709

Pressure Measurement

5

5.1SELECTION AND APPLICATION 712

Introduction 712Orientation Table 712Reference Pressures 715An Example 715

Selecting the Pressure Detector 716Accessories 716Intelligent Transmitters 717

Bibliography 717

5.2ACCESSORIES (SEALS, SNUBBERS, CALIBRATORS, MANIFOLDS) 718

Pulsation Dampeners and Siphons 719Freeze Protection 720Chemical Seals 720

Limitations 721Standard Seals 721

Process Connections 722Self-Cleaning Seals 722Volumetric Seal Elements 723

Valve Manifolds 724Calibrations and Communicators 724Bibliography 725

5.3BELLOWS-TYPE PRESSURE SENSORS 726

Introduction 726Basic Designs 727

Absolute Pressure Sensors 727

Motion Balance 727Force Balance 727

Atmospheric Reference Sensors 728Motion Balance 728Single Bellows 728Dual Bellows 728Force Balance 728

Bibliography 728

5.4BOURDON AND HELICAL PRESSURE SENSORS 731

The Bourdon Tube 732C-Bourdon Pressure Sensors 732Spiral Bourdon Pressure Sensors 734Helical Bourdon Pressure Sensors 734

Fused Quartz Helix Sensors 735Bibliography 735

5.5DIAPHRAGM OR CAPSULE-TYPE SENSORS 736

Introduction 737Diaphragm Elements 737

Materials and Configurations 737Sensor Configurations 738

Absolute Pressure Sensors 738Motion Balance 738Force Balance Design 739Transmitter Ranges and Materials 739

Atmospheric Reference 739Motion Balance Design 739Materials of Construction and Spans 740Slack Diaphragms 740

© 2003 by Béla Lipták

710

Pressure Measurement

Force Balance Designs 741Features and Construction Choices 741

Pressure Repeaters 741Bibliography 742

5.6DIFFERENTIAL PRESSURE INSTRUMENTS 743

Introduction 744Measurement Error 744

Example 745Smart Transmitters Reduce Error 745

D/P Instrument Designs 745Filter Status Indicators 745D/P Switches 745D/P Indicators 745

Temperature Compensation and Over-Range Protection 746

Ranges and Materials 746Liquid Manometers 747

D/P Transmitters 747Dry, Force Balance Design 747Pneumatic Version 747Suppression and Elevation 748Flat and Extended Diaphragms 748Ranges and Pressure/Temperature

Ratings 748Wafer Elements 748

Torque Tube Sensors 749Low-Differential Transmitters 750

Membrane Type Design 750Bibliography 750

5.7ELECTRONIC PRESSURE SENSORS 751

Introduction 752Electrical Safety 752Strain Gauge Transducers 753

Historical Development 753The Bonded Strain Gauge 754Temperature Compensation 754

Detecting the Change in Resistance 754Transducer Designs 755

Bonded Designs 755Force Balance 755

Capacitance Transducers 756Potentiometric Transducers 757Resonant Wire Transducers 757Piezoelectric Pressure Sensors 757Magnetic Transducers 758

Inductive Elements 758Linear Variable Differential

Transformer 758Inductive Transmitters 758

Reluctive Elements 759

Optical Transducers 760Smart Transmitters 760

References 761Bibliography 761

5.8HIGH-PRESSURE SENSORS 762

Introduction 763Mechanical High Pressure Sensors 763

Dead-Weight Piston Gauges 763Button-Type Pressure Repeater 764Helical Bourdon 764

Bulk Modulus Cells 764Pressure-Sensitive Wires 765Change-of-State Detection 765Dynamic Sensors 765Reference 765Bibliography 765

5.9MANOMETERS 766

Introduction 767Liquid-Sealed Designs 767

Inverted Bells 767Cylindrical and Ledoux Bells 767Double Bell Unit 768

Ring Balance Manometers 768McLeod Vacuum Gauges 768

90

°

Rotation Type 768Piston-Type McLeod Gauge 769Micromanometer With Precision Needle 770

Visual Manometers 770Indicator Fluids 770Liquid Barometers 770Glass Tube Manometers 770

Well-Type Design 771Vacuum Measurement 771Multitube and Interface Manometers 771Inclined Tube Designs 772

Micromanometers 772Float Manometers 772

Servomanometers 773Conclusions 773Bibliography 773

5.10MULTIPLE PRESSURE SCANNERS 774

Introduction 774Rotary Pressure Scanners and Distributors 775

Scanning Frequency 775Differential Pressure Scanning 775Digital Interface Units 776

© 2003 by Béla Lipták

Contents of Chapter 5

711

Rotary Air Signal Distributors 776Air Signal Distributor Manifolds 776Ramping Pressure Scanners 777Electronic Pressure Scanners 777

Low Level Multiplexer 777High Level Multiplexer 778

Bibliography 778

5.11PRESSURE GAUGES 779

Introduction 780The Gauge Components 780

The Gauge Housing 780Gasket and Lens 780Sensing Element 781Safety Features 781

Dials and Pointers 781Maximum Pointer 782Digital Displays 782

Diaphragm-Type Vacuum Gauges 782Special Features 782

Illumination 782Temperature Compensation 782Duplex and Differential Gauges 782Failure Causes and Reliability 783

Conclusions 783Reference 783Bibliography 783

5.12PRESSURE REPEATERS 785

Booster Relays 785Flanged Repeaters 785Force Balance Pressure Repeaters 786

Vacuum Repeaters 787Button-Diaphragm Repeaters 787Pressure Repeater Applications 788

Repeating a Vacuum 788Level Measurement Applications 788

Bibliography 789

5.13PRESSURE AND DIFFERENTIAL PRESSURE SWITCHES 790

Pressure Switch Features 790Features and Configurations 790Terminology 791Selection 791Differentials 791Designs for Hazardous Area Applications 792Differential Pressure Switches 792

Bibliography 793

5.14VACUUM SENSORS 795

Introduction 796Vacuum Gauge Classifications 796Mechanical Vacuum Gauges 797

Manometers 797Capacitance Manometers 798Quartz Helix Vacuum Gauge 798Viscous Friction of Spinning Ball 799Molecular Momentum Vacuum Gauges 799Mechanical Linkage Vacuum Gauge 799

Thermal Vacuum Gauges 799Pirani Vacuum Gauge 800Thermocouple Vacuum Gauge 800Thermopile Vacuum Gauge 801Convectron Vacuum Gauge 801

Ionization Vacuum Gauges 802Hot Cathode Ionization Gauges 802Cold Cathode Ionization Gauges 803

Vacuum Gauge Calibration 804McLeod Vacuum Gauges 804Calibration Reference Tubes 804

Vacuum Controllers 804Aneroid Manostats 805Cartesian Diver Regulators 805Analog Electronic Controllers 805Mass Flow Controllers 805

References 806Bibliography 806

© 2003 by Béla Lipták

712

5.1 Selection and Application

B. G. LIPTÁK

(1969)

J. T. HALL

(1982)

J. A. NAY

(1995, 2003)

Partial List of Suppliers:

ABB Instrumentation (www.abb.com/us/instrumentation) Barton Instruments (www.barton-instruments.com)Brooks Instrument (www.brooksinstrument.com)Dresser Instrument (www.dresserinstruments.com)Endress

+

Hauser Inc. (www.us.endress.com)Fisher controls (www.fisher.com)Foxboro/Invensys (www.foxboro.com)Honeywell (www.iac.honeywell.com)Marsh Bellofram (marshbellofram.com)Moore Industries (www.miinet.com)Omega Engineering (www.omega.com)Rosemount/Emerson (www.rosemount.com)Siemens (www.sea.siemens.com)United Electric (www.ueonline.com)Youkogawa Corp. of America (www.yca.com)

Pressure detection devices can be classified on the basis ofthe pressure ranges they can measure, on the basis of thedesign principle involved in their operation, or on the basisof their application. In this chapter, the various categories arenot separated in any strict manner. Industrial instruments arediscussed in detail, with emphasis on the most commonly useddevices; laboratory instruments are covered in less detail.

INTRODUCTION

Each section starts with a brief summary of basic featuresapplicable to the group of instruments discussed in that sec-tion. This information allows readers to quickly determinewhether that category of instrumentation is suitable for theirapplication.

This chapter covers a wide range of pressure sensors,which can measure pressures from ultrahigh vacuums, suchas 10

−

13

mmHg, to ultrahigh gauge pressures approaching400,000 PSIG (2,800 MPa).

The range of the costs and inaccuracies of these instru-ments are equally broad. A simple, 1.5 in. diameter, 5% inac-curate gauge might cost only $10, while a fused-quartz helixsensor with an error of 0.01% and a digital readout could cost$6000. The cost of pressure transmitters range from a fewhundred dollars or less for the disposable models that havelimited features, to $2000 for smart models with built-indigital PID control algorithms and/or digital networkingcapabilities.

Silicon microchip technology continues steadily to reducethe cost of advanced features, reducing the size and weightof hardware and improving their availability and accuracy,while extending the long-term stability their calibration.Many of the sensors are available with digital communicationcapability, which can serve calibration, adjustment, andreporting of process variables, allowing for complete plant-wide integration.

With so many types of sensors, it might seem that makingthe proper selection for a particular installation would bedifficult and time consuming. Actually, this is not the case.A multitude of devices are covered here for the purpose ofcompleteness, but for a typical industrial installation, theselection is fairly simple, and often repetitive.

Orientation Table

The reader should find Table 5.1a, the Orientation Table forPressure Detectors, of value in narrowing the choices. For eachcategory of sensors, this table indicates the overall pressurerange that the category is capable of detecting. The table alsonotes whether the unit is available for industrial on-line instal-lation or for laboratory use only. Although any transmittinginstrument can easily be provided with an inexpensive analogor digital local indicator, the table differentiates those sensorcategories, which primarily serve as local gauges or indicators.Also distinguished are the sensor categories, which are com-monly available in microprocessor-based smart configurations.

The table also indicates the type of pressure reference used.When the environmental pressure surrounding the instrument

© 2003 by Béla Lipták

5.1Selection and A

pplication

713

TABLE 5.1a

Orientation Table for Pressure Detectors

Features Applicable Pressure Ranges

Inli

ne D

evic

e

Lab

orat

ory

or P

ilot

Pla

nt D

evic

e

Loc

al R

eado

ut (

Gau

ge)

Rem

ote

Rea

dout

Tra

ns.

Smar

t Uni

ts A

vail

able

mmHg absolute (1 mmHg = 133 Pa)

“H2O(1"H2O = 250 Pa)

PSIG(1 PSIG = 6.9 kPa)

10 -14 10 -10 10 -6 10-3 10 -1 1 50 200 400 600 4 7 11 10 2 10 3 10 4 10 5 10 6

Type of Design −300 -200 -100 -10 -5 -1 ±0.1 +1 +5 +10 +100 +200 +300

Abs. Press. Motion Balance Abs. Press. Force Balance Atm. Press. Ref. Motion Bal.

Atm. Press. Ref. Force Bal. Aneroid Manostats

C-Bourdon Spiral Bourdon Helical BourdonQuartz Helix

Abs. Press. Motion Balance Abs. Press. Force Balance Atm. Press. Ref. Motion Bal. Atm. Press. Ref. Force Bal.

Strain Gauge Capacitive Sensors Potentiometric Resonant Wire Piezoelectric Magnetic Optical

Ele

ctro

nic

Dia

phra

gmB

ourd

onB

ello

ws

Dead Weight Piston Gauge Bulk Modulus Cell Manganin Cell

Inverted Bell Ring Balance Float Manometer Barometers Visual Manometers Micromanometers

D/P Cell

Std. Diaphragm

Button Diaphragm

Hot Cathode Cold Cathode

Hig

h-Pr

ess.

Se

nsor

s

Man

omet

ers

Pres

sure

R

epea

ters

. Io

niza

- tio

n

© 2003 by Béla Lipták

714

Pressure M

easurement

TABLE 5.1a Continued

Orientation Table for Pressure Detectors

Features

Type of Design

Applicable Pressure Ranges

Inli

ne D

evic

e

Lab

orat

ory

or P

ilot

P

lant

Dev

ice

Loc

al R

eado

ut (

Gau

ge)

Rem

ote

Rea

dout

Tra

ns.

Smar

t Uni

ts A

vail

able

mmHg absolute (1 mmHg = 133 Pa)

“H2O (1“H2O = 250 Pa)

PSIG(1 PSIG = 6.9 kPa)

10-14 10-10 10-6 10-3 10-1 1 50 200 400 600 4 7 11 10 2 10 3 10 4 10 5 10 6

-300 -200 -100 -10 -5 -1 ±0.1 +1 +5 +10 +100 +200 +300

Thermocouple Thermopile Resistance Wire-Pirani Convectron

Quartz Helix McLeod Molecular Momentum Capacitance Spinning Ball

- Indicates that the device uses full-vacuum reference in its operation.

- Indicates that the device uses atmospheric pressure reference.

- Indicates that the operating priciple used does not involve the use of reference pressure.

The

rmal

Mec

hani

cal

© 2003 by Béla Lipták

5.1 Selection and Application

715

is used as reference (for the measurement of gauge pressures),it is noted by for atmospheric. When the reference is anevacuated chamber with almost perfect vacuum sealed inside,which is used for the measurement of absolute pressures, thistype of reference is noted by . There is no reference fordifferential pressure devices because they just compare onepressure against another. A few measurement technologies,such as the thermal and ionization type vacuum gauges, donot depend on the use of reference pressures or vacuumswithin the instrument itself.

Table 5.1b gives a variety of pressure conversion multi-pliers.

In addition to the general orientation provided by Table 5.1a,a summary of the features of some of the pressure instrumentsthat are commonly used in the manufacturing, petrochemical,and power industries is given in Table 5.1c. Note that in actualpractice the metric ranges are also rounded.

Reference Pressures

Reliable reference pressures are important, because they canbe a source of error just as much as an error on the measure-ment side can. In the case of absolute pressure sensors, thereference chamber cannot be fully evacuated to absolute zeropressure, but full vacuum is only approached within a fewthousands of a millimeter of mercury (torr). This means thata nonzero value is being treated as zero, which, when mea-suring higher vacuums, can cause significant errors. The otherpotential error source is the possibility of in-leakage of atmo-spheric air into the vacuum reference chamber of absolutepressure detector.

In case of positive pressure detectors, if the barometricpressure is the reference, atmospheric pressure variationscause a problem. As the atmospheric pressure can vary, byabout 1 in. of mercury (13.6 in. or 0.345 m of water), theresulting error can be significant if the process pressure isnear atmospheric. In addition, the output signal of the sensorcan change even when the process pressure is constant. Theresulting error might not be significant when detecting highgauge pressures, but it can be a problem with compounddetectors.

A compound pressure sensor is one that operates at nearatmospheric pressures and can detect the pressures on bothabove and below atmospheric. In controlling the pressure insealed rooms (clean rooms, biohazard containment chambers,ordinary tight buildings, etc.) where the ventilation systemsmay purposely hold the pressure above or below atmosphericpressure, this variable reference can be a source of problems.In selecting pressure measurement devices for such applica-tions, the design engineer must not ignore this and must eitherdetermine that the effect of barometric pressure variation canbe safely neglected, or must measure and correct for thisvariation in the measurement and control systems.

An Example

Take the example of a chemical reactor that has to be evac-uated to 10 mmHg before being purged at 1 in. (25.4 mm)H

2

O positive pressure. After purging, when the reactionstarts, this reactor operates at a higher positive pressure.

Due to the problem with the references, there is no singlepressure transmitter that can detect all these pressures. If avacuum reference is used, the purge pressure of 1 in. H

2

Oover atmospheric cannot be reliably detected. If an atmo-spheric reference is used, the 10 mmHg vacuum cannot beaccurately detected, because it could be subject to a 25 mmHgerror. In the past, the logical solution was to use multiplesensors. Today, we can also use intelligent transmitters,which are provided with multiple references and are capableof switching them on the basis of the batch sequence of thereactor.

TABLE 5.1b

Pressure Conversion

To Convert to Pascals (Pa) From Multiply by

atmosphere (standard) 1.01

×

10

5

atmosphere (technical

=

1 kgf/cm

2

) 9.81

×

10

4

bar 1.00

×

10

5

centimeter of mercury (0

°

C) 1.33

×

10

3

centimeter of water (4

°

C) 98

dyne/cm

2

0.1

foot of water (39.2

°

F) 2.98

×

10

3

gram-force/cm

2

98

inch of mercury (32

°

F) 3.39

×

10

3

inch of mercury (60

°

F) 3.37

×

10

3

inch of water (39.2

°

F) 249

inch of water (60

°

F) 248

kgf/cm

2

9.81

×

10

4

kgf/m

2

9.81

kgf/mm

2

9.81

×

10

6

kip/in

2

(ksi) 6.89

×

10

6

millibar 100

millimeter of mercury (0

°

C) 133

lbf/ft

2

47.8

lbf/in

2

(psi) 6.89

×

10

3

psi 6.89

×

10

3

torr (mmHg, 0

°

C) 133

Example: 1 PSI

=

6.89

×

10

3

Pa/PSI

=

6890 Pa

=

27.7 in. water

=

2.31 ft. water

=

2.04 in. Hg

=

0.07 kgf/cm

2

A

V

© 2003 by Béla Lipták

716

Pressure Measurement

SELECTING THE PRESSURE DETECTOR

When local pressure indication is required and the processpressure range is between 0 to 10 in. H

2

O (2.6 kPa) and 0 to100,000 PSIG (690 MPa), the conventional pressure sensors,which are described in Sections 5.3, 5.4, 5.5, and 5.11 canbe considered. The local pressure gauges, described in Section5.11, can have ranges from 10 in. H

2

O (2.6 kPa) up to 100,000PSIG (690 MPa).

For the measurement of near-atmospheric pressures,the bellows diaphragm sensors and manometers (Sections5.3, 5.4 and 5.9) are the most likely choices. Similarly, forlocal vacuum measurement down to 1 mmHg (0.13 kPa),the diaphragm, the bellows-type, and the vacuum manom-eters (Sections 5.3, 5.5, and 5.9) will give satisfactoryperformance. Vacuum sensors, ranging from 10

−

12

to 760mmHg, are discussed in Section 5.14. See Figure 5.14a fora summary of all the available vacuum sensors and theirranges.

Where remote transmission is required, the force balanceor motion balance transmitters (Sections 5.3, 5.4, 5.5, and5.7) will handle most applications. They can detect vacuumsdown to 1 mmHg (0.13 kPa) absolute and gauge pressuresup to 100,000 PSIG (690 MPa). When small, near-atmosphericor high pressures up to 200,000 PSIG (1,400 MPa) are to be

transmitted, the differential pressure or the electronic sensorsdescribed in Sections 5.6 and 5.7 should be considered. Thehigh-pressure sensors, described in Section 5.8, are the rec-ommended choices for pressures from 20,000 PSIG (140MPa) up to 400,000 PSIG (2,800 MPa).

Multiple pressure sensors, including scanners andmultiplexers, are discussed in Section 5.10. Pressure rep-eaters capable of repeating pressures from full vacuum to10,000 PSIG (69 MPa) are described in Section 5.12.Pressure and differential pressure switches for applicationsat up to 20,000 PSIG (138 MPa) pressures can be foundin Section 5.13.

Accessories

The pressure detectors are often provided with various acces-sory items (discussed in Section 5.2), which serve to protectthem from process conditions and environmental effects, areprovided to reduce maintenance. The most common causes offailure or maintenance problems include plugging, vibration,freezing, corrosion, excessive temperatures, and hard-to-handle process materials. The various protection devices dis-cussed in Section 5.2 can assist in making the installation

TABLE 5.1c

Pressure Detector Errors, Ranges, and Costs

Type Range Inaccuracy Approx. Cost

General-purpose Bourdon-tube indicator

15–10,000 PSIG(1–690 bars)

2% $100

High-accuracy test gauge Low vacuum to 3000 PSIG(Low vacuum to 207 bars)

0.1% to 0.01% $300–$6,000

Bourdon/spiral case-mounted indicator/recorder

Low vacuum to 50,000 PSIG(Low vacuum to 3450 bars)

0.50% $1,200

Spring-and-bellow case-mounted recorder

Low vacuum to 50 PSIG(Low vacuum to 3.5 bars)

0.50% $1,600

Nested capsular case-mountedrecorder

10–90 PSIG,(0.7–6.2 bars)

0.50% $1,600

Low-pressure bell case-mounted indicator

−

0.1 to 0.1 in. H

2

O(

−

3 to 3 mm H

2

O)2% $2,200

Beam-mounted strain gauge(sensor only) 4–20-mA DC output

0–1000 PSIG(0–69 bars)

0.25% $800

Piezoresistive transducer4–20-mA DC output

0–5000 PSIG(0–365 bars)

0.50% $500

“Smart” piezoresistive transmitter4–20-mA DC output

0–6000 PSIG(0–414 bars)

0.10% $1,200–$2,000

“Smart” field communicator for remotecalibration and configuring of “smart”transmitter

— $1000–$3,000

Capacitive sensor/transmitter 1 in. H

2

O–6000 PSIG(25 mm H

2

O–414 bars)0.2% 1000

© 2003 by Béla Lipták

5.1 Selection and Application

717

less sensitive to such effects and can reduce the required tasksof periodic servicing, testing, calibration, and maintenance.

Intelligent Transmitters

Microprocessor based pressure and differential pressuretransmitters are widely available today. Such a transmitterincludes an input circuit referred to as an analog-to-digital(A/D) converter that converts the sensor input into a digitalsignal before it is sent to the microprocessor. The micro-processor performs the manipulations of ranging, lineariza-tion, error checking, and conversion and either transmitsthe reading digitally or sends the resulting value to theoutput digital-to-analog converter (D/A), which converts thesignal back to an analog signal of 4–20 mA DC, 0–1 V DC,or 0–10 V DC.

Just as microprocessors have evolved in sophistication,so have A/D and D/A converters, increasing their resolutionfrom 8-bit up to the 18-bit, which has been used in the bettertransmitters since the beginning of 2000. These advancedtransmitters also check their own calibration on every mea-surement cycle and incorporate self-diagnostics, while beingconfigurable by using simple personal computer (PC) software.The reconfiguration process is not only quick and convenient,but also tends to lower inventories by making the transmittersinterchangeable.

The benefits of remote setup, configuration, and accessto diagnostics has resulted in a dramatic increase in the useof proprietary protocols that are supported by many of thelarger manufacturers. A wide variety of intelligent pressuretransmitters are available on the market today. Some commonfeatures of the leading models include not only digital andanalog outputs, but also multiple ranges, remote zero andspan settings, configuration push buttons, PC software andHand Held Configurators. The available bus and networkprotocols include Highway Addressable Remote Transducer(HART), Foundation Fieldbus, Profibus, Ethernet, or just4–20 mA. Some field locations will benefit from local indi-cation and this feature is optional with most manufacturers.

Bibliography

Benedict, R.P.,

Fundamentals of Temperature Pressure and Flow Measure-ments

, 3rd ed., New York: John Wiley & Sons, 1984.Cooper, F.G., “Low Pressure Sensing and Control,”

InTech

, September 1992.Dally, J.W., Riley, W.F., and McConnell, K.G.,

Instrumentation for Engi-neering Measurements

,

2nd ed., New York: John Wiley & Sons, 1993.Dyer, S.A.,

Wiley Survey of Instrumentation and Measurement

, New York:John Wiley & Sons, 2001.

Faust, E., “Digital Pressure Indicators versus Manometers,”

Measurements& Control

, April 1993.Ferree, S.R., “Sensors: Simple to Smart to Systems,”

InTech

, November 1991.Figliola, R.S. and Beasley, D.E.,

Theory and Design for Mechanical Mea-surements

,

3rd

ed., New York: John Wiley & Sons, July 2000.Gray, H., “Capacitive Pressure Transducers,”

Measurements and Control

,June 1991.

Hughes, T.A., Pressure Measurement,

EMC series, downloadable PDF,Instrumentation, Systems, and Automation Society, Research TrianglePark, NC, 2002.

Instrumentation, Systems, and Automation Society, Pressure (video, VHS,PAL & NTSC), Research Triangle Park, NC, 2002.

Instrumentation, Systems, and Automation Society, Pressure: Indicators andTransmitters, CD-ROM, Research Triangle Park, NC, 2002.

Instrumentation, Systems, and Automation Society, Process MeasurementSeries, CD-ROM, Research Triangle Park, NC, 2002.

Jager, R., “A Smart Transmitter for Differential Pressure Measurement,”

M&C News

, September 1992.Johnson, D., “Pressure Sensing: It’s Everywhere,”

Control Engineering

,April 2001

Juanarena, D.B., “Pressure Measurement in Cryogenic Environments,”Instrumentation, Systems, and Automation Society Conference, Houston,TX, October 1992.

Lewis, J.C., “Pressure Sensing: A Practical Primer,”

InTech

, December 1989.Loth, S.A., “Picking the Perfect Pressure Transducer,”

InTech

, July 1988.Nishikawa, T. et al., Intelligent DP Transmitter Using a Resonant Single-

Crystal Sensor, Instrumentation, Systems, and Automation Society1993 Technical Conference, Chicago, September 19–24, 1993.

Paros, I.M., “Fiber-Optic Resonator Pressure Transducers,”

M&C News

,September 1992.

“Reluctive and LVDT Pressure Transducers,”

Measurements and Control

,September 1992.

Stockdale, R., “Advanced Pressure Sensor Designs Benefit Control Sys-tems,”

Control Engineering

, June 1991.Studebaker, P., “Engineers Find New Uses for Pressure Transmitters,”

Con-trol

, February 1994.Weiss, J., “Sensors: Toward the Next Generation,”

InTech

, September 1993.

© 2003 by Béla Lipták