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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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.

Boyes Walt, Low Pressure Measuring, Butterworth: Heinemann, 2008, pg 113.

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How Does a Strain Gauge Work? . 13th October 2010. http://www.ehow.com/how-does_500869_strain-gauge-work.html#ixzz12Dxzowuw

Instruments Of Dead Weight Tester. 10th Ocober 2010. http://www.sensorland.com/Ho wPage001.html

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Manometer. 9th October 2010. http://www.chm.davidson.edu/vce/gaslaws/pressure.html

Manometer. 8th October 2010. http://www.efunda.com/formulae/fluids/manometer.cfm

Middleton, W.E. Knowles. (1964). The history of the barometer. Baltimore: Johns Hopkins Press. New edition (2002), ISBN 0801871549.

Piezo Electric sensor. 12th October 2010. http://www.dytran.com/img/tech/a5.pdfPiezo Electric sensor. 11th October 2010. http://zone.ni.com/devzone/cda/tut/p/id/3639

Piezoelectric Transducer. 13th October 2010.http://www.globalspec.com/learnmore/motion_c ontrols/ piezoelectric_device s/piezoelectric_sensors_transducers

Pressure Measurement. 2nd October 2010. http://en.wikipedia.org/wiki/Pressure_measurement

Pressure Measurement. 7th October 2010. http://en.wikipedia.org/wiki/Pressure_measurement

Robert M. Besançon, ed (1990). "Vacuum Techniques" (3rd edition ed.). Van Nostrand Reinhold, New York. pp. 1278–1284. ISBN 0-442-00522-9.

Robert M. Besancon, Vacuum Techniques, New York: Van Nostrand, 1990, pg. 45.

Strain Gauge. 13th October 2010. http://www.brighthub.com/engineering/mechanica l/articles/48653.a spx#ixzz12E01vsxq