FACULTY OF MECHANICAL ENGINEERING
SKMM 1912 EXPERIMENTAL METHODS
UNIVERSITI TEKNOLOGI MALAYSIA
MEASUREMENT OF
PRESSURE
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MEASUREMENT OF PRESSURE
- SOME DEFINITIONS
When a force is applied perpendicular to a surface
area, it exerts pressure on that surface equal to the ratio
of F to A, where F is the force and A is the surface
area.
Hence, the formula for pressure (p) is p = F/A.
The principle SI unit is called a Pascal (Pa),
(Pa) or 1 N/m2.
Other units used – lbf/in2, kgf/cm2, tonf/in2, kp/cm2,
i H2O,
inH
O inHg,
i H dyne/cm
d / 2, torr
t
3
MEASUREMENT OF PRESSURE
-UNITS
UNITS FOR PRESSURE
There are also two other specialized units of pressure
measurement in the SI system:
y
the bar, equal to 105 Pa, and
the torr, equal to 133 Pa.
The torr,, once known as the "millimeter of mercury,"
y, is
equal to the pressure required to raise a column of
mercury (chemical symbol Hg) 1 mm. It is named for
the
h Italian
li physicist
h i i Evangelista
li Torricelli
i lli (1608-1647),
(
)
who invented the barometer.
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MEASUREMENT OF PRESSURE
-UNITS
UNITS FOR PRESSURE
5
MEASUREMENT OF PRESSURE
-Atmospheric
At
h i P
Pressure
Atmospheric pressure is pressure caused by the weight of
the atmosphere.
At sea level it has a mean value of one atmosphere (1 atm)
= 760 mm of mercuryy = 14.70 lbs per
p square
q
in. = 101.35
kilopascals = 100 kN/m2 = 1 bar
Pressure reduces with increasing altitude
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MEASUREMENT OF PRESSURE
-Pressure Change
This plastic bottle was closed at approximately 2000m
altitude, then brought back to sea level. As a result, air
pressure crushes
h it.
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MEASUREMENT OF PRESSURE
- Vacuum
V
An outer-space
outer space in which there is no matter or in which the pressure is so
low that any particles in the space do not affect any processes being
carried on there. It is a condition well below normal atmospheric
pressure
The most nearly perfect vacuum exists in intergalactic space, where it is
estimated that on the average there is less than one molecule per cubic
meter
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MEASUREMENT OF PRESSURE
- Absolute,, Gauge
g and Differential
Pressure
Absolute pressure of a fluid is referenced against a
perfect
f vacuum
Gauge
g p
pressure ((eg.
g as read by
y barometer)) is
referenced against ambient air pressure, so it is equal
to absolute pressure minus atmospheric pressure.
Differential pressure is the difference in pressure
between two points.
points
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MEASUREMENT OF PRESSURE
- PRESSURE DIAGRAM
10
MEASUREMENT OF PRESSURE METHOD TO MEASURE
By elastic deformation
„
Pressure creates force which in turn deforms
the elastic material
By
y liquid
q
column
Hydrostatic pressure is the pressure due to the
weight
g of a fluid.
„ p = ρgh
„
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ELASTIC DEFORMATION –
B
Bourdon
d T
Tube
b
12
A Bourdon gauge uses a coiled tube which as it
expands
p
due to ppressure increase causes a rotation
of an arm connected to the tube.
Coiled tube made of
copper
pp
Mechanical
movement linked by
rack and pinion
Range = 100kN/m2
– 300MN/m2
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ELASTIC DEFORMATION –
Di h
Diaphragm
((membrane)
b
)B
Based
d
LVDT = linear variable
displacement transducer
Spring
p g can be used
instead of LVDT and
attached to force (F)
measuring device
p = kx/A
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ELASTIC DEFORMATION –
Diaphragm
i
Based
INDUSTRIAL PRESSURE DIAPHRAGM ASSEMBLY
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ELASTIC DEFORMATION –
Di h
Diaphragm
B
Based
d
Diaphragm Shape
¾fl t for
¾flat
f low
l pressure measurementt or
¾Corrugated for high pressure measurement
Material = Copper-beryllium
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ELASTIC DEFORMATION –
B ll
Bellows
Based
B d
For extremely low
ppressure measurement
(0.5 – 75psig to a
1000psigmax)
psig = pound per square
inch gauge
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ELASTIC DEFORMATION –
A
Aneroid
id G
Gauge
FITTED
WITH
BELLOW
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ELASTIC DEFORMATION –
Aneroid Concept
An enclosed and sealed bellows chamber, called an
aneroid, which means "without liquid".
q
The important component is a sealed chamber made of
thin metal in semi vacuum state
The chamber shrinks when atmospheric pressure
increases and expands when atmospheric pressure
increases,
reduces
Can be
b used
d as altimeter.
li
19
LIQUID COLUMN –
C
Concept
t
Liquid column gauges consist of a
vertical column of liquid in a tube whose
ends are exposed to different pressures.
The column will rise or fall until its
weight is in equilibrium with the
pressure differential between the two
ends of the tube
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LIQUID COLUMN –
H d t ti Pressure
Hydrostatic
P
21
LIQUID COLUMN –
Ut b M
U-tube
Manometer
t
22
LIQUID COLUMN –
U Tube Manometer
U-Tube
Equating the pressure at the level X'(pressure
at the same level in a continuous body of fluid
is equal)
LHS Px = P1 + ρg(a+h)
LHS;
( +h)
RHS; Px' = P2 + ρga + ρmgh
Since Px = Px'
P1 + ρg(a+h)
( +h) = P2 + ρga + ρmghh
P1 - P2 = ρmgh - ρgh
i.e. P1 - P2 = (ρm - ρ)gh.
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LIQUID COLUMN –
W ll T
Well-Type
Manometer
M
t
24
LIQUID COLUMN –
Well-Type Manometer
p2
p1
hc
hw
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LIQUID COLUMN –
Well-Type Manometer
p2
LHS: p1 – ρg
ρghw
RHS: p2 + ρghc
p1
hc
hw
(Measuring range = 1bar – 1.5bar)
LHS = RHS
p1 – ρg
ρghw = p2 + ρg
ρghc
p1 – p2 = ρghc + ρghw
By constant volume, hwAw = hcAc ⇒ hw/hc = Ac/Aw
Therefore
p1 – p2 = ρghc (1+Ac/Aw)
= ρghc when Aw>>Ac
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LIQUID COLUMN –
Inclined Manometer
27
LIQUID COLUMN –
I li d M
Inclined
Manometer
t
28
LIQUID COLUMN –
I li d M
Inclined
Manometer
t
29
LIQUID COLUMN –
I li d M
Inclined
Manometer
t
Th inclined
The
i li d version
i is
i usedd for
f better
b tt sensitivity.
iti it
Measuring range = 0.1bar – 30mbar
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PIEZOMETER
31
MERCURY BAROMETER
A barometer
b
t is
i an instrument
i t
t usedd to
t
measure atmospheric pressure.
A standard mercury barometer has a
glass column of about 30 inches (about
76 cm) in height,
height closed at one end,
end with
ith
an open mercury-filled reservoir at the
base.
base
32
MERCURY BAROMETER
Mercury iin the
M
th tube
t b adjusts
dj t until
til the
th weight
i ht off
the mercury column balances the atmospheric
force exerted on the reservoir.
reservoir
High atmospheric pressure places more force on
the reser
reservoir,
oir forcing mercury
merc r higher in the
column.
Low pressure allows the mercury to drop to a
lower level in the column by lowering the force
placed on the reservoir.
reservoir
33
MERCURY BAROMETER
34
MANOMETER LIQUID
SELECTION
Low viscosity
Low
ow coe
coefficient
c e of
o expansion
e p so
Low vapour pressure
L cohesiveness
Low
h i
Non corrosive
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MANOMETER LIQUID IN USE
Mercury
W e
Water
Kerosene
Al h l
Alcohol
Ethyl
Benzene
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FACULTY OF MECHANICAL ENGINEERING
THE END
UNIVERSITI TEKNOLOGI MALAYSIA
37