chapter -5- pressure measurements students.ppt

8/14/2019 Chapter -5- Pressure Measurements Students.ppt

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Pressure definitionPressure is the action of one force againstanother over, a surface. The pressure P of a

force F distributed over an area A is defined as:P = F/A


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Units of MeasureSystem Length Force Mass Time Pressure

MKS Meter Newton Kg Sec N/M 2 =Pascal

CGS CM Dyne Gram Sec D/CM 2

English Inch Pound Slug Sec PSI


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How Much is a Pascal (Pa) A Newton is the force necessary to accelerate a mass of1 kg at a rate of 1 meter per second per second.

The acceleration of gravity is 9.8 m/sec2

The force due to gravity on a 1 kg mass is 9.8 N is 1 kg weight.1 Newton is 0.102 kg weight.


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How Much is a Pascal (Pa)1 N/m2 is a very small pressureTherefore kilopascal (kPa)

1 atmosphere (14.7 psi, 750mmHg) is approximately100 kPa = 1 bar1 kPa is about 7 mmHg1% of a gas at our altitude is about 7 mmHg


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How is pressure generated?Collision of molecule with wallMomentum is mass x velocity

Change of momentum is doubleCollision is isothermal = perfectly elasticSum collisions over area to get force


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Static, dynamic, and impact pressures

Static pressure is the pressure of fluids or gases that are stationary or notin motion.

Dynamic pressure is the pressure exerted by a fluid or gas when itimpacts on a surface or an object due to its motion or flow. In Fig., thedynamic pressure is ( B A).Impact pressure (total pressure) is the sum of the static and dynamicpressures on a surface or object. Point B in Fig. depicts the impactpressure.


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Definition Of Pressure


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Definition Of PressureAbsolute pressureThe pressure is referenced to zero absolute pressure and has units ofpsia. Absolute pressure can only have a positive value.

Gauge pressure

The pressure is referenced to atmospheric pressure and by conventionis measured in the positive direction, i.e. 7 psig.

Vacuum pressureThe pressure is referenced to atmospheric pressure and by conventionis measured in the negative direction, i.e. -50 mm Hg.


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Standard Atmospheric Pressure


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Pressure Measurement

A number of measurement units are used for pressure.They are as follows:

1. Pounds per square foot (psf) or pounds per squareinch (psi)

2. Atmospheres (atm)3. Pascals (N/m 2) or kilopascal (1000Pa)*4. Torr = 1 mm mercury5. Bar (1.013 atm) = 100 kPa

6. 14.696 lbf/in2 equals 33.9 feet of H2O 7. 14.696 lbf/in2 equals 29.921 inches of of Hg


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Pressure Units

As previously noted, pressure is force per unit areaand historically a great variety of units have beenused, depending on their suitability for the application.

For example, blood pressure is usually measured inmmHg because mercury manometers were usedoriginally.

Atmospheric pressure is usually expressed in in mmHgfor the same reason.

Other units used for atmospheric pressure are bar andatm.


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Pressure UnitsThe following conversion factors should help in dealing

with the various units:

1 psi= 51.714 mmHg= 2.0359 in.Hg= 27.680 in.H2O= 6.8946 kPa

1 bar= 14.504 psi1 atm. = 14.696 psi


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Wet Meters (Manometers)


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Manometer basics

Characterized by its inherentaccuracy and simplicity ofoperation.

Its the U -tube manometer, whichis a U-shaped glass tube partiallyfilled with liquid.

This manometer has no movingparts and requires no calibration.

Manometer measurements arefunctions of gravity and theliquids density, both physicalproperties that make the U-tubemanometer a NIST standard foraccuracy.


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ManometerWith both legs of a U-tubemanometer open to theatmosphere or subjected to the

same pressure, the liquidmaintains the same level ineach leg, establishing a zeroreference.


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ManometerWith a greater pressureapplied to the left side of aU-tube manometer, theliquid lowers in the left legand rises in the right leg.

The liquid moves until theunit weight of the liquid,as indicated by h, exactlybalances the pressure.


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ManometerWhen the liquid in the tube is mercury, forexample, the indicated pressure h is usuallyexpressed in inches (or millimeters) of mercury.To convert to pounds per square inch (orkilograms per square centimeter), P 2 = h

WhereP 2 = pressure, (kg/cm 2) = density, (kg/cm 3) h = height, (cm)


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Manometer Gauge pressure is ameasurement relative toatmospheric pressureand it varies with thebarometric reading.

A gauge pressuremeasurement is positivewhen the unknownpressure exceedsatmospheric pressure(A), and is negative whenthe unknown pressure isless than atmosphericpressure (B).


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Reservoir (Well) Manometer In a well-type manometer,

the cross-sectional area ofone leg (the well) is muchlarger than the other leg.When pressure is appliedto the well, the fluid lowersonly slightly compared tothe fluid rise in the other

leg.


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Reservoir (Well) ManometerIn this design one leg is replaced by alarge diameter well so that thepressure differential is indicated onlyby the height of the column in thesingle leg.

The pressure difference can be readdirectly on a single scale. For staticbalance,

P 2 P 1 = d (1 + A 1/ A 2) h Where

A 1 = area of smaller-diameter leg

A 2 = area of well

If the ratio of A 1/ A 2 is small comparedwith unity, then the error in neglectingthis term becomes negligible, and thestatic balance relation becomes P 2 P

1 =dh


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Typical pressure sensor functional blocks.


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Sensing ElementsThe main types of sensing elements are

Bourdon tubes,diaphragms,capsules, andbellows .

All except diaphragms provide a fairly large displacementthat is useful in mechanical gauges and for electricalsensors that require a significant movement.


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Sensing Elements

The basic pressure sensing element can be configured as a C-shapedBourdon tube (A); a helical Bourdon tube (B); flat diaphragm (C); aconvoluted diaphragm (D); a capsule (E); or a set of bellows (F) .


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Primary Pressure ElementsCapsule, Bellows & Spring Opposed Diaphragm


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Bellows

Made of Bronze, S.S., BeCu, Monel etc..

The movement is proportional to number of convolutionsSensitivity is proportional to sizeIn general a bellows can detect a slightly lower pressure than adiaphragmThe range is from 0-5 mmHg to 0-2000 psi

Accuracy in the range of 1% span


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Bellows


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Bourdon Tube


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Bourdon Tubes

(a) C-type tube.(b) Spiral tube.(c) Helical tube


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Bourdon Tubes


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Diaphragm

A diaphragm usually is designed so that the deflection- versus-pressure characteristics are as linear as possible overa specified pressure range, and with a minimum ofhysteresis and minimum shift in the zero point.

(a) flat diaphragm; (b) corrugated diaphragm


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Diaphragm


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Capsule A capsule is formedby joining theperipheries of two

diaphragms throughsoldering or welding.

Used in someabsolute pressuregages.


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Use of capsule element in pressure gage


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Range of Elastic-Element Pressure Gages


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Pressure Gauges

In C type Bourdon tube, a section of tubing that is closed at one end is

partially flattened and coiled. When a pressure is applied to the open end, the tube uncoils.This movement provides a displacement that is proportional to theapplied pressure.The tube is mechanically linked to a pointer on a pressure dial to give acalibrated reading.

Bourdon tube pressure gauge


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Pressure Gauges

To amplify the motion that a diaphragm capsule produces, several

capsules are connected end to end.Diaphragm type pressure gauges used to measure gauge, absolute,or differential pressure.They are normally used to measure low pressures of 1 inch of Hg,but they can also be manufactured to measure higher pressures inthe range of 0 to 330 psig.They can also be built for use in vacuum service.

Diaphragm-type pressure gauge


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Dead-weight pressure gauge

A cylindrical piston 1 is placed inside a stainless-steel cylinder 2.The measuring pressure is supplied through the vent 8 to the fluid 4.The gravitational force developed by calibrated weights 3 can balance thisforce and the piston itself..The balance should be achieved for a certain position of the piston againsta pointer 9 of the stainless-steel cylinder. A manual piston pump 5 is used to achieve approximate force balance (toincrease pressure in the system), whereas a wheel-type piston pump 6 serves for accurate balancing. A Bourdon-type pressure gauge 7 is used for visual reading of pressure.

1

2

3

4

5

6

7

8

2

9


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Calibration of Pressure Sensing

Devises


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From Mechanical to Electronic

The free end of a Bourdon tube (bellows or diaphragm)no longer had to be connected to a local pointer, butserved to convert a process pressure into a transmitted

(electrical or pneumatic) signal. At first, the mechanical linkage was connected to apneumatic pressure transmitter, which usually generateda 3-15 psig output signal for transmission over distances

of several hundred feet,The force-balance and later the solid state electronicpressure transducer were introduced.


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Potentiometric type sensor A mechanical device such as adiaphragm is used to move the wiperarm of a potentiometer as the inputpressure changes. A direct current voltage (DC) V is appliedto the top of the potentiometer (pot),

and the voltage that is dropped from the wiper arm to the bottom of the pot issent to an electronic unit.It normally cover a range of 5 psi to10,000 psi.Can be operated over a wide range of

temperatures.Subject to wear because of themechanical contact between the sliderand the resistance element.Therefore, the instrument life is fairlyshort, and they tend to become noisier asthe pot wears out.


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Strain GageIf a wire is held undertension, it gets slightly longerand its cross-sectional area isreduced. This changes itsresistance (R) in proportionto the strain sensitivity (S) ofthe wires resistance.

The strain sensitivity, whichis also called the gage factor(GF), is given by: GF = ( R/R)/( L/L) = ( R/R)/Strain


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Strain Gauge Used in a Bridge Circuit


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Bellows Resistance TransducerBellows or a bourdon tube with a variable resistor.Bellow expand or contract causes the attached sliderto move along the slidewire.This increase or decrees the resistance.

Thus indicating an increase or decrease in pressure.


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Inductance-Type Transducers

The inductance-type transducer consists of three parts: acoil, a movable magnetic core, and a pressure sensingelement.

An AC voltage is applied to the coil, and, as the core moves,the inductance of the coil changes.


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LVDT Another type ofinductance transducer,utilizes two coils wound ona single tube and is

commonly referred to as aDifferential Transformer orsometimes as a Linear Variable Differential

Transformer (LVDT).


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Capacitance


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PiezoelectricWhen pressure, force or acceleration is applied to aquartz crystal, a charge is developed across the crystalthat is proportional to the force applied.

Piezoelectric devices can further be classified accordingto whether the crystals electrostatic charge, its resistivity,or its resonant frequency electrostatic charge ismeasured.

Depending on which phenomenon is used, the crystalsensor can be called electrostatic, piezoresistive, orresonant.


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Electronic Pressure Sensor Range

chapter -5- pressure measurements students.ppt

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