mmm pressure measurements

8/10/2019 MMM Pressure Measurements

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Pressure Measurements


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Pressure definition

Pressure is action of force against some opposite

force.

Pressure is Force in nature of thrust distributedover a surface.

Pressure is force acting against a surface within aclosed container.


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Pressure measurement terms.

Atmosphere pressure (Pat): The pressure exertedby the envelop of air surrounding earthssurface.

Usually determined by mercury column barometer.

Pat Pvp = gh

Pvp = 1.6 x 10(-6) kgf/cm2

So it can be neglected.

Pat = gh


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Absolute pressure (Pabs): pressure intensity measuredfrom the state of vacuum or zero pressure is calledabsolute pressure.

Gauge pressure(Pg) : Instruments and gauges used tomeasure the fluid pressure generally measure thedifference between unknown pressure p and theexisting atmosphere pressure.

-If unknown pressure is more than atmospheric

pressure than it is known as gauge pressure.-If unknown pressure is less than atmospheric

pressure, than it is known as vacuum pressure.


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Static pressure (Ps): Force per unit area acting onthe wall by at fluid at rest or flowing parallel to thewall.

Difference between total (stagnation) and staticpressure is dynamic pressure.


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Instruments for pressure measurements

For low pressures (below 1mm of Hg) :- Manometersand low pressure gauges.

For medium and high pressure gauges (between 1mmof Hg to 1000 atm) : Bourdon tube and diaphragmgauges.

For low vacuum and ultralow vacuum : McLeod,thermal conductivity and ionization gauges.

For very high pressures : Bourdon tube, diaphragm

and electrical resistance gauge. For varying pressure : engine indicator and cathode

ray oscilloscope.


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Propositions for pressure measurements

1) Pressure at any point in a body of liquid at rest isproportional to depth of point below the freesurface of liquid.

2) Pressure is same at two pints which lie in ahorizontal plane.


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Piezometer

Vertical transparent glass tube, upper end of which isopen to atmosphere.

Lower end is in communication with the gage point.


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1. Elastic pressure transducers

2. Manometer method

3. Pressure measurement by measuring vacuum

4. Electric pressure transducers

5. Pressure measurement by balancing forces produced on a known

area by a measured force

Pressure Measurement Methods


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Wet Meters (Manometers)


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Manometer basics

Characterized by its inherent

accuracy and simplicity of operation.

Itsthe U-tube manometer, which is a

U-shaped glass tube partially filledwith liquid.

This manometer has no moving parts

and requires no calibration.

Manometer measurements arefunctions of gravity and the liquids

density, both physical properties that

make the U-tube manometer a

standard for accuracy.


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ManometerWith both legs of a U-tubemanometer open to theatmosphere or subjected tothe same pressure, the liquid

maintains the same level ineach leg, establishing a zero

reference.


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Manometer With a greater pressure

applied 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, asindicated by h, exactlybalances the pressure.


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Manometer

When the liquid in the tube is mercury,for example, the indicated pressure h isusually expressed in inches (ormillimeters) of mercury.

To convert to pounds per square inch(or kilograms per square centimeter), P2= h

Where

P2= pressure, (kg/cm2)

= density, (kg/cm3)

h = height, (cm)


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Manometer

Gauge pressure is ameasurement relative toatmospheric pressure andit varies with thebarometric reading.

A gauge pressuremeasurement is positivewhen the unknownpressure exceedsatmospheric pressure (A),and is negative when the

unknown pressure is lessthan atmosphericpressure (B).


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Variations on the U-Tube Manometer The pressure reading is

always the difference betweenfluid heights, regardless of thetube sizes.

With both manometer legsopen to the atmosphere, thefluid levels are the same (A).

With an equal positivepressure applied to one leg of

each manometer, the fluidlevels differ, but the distancebetween the fluid heights isthe same (B).


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Manometer


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Manometer


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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 otherleg.


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Reservoir (Well) Manometer

In this design one leg is replaced by a largediameter well so that the pressure differential isindicated only by the height of the column in thesingle leg.

The pressure difference can be read directly on a

single scale. For static balance,P2 P1 = d (1 +A1/A2)h

Where

A1 = area of smaller-diameter leg

A

2 = area of wellIf the ratio ofA1/A2 is small compared with unity,then the error in neglecting this term becomesnegligible, and the static balance relation becomesP2 P1 = dh


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Sensing Elements

The main types of sensing elements are

Bourdon tubes,

diaphragms,

capsules, and

bellows .

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


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Primary Pressure Sensing 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 convolutions Sensitivity is proportional to size

In general a bellows can detect a slightly lower pressure than adiaphragm

The range is from 0-5 mmHg to 0-2000 psi

Accuracy in the range of 1% span


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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 the appliedpressure.

The tube is mechanically linked to a pointer on a pressure dial to give acalibrated reading.

Bourdon tube pressure gauge


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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


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Capsule

A capsule is formedby joining theperipheries of two

diaphragms throughsoldering or welding.

Used in some

absolute pressuregages.


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Range of Elastic-Element PressureGages



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2. Manometer method



5. Pressure measurement by balancing forces produced on a knownarea by a measured force


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Dead weight tester


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Material Selection

Ashby Chart

MaterialPoisson's

Ratio

Upper limit 0.5

Aluminum 0.334

Aluminum,

6061-T60.35

Aluminum,

2024-T40.32

Beryllium

Copper0.285

Brass, 70-

300.331

Brass, cast 0.357

Bronze 0.34

Copper 0.355

Cork 0

Inconel 0.27 - 0.38

Iron, Cast -

gray 0.211

Iron, Cast 0.22 - 0.30

Iron,

Ductile0.26 - 0.31

Iron,

Malleable0.271

Lead 0.431

Magnesium 0.35

Magnesium

Alloy0.281

Marble 0.2 - 0.3

Molybdenum 0.307

Monel metal 0.315

Nickel Silver 0.322

Nickel Steel 0.291

Polystyrene 0.34

Phosphor

Bronze

0.359

Rubber0.48 -

~0.5

Stainless

Steel 18-80.305

Steel, cast 0.265

Steel, Cold-

rolled0.287

Steel, high

carbon 0.295

Steel, mild 0.303

Titanium

(99.0 Ti)0.32

Wrought iron 0.278

Z-nickel 0.36

Zinc 0.331


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Ring balance manometer

rotating moment = (p1-p2)Ar1

restoring moment = mgr2Sin

So,

(p1-p2)Ar1 = mgr2Sin

(p1-p2)= mgr2Sin/Ar1(p1-p2)= K Sin

K= mgr2/Ar1

Data: d=100mm, r1=200mm

R2=275mm; m=? For =300 and

dp= 200 mm of water



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2. Manometer method





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V1 = V+ahc

P2= P1+h in mm of

P1V1 = P2V2 (Boyles La

P1V1= (P1+h)ah

P1V1 = P1ah + ah2

P1V1-P1ah = ah2

P1 = ah2/(V1-ah)

P1 = ah2/V1

V1=250 cc; a=0.1 s cm; h=1 cm 1=?


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Pirani gage (1 wire)

Two filaments, platinum, are usedas two arms of a Wheatstone bridge.

Reference filament is immersed in afixed-gas pressure, while themeasurement filament is exposed to

the system gas. Both filaments are heated by the

current through the bridge atconstant filament temperature.

Gas molecules hitting the immersedelement conduct energy away that is

detected and replaced by thefeedback circuit to the powersupply.

Cover a pressure range from about10 Torr to 1 x 10-5Torr.


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Thermal Designs:

The thermal conductivity of a gas changes

with its pressure in the vacuum range.

If an element heated by a constant power

source is placed in a gas, the resulting

surface temperature of the element will be

a function of the surrounding vacuum.

Because the sensor is an electrically

heated wire, thermal vacuum sensors are

often called hot wire gauges. Typically, hot

wire gauges can be used to measure downto 10-3mm Hg. Pirani:In this design, a sensor wire is heated

electrically and the pressure of the gas

is determined by measuring the current

needed to keep the wire at a constant

temperature


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Thermocouple gage

Works 10 Torr and 10-3

Torr Filament, fed from a constant

current supply, reaches atemperature determined by theamount of energy extracted by thegas.

At higher pressures, moremolecules hit the filament andextract more energy than at lowtemperatures.

The filament temperatures induce

thermocouple voltage changes.

Used extensively in fore linemonitoring and to provide thesignal to automatically switch themain chamber from backing and

high-vacuum pumps at thecrossover pressure.


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Ionization gage

Energetic electrons ionize theresidual gases

The positive ions are collected atan electrode and the current isconverted to a pressure

indication. Hot filament gauges (Bayard-

Alpert, Schulz-Phelps) usethermionic emission of electronsfrom a hot wire,

while cold cathode gauges(Penning, Inverted Magnetron)use electrons from a glowdischarge or plasma.

Gauge sensitivities (S) rangingfrom 0.6 Torr-1to 20 Torr-1

Gas pressure (P): ip= S x iex P

or P = 1/S x ip/ie


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Ionization Gauges


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Ionization Gauges


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Pressure range for gages



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2. Manometer method




Electric Pressure Transducers


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Why Electrical Pressure Transducers?

Transmission requirements for remote display viaCable or Cordless

Quicker responses and high accuracy

The linearity property

Immune to hysteresis, shock and mechanicalvibrations.

They can be used for extreme pressure applications,i.e. high vacuum and pressure measurements.

Electric Pressure Transducers


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1. Pressure sensing element such as a bellow , a diaphragm or a bourdon tube

2. Primary conversion element e.g. resistance or voltage

3. Secondary conversion element


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Strain Gage

If a wire is held under tension, itgets slightly longer and itscross-sectional area is reduced.This changes its resistance (R)in proportion to the strain

sensitivity (S) of the wiresresistance.

The strain sensitivity, which isalso called the gage factor (GF),

is given by: GF = ( R/R)/(L/L) = (R/R)/ Strain

Strain Gauge Pressure Transducer


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Construction and Working

Strain Gauge Pressure Transducer


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-- Strain gauge pressure transducers are used for narrow pressure

span and for differential pressure measurments

-- Available for pressure ranges as low as 3 inches of water to as high

as 200,000 psig

-- Inaccuracy ranges from 0.1 % of span to 0.25 % of full scale


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LVDT

Another type of inductancetransducer, utilizes two coilswound on a single tube and iscommonly referred to as aDifferential Transformer orsometimes as a LinearVariable DifferentialTransformer (LVDT).

Capacitive Pressure Transducer


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C=0 rA/d

Where,

C = the capacitance of a capacitor in farad

A = area of each plate in m2

d = distance between two plates in m

r= dielectric constant

0= 8.854*10^-12 farad/m2

Thus, capacitance can be varied by changing distance

between the plates, area of the plate or value of the

dielectric medium between the plates. Any change in

these factors cause change in capacitance.

In capacitive transducers, pressure is utilized to vary any of the above mentioned

factors which will cause change in capacitance and that is a measureable by any

suitable electric bridge circuit and is proportional to the pressure.



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When the two input pressures are equal the diaphragm is positionedcentrally and the capacitance are equal.

A difference in the two input pressure causes displacement of the sensingdiaphragm and is sensed as a difference between the two capacitances



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-- Originally developed for use in low vacuum research

-- Wide rangeability from high vacuum in the micron range to 10,000 psig

-- Differential pressure as low as 0.01 inch can be readable

-- Accurate within 0.1 % of reading or 0.01 % of full scale

-- More Corrosion resistant

Potentiometer Pressure Transducer


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The device consists of a precision potentiometer whose wiper arm

is mechanically linked to bourdon tube or bellow.

The movement of wiper alarm across the potentiometer converts

the mechanically detected sensor deflection into a resistance

measurement using a Wheatstone bridge circuit.


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Piezoelectric Pressure Transducer


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When a pressure , force or acceleration is applied to a quartz crystal ,

a charge is developed across the crystal that is proportional to the

force applied

Piezoelectric Pressure Transducer


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Signals generated by crystals decays rapidly so unsuitable for

static force or pressure measurements

Measure rapidly changing pressure resulting from blasts,

explosions or pulsation pressures

Range : 5,000 to 10,000 psi

Rugged construction, small size and high speed


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mmm pressure measurements

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