of operation. It's the U-tube manometer, which is a
U-shaped glass tube partially filled with liquid
An example of U tube manometer is presented in the figure
below. It is located to measure the pressure at point A.
h1= Height of light liquid above the datum line.
h2 = Height of heavy liquid above the datum line
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G
AU
G
EAN
DV
AC
U
U
MPRESSU
RE
Bis the point at which pressure is to be measured, whose
value is P
.
datum line is A-A:
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U– T
U
BEMAN
OMET
ER
Gauge Pressure:
Pressure above A-A line in the left column = P+p1 x g x h1
Pressure above A-A line in the right column = p2 x g x h2
By equating the two pressures P+p1 x g x h1= p2 x g x h2
P= (p2 x g x h2 - p1 x g x h1)
Vacuum Pressure:
Pressure above A-A line in the left column = p2 x g x h2 + p1 x g
x h1+ P
Pressure above A-A line in the right column= 0
By equating the two pressures p2 x g x h2 + p1 x g x h1+ P=0
P= - (p2 x g x h2 + p1 x g x h1) L
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SIN
G
LEC
OL
U
MNMAN
OMET
ER
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Single column manometer is a modified form of a U-tube
manometer in which one side is a large reservoir and the
other side is a small tube, open to the atmosphere.
There are two types of single column manometer:
1. Vertical single column manometer.
2. Inclined single columnmanometer.

VER
T
IC
ALSIN
G
LEC
OL
U
MNMAN
OMET
ER
h1 = Height of the centre of pipe above XX.
h2 = Rise of heavy liquid in the right limb.
Δ h = Fall of mercury in a reservoir.
S1 = Specific gravity of the liquid in pipe.
S2 = Specific gravity of heavy liquid in reservoir and
right limb.
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• ρ1 = Density of liquid in pipe.
• ρ2 = Density of heavy liquid in reservoir and right limb.
• P= Pressure to measured at point A.
• A= Cross sectional area of the reservoir.
• a = Cross sectional area of the right limb.
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V
ER
T
IC
ALSIN
G
LEC
OL
U
MNMAN
OMET
ER

• Fall of heavy liquid in reservoir will cause a rise of heavy liquid
level in right limb
A x Δ h = a h2
Δ h = a h2
A
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• Considering the datum line YY
• Pressure in the right limb above YY
• Pressure in the left limb above YY
= ρ2 * g * (Δ h + h2 )
= P + ρ1 * g *(Δ h + h1 )
• Equating both and substituting Δ h from the above equation and
neglecting a/A ratio, the Pressure at the point A is given as
P = (ρ2 * g x * h2) - (ρ1 * g * h1)

INCLINESSINGLEC
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NMANOMETER
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DIFFEREN
T
IALMAN
OMET
ER
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Differential Manometersare devices usedform measuring
the difference of pressure between two points in a pipe or
in two different pipes. A differential manometer consists of
a U- tube, containing a heavy liquid, whosetwo endsare
connectedto thepoints,whichdifference of pressureis to
be measure.
Most commonly types of differential manometers are:
1. U-tube differential manometer.
2. Inverted U-tube differential manometer.

U
-T
U
BEDIFFEREN
T
IALMAN
OMET
ER
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U– T
U
BEDIFFEREN
T
IALMAN
OMET
ER
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U– T
ubeDifferentialManometer
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IN
V
ER
T
EDU
-T
U
BEMAN
OMET
ER
It consists of inverted U-tube, containing a light liquid. The
two endsof thetube are connectedto thepoints whose
difference of pressure is to be measured. It is used for
measuring differences of low pressures.
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InvertedU
-T
ubeDifferentialManometer
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Lecture 6 Manometers.pptx

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