UNIVERSITY MALAYSIA SABAH
SCHOOL OF ENGINEERING & INFORMATION TECHNOLOGYCHEMICAL ENGINEERING PROGRAM
SEMESTER 1, 2010 / 2011
KC 20503 CHEMICAL PROCESS PRINCIPLES
TITLE :\
Assignment 1
Pressure Measurements Devices
GROUP 1 MEMBERS : Kenny Then Soon Hung
(BK09110098)Chin Chung Fui
(BK09110026)Scott Biondi R Valintinus
(BK09110151)Jenefer Tan Phaik Yee
(BK09110120)Ermieza Sinin
(BK09160211)Clarice Vencislaus Binjinol
(BK09110005)Norhayati Binti Asgan
(BK09110204)
DATE : 18th October 2010
LECTURER : Dr. S.M. Anisuzzaman
TABLE OF CONTENTS :
Contents Page
1.0 Elastic-element method
(a) Aneroid Gauge
(b) Bourdon Gauge
(c) Diaphragm Gauge
2.0 Liquid Column method
(a) Barometer
(b) Manometer
(c) McLeod Gauge
3.0 Electric method
(a) Dead Weight Tester
(b) Piezoelectric
(c) Strain Gauge
4.0 References
ASSIGNMENT 1 :
Show images / pictures of pressure measurement devices and explain how they
work.
1.0 Elastic-element method
(a) Aneroid Gauge
Aneroid means “with no fluid” which means aneroid gauges can be used for
liquid or gas pressure measurement even without the presence of liquid itself.
They are based on a metallic pressure sensing element which flexes elastically
under the effect of a pressure difference across the element. Another name for
aneroid gauge is mechanical gauge. 1
Although aneroid gauges are
mostly known as mechanical gauges
in the modern world, they are still
basically the same thing.
Aneroid gauge does not affected by the type of gas that is being measured
and less probable to contaminate the system. There are many types of pressure
sensing element for aneroid gauges such as Bourdon tube, a diaphragm, a
capsule, or a set of bellows which has different function according to the desired
region. 2
1 Aneroid. 1st October 2010. http://dictionary.reference.com/browse/aneroid2 Pressure Measurement. 2nd October 2010. http://en.wikipedia.org/wiki/Pressure_measurement
An example of aneroid gauge with a
bourdon type of pressure sensing
element
The pressure sensing element is connected with a needle as an indicator
which it moves when the pressure sensing element deflected as a result of a
pressure change and this deflection is mechanically amplified, by using a suitable
gear and linkage mechanism, and indicated on the calibrated dial. 3
The needle deflects to the corresponding
pressure making pressure measurement
easier.
Some may have a secondary transducer; a device that converts one type of
energy to another. The most popular secondary transducers in current vacuum
gauges evaluate a change in capacitance due to the mechanical deflection. 4
The cuff interface connects may
connect to many pressure measurement
resources such as secondary
transducers, pressure bladder, gas
connector etc.
3 Aneroid Gauge. 1st October 2010. http://www.brighthub.com/engineering/civil/articles/43777.aspx4 Aneroid Gauge. 1st October 2010. http://lsda.jsc.nasa.gov/books/skylab/appAIc8.html
(b) Bourdon Gauge
Figure 1: The Bourdon Gauge
The Bourdon gauge is shown in figure 1. It works on the same principle as
that of the snakelike, paper party whistle you get at a New Year party, which
straightens when you blow into it. Within the Bourdon gauge is a thin-walled metal
tube, somewhat flattened and bent into the form of a C. Attached to its free end is
a lever system that magnifies any motion of the free end of the tube. On the fixed
end of the gauge is a fitting you thread into a boiler system. As pressure increases within
the boiler, it travels through the tube. Like the snake like paper whistle, the metal
tube begins to straighten as the pressure increases inside of it. As the tube
straightens, the point moves around a dial that indicates the pressure in psi.
(c) Diaphragm Gauge
Diaphragm gauges is a device generally used to measure air pressure in the
space between the inner and outer boiler casings. It used a diaphragm with a
known pressure to measure pressure in a fluid. Diaphragm gauges are very
sensitive and give reliable indication of small differences in pressure. Thus, it
has many uses, such as monitoring pressure of a canister of gas, measuring
atmospheric pressure, or recording the strength of the vacuum in a vacuum pump.
This mechanism consists of a tough, pliable, neoprene rubber membrane
connected to a metal spring that is attached by a simple linkage system to the
gauge pointer. The diaphragm has a flexible membrane with two sides. One side is
an enclosed capsule containing air or some other fluid at a predetermined
pressure. The other side can be left open to the air or screwed in to whatever
system the gauge is meant to measure. Besides that, the diaphragm also attaches
to some sort of meter, which shows the intensity of pressure. - When pressure is
applied to the diaphragm, it moves and, through a linkage system, moves the
pointer to a higher reading on the dial.5
A fluid in contact with a flexible membrane pushes on that membrane,
bending it. The pressure is a measure of how hard it pushes. When the outside
preference is low, the reference pressure bends the membrane out. However, as
the outside pressure increases, it pushes back on the membrane, bending it back
the other way. By measuring how far the membrane bends, the gauge can detect
the outside pressure.
Actually, there are many different ways to measure the pressure from a
dynamic pressure gauge.The simplest ones is to attach a needle to the gauge. As
the pressure increases, it pushes on the needle, moving it up and down along a
dial which shows the pressure. Another way is to use an electric resistance strain
gauge. An electric resistance strain gauge uses a long strip of an electric resistor
(a device that resists the flow of electricity). The resistor is attached to the
diaphragm. When the diaphragm bends, it stretches out the resistor, increasing
the resistance. The resistor has an electric current running through it. The more
the diaphragm bends and increases the resistance, the more the current drops. By
measuring the electric current, the gauge can determine how far the diaphragm
has bent, and thus, how much pressure the outside air is creating.6
5 Diaphragm Gauge. 5th October 2010. http://www.tpub.com/content/fc/14104/css/14104_234.htm6 Diaphragm Pressure Sensors. 5th October 2010. http://www.efunda.com/DesignStandards/sensors/diaphragm \ diaphragm_intro.cfm
2.0 Liquid Column method
(a) Barometer
A barometer is a scientific instrument used to measure atmospheric pressure. It
can measure the pressure exerted by the atmosphere by using water, air, or
mercury. Pressure tendency can forecast short term changes in the weather.
Numerous measurements of air pressure are used within surface weather analysis
to help find surface troughs, high pressure systems, and frontal boundaries.
There are two main types of barometers. The most widely available and
reliable Mercury Barometers, or the newer digital friendly Aneroid Barometer.
The classic mercury barometer is typically a glass tube about 3 feet high
with one end open and the other end sealed. The tube is filled with mercury. This
glass tube sits upside down in a container, called the reservoir, which also contains
mercury. The mercury level in the glass tube falls, creating a vacuum at the top.
The first barometer of this type was devised by Evangelista Torricelli in 1643. 7
The barometer works by balancing the weight of mercury in the glass tube
against the atmospheric pressure just like a set of scales. If the weight of mercury
is less than the atmospheric pressure, the mercury level in the glass tube rises. If
the weight of mercury is more than the atmospheric pressure, the mercury level
falls.
Atmospheric pressure is basically the weight of air in the atmosphere above
the reservoir, so the level of mercury continues to change until the weight of
mercury in the glass tube is exactly equal to the weight of air above the reservoir.
7 Barometer. 3rd October 2010. http://en.wikipedia.org/wiki/Barometer
In areas of low pressure, air is rising away from the surface of the earth
more quickly than it can be replaced by air flowing in from surrounding areas. This
reduces the weight of air above the reservoir so the mercury level drops to a lower
level. 8
In contrast, in areas of high pressure, air is sinking toward the surface of the
earth more quickly than it can flow out to surrounding areas. There is more air
above the reservoir, so the weight of air is higher and the mercury rises to a
higher level to balance things out.
Changes in atmospheric pressure are one of the most commonly used ways
to forecast changes in the weather because weather patterns are carried around in
regions of high and low pressure. Weather maps use lines of equal pressure called
isobars to indicate areas of equal pressure. (Learn more about weather map
symbols).
A slowly rising atmospheric pressure, over a week or two, typically indicates
settled weather that will last a long time. A sudden drop in atmospheric pressure
over a few hours often forecasts an approaching storm, which will not last long,
with heavy rain and strong winds. 9
By carefully watching the pressure on a barometer, you can forecast local
weather using these simple guidelines. The decrease in barometric pressure
indicates storms, rain and windy weather, whereas the rise in barometric pressure
indicates good, dry, and colder weather. For slow, regular and moderate falls in
pressure, it may suggest a low pressure area is passing in a nearby region. Marked
changes in the weather where you are located are unlikely. A small rapid
decreases in pressure indicate a nearby change in weather. They are usually
followed by brief spells of wind and showers. As quick drop in pressure over a short
time occur, it indicates that a storm is likely in 5 to 6 hours. A long period of poor
weather is forecast by large, slow and sustained decreasing pressure. The weather
will be more pronounced if the pressure started rising before it began to drop.
Contrast to that, a rapid rise in pressure, during fair weather and average, or
above average pressure, indicates a low pressure cell is approaching. The pressure
will soon decrease forecasting poorer weather. Last but not least, quickly rising
8 Barometer. 4th October 2010. http://weather.about.com/od/weatherinstruments/a/barometers.htm9 Burch, David F. The Barometer Handbook; a modern look at barometers and applications of barometric pressure. Seattle: Starpath Publications (2009), ISBN 978-0-914025-12-2.
pressure, when the pressure is low, indicates a short period of fair weather is likely
while, a large, slow and sustained rise in pressure forecasts a long period of good
weather is on its way. 10
Fig 1.1 Mercury barometer measures atmospheric pressure by balance the weight
of mercury in a glass tube against the weight of air in the atmosphere.
Fig 1.2 Modern aneroid barometer
10 Middleton, W.E. Knowles. (1964). The history of the barometer. Baltimore: Johns Hopkins Press. New edition (2002), ISBN 0801871549.
Fig 1.3 Old aneroid barometer
(b) Manometer
A manometer is a device employed to measure pressure. There are a variety of
manometer designs. A simple, common design is to seal a length of glass tubing
and bend the glass tube into a U-shape. The glass tube is then filled with a liquid,
typically mercury, so that all trapped air is removed from the sealed end of the
tube. The glass tube is then positioned with the curved region at the bottom. The
mercury settles to the bottom. 11
After the mercury settles to the bottom of the manometer, a vacuum is
produced in the sealed tube. The open tube is connected to the system whose
pressure is being measured. In the sealed tube, there is no gas to exert a force on
the mercury (except for some mercury vapor). In the tube connected to the
system, the gas in the system exerts a force on the mercury. The net result is that
the column of mercury in the left (sealed) tube is higher than that in the right
(unsealed) tube. The difference in the heights of the columns of mercury is a
measure of the pressure of gas in the system. 12
For example, let’s say the top left is the sealed end of the tube and the top
right is the unsealed end of the tube. If the top of the left column of mercury
corresponds to 875 mm on the scale and the top of the right column of mercury
corresponds to 115 mm, the difference in heights is 875 mm - 115 mm = 760.
mm, which indicates that the pressure is 760. mm Hg or 760. torr. 13
11 Manometer. 9th October 2010. http://www.chm.davidson.edu/vce/gaslaws/pressure.html12 Pressure Measurement. 7th October 2010. http://en.wikipedia.org/wiki/Pressure_measurement13 Manometer. 8th October 2010. http://www.efunda.com/formulae/fluids/manometer.cfm
This method for measuring pressure led to the use of millimeters of mercury
(mm Hg) as a unit of pressure. Today 1 mm Hg is called 1 torr. A pressure of 1 torr
or 1 mm Hg is literally the pressure that produces a 1 mm difference in the heights
of the two columns of mercury in a manometer. 14
To understand how the height of a column of mercury can be used as a unit
of pressure and how the unit of torr is related to the SI unit of pascal (1 Pa = 1
N/m2), consider the following mathematical analysis of the behavior of the
manometer.
The force exerted by the column of mercury in a tube arises from the
gravitational acceleration of the column of mercury. Newton's Second Law
provides an expression for this force:
F = m g
In this equation, m is the mass of mercury in the column and g = 9.80665
m/sec2 is the gravitational acceleration. This force is distributed over the cross-
sectional area of the column ( A ). The pressure resulting from the column of
mercury is thus
P=mgA
The mass of mercury is given by the product of the density of mercury (
dHg ) and the volume of mercury ( V ). For a cylindrical column of mercury, the
volume of mercury is the product of the cross-sectional area and the height of the
column ( h ). These relationships produce the following equation.
P=mgA
=dhgVgA
=dhgVhgA
=dhgV g
This equation clearly shows that the height of a column of mercury is directly
proportional to the pressure exerted by that column of mercury. The difference in
heights of the two columns of mercury in a manometer can thus be used to
measure the difference in pressures between the two sides of the manometer. 15
14 Beckwith, Thomas G.; Roy D. Marangoni and John H. Lienhard V (1993). "Measurement of Low Pressures". Mechanical Measurements (Fifth ed.). Reading, MA: Addison-Wesley. pp. 591–595. ISBN 0-201-56947-7.15 Robert M. Besançon, ed (1990). "Vacuum Techniques" (3rd edition ed.). Van Nostrand Reinhold, New York.
The relation between torr and Pa is also clearly evident. Using dHg = 13.5951
g cm-3, one finds that 1 torr = 133 Pa or 1 atm = 760 torr = 101 kPa.
Fig 2.1 Manometer pressure. The difference in fluid height in a liquid column
manometer is proportional to the pressure difference.
Fig 2.2 This manometer is design to measure relative pressure under water.
(c) McLeod Gauge
A McLeod gauge is a scientific instrument used to measure very low pressures,
down to 10-6 mbar. It was invented in 1874 by Herbert G. McLeod (1841–1923).
McLeod gauges were once commonly found attached to equipment that operates
under a vacuum. Today, however, these gauges have largely been replaced by
electronic vacuum gauges. Compared to digital gauges, the McLeod gauge is
pp. 1278–1284. ISBN 0-442-00522-9.
somewhat unwieldy to use. Its use requires some calculation, and a liquid nitrogen
bath may be required to prevent interference from the mercury's vapor pressure.
A glass McLeod Gauge, drained of mercury McLeod Gauge symbol
The design of a McLeod gauge is somewhat similar to that of a mercury
column manometer. Typically it is filled with mercury. If used incorrectly, this
mercury can escape and contaminate the vacuum system attached to the gauge.
A slug of mercury moving in a tube is used to isolate a volume of gas at the
pressure to be measured. The gas in the volume is then compressed by a known
amount, and the final pressure is obtained with a manometer16.
In order to take a pressure reading of a vacuum chamber, the McLeod
gauge must take in a sample from the chamber. Caution during this operation is
crucial, as errors could cause accidental release of the mercury into the test
chamber. After the gauge takes in the sample volume of gas, it is tilted again such
that the mercury applies pressure to the gas. A manometer then measures the
pressure applied by the gas using movement of mercury in the manometer. Using
the final pressure, final volume, and initial volume, the initial pressure can be
calculated with the help of Boyle's Law. Boyle's law states that p1V1 = p2V2. The
McLeod gauge calculates pressure in absolute terms, rather than relative (relative
pressure is difference from atmospheric pressure)17.
16 Boyes Walt, Low Pressure Measuring, Butterworth: Heinemann, 2008, pg 113. 17 Robert M. Besancon, Vacuum Techniques, New York: Van Nostrand, 1990, pg. 45.
The calibration of the gauge is based upon Boyles/Charles physical chemistry
gas laws p1V1 = p2V2 and therefore measurement of the volume of the glass bulb and
the volume per unit length or bore of the capillary tubes is made with high
precision. It is based upon dimensions during manufacture so that once it is correct,
very little can go wrong to change its calibration, and so it can be reliably used as a
reference standard.
A McLeod gauge is an absolute pressure standard to which many other
vacuum gauges are calibrated. It will accurately measure the total pressure of non-
condensable permanent gases (i.e. hydrogen, nitrogen, oxygen, etc.) in a vacuum
system, but will not correctly measure condensable vapors if present. Many
condensable vapors will be condensed during compression of the gas sample in the
capillary tube and not contribute to depression of the gauge liquid. If condensable
vapors may be present while calibrating a vacuum gauge against the McLeod gauge,
then a liquid nitrogen cold trap should be used to ensure that only non-condensable
gases are being measured18.
Operating Schemes of McLeod Gauge
McLeod gauge is substantially less accurate for compressible gases than for
incompressible ones. This is because Boyle's law assumes an incompressible gas.
Condensable gases, such as water vapour, ammonia, carbon dioxide and pump oil
vapors, may be in gaseous form in the low pressure of the vacuum chamber, but
will condense when compressed by the McLeod gauge. The result is an erroneous
reading, showing a pressure much lower than actually present.
It has the advantage that it is simple to use and that its calibration is the
same nearly for all non-condensable gases. Modern electronic vacuum gauges are
simpler to use, less fragile, and do not present a mercury hazard, but their reading
18 Callen Herbert, The Dynamics of Pressure, London: Leeds & Sons, 1995, pg. 165.
is highly dependent on the chemical nature of the gas being measured and their
calibration is unstable. For this reason McLeod gauges continue to be used as a
calibration standard for electronic gauges.
Example of McLeod Gauge - HyVac Oil McLeod Gauge
3.0 Electric method
(a) Dead Weight Tester
1 - Handpump2 - Testing Pump3 - Pressure Gauge to be calibrated4 - Calibration Weight5 - Weight Support6 - Piston7 - Cylinder8 - Filling Connection
One of the pressure measurement devices is deadweight tester. First of all,
what is deadweight tester? Deadweight tester actually can be considered as a
master gauge which is used to calibrate pressure gauges. In the aspect of
instrumentation, a deadweight tester (DWT) is a calibration standard which uses a
piston cylinder on which a load is placed to make an equilibrium with an applied
pressure underneath the piston. Deadweight tester is also known as primary
standards. It is due to the pressure measured by a deadweight tester is defined
through other quantities, such as the length, mass and time. The deadweight
tester was invented by Albert Einstein. A deadweight tester is being called as
deadweight tester because it uses those cylinders weights which are called dead
weights. At dead weight, the mass cannot move, it is constant. Opposite of that is
a living weight which is any weight that could change.
Nowadays, the deadweight testers are more accurate and more complex,
but the essential operating principles are the same as the one used before. In the
United States, the National Institute of Standards & Technology (NIST) provides
certified weights and calibrates laboratory piston gauges by measuring the
diameter of the piston. Deadweight testers can be used to calibrate at pressure
levels as low as 5 psig (35 kPa) and as high as 100,000 psig (690 MPa). NIST
calibrated deadweight testers can be accurate to 5 parts in 100,000 at pressures
below 40,000 psig (280 MPa). For an industrial quality deadweight tester, error is
typically 0.1% of span.
How dead weight tester work ? Firstly, the testing pump (2) is connected to
the instrument to be tested (3), to the actual measuring component and to the
filling socket. Then, a special hydraulic oil or gas such as compressed air or
nitrogen is used as the pressure transfer medium. The measuring piston is then
loaded with calibrated weights (4). The pressure is applied via an integrated pump
(1) or, if an external pressure supply is available, via control valves in order to
generate a pressure until the loaded measuring piston (6) rises and 'floats' on the
fluid. This is the point where there is a balance between pressure and the mass
load. The piston is rotated to reduce friction as far as possible. Since the piston is
spinning, it exerts a pressure that can be calculated by application of a derivative
of the formula P = F/A. 19
In a deadweight tester, there consists of a pumping piston with a screw that
presses it into the reservoir, a primary piston that carries the dead weight, and the
gauge or transducer to be tested. It works by loading the primary piston with the
amount of weight. Then, more fluid is pressed into the reservoir cylinder which will
result in the pumping piston pressurizes the whole system. It is done until the
dead weight lifts off its support. Finally, the pressure can be calculated. 20
19 Instruments Of Dead Weight Tester. 10th Ocober 2010. http://www.sensorland.com/HowPage001.html20 Dead Weight Tester. 12th October 2010. http://www.minervaipm.com/
The formula on which the design of a DWT is based basically is expressed as follows :
p = F / A [Pa]
There are many types of deadweight tester. According to AMETEK
calibration instruments, there are MODEL PK II, MODEL RK, MODEL HK, T&R
Hydraulic, T&R Hydraulic Dual Column and HydraLite (HL) portable deadweight
tester series.
For the PK II tester, it is used for low pressure applications up to 30 psi (2
bar). It is available in 7 engineering units: psi, g/cm2, kPa, bar, inH2O, cmH2O, and
mmHg. This industry standard has an accuracy of up to ±0.015% of reading. It is
put in a rugged case made for 'closed case' operation to protect from wind
conditions. It is used for optional tripod and also available for medical applications
with oxygen. 21
For RK, it is accurate up to ±0.015% of indicated reading. This primary
standard is ideal for pressure ranges from 1 to 300 psi (0.01 to 20 bar). It provides
incremental pressures down to only 0.1 psi (1 mbar). It is also available in seven
different engineering units. It features a cast metal base with quick leveling for
field or laboratory use. Same as PK II, it also operates with cover closed.
21 Dead Weight Tester. 12th October 2010. http://www.euramet.org/index.php?id=calibration-guides
For HK, it is a high pressure tester up to 1,500 psi (100 bar), AMETEK HK
series testers operate in the same easy manner as the Model PK II and RK testers.
It features an accuracy up to ±0.025% and a repeatability up to ±0.005% of
reading. It is only available in psi, kg/cm2, kPa and bar engineering units.
For T&R Hydraulic, it is an ideal tester for laboratory or field use. It uses
distilled water or fluid compatible with 300 series stainless steel and MONEL. It is
suitable for applications up to 15,000 psi (1,000 bar).
For T&R Hydraulic Dual Column, it provides separate columns for high and
low pressure measuring piston/ cylinder assemblies. Its range changes are
achieved using a built-in crossover valve. It allows three-point calibrations to be
performed in seconds.
For HydraLite (HL), it is designed for pressure ranges from 10-200 psi (1-15
bar) up to 50-3,000 psi (5-225 bar). Its accurate reading is up to ±0.05%. The
weights and piston assemblies are interchangeable. It is design in 9 x 9 x 10 in
(23 x 23 x 24.5 cm).
According to Mensor, there are Pneumatic, Hydraulic, Portable Hydraulic,
Automatic Calibrator Unit, Piston / Cylinder Assembly and Masses.
For Pneumatic Deadweight Tester from Mensor, it operates on clean gas for
ranges up to 1500 psi. The ConTectTM System allows quick and easy cleaning and
range changes without the need for special tools.
For Hydraulic Deadweight Tester from Mensor, it operates with a hydraulic
fluid media producing ranges up to 15,000 psi. The ConTectTM System works the
same way as the pneumatic system.
For Portable Hydraulic Deadweight Tester from Mensor, it uses a hydraulic
fluid media producing ranges up to 15,000 psi with an accuracy up to 0.025%. Its
masses stacked directly on the base making it suitable for fields use.
For Automatic Calibrator unit, it is used to achieve the ultimate performance
with either the pneumatic or the hydraulic Deadweight Testers. The unit
automatically calculates the pressure based on current environmental influences
such as gravity, piston / cylinder temperature, Barometric Pressure, ambient
temperature and relative humidity.
For Piston / Cylinder Assembly, it utilizes a unique design for the piston /
cylinder. This system protects the piston / cylinder in a housing. This enables the
user to quickly change ranges while protecting the piston and cylinder from
accidental damage.
For Masses, they are manufactured from non-magnetic Series 303 Stainless
Steel for long term stability and durability. It is designed with a bell mass to lower
the center of gravity and improving stability. In order to generate very low
pressures, an aluminum plate allows small masses to be applied directly to the top
of the piston.
There are two main source of error which is the weight combination and
gravity variations. To eliminate those errors, order it calibrated to local gravity
when buying a new tester. Besides, find the local gravity and calculate the
corrected pressure values for each weight combination. 22
22 Instruments Of Dead Weight Tester. 10th Ocober 2010. http://www.dhinstruments.com/
As a conclusion, deadweight testers are used to measure the pressure
exerted by gas or liquid. They can also generate a test pressure for the calibration
of numerous pressure instruments. This pressure measurement device works by
placing the known weight on a rotating plate on top of a calibrated piston. It is
then connected by tubing to the pressure sensor and is being tested. This puts a
known force (weights) on a known surface area (piston). The rotation eliminates
any static friction that would affect the reading.
(b) Piezoelectric
Piezoelectric pressure sensor are designed to measure pressure changes in liquids
and gases such as in shock tube studies, in-cylinder pressure measurements, field
blast tests, pressure pump perturbations and in other pneumatic and hydraulic
processes23. It consists of naturally occurring crystals such as quartz. The quartz
generates an electrical charge when they are strained. The piezoelectric pressure
sensors do not require an external excitation source but requires charge
amplification circuitry. The obtained electrical charge is converted into actual units
of pressure by using a typical conversion formula. 24
23 Piezo Electric sensor. 12th October 2010. http://www.dytran.com/img/tech/a5.pdf2424 Piezo Electric sensor. 11th October 2010. http://zone.ni.com/devzone/cda/tut/p/id/3639
Generally, a piezoelectric sensor works on the principle of conversion of
energy in mechanical and electrical energy forms. When a polarized crystal is put
under pressure, some mechanical deformation takes place in the polarized crystal.
So this will leads in the generation of the electric charge. Then piezo sensor is
used to measured the generated electric charge or the mechanical deformation.
There are many types of piezoelectric sensors. For examples, piezoelectric
accelerometer, piezoelectric force sensors, and piezoelectric pressure sensors. A
piezoelectric accelerometer is widely used for OEM applications and is suitable for
working at a lower power consumption and wider frequency range. Piezoelectric
force sensors are low impedance voltage force sensors designed for generating
analog voltage signals when a force is applied on the piezoelectric crystal and are
widely used in machines for measuring force. A piezoelectric pressure sensor is
also known as piezoelectric sensor pressure. Piezoelectric pressure sensors are
used for measuring change in liquid and gases pressure. Other piezoelectric
sensors are commonly available.
Actually, there are several ways in which piezoelectric sensors function.
Piezoelectric material consists of polarized ions within the crystal. As a
piezoelectric sensor applies pressure on the piezoelectric crystal in proportion to
the charge output. The resultant displacement in the ions within the crystal
position is measured and recorded using piezoelectric vibration sensors. A
piezoelectric accelerometer has a charge frequency response capacity ranging
from 20 Hz to 10 KHz. A piezoelectric accelerometer can have electromagnetic
sensitivity of 0.0009 equiv.gm/gm and base strain sensitivity of 0.008
equiv.gm/micro strain. Piezoelectric force sensors should display a 5-volt full
display signal. Piezoelectric force sensors should have sensitivity of approximately
105 pC/N. Apart from that, piezoelectric pressure sensors should have rise time
less than 2.0 micro seconds. The maximum pressure applied by piezoelectric
sensors can be 1000psi and the voltage measurement range can be up to 5 volts.
Piezoelectric sensors are designed and manufactured to meet most industry
specifications.
Basically, Piezoelectric sensors are used in many applications. Piezoelectric
sensors are used in shock detection and machine monitoring applications. Besides
that, piezoelectric sensors are also used in structural dynamics, vehicle dynamics,
and low power applications. Piezoelectric sensors should adhere to American
National Standards Institute (ANSI) and Institute of Electrical and Electronics
Engineers (IEEE) standards. 25
(c) Strain Gauge
It is often easy to
measure the parameters like length, displacement, weight etc that can be felt
easily by some senses. However, it is very difficult to measure the dimensions like
force, stress and strain that cannot be really sensed directly by any instrument.
For such cases special devices called strain gauges are very useful. There are
some materials whose resistance changes when strain is applied to them or when
they are stretched and this change in resistance can be measured easily. For
applying the strain you need force, thus the change in resistance of the material
can be calibrated to measure the applied force. Thus the devices whose resistance
changes due to applied strain or applied force are called as the strain gauges. The
strain gauge has been in use for many years and is the fundamental sensing
element for many types of sensors, including pressure sensors, load cells, torque
sensors and position sensors.
Stress is a measure of the amount of internal pressure acted on a certain
material. Deformation occurs when a greater force is supplied to a smaller body.
The effect of stress is what we call strain. Any material being stressed is more
likely to be stretched into a longer shape when pulled apart. It may also become
shorter when it is pushed together. Strain gauges are classified into three types.
These are mechanical, electrical resistance and piezoelectric. Mechanical strain
2525 Piezoelectric Transducer. 13th October 2010. http://www.globalspec.com/learnmore/motion_controls/piezo electric_device s/piezoelectric_sensors_transducers
gauges act as strain sensors and strain amplifiers on the wall. Electric resistance
gauges record the deformities of vehicles, primarily aircraft. A piezoelectric gauge
is used in recording timekeeping signals for watches. 26.
Here are some ways on how a strain gauge works.
1. A strain gauge is usually made of foils. It comes in a variety of shapes. It
also plays different functions. It is first aligned to a Wheatstone bridge
circuit. It is then joined with other four full bridges, two half bridges and a
quarter bridge. A precision resistor completes half and quarter circuits.
2. The Wheatstone bridge is activated by a power supply of electricity and an
added electronic device. It undergoes unreceptive changes and unbalance
when stress is applied to the strain gauge.
3. A signal output is released corresponding to the stress value exerted on the
strain gauge. An amplification of 5 to 10 volts is supplied by the conditioning
electronic device. This happens when the signal value is small. This signal
level is adequate to cater to the external data collection systems.
A strain gauge works in a lot of beneficial ways. It does not only help professional
engineers do their job, it can also be vital in our everyday lives. Strain gauges can
also be used as a way to help our homes stay safe and secure. 27.
Example Of Strain Gauge Pressure Measurements Device
4.0 REFERENCES
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Middleton, W.E. Knowles. (1964). The history of the barometer. Baltimore: Johns Hopkins Press. New edition (2002), ISBN 0801871549.
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Robert M. Besançon, ed (1990). "Vacuum Techniques" (3rd edition ed.). Van Nostrand Reinhold, New York. pp. 1278–1284. ISBN 0-442-00522-9.
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