Pressure Std.ppt

7/23/2019 Pressure Std.ppt

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12 March PMI Revision 00 1

PRESSURE

MEASUREMENT


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nstrument

A device for measuring the magnitude of a physicalvariable.

All sensing is using physical measurement of

phenomena. Sensor developments have been greatly influencedby the progress in solid state electronics.

Solid state electronics and chip technology have

assisted in miniaturization of various sensors.


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For example, gas chromatography column isavailable on a small silicon wafer.

Human udgment is always the hardest thing to

duplicate in any machine. However, advantages of inline sensors includereal!time analysis and process control, improvedproduct uniformity and "uality, and conversion ofbatch operations to semi!continuous or fully

continuous processes.


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Terminology

Span: the range of measured variable that a sensorcan measure.

Least-count: the smallest difference of measuredvariable that can be detected by a sensor.

Readability: the closeness with which the scale of asensor can be read in analog output.

A sensor with a #$!cm scale would have a higherreadability than a sensor with a %&!cm scale and the

same span. %n digital output, readability will be therelative size of the letters.


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Terminology

Sensitivity: the change in output of the

sensor with the unit change in input

variable to be measured' e.g., if a %!m(recorder has a &!cm scale length, its

sensitivity would be & cm) m(.


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Sensor Accuracy

Accuracy: the deviation of the output of a sensorfrom a *nown measured input. Accuracy is usuallyexpressed as a percentage of full scale reading

e.g., a +$$!*a pressure transducer having anaccuracy of + - would be accurate within+ *aover the entire range.


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Sensor Accuracy

Measured variable: absolute

measured value

Percentage of full-scale(FS) reading

Percentage of instrument span

Percentage of actual reading

Precision: the ability of a sensor to reproduce acertain output with a given accuracy.



Sensor Accuracy

/he difference between precision and accuracy

0onsider the measurement of a *nowntemperature of +$$10 with a certain transducer.

Five observations are recorded, and the indicatedvalues are +$#, +$&, +$#, +$&, and +$#10. /hesevalues show that the accuracy of the transducer is&- 2&103,



Contd

and its precision is 4+-, since the maximumdeviation from the average reading +$510 isonly +10.

/he transducer can be calibrated and thencan measure temperature within 4 +10. /husaccuracy can be improved by calibration up

to, but not beyond, the precision of thetransducer



Contd

Threshold: %f the input of a sensor is very graduallyincreased from zero, there will be some minimumvalue below which no output can be detected.

/his minimum value is the threshold of the sensor.

Resolution: the input increment that gives somesmall but definite numerical change in the sensoroutput.

/hus resolution is the smallest measurable change,

while threshold is the smallest measurable input.


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Contd

Hysteresis:A sensor exhibits hysteresis when thereis a difference in readings depending on whetherthe values of the measured variable are

approached from above or below. Linearity: the maximum deviation of any calibration

point from the linear relationship. /his may beexpressed as a percent of the actual reading or as

a percent of the full scale reading.


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Contd

Error: a deviation of the measured value from thetrue value. 6rrors are divided into three broadclasses!gross, systemic, and random.

/he gross errors arc mostly human errors such asincorrect reading, adustment, and application ofinstruments.

7oading effects, due to improper circuitimpedance, also give this type of error.


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Contd

Systemic errors are related to the functioning of the

instruments and their mechanical or electrical

structure and calibration errors, and to the effect of

environment on the e"uipment performance. /hese errors related to the instrument can be

avoided or reduced by proper calibration of the

instrument, and by selecting a suitable instrument


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Contd

6nvironmental errors are reduced by using theinstrument at the recommended conditions oftemperature, relative humidity, pressure, etc.

8andom errors are due to un*nown causes. /heycan be reduced by proper design of the instrument,and by ta*ing more readings and using statisticalmethods to correct the observations.


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Pressure

9efinition Force per unit area

Absolute pressure

Atmospheric pressure

9ifferential pressure

:auge pressure

Head pressure


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Pressure Definition


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Standard pressure

ressure of one normal 2standard3

atmosphere

; +$+#


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Sensor Properties

8epeatability refers to the ability ofa pressure sensor to provide the

same output with successive applications of the same pressure.

Hysteresis is a sensorCs ability togive the same output at a given te

mperature before and after a temperature cycle.


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Pressure Sensor

/he basic pressure sensingelement

A 0!shaped Dourdon tube

D a helical Dourdon tube

0 flat diaphragm

9 convoluted diaphragm

6 capsule

F a set of bellows


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Bourdon tube

A sealed tube that deflects in response to applied

pressure.

rovide a fairly large displacement 2except

diaphragms3

Eseful in mechanical gauges and for electrical

sensors that re"uire a significant movement.


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Potentiometric Pressure

Sensors

Ese a Dourdon tube, capsule, or

bellows to drive a wiper arm on

a resistive element.

%nexpensive, but subect to

repeatability and hysteresis error

s


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Inductive Pressure Sensors

7inear (ariable

9ifferential /ransformer p

ressure sensor drives a

moving core that varies th

e inductive coupling betw

een the transformer prim

ary and secondary


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Capacitive Pressure Sensors/ypically use a thin diaphragmas one plate of a capacitor.

Applied pressure causes the diaphragm to deflect and the ca

pacitance to change./he change in capacitancemay be used to control the fre"uency of an oscillator or to va

ry the coupling of an A0 signalthrough a networ*.


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Piezoelectric Pressure

Sensors

Di!directional transducerscapable of converting stress intoan electric potential and vice versa.

0onsist of metallized "uartz orceramic materials.

9ynamic effect, providing anoutput only when the input is cha

nging


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Strained Gauge Sensors

Esed a metal diaphragmwith strain gauges bonded to

it.

A strain gauge measures the

strain in a material subected to

applied stress

Signal due to deformation of

the material is small, on the or

der of $.+- of the base resista

nce.


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Semiconductor strain

gauges

idely used, both bonded and integrated into a

silicon diaphragm, because larger 2one order of

magnitude3 response than metallic strain gauge

.

iezoresistive effect 0hange of resistance,

when the crystal lattice structure of silicon is def

ormed by applied stress.


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A silicon bar can be bonded toa diaphragm to yield a strain gauge sensor with a relatively high output.

%0 processing is used to form thepiezoresistors on the surface of a siliconwafer to fabricate an integrated piezores

istive pressure sensor


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Manometer

A simple pressure standard

Gay be used for gauge, differential, and

absolute measurements with a suitable refer

ence.

Eseful mainly for lower pressure wor*because the height of the column of mercury

will otherwise become unwieldy.

/he difference in column heights gives the

pressure reading


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TA!" #$%

Pressure Std.ppt

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