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Pressure Metrology andCalibration

S.Srinivasa Pai

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INTRODUCTION

Mechanical methods of measuring pressure havebeen known for centuries. U-tube manometerswere among the first pressure indicators.Originally, these tubes were made of glass, andscales were added to them as needed. Butmanometers are large, cumbersome, and not wellsuited for integration into automatic control loops.Therefore, manometers are usually found in thelaboratory or used as local indicators. Dependingon the reference pressure used, they couldindicate absolute, gauge, and differential

pressure.

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INTRODUCTION

Differential pressure transducers often are used in flowmeasurement where they can measure the pressuredifferential across a venturi, orifice, or other type of primaryelement. The detected pressure differential is related to

flowing velocity and therefore to volumetric flow. Manyfeatures of modern pressure transmitters have come fromthe differential pressure transducer. In fact, one mightconsider the differential pressure transmitter the model forall pressure transducers.

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Various pr essur e Mod es

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Gaug e Pr essur e "G auge " pressure is defined relative to

atmospheric conditions. In those parts of theworld that continue to use English units,gauge pressure is indicated by adding a " g " tothe units descriptor. Therefore, the pressureunit " pounds per square inch gauge " isabbreviated psig. When using SI units, it isproper to add " gauge " to the units used, suchas "P a gauge. " When pressure is to bemeasured in absolute units, the reference isfull vacuum and the abbreviation for " poundsper square inch absolute " is psia.

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Te r ms def ined in IS 36 24In the IS 3624, the pressure gauges areclassified based on Service or specific

application they are used like industrialOxygen, Acetylene, Ammonia, C hemicalG auges etc. Information such asconstructional details, mounting details,the materials of construction,metrological characteristics, safetyprecautions, calibration methods etc.are also dealt with in detail.

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Other Te chnical t er msOften, the terms pressure gauge, sensor, transducer,and transmitter are used interchangeably. The termpressure gauge usually refers to a self-containedindicator that converts the detected process pressureinto the mechanical motion of a pointer. A pressuretransducer might combine the sensor element of agauge with a mechanical-to-electrical or mechanical-to-pneumatic converter and a power supply. A pressuretransmitter is a standardized pressure measurementpackage consisting of three basic components: a

pressure transducer, its power supply, and a signalconditioner/retransmitter that converts the transducersignal into a standardized output.

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Bourdon Pr essur e Gaug es A Bourdon tub e is C-shap ed and has an oval

cross-s ection with on e end o f the tube conn ected

to th e proc ess pr essur e (Figur e 3-1A). The other end is s ealed and conn ected to th e point er or trans mitter mechanis m. To incr ease their sensitivity, Bourdon tub e eleme nts can b eextended into spirals or h elical coils (Figur es 3-1Band 3-1 C). This incr eases their effe ctiv e angular length and th er ef or e incr eases th e moveme nt attheir tip, which in turn incr eases th e r esolution o f the transduc er.

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Pr essur e Sensor Diaphrag m Designs

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F igure 2: Bourdon Tube Designs

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Accuracy, Re peatability, Lin earity &Hyst er esis o f Pr essur e Transduc er

P ressure transducer performance-related terms alsorequire definition. Transducer accuracy refers to thedegree of conformity of the measured value to anaccepted standard. It is usually expressed as apercentage of either the full scale or of the actualreading of the instrument. In case of percent-full-scaledevices, error increases as the absolute value of themeasurement drops.

R epeatability refers to the closeness of agreementamong a number of consecutive measurements of thesame variable.

Linearity is a measure of how well the transduceroutput increases linearly with increasing pressure.

H ysteresis error describes the phenomenon whereby thesame process pressure results in different output signalsdepending upon whether the pressure is approachedfrom a lower or higher pressure.

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Pr es.trans mitters & th eir o/p Pr essur e trans mitters can s end th e proc ess pr essur e

of inter est using an analog pn eumatic (3-15 psig),

analog electronic (4-20 mA dc), or digitalelectronic signal. Wh en transduc ers ar e dir ectlyinter f aced with digital data acquisition syst emsand ar e locat ed at so me distanc e f rom the dataacquisition hardwar e, high output voltag e signalsar e pr efe rr ed. These signals must b e prot ectedagainst both electro magn etic and radio f r equencyinter fer ence (EMI/R FI) wh en trav eling long er

distanc es.

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Transducer Types

Strain gaug eCapacitanc e

Pot entio metricResonant wir e

Pi ezoelectric Magn eticO ptical

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Transducer Types

Figur e 4 provid es an ov erall ori entation to th e scientist or engin eer who might b e f aced with th e task o f selecting a

pr essur e detector f rom among th e many d esigns availabl e.This tabl e shows th e rang es of pr essur es and vacuu ms thatvarious s ensor typ es ar e capabl e of detecting and th e typesof internal r efe r ences (vacuu m or at mosph eric pr essur e)used, i f any.

Becaus e electronic pr essur e transduc ers ar e of gr eatestutility f or industrial and laboratory data acquisition andcontrol applications, th e operating principl es and pros andcons o f each o f these is f urth er elaborat ed in this s ection.

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Electronic Pr essur e Sensor R ang es

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St rain Gage S ensors

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St rain Gage S ensors Wh en a strain gag e is us ed to measur e the

def lection o f an elastic diaphrag m or Bourdontube, i t b ecomes a c o m pon ent in a pr essur e

transduc er. Strain gag e-typ e pr essur e transduc ersar e wid ely us ed.

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Strain Gage

Sensors

Strain-gag e transduc ers ar e used f or narrow-span pr essur eand f or di ffe r ential pr essur e measur eme nts. Essentially, th estrain gag e is us ed to measur e the displac eme nt o f anelastic diaphrag m due to a di ffe r ence in pr essur e across th ediaphrag m. These devices can d etect gaug e pr essur e if thelow pr essur e port is l ef t op en to th e atmosph er e or diffe r ential pr essur e if conn ected to two proc ess pr essur es.

If the low pr essur e side is a s ealed vacuu m r efer ence, th etrans mitter will act as an absolut e pr essur e trans mitter.

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C apaci t ance t ype sensorsCapacitanc e pr essur e transduc ers w er e originallydeveloped f or us e in low vacuu m r esearch. Thiscapacitanc e chang e r esults f rom the moveme nt o f a diaphrag m eleme nt (Figur e 6). The diaphrag m isusually me tal or me tal-coat ed quart z and isexpos ed to th e proc ess pr essur e on on e side and tothe r efe r ence pr essur e on th e other. De pending onthe type of pr essur e, the capacitiv e transduc er can

be either an absolut e, gaug e, or di ffe r ential pr essur e transduc er.

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C apaci t ance t ype sensors Stainl ess st ee l is th e most co mm on diaphrag m

material us ed, but f or corrosiv e servic e, high-nick el stee l alloys, such as Incon el or Hast elloy,give better p er f or mance. Tantalu m also is us ed f or

highly corrosiv e, high t em peratur e applications.As a sp ecial cas e, silv er diaphrag ms can b e used tomeasur e the pr essur e of chlorin e, f luorin e, andother halog ens in th eir eleme ntal stat e.

In a capacitanc e-typ e pr essur e sensor, a high-f r equency, high-voltag e oscillator is us ed tocharg e the sensing electrod e eleme nts. In a two-

plat e capacitor s ensor d esign, th e moveme nt o f thediaphrag m between th e plat es is d etected as anindication o f the chang es in proc ess pr essur e.

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Capacitanc e-Bas ed Pr essur e Ce ll

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Capacitanc e-typ e sensors As shown in Figur e 6, th e def lection o f the

diaphrag m caus es a chang e in capacitanc e that isdetected by a bridg e circuit. This circuit can b eoperated in either a balanc ed or unbalanc ed mode.In balanc ed mode, the output voltag e is fed to anull d etector and th e capacitor ar ms ar e vari ed tomaintain th e bridg e at null. Ther ef or e, in th e

balanc ed mode, the null s etting its elf is a measur e

of proc ess pr essur e. Wh en op erated in unbalanc edmode, th e proc ess pr essur e measur eme nt is r elatedto th e ratio b etween th e output voltag e and th eexcitation voltag e.

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Capacitanc e-typ e sensors Singl e-plat e capacitor d esigns ar e also co mm on.

In this d esign, th e plat e is locat ed on th e back sid e

of the diaphrag m and th e variabl e capacitanc e is af unction o f def lection o f the diaphrag m.Ther ef or e, the detected capacitanc e is anindication o f the proc ess pr essur e. The capacitanc eis conv erted into either a d i r ect curr ent or avoltag e signal that can b e r ead dir ectly by pan elme ters or microproc essor-bas ed input/output

boards.

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Capacitanc e-typ e sensorsCapacitanc e pr essur e transduc ers ar e widespr eadin part b ecaus e of their wid e rang eability, f rom

high vacuu ms in th e micron rang e to 1 0,000 psig(70 MPa). Diffe r ential pr essur es as low as 0.01inch es o f water can r eadily b e measur ed. And,com par ed with strain gag e transduc ers, th ey do notdri f t much. B etter designs ar e availabl e that ar eaccurat e to within 0.1% o f r eading or 0.01% o f f ull scal e. A typical t em peratur e effe ct is 0.25% o f f ull scal e per 1000 F.

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Capacitanc e-typ e sensors

Capacitanc e-typ e sensors ar e of ten us ed assecondary standards, especially in low-di ffe r ential

and low-absolut e pr essur e applications. They alsoar e quit e r esponsiv e, becaus e the distanc e thediaphrag m must physically trav el is only a fewmicrons. Newer capacitanc e pr essur e transduc ersar e mor e r esistant to corrosion and ar e lesssensitiv e to stray capacitanc e and vibration effe ctsthat us ed to caus e "r eading jitt ers" in old er designs.

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P oten

tiome

tric pressure sensor

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P ot en t iome t ric pressure sensor

The potentio me tric pr essur e sensor provid es asim ple me thod f or obtaining an electronic output

f rom a mechanical pr essur e gaug e. The deviceconsists o f a pr ecision pot entio me ter, whos e wiper ar m is mechanically link ed to a Bourdon or

bellows eleme nt. The moveme nt o f the wip er ar macross th e potentio me ter conv erts th emechanically d etected sensor d ef lection into ar esistanc e measur eme nt, using a Wh eatston e

bridg e circuit (Figur e 7).

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P oten

tiome

tric pressure sensor

The mechanical natur e of the linkag es conn ectingthe wiper ar m to th e Bourdon tub e, bellows, or

diaphrag m eleme nt introduc es unavoidabl e errorsinto this typ e of measur eme nt. Tem peratur eeffe cts caus e additional errors b ecaus e of thediffe r ences in th er mal expansion co eff icients o f the me tallic co m pon ents o f the syst em . Errors alsowill d evelop du e to mechanical w ear o f thecom pon ents and o f the contacts.

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P ot en t iome t ric pressure sensor

Pot entio me tric transduc ers can b e made extr eme lysmall and install ed in v ery tight quart ers, such as

insid e the housing o f a 4.5-in. dial pr essur e gaug e.They also provid e a strong output that can b e r eadwithout additional a m pli f ication. This p er mitsthem to b e used in low pow er applications. Theyar e also in expensiv e. Pot entio me tric transduc erscan d etect pr essur es between 5 and 1 0,000 psig(35 KPa to 70 MPa). Their accuracy is b etween0.5% and 1% o f f ull scal e, not including dri f t andthe effe cts o f tem peratur e.

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Resonant wir e sensor

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esonant

WireThe r esonant-wir e pr essur e transduc er was introduc ed inthe late 1970s. In this d esign (Figur e 8), a wir e is gripp ed

by a static mem ber a t on e end, and by th e sensingdiaphrag m at th e other. An oscillator circuit caus es th ewir e to oscillat e at its r esonant f r equency. A chang e in

proc ess pr essur e chang es th e wir e tension, which in turnchang es th e r esonant f r equency o f the wir e. A digitalcount er circuit d etects th e shi f t. B ecaus e this chang e in

f r equency can b e detected quit e pr ecisely, this typ e of transduc er can b e used f or low di ffe r ential pr essur eapplications as w el l as to d etect absolut e and gaug e

pr essur es.

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esonant

WireThe most signi f icant advantag e of the r esonant

wir e pr essur e transduc er is that it g enerates an

inher ently digital signal, and th er ef or e can b e sentdir ectly to a stabl e crystal clock in amicroproc essor. Li mitations includ e sensitivity totem peratur e variation, a nonlin ear output signal,and so me sensitivity to shock and vibration. Theselimitations typically ar e minimized by using amicroproc essor to co m pensat e f or nonlin eariti es aswell as a m bient and proc ess tem peratur e

variations.

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esonant

WireResonant wir e transduc ers can d etectabsolut e pr essur es f rom 10 mm Hg,differ ential pr essur es up to 750 in. wat er,and gaug e pr essur es up to 6, 000 psig (42MPa). Typical accuracy is 0.1% o f

calibrat ed span, with six- month dri f t of 0.1% and a t em peratur e effe ct o f 0.2% p er 1000 F.

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Construction o f anoth er Resonant

Sensor

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Relation b et. Resonant f r equency &

Pr essur e a typical d esignThe r elationship b etween pr essur e and r esonant f r equency is expr essedwith th e f ollowing equation:

= k (a2/t2) P

f = 4.732h/2 l2(E /12 (1 + 0.2366 (1/h) 2 ))1/2wher eE = tensionk = a constanta, t = radius and thickn ess o f the diaphrag mP = pr essur e to b e appli ed

f = r esonant f r equencyh, l = thickn ess and l ength o f r esonant eleme ntE = elastic modulus o f silicon

= density o f silicon

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esonan t WirePrinciple

The pr essur e f rom an obj ect to b e measur ed is appli ed to ametal s eal diaphrag m and introduc ed to th e botto m of theSi diaphrag m shown in Figur e 2 through silicon e oil insid e

the seal diaphrag m. By this pr essur e input, th e Sidiaphrag m distorts elastically, causing strain in th er esonator and changing its r esonant f r equency. Ther esonator oscillat es vertically according to th e principl e of electro magn etic induction and th e magn etic f ield is giv en

hori zontally with a p er manent magn et. The entir e sensor ismagn etically shi elded and thus th e sensor is co m pletelyuna ffe cted by external magn etic f ields.

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P iezoelec t ric sensors Wh en pr essur e, f orce or acc eleration is appli ed to

a quart z crystal, a charg e is d eveloped across th ecrystal that is proportional to th e f orce appli ed(Figur e 9). The f unda mental di ffe r ence betweenthese crystal s ensors and static- f orce devices suchas strain gag es is that th e electric signal g enerated

by th e crystal d ecays rapidly. This charact eristicmak es these sensors unsuitabl e f or th e

measur eme nt o f static f orces or pr essur es butusef ul f or dyna mic measur eme nts. (This phenomenon also is discuss ed in lat er chapt ersdevoted to th e measur eme nt o f dyna mic f orce,im pact, and acc eleration.)

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Piezoelectric s ensors

Piezoelectric d evices can f urth er be classi f ied according to- the crystal's electrostatic charg e,

- its r esistivity, or - its r esonant f r equency electrostatic charg e is measur ed.

De pending on which ph enomenon is us ed, th e crystal s ensor can b e call ed electrostatic, piezor esistiv e, or r esonant.

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P iezoelec t ric sensors

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P iezoelec t ric sensors Wh en pr essur e is appli ed to a crystal, it is

elastically d ef or med. This d ef or mation r esults in af low o f electric charg e (which lasts f or a p eriod o f a few seconds). The r esulting electric signal can

be measur ed as an indication o f the pr essur e which

was appli ed to th e crystal. These sensors cannotdetect static pr essur es, but ar e used to measur erapidly changing pr essur es r esulting f rom blasts,explosions, pr essur e pulsations (in rock et motors,engin es, co m pr essors) or oth er sourc es o f shock or vibration. So me of these rugg ed sensors can d etect

pr essur e events having "rise times" of the order o f a millionth o f a second

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Dyna mic pr essur e sensorsThe output o f such dyna mic pr essur e sensors isof ten expr essed in "r elativ e" pr essur e units (suchas psir inst ead o f psig), th er e by r efe r encing th emeasur eme nt to th e initial condition o f the crystal.The maximum rang e of such s ensors is 5, 000 or 10,000 psir. The desirabl e featur es o f piezoelectricsensors includ e their rugg ed construction, s mallsize, high sp eed, and s elf -generated signal. On th eother hand, th ey ar e sensitiv e to tem peratur evariations and r equir e special cabling and

am pli f ication.

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P iezoelec t ric sensors

They also r equir e special car e during installation: One suchconsid eration is that th eir mounting torqu e shouldduplicat e the torqu e at which th ey w er e calibrat ed (usually30 in.-lbs). Anoth er f actor that can har m their p er f or mance

by slowing r espons e speed is th e de pth o f the em pty cavity below th e cavity. The larg er th e cavity, th e slow er th er espons e. Ther ef or e, it is r ecomme nded that th e de pth o f

the cavity b e minimized and not b e dee per than th ediameter of the prob e (usually about 0.25-in.).

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P iezoelec t ric sensors

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Calibration o f Pr essur e

Measuring instru mentsS.Srinivasa Pai

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Calibration o f Pr essur e Measuring instru ments

High Accuracy Pressure CalibrationHigh accuracy pressure calibration is

required to achieve a total uncertainty of

less than 0.05 % of the full scale pressure.Many calibration methods and pressurestandards equipment are available.However, the method used normally

depends on the equipment available toperform the test. The constraints usuallyrelate to the budget for equipment and thevolume of testing to be performed.

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Calibration o f Pr essur e Measuring instru ments

The first decision to make is to determinewhat type of standard is appropriate for theapplication. The choice is between primaryand secondary standards. A primarypressure standard is a pressure measuringor generating instrument, which can reducepressure measurements intomeasurements of mass, length and

temperature and gravity. Examples aredead weight testers and mercurymanometers. A secondary standard is aninstrument which must be calibrated torelate the output to pressure. Bothstandards have advantages and

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Calibration o f Pr essur e Measuringinstru ments-Pri mary Pr essur e Standards

Since there are a variety of pressure standards available, onewith appropriate quality should be selected.

Primary Pressure Standard Advantages-Pressure measurements are traceable to measurements of

mass and length and therefore more directly to national

standards.-They have good long term stability-Each measurement must be corrected for temperature, local

gravity and in some instances air buoyancy. Thesecorrections make this type of standard slower to use andthe probability for errors increases as the corrections are

applied-Dead weight testers generate incremental pressures. There is

a combination of weights for each pressure to begenerated. This type of standard cannot be used tomeasure unknown pressures except within the incrementsof the weights.

P i P St d d

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Primary Pressure StandardDisadvantages

Dead weight testers are difficult to use to generate absolutepressure (that is below ambient pressure) and then thepressures must be corrected for the loss of air buoyancy on theweights. The piston and weights are in a bell jar which must beevacuated each time the weights are changed to generate a newpressure. Since the pressure in the bell jar is the reference

pressure it must be measured and added to the generatedpressure to obtain the absolute pressure. Measuring thevacuum introduces another measurement uncertainty.Mercury manometers, since they contain mercury are a healthand environmental hazard. Over pressuring a mercury

manometer can result in the mercury being blown out of thetube, and should the mercury contaminate the device beingtested it would probably be ruined due to mercury's tendenciesto amalgamate with other metals.

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Secondary Pr essur e Standardsf or Calibration

Today pressure transducers are manufactured inlarge quantities and with high accuracy.Automated pressure standards are requiredduring the manufacturing process and for final

calibration. Because of corrections, manipulatingthe weights and controlling the environment,primary pressure standards are difficult andexpensive to adapt to production testing whereautomatic calibration is necessary.

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Advantagesof Secondary Pressure Standards as Calibration

Standards

F aster and easier to use.Usually no measurement corrections.

Non-incremental measurements. Easier to adaptto automatic operation.Generally less expensive.

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D isadvantagesof Secondary Pressure Standards as Calibration

Standards

Must be periodically recalibrated by a standardtraceable to national standards.Pressure measurements cannot be reduced tomeasurements of mass, length or temperature.

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Traceability, Precision and Accuracy

The most important considerations in selecting aprecision measuring instrument are traceability,precision and accuracy.In pressure terms, traceability is defined as theability to trace the calibration of a givenmeasurement either directly or indirectly tonational Standards of mass and length, such asthe NPL, New Delhi in India and, National Instituteof Standards and Technology (NIST) in the UnitedStates.Precision is defined, in the metrological sense, asthe limit of error or agreement within which theinstrument will reproduce measurements whenthe same input (pressure in this instance) isrepeatedly applied to it under the sameenvironmental conditions.

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Measurement accuracy

Measurement accuracy is the degree of conformityto some standard and combines traceability andinstrument precision. Accuracy is the differencebetween the true value and the measured value. Aninstrument may be traceable to a given referencesource yet not have sufficient precision to satisfythe measurement accuracy required by theapplication. The ability of an instrument or transducer to make accurate measurements is onlypartially defined by the accuracy specification.Specifications relate the accuracy to specifiedenvironmental conditions and within a certain timeperiod after calibration.The accuracy specification is an aid in the initialselection of the instrument or transducer, but it tellslittle of the actual performance in a particular situation.

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Methods of Pressure Calibration

Pressure calibration always involves applying aknown pressure to the device under test and

recording a minimum of two values --the knownpressure and the output signal or reading of thedevice under test. Additionally, other variables

may be recorded such as the temperature of thedevice under test, time of reading and whether thepressure is ascending or descending.There are many variations of calibration --from themost simple manual methods, to completelyautomated calibration systems. However, the heartof each system is a pressure measuringinstrument.

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Dead Weight Pressure tester

Also called Dead Weight piston gauge or Pressure balance. O ne of the fundamental method - force per unit area of thepiston.consists of an accurately machined piston of known weightwhich is inserted into a closed fitting cylinder (clearancebetween piston and cylinder will be an order of few microns),both of known cross-sectional area.Weights of known mass (various sizes depending on pressuredenominations) loaded on one end of the piston and fluidpressure applied to the other end of the piston until enoughforce is developed to lift the piston-weight combination

When the piston is floating freely within the cylinder (betweenlimit stops), the piston is in equilibrium with the unknown systempressure. Applied pressure = the ratio of force due to theweights-piston and the area of cross section of the piston-cylinder.

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Metrological and technicalrequirements of Dead Weight Tester as

per OIML R110 standard .(1) Measuring Range: The maximum pressure to bemeasured by dead weight tester to be selected from thefollowing two series

(i) 1x10n

, 1.6x10n

, 2.5x10n

, 4x10n

, 6x10n

(MPa)(ii) 1x10 n, 2x10 n , 5x10 n(MPa)

(2) Accuracy classes: Dead Weight Testers are classified intosix accuracy classes as 0.005, 0.01, 0.02, 0.05 , 0.1 and 0.2

and the class is determined by calibration.(3) Free Rotation Time and Fall rate of the piston: It shouldnot be less than the value mentioned in the following tables.

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Free Rotation Time of the iston as per Accuracy lass and measuring range

Upper limit of the Free Rotation Time ( minutes) for Accuracy Ca lssMeasuring range (MPa) 0.005 0.01 0.02 0.05 0.1 0.2

0.1 to 6 4 4 3 2 2 26 to 500 6 6 5 3 3 3

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Fall R at of t Pi st on a s per A racy Cla ss and Mea s ring rangePr ss r U r limit o f Maximum Pi ston Fall r ate (mm/minute s) medium the mea sur ing f or A ur acy cla ss

r ange (MPa ) 0.005 0.01 0.02 0.05 0.1 0.2gas 0.1 to 1 1 1 1 2 2 -gas mor e than 1 2 2 2 3 3 -

liquid 0.6 to 6 0.4 0.4 0.4 1 2 3liquid 6 to 500 1.5 1.5 1.5 1.5 3 3

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Calibration of Dead Weight pressure Tester

Dead weight pressure testers are normally calibratedagainst reference standard dead weight pressuretester by cross float method.

B asic principle followed in this method is that, boththe reference and test testers are connected througha differential pressure cell ( null indicator) as shownin figure. For high accurate work, the capacitancesensor can be used for monitoring the verticalmovement and position of the floating piston. The

dead weight pressure testers are normally calibratedeither in terms of pressure value itself or in terms of effective area. The uncertainty of the referencestandard is known and is calibrated with traceabilityto National/International Standard.

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Absolute Pressure Standard

Absolute pressure standards have a permanentvacuum in the reference chamber. They arezeroed by evacuating the pressure chamber to apressure less than the resolution of theinstrument or by measuring the residual pressure

with a vacuum gauge and setting that pressure byadjusting the zero. Although vacuum gaugeshave low accuracy as a percent of full scale, theerror is small relative to the resolution of thestandard, unless the standard has a very low

pressure range. Absolute pressure standards aresometimes "zeroed"at higher pressures byapplying a pressure from another standard or bymeasuring atmospheric pressure with bothinstruments and setting the reading with the zero

adjustment of the instrument being set up for test.

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Gauge Pressure Standards

Gauge pressure standards useatmospheric pressure as the reference

pressure. These instruments are easy tozero. Atmospheric pressure is applied tothe pressure chamber and the instrumentis adjusted to zero output.

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Typical Uncertainti es in pr essur e calibration

Type A Re peatabilityType B

- Due to Accuracy o f the Refe r ence Standard

- Due to th e Calibration unc ertainty o f the Refe r ence Standard- Due to th e Resolution o f the Ref . / Te st gaug e- Due the acceleration du e to gravity