pressure measurements

8/11/2019 Pressure Measurements

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Pressure

Measurements


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Pressure

Pressure is the effect of a force applied to

a surface. Pressure is the amount of force

acting per unit area. The symbol of

pressure is p

Mathematically:

P = F/A.


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WHY MEASURE PRESSURE?

Pressure negates the properties of a fluid:

State, flow, forces

Quality and Safety of Operation:

Tire, compressors, etc

Pressure measurements is used in various

general, industry and research applications


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INTRODUCTION

Types of medium pressure sensors

Manometer & Barometer

Types of high pressure sensorsBourdon-tube Gauge


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BOURDON-TUBE GAUGE


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Invented by Eugene Bourdon in 1849

Can be used to measure pressuresup to 100,000 psi

Uses an elastic tube as its primary

element

The tube straightens out with

increasing pressure,

moving the pointer via mechanicallinks


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a flattened thin-wall, closed-end tube is connected at thehollow end to a fixed pipe containing the fluid pressureto be measured. As the pressure increases, the closed

end moves in an arc, and this motion is converted intothe rotation of a (segment of a) gear by a connectinglink that is usually adjustable. A small-diameter piniongear is on the pointer shaft, so the motion is magnifiedfurther by the gear ratio. The positioning of theindicator card behind the pointer, the initial pointershaft position, the linkage length and initial position, all

provide means to calibrate the pointer to indicate thedesired range of pressure for variations in the behaviourof the Bourdon tube itself. Differential pressure can bemeasured by gauges containing two different Bourdon

tubes, with connecting linkages
http://en.wikipedia.org/wiki/Gear_ratiohttp://en.wikipedia.org/wiki/Gear_ratio


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Advantages

High Pressure range

Low cost

Simple constructionGood accuracy except at low

pressure

They are easily adopted for designsfor obtaining electrical output


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Disadvantages

They are susceptible to shocks and

vibration and are subject to hysteresis

Limited use


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Diaphragm gauge


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The Diaphragm Pressure Gage

uses the elastic deformation of a

diaphragm (i.e. membrane) instead of

a liquid level to measure thedifference between an unknown

pressure and a reference pressure.


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A typical Diaphragm pressure gage

contains a capsule divided by a diaphragm,

as shown in the schematic below. One side

of the diaphragm is open to the externaltargeted pressure, PExt, and the other side

is connected to a known pressure, PRef,.

The pressure difference, PExt- PRef,

mechanically deflects the diaphragm.


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Advantages

Minimum hysteresis

Can withstand high overpressure

Can maintain linearity over a widerange

Relatively small size and moderate

cost


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Limitations

Difficult to repair

Needs protection from shock and

vibration

Cannot be used to measure high

pressure


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U tube manometer


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A U tube manometer is a pressuremeasuring instrument, usually limited tomeasuring pressure less than atmospheric.It is often use to refer specifically to liquidcolumn hydrostatic instrument containswater or mercury in a U shaped tube and isusually used to measure pressure. Oneend of the tube exposed to the unknown

pressure field and other end is connectedto a reference pressure source(usuallyatmospheric pressure)


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Working of U-tube Manometer:

The unknown pressure is applied in the

one arm of the tube and the mercury in the

tube or manometeric liquid filled in the tube

moves in the tube or rises to the constantregion and then the movement is stopped.

The height of the liquid is measured and

noted. The pressure is calculated by using

the formula,P1-P2 = Pmhg


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The above equation is arrived by

P1 = P2+Pmhg

P1-P2 = hg(Pt

Pm)

P1 = applied pressure

P2 = Reference pressure

Pt = specific gravity of the liquid or waterg = acceleration due to gravity.

P1 P2 is approximately equal to Pmhg.


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Advantages of U-tube

Manometer:

Simple in construction

Low cost

Very accurate and sensitiveIt can be used to measure other

process variables.


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Disadvantages of U-tube

Manometer:

Fragile in construction.

Very sensitive to temperature

changes.

Error can happen while measuring the

h.


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Characteristics of liquid used in

U-tube Manometer

Viscosity should be low.

Low surface tension is required.

Should not get vaporized


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Dead Weight Tester

Dead weight testers are a piston-cylinder type measuring device.As primary standards, they are the most accurate instruments forthe calibration of electronic or mechanical pressure measuringinstruments.

They work in accordance with the basic principle that P= F/A,where the pressure (P) acts on a known area of a sealed piston (A),

generating a force (F). The force of this piston is then comparedwith the force applied by calibrated weights. The use of high qualitymaterials result in small uncertainties of measurement andexcellent long term stability.

Dead weight testers can measure pressures of up to 10,000 bar,attaining accuracies of between 0.005% and 0.1% although mostapplications lie within 1 - 2500 bar. The pistons are partly made of

tungsten carbide (used for its small temperature coefficient), andthe cylinders must fit together with a clearance of no more than acouple of micrometers in order to create a minimum friction thuslimiting the measuring error. The piston is then rotated duringmeasurements to further minimise friction.


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Dead Weight Tester


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1 - Handpump

2 - Testing Pump

3 - Pressure Gauge to be calibrated

4 - Calibration Weight5 - Weight Support

6 - Piston

7 - Cylinder8 - Filling Connection


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The testing pump (2) is connected to the instrument to betested (3), to the actual measuring component and to thefilling socket. A special hydraulic oil or gas such ascompressed air or nitrogen is used as the pressure transfer

medium. The measuring piston is then loaded with calibratedweights (4). The pressure is applied via an integrated pump(1)or, if an external pressure supply is available, via controlvalves in order to generate a pressure until the loadedmeasuring piston (6) rises and 'floats' on the fluid. This is thepoint where there is a balance between pressure and themass load. The piston is rotated to reduce friction as far as

possible. Since the piston is spinning, it exerts a pressurethat can be calculated by application of a derivative of theformula P = F/A.

The accuracy of a pressure balance is characterised by thedeviation span, which is the sum of the systematic error andthe uncertainties of measurement.


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McLeod gauge


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A type of instrument used to measurevacuum by application of the principle ofBoyle's law. A known volume of a gas

whose pressure is to be measured istrapped by raising the level of a fluid(mercury or oil) by means of a plunger, bylifting a reservoir, by using pressure, or by

tipping the apparatus. As the fluid level isfurther raised, the gas is compressed intothe capillary tube . Obeying Boyle's law,the compressed gas now exerts enough

pressure to support a column of fluid highenough to read. Readings are somewhatindependent of the composition of the gasunder pressure


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Advantages

Simple and reliable


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Limitations

Limited to static measurements

Accuracy may not be high enough insome applications

Cannot be used in weightlessenvironment

Condensed of low pressure gas to the

liquid may occurContamination by mercury vapoursmay occur


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Measuring pressure at high

temperatures


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These 'melt pressure' sensors have been used formany years. Usually regarded as commodityitems, they are manufactured by a number of

different companies, and they do their work verywell. But, they have two major drawbacks: first,using mercury as a transmission fluid isconsidered environmentally unsound andgovernmental agencies have demanded that thepractice be discontinued. Secondly, the thinmembrane (which is only about 0.1 mm thick)separating the transmission fluid from the processfluid is prone to rupture. This is caused by theabrasion due to charged polymers on themembrand. Newer coatings have made it less

vulnerable to failure, and this has improved itsperformance. Still, 90% of melt pressure sensorfailures are due to the collapse of the membrane


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Impact


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pressure measurements

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