Albaha University
Faculty of Engineering
Mechanical Engineering
g
g
Department
MEASURING INSTRUMENTS
AND CALIBRATION
Lecture (10)
Pressure measurement
By:
Ossama Abouelatta
o abouelatta@yahoo com
[email protected]
Mechanical Engineering Department
Faculty of Engineering
Albaha Universityy
2013
Measuring Instruments and Calibration
Dr. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
AIMS
This lecture aims:

to explain some terms used in pressure measurement.

to define the difference between absolute pressure, gauge
pressure and differential pressure.

to identify the principles of pressure measurement.

to differentiate between Diaphragms,
p
g , Capacitive
p
pressure
p
sensor, Fibre-optic pressure sensors, Bellows, Bourdon
tube,, Manometers,, Resonant-wire devices,, and Deadweight gauge.

to select the suitable pressure sensors.
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(2)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
OUTLINE

Introduction

Diaphragms

Capacitive pressure sensor

Fibre-optic pressure sensors

Bellows

Bourdon tube

Manometers

Resonant-wire devices

Dead weight gauge
Dead-weight

Selection of pressure sensors
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(3)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
BELLOWS


The bellows, schematically
Scale
illustrated in the figure, is
another elastic-element type of
pressure sensor that operates Helical Spring
on very similar principles to
the
diaphragm
pressure
sensor.
Pressure changes within the
Pinion
bellows, which is typically
Gear
fabricated as a seamless tube
of either metal or metal alloy,
produce translational motion
of the end of the bello
bellowss that
can
be
measured
by
Pressure
capacitive, inductive (LVDT)
Connection
or p
potentiometric transducers.
Sector
Connecting
Link
Spring
Bellows
Cast
Absolute, gauge,
vacuumed pressure.
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(4)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
BELLOWS





Different versions can measure either
absolute pressure (up to 2.5 bar) or gauge
pressure (up to 150 bar).
Double-bellows versions also exist that are
designed to measure differential pressures
off up to 30 bar.
b
Bellows have a typical measurement
uncertainty of only ±0.5%, but they have a
relatively high manufacturing cost and are
prone to failure.
Their principal attribute in the past has
been
their
greater
g
measurement
sensitivity compared with diaphragm
sensors.
However, advances in electronics mean
that
h the
h high-sensitivity
hi h
i i i requirement
i
can
usually be satisfied now by diaphragmtype devices, and usage of bellows is
therefore falling.
Electrical output
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(5)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
BOURDON TUBE






The Bourdon tube is also an elastic
element type of pressure transducer.
It is relatively cheap and is commonly used
for measuring the gauge pressure of both
gaseous and liquid fluids.
I consists
It
i
off a specially
i ll shaped
h
d piece
i
off
oval-section, flexible, metal tube that is
fixed at one end and free to move at the
other end.
When pressure is applied at the open,
fixed end of the tube, the oval cross-section
becomes more circular.
In consequence, there is a displacement of
the free end of the tube. This displacement
is measured by some form of displacement
t
transducer,
d
which
hi h iis commonly
l
a
potentiometer or LVDT.
Capacitive and optical sensors are also
sometimes
used
to
measure
the
displacement.
Bourdon tube
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(6)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
BOURDON TUBE




The three common shapes of Bourdon tube are shown in the figure.
C-type
C
type tubes are available for measuring pressures up to 6000 bar.
bar
Measurement inaccuracy is typically quoted at ±1% of full-scale
deflection.
Similar
Si
il
accuracy
is
available from
helical
and
spiral types, but
whilst
the
measurement
resolution
is
higher,
the
maximum
pressure
measurable
is
only 700 bar.
Bourdon tube
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(7)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
MANOMETERS
Manometers are passive instruments that give
a visual indication of pressure values.
yp exist are:
Various types
 U-tube manometer.
 Well-type
Well type or cistern manometer.
manometer
 Inclined type.

Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(8)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
MANOMETERS: U
U-TUBE MANOMETER

The U-tube manometer, shown in the figure, is the most common form of
pp
p
pressure causes a displacement
p
of liquid
q
inside the
manometer. Applied
U-shaped glass tube, and the output pressure reading P is made by
observing the difference h between the level of liquid in the two halves of
the tube A and B, according to the equation P = hg, where  is the
specific gravity of the fluid. If an unknown pressure is applied to side A,
and side B is open to the atmosphere, the
output
reading
is
gauge
pressure.
Alternatively, if side B of the tube is sealed
and evacuated, the output reading is absolute
pressure. The U-tube manometer also
measures the differential pressure (p1 - p2),
according to the expression (p1 - p2) = hg, if
two unknown pressures p1 and p2 are applied
respectively
i l to sides
id A and
d B off the
h tube.
b
U t b manometer
U-tube
t
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(9)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
MANOMETERS: U
U-TUBE MANOMETER


Output readings from U-tube manometers are subject to error,
principally
p
p y because it is very
y difficult to jjudge
g exactly
y where the
meniscus levels of the liquid are in the two halves of the tube. In
absolute pressure measurement, an addition error occurs because it is
impossible to totally evacuate the closed end of the tube.
U-tube manometers are typically used to
measure gauge and differential pressures up
to about 2 bar.
U t b manometer
U-tube
t
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(10)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
MANOMETERS: U
U-TUBE MANOMETER


Advantages:
Pb
Pa
 Easy
E
tto make
k
 Best suited to static pressure measurement
 Reasonably precise.
precise
 Cheap
Disadvantages:
ρm
 Depends on available fluid densities SpG from 0.7 to
13.456 (Mercury).
 Liquid may squirt out with step changes in pressure.
 Mercury is toxic!.
 Difficult to use for small pressure changes, unsuitable
for very large pressures.
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(11)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
MANOMETERS: U
U-TUBE MANOMETER
Example (1):
The shown
Th
h
U-Tube
UT b M
Manometer employs
l
a
3
Mercury (ρmercury =13600 kg/m ) was used Water
for measuring the differential pressure
across a Venturi
V t i meter
t
th
through
h which
hi h
water is flowing. Calculate the pressure
difference. Assuming density of the water
ρwater= ρf = 1000 kg/m3.
h (cm)
Manometer
Solution:
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(12)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
MANOMETERS: U
U-TUBE MANOMETER
Example (1):
The shown
Th
h
U-Tube
UT b M
Manometer employs
l
a
3
Mercury (ρmercury =13600 kg/m ) was used Water
for measuring the differential pressure
across a Venturi
V t i meter
t
th
through
h which
hi h
water is flowing. Calculate the pressure
difference. Assuming density of the water
ρwater= ρf = 1000 kg/m3.
h (cm)
Manometer
Solution:
Pa –Pb = (ρm – ρf) g.h
= (13600-1000)*9.8*(6/100)
(13600 1000)*9 8*(6/100)
= 7408.8 N/m2 (Pa)
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(13)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
MANOMETERS: U
U-TUBE MANOMETER
Example (2):
The shown
Th
h
U-Tube
UT b M
Manometer
t employs
l
a special
i l
oil having a specific pressure of 0.82 for the
manometer fluid. One side of the manometer is
open to local atmosphere
ope
at osp e e pressure
p essu e and
a d tthee
difference in column height is measured as 20 cm.
Standard acceleration gravity is present. Calculate
the pressure of the air source in Pascals.
Solution:
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(14)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
MANOMETERS: U
U-TUBE MANOMETER
Example (2):
The shown
Th
h
U-Tube
UT b M
Manometer
t employs
l
a special
i l
oil having a specific pressure of 0.82 for the
manometer fluid. One side of the manometer is
open to local atmosphere
ope
at osp e e pressure
p essu e and
a d tthee
difference in column height is measured as 20 cm.
Standard acceleration gravity is present. Calculate
the pressure of the air source in Pascals.
Solution:
ρm = 0.82 ρwater = 0.82 * 1000 = 820 kg/m3
The fluid in this problem is the ρair 1 kg/m3
The local atmosphere pressure is 1 Patm = 1.013 x 105 Pa
Pa –Pb = (ρm – ρf) g.h
(P-Pa) = (820-1)*9.8*0.2 = 1605.24 Pa
P = 1605.24 + Pa = 1605.24 + 1.013 x 105 Pa
= 1.029 x 105 Pa
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(15)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
MANOMETERS: WELL-TYPE OR CISTERN MANOMETER

The well-type or cistern manometer, shown in the figure, is similar to a Utube manometer but one half of the tube is made very large so that it
forms a well. The change in the level of the well as the measured
pressure varies is negligible. Therefore, the liquid level in only one tube
has to be measured,, which makes the instrument much easier to use
than the U-tube manometer. If an unknown pressure p1 is applied to port
A, and port B is open to the atmosphere, the gauge pressure is
given by
g
y p1 = h. It might
g appear
pp
that the instrument would give a
better measurement accuracy
than the U-tube manometer
because the need to subtract two
liquid level measurements in
order to arrive at the pressure
value is avoided.
Manometers: well type.
type
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(16)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
MANOMETERS: INCLINED TYPE

The inclined manometer or draft gauge, shown in the figure, is a
variation on the well-type manometer in which one leg of the tube
is inclined to increase measurement sensitivity. However, similar
comments to those above apply about accuracy.
Pa –Pb = (ρm – ρf) g.L.sin 
L
Manometers: well type.
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(17)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
RESONANT-WIRE DEVICES


A typical resonant-wire device is shown schematically in the figure. Wire
is stretched across a chamber containing fluid at unknown pressure
subjected to a magnetic field.
The wire resonates at its natural frequency according to its tension, which
varies with pressure.
pressure Thus pressure is calculated by measuring the
frequency of vibration of the wire. Such frequency measurement is
normally carried out by electronics
integrated into the cell.
cell These
devices are highly accurate, with a
typical inaccuracy figure being
±0 2% full-scale reading
±0.2%
reading. They are
also particularly insensitive to
ambient condition changes and
can measure pressures between
5 mbar and 2 bar.
Resonant-wire device.
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(18)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
DEAD-WEIGHT GAUGE


The dead-weight gauge, as shown
in the figure, is a null
null-reading
reading
type of measuring instrument in
which weights are added to the
piston p
p
platform until the p
piston is
adjacent to a fixed reference mark,
at which time the downward force
of the weights on top of the piston
is balanced by the pressure
exerted by the fluid beneath the
piston.
The fluid pressure is therefore
calculated in terms of the weight
added to the platform and the
known area of the piston.
Deadweight pressure gauge.
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(19)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
DEAD-WEIGHT GAUGE

The instrument offers the ability to measure pressures to a high degree of
accuracy but is inconvenient to use. Its major application is as a reference
instrument against which other pressure-measuring devices are
calibrated. Various versions are available that allow measurement of
gauge
g
g p
pressures up
p to 7000 bar.
Deadweight pressure gauge.
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(20)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
DEAD-WEIGHT GAUGE
1 - Hand pump
2 - Testing pump
3 - Pressure
P
gauge to be
b calibrated
lib
d
4 - Calibration weight
5- W
Weight
g support
pp
6 - Piston
7 - Cylinder
8 - Filling connection
Deadweight pressure tester.
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(21)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
SELECTION OF PRESSURE SENSORS




Choice between the various types of instrument available for
measuring mid-range
mid range pressures (1
(1.013–7000
013 7000 bar) is usually
strongly influenced by the intended application.
Manometers are commonly used when just a visual indication of
pressure level is required.
Deadweight gauges are commonly used because of their superior
accuracy are used in calibration procedures of other pressure
accuracy,
pressuremeasuring devices.
When an electrical form of output is required, the choice is
usually
ll either
ith one off th
the severall ttypes off di
diaphragm
h
sensor (strain
( t i
gauge, capacitive or fibre optic) or, less commonly, a Bourdon
tube.
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(22)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
SELECTION OF PRESSURE SENSORS



Bellows-type instruments are also sometimes used for this
purpose but much less frequently.
purpose,
frequently If very high measurement
accuracy is required, the resonant-wire device is a popular choice.
In the case of pressure measurement in the vacuum range (less
than atmospheric pressure, i.e. below 1.013 bar), adaptations of
most of the types of pressure transducer can be used.
Special forms of Bourdon tubes measure pressures down to 10
mbar, manometers and bellows-type instruments measure
pressures down to 0.1 mbar, and diaphragms can be designed to
measure pressures down to 0.001
0 001 mbar
mbar.
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(23)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University
THANK YOU
Ossama Abouelatta
Mechanical Engineering Department
F
Faculty
lt off Engineering
E i
i
Albaha University
Albaha, KSA
email: [email protected]
Lecture (10): Pressure measurement
Experimental Engineering and Measurement
(24)
Assoc. Prof. Ossama Abouelatta, Department of Mechanical Engineering , Faculty of Engineering, Albaha University