handbook electronic pressure

8/13/2019 Handbook Electronic Pressure

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VERLAG MODERNE INDUSTRIE

Electronic Pressure

Measurement

Basics, applications and instrument selection

Electronic

PressureMeasurement


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verlag moderne industrie

Electronic PressureMeasurement

Basics, applications and

instrument selection

Eugen Gamann, Anna Gries


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This book was produced with the technical collaboration ofWIKA Alexander Wiegand SE & Co. KG.

The authors would like to extend their particularthanks for the careful checking of the books contents toDr Franz-Josef Lohmeier.

Translation: RKT bersetzungs- und Dokumentations-GmbH,Schramberg

2010 All rights reserved withSddeutscher Verlag onpact GmbH, 81677 Munichwww.sv-onpact.de

First published in Germany in the seriesDie Bibliothek der TechnikOriginal title:Elektronische Druckmesstechnik 2009 by Sddeutscher Verlag onpact GmbH

Illustrations: No. 19 Phoenix Testlab GmbH, Blomberg;No. 22 M+W Zander, Stuttgart; all othersWIKA Alexander Wiegand SE & Co. KG, KlingenbergTypesetting: abavo GmbH, 86807 BuchloePrinting and binding: Sellier Druck GmbH, 85354 FreisingPrinted in Germany 889539


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Contents

Introduction 4

Pressure and pressure measurement 6

International pressure units ..................................................................... 6

Absolute, gauge and differential pressure ............................................... 7

Principles of electronic pressure measurement ....................................... 8

Sensor technology 14

Metal thin-film sensor ............................................................................. 14

Ceramic thick-film sensor ....................................................................... 15

Piezo-resistive sensor .............................................................................. 17

Sensor principles by comparison ............................................................ 19

Pressure measuring instruments 21

Instrument types at a glance .................................................................... 21

Instrument qualification and reliability ................................................... 26

Environmental influences and special requirements ............................... 32

Standard applications and requirements 39

Critical value monitoring ........................................................................ 39

Pressure control ....................................................................................... 41Indirect measurement of process values ................................................. 42

Criteria for the instrument selection 49

Pressure range ......................................................................................... 49

Pressure connection ................................................................................ 52

Electrical connection ............................................................................... 54

Output signals ......................................................................................... 55

Characteristic curve, accuracy and measuring error ............................... 58

Prospects 68

Glossary 69

The company behind this book 71


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4

Introduction

Electronic pressure measurement contributesto the safe, accurate and energy-saving controlof processes. Alongside temperature measure-ment, it is the most important and most com-monly-used technology for monitoring andcontrolling plants and machinery. Particularlyin pneumatics and hydraulics (Fig. 1), meas-

urement and control of the system pressure isthe most important prerequisite for safe andeconomic operation.During the past 20 years, electronic pressuremeasurement has been introduced in a multi-tude of applications, and new applications areadded every day. However, the demands on theinstruments are as diverse as the applications.This fact is also reflected in the very largenumber of products. In the early days of elec-tronic pressure measurement the user couldonly choose from a small number of variants,manufactured by a handful of providers. Todaythe user is confronted with a multitude of tech-nical solutions by numerous providers, and

must therefore rely on competent help with theselection.This selection is a classic optimisation pro-cess, including the comparison of numerousparameters and weighing of requirements rela-tive to each other. This is needed in order toachieve diverse objectives in the application,

to ensure maximum safety of operation, toreach or increase the planned performance ofthe plant and machinery and to reduce the totalcosts. Incorrect decisions not only have eco-nomic consequences, but can also bear a poten-tial safety risk.In order to be able to make a proper selectionof the suitable electronic pressure measuringinstrument, the users or engineers should have

Variety ofapplications andinstruments

Instrumentselection

Suitability


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Introduction 5

knowledge about the physical principles ofpressure measurement, the advantages and dis-advantages of different sensor technologies inrelation to the particular application, and alsoabout the key basics of instrument technology.The selection of the suitable pressure measur-ing instrument is based, among other things,on such criteria as the pressure range, the pres-sure or process connection, the electrical con-nection, the output signal and the measuringaccuracy. This book presents the backgroundknowledge required to understand and com-pare data in the data sheets in an easy-to-understand and clear way.

Fig. 1:Typical applicationof pressure measure-ment: pneumatic andhydraulic applica-tions in factory auto-mation


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6

Pressure and pressure

measurementIn process systems two of the most importantprocess variables to measure are temperatureand pressure. The common pressures measuredare the hydrostatic pressure of a liquid columnand the atmospheric pressure.In general, pressure is defined as follows: if a

force per unit area is applied in a direction per-pendicular to a surface, then the ratio of theforce value F to the surface area A is calledpressurep:

p F

A=

(1)

To transmit pressure, incompressible mediasuch as liquids are suitable. To store energy inthe form of pressure work, compressible mediasuch as gases are used.

International pressure units

The derived SI unit for pressure is Pascal (unitsymbol Pa), which can also be represented, ac-cording to the equation above, in the SI unitsNewton (unit symbol N) and metre:

1 1 102

5Pa

N

mbar= =

(2)

The bar is the most common unit of pressurein Europe. This legitimate, SI-compliant unitenables large pressure values, common in dailylife and in technology, to be expressed usingsmall numerical values. In North America, onthe other hand, the pressure unit pound(-force) per square inch (psi) is common.Specifically in Asia, the common units are

Definition

of pressure

SI unit

Europe

North America


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Absolute, gauge and differential pressure 7

Megapascal (MPa) and kilogram(-force) persquare centimetre (kg/cm2). Table 1 displaysthe correlation of these pressure units.

Absolute, gauge and differentialpressure

Absolute, gauge and differential pressure arethree measurement parameters that differ intheir reference points, i.e. in the correspondingzero point of the pressure scale. The zero point

of absolute pressure is always the pressure inan evacuated space, i.e. in a vacuum (Fig. 2).The zero point of gauge pressure, on the otherhand, is provided by the prevailing local at-mospheric pressure. This atmospheric pressureequals approximately 1 bar at sea level and de-creases continuously with increasing height. In

addition, it depends on the weather conditions.

Asia

Pressure unit Conversion

1 bar 105Pa 1000 mbar

1 psi 6895 Pa 68.95 mbar

1 MPa 106Pa 10 bar

1 kg/cm2 0.0981 MPa 0.981 bar

Process pressure 3 bar absolute 2 bar gauge

Atmospheric pressure 1 bar absolute 0 bar gauge

Vacuum pressure 0 bar absolute 1 bar gauge

Absolute pressure Gauge pressure

Fig. 2:Pressure types

Table 1:Internationalpressure units


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8 Pressure and pressure measurement

For some applications, the difference betweentwo variable system pressures is the actualmeasurement value. This is known as the

measurement of differential pressure. A prac-tical example of this is differential pressuremonitoring upstream and downstream of afilter element (see Critical value monitoring,p. 39 f.).

Principles of electronic pressure

measurement

For electronic pressure measurement a sensoris required to detect the pressure and/or itschange, and to convert it accurately and re-peatably into an electrical signal utilising aphysical operating principle. The electrical sig-

nal is then a measure of the magnitude of theapplied pressure or change in pressure. Fourkey measuring principles and their technicalrealisation are shown below.

Resistive pressure measurementThe principle of resistive pressure measure-ment is based on the measurement of thechange in resistance of electric conductorscaused by a pressure-dependent deflection.The following equation applies for the resist-ance of an electric conductor:

R l

A=

(3)

R Electrical resistance Resistivityl LengthA Cross-sectional area

If a tensile force is applied to the conductor, itslength increases and its cross-sectional areadecreases (Fig. 3). Since the resistivity of a

From pressureto electricalsignal


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Principles of electronic pressure measurement 9

metallic conductor is a (temperature-depend-ent) constant for a particular material and,therefore, independent of the geometry, theelectrical resistance increases as a result of theelongation. In the case of compression, the op-posite applies.The principle of resistive pressure measure-ment is realised using a main body which ex-hibits a controlled deflection under pressure.This main body frequently has a (thin) area re-ferred to as the diaphragm, which is weakenedintentionally. The degree of deflection causedby the pressure is measured using metallic

strain gauges.

Diaphragm

I

I + I

A AA

Fig. 3:Change of thedimensions of acylindrical conductorby elongation

Strain gauges

StrainingCompression

Pressure

Fig. 4:Deflection of thesensor diaphragm

under pressure


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Usually four strain gauges are applied to a dia-phragm. Some of them are located on elong-ated and others on compressed areas of the

diaphragm. If the diaphragm deflects under theaction of a pressure, the strain gauges are de-flected correspondingly (Fig. 4). The electricalresistance increases or decreases proportion-ally to the deflection (elongation or compres-sion). To accurately measure the resistancechange, the strain gauges are wired to a Wheat-

stone measuring bridge.

Piezo-resistive pressure measurementThe principle of piezo-resistive pressuremeasurement is similar to the principle of re-sistive pressure measurement. However, sincethe strain gauges used for this measuringprinciple are made of a semiconductor ma-terial, their deflection due to elongation orcompression results primarily in a change inresistivity. According to equation 3 (see page8), the electrical resistance is proportional tothe resistivity. While the piezo-resistive effectin metals is negligible and thus effectivelyinsignificant within resistive pressure meas-

urement, in semiconductors such as silicon itexceeds the effect of the variation of lengthand cross-section by a factor between 10 and100.Unlike metallic strain gauges, which can beattached to nearly any material, the semicon-ductor strain gauges are integrated into the

diaphragm as microstructures. Thus, the straingauges and the deflection body are based onthe same semiconductor material. Usually fourstrain gauges are integrated into a diaphragmmade of silicon and wired to a Wheatstonemeasuring bridge.Since the microstructures are not resistant tomany pressure media, for most applicationsthe sensor chip must be encapsulated. The

with metallicstrain gauges

Silicondiaphragmwith integrated

strain gauges

Encapsulationof the sensorelement


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pressure must then be transmitted indirectly tothe semiconductor sensor element, e.g. using ametallic diaphragm and oil as a transmission

medium.Due to the magnitude of the piezo-resistiveeffect, piezo-resistive sensors can also beused in very low pressure ranges. However,due to strong temperature dependency andmanufacturing process-related variation, indi-vidual temperature compensation of every sin-

gle sensor is required.

Capacitive pressure measurementThe principle of capacitive pressure measure-ment is based on the measurement of the cap-acitance of a capacitor, which is dependentupon the plate separation. The capacitance of a

dual-plate capacitor is determined using thefollowing equation:

C A

d=

(4)

C Capacitance of the dual-plate capacitor PermittivityA Area of the capacitor plated Plate separation

The principle of capacitive pressure measure-ment is realised using a main body with ametallic diaphragm, or one coated with a con-ductive material, which forms one of the two

plates of a dual-plate capacitor. If the dia-phragm is deflected under pressure, the plateseparation of the capacitor decreases, whichresults in an increase in its capacitance whilethe plates surface area and permittivity remainconstant (Fig. 5).In this way, the pressure can be measured withhigh sensitivity. Therefore, capacitive pressuremeasurement is also suitable for very low pres-

Diaphragmas a movingcapacitor plate


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sure values, even down in the one-digit milli-bar range. The fact that the moving diaphragmcan be deflected until it reaches the fixed plateof the capacitor ensures a high overload safetyfor these pressure sensors. Practical restric-tions on these sensors arise from the dia-phragm material and its characteristics, andalso from the required joining and sealingtechniques.

Piezo-electric pressure measurement

The principle of piezo-electric pressure meas-urement is based on the physical effect of thesame name, only found in some non-conduct-ive crystals, e.g. monocrystalline quartz. Ifsuch a crystal is exposed to pressure or tensileforce in a defined direction, certain opposedsurfaces of the crystal are charged, positive

and negative, respectively. Due to a displace-ment in the electrically charged lattice elem-ents, an electric dipole moment results whichis indicated by the (measurable) surfacecharges (Fig. 6). The charge quantity is pro-portional to the value of the force, its polaritydepends on the force direction. Electrical volt-age created by the surface charges can bemeasured and amplified.

Piezo-crystallinediaphragm

Fixed plate

Movingplate

Pressure

A

A

d

Fig. 5:Capacitivemeasuring principle


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The piezo-electric effect is only suitable forthe measurement of dynamic pressures. Inpractice, piezo-electric pressure measurementis therefore restricted to specialised applica-tions.

Measurementof changes inpressure

+

+ +

+

+ +

+

+ +

Unstressed:no dipole moment

Pressure

Pressure

Pressure Pressure

+ +

+ +

Fig. 6:Piezo-electric effect


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14

Sensor technology

The three most common sensor principles aredescribed below (Fig. 7). Metal thin-film andceramic thick-film sensors are the two mostcommon implementations of resistive pressuremeasurement. The significant differences be-

Fig. 7:Metal thin-filmsensor (left), ceramicthick-film sensor(centre) and open

piezo-resistive sensor(right)

tween them result from the different materialsused and their properties. The third sensorprinciple described is the piezo-resistive pres-sure sensor.

Metal thin-film sensor

The main body and the diaphragm of a metalthin-film sensor are usually made of stainlesssteel. They can be manufactured with the re-quired material thickness via machining thediaphragm in automatic precision lathes andthen grinding, polishing and lapping it. On theside of the diaphragm not in contact with the

medium, insulation layers, strain gauges, com-pensating resistors and conducting paths areapplied using a combination of chemical(CVD) and physical (PVD) processes and arephotolithographically structured using etching(Fig. 8). These processes are operated undercleanroom conditions and in special plants, insome parts under vacuum or in an inert atmos-phere, in order that structures of high atomic

Production


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Ceramic thick-film sensor 15

purity can be generated. The resistors and elec-trical conducting paths manufactured on thesensor are significantly smaller than a micro-metre and are thus known as thin-filmresistors.The metal thin-film sensor is very stable as aresult of the materials used. In addition, it is

resistant to shock and vibration loading aswell as dynamic pressure elements. Since thematerials used are weldable, the sensor canbe welded to the pressure connection her-metically sealed and without any additionalsealing materials. As a result of the ductilityof the materials, the sensor has a relatively

low overpressure range but a very high burstpressure.

Ceramic thick-film sensor

The main body and the diaphragm of the cer-amic thick-film sensor are made of ceramic.Aluminium oxide (Al

2

O3

) is widely used dueto its stability and good processability. The

Special features

Fig. 8:Photomask in orderto produce resistorstructures on thesensor diaphragms


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16 Sensor technology

four strain gauges are applied as a thick-filmpaste in a screen-printing process onto theside of the diaphragm which will not be in

contact with the pressure medium, and thenburned in at high temperatures and passiv-ated through a protective coating. No impur-ities are permitted during the screen-printingand the burn-in processes. Therefore, manu-facturing is usually performed in a cleanroom(Fig. 9). Only the leading manufacturers are

able to operate their plants with the propersegregation in order to avoid any cross-con-tamination and thus maintain the high pro-cess stability.

Productionprocesses

Fig. 9:Sensor production incleanroom

The ceramic used for the sensor is very corro-sion-resistant. However, installation of thesensor into the pressure measuring instrumentcase requires an additional seal for thepressure connection, which will not be resist-ant against all media. In addition, the ceramicis brittle and the burst pressure is thereforelower in comparison to a metal thin-filmsensor.

Special features


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Piezo-resistive sensor 17

Piezo-resistive sensor

A piezo-resistive sensor has a far more com-

plex structure than the sensors describedabove. The sensor element is made of a siliconchip. This chip consists of a diaphragm, struc-tured with piezo-resistive resistors, which de-flects under pressure. The chip has a surfacearea of only a few square millimetres and isthus much smaller than, for example, the dia-phragms of metal thin-film or ceramic thick-film sensors.The piezo chip is very susceptible to environ-mental influences and, therefore, must be her-metically encapsulated in most cases (Fig. 10).For this reason it is installed into a stainlesssteel case which is sealed using a thin flushstainless-steel diaphragm. The free volume be-

tween the piezo chip and the (external) dia-phragm is filled with a transmission fluid. Asynthetic oil is usually used for this. In an en-capsulated piezo-resistive sensor, the pressuremedium is only in contact with the stainless-

Structure

Encapsulation

Transmissionuid

Ventilation tube

Diaphragm

Piezochip/sensor

Displacementbody

Bond wires

Header

Pin

Fig. 10:Design of an encap-sulated piezo-resistivesensor


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steel diaphragm, which then transmits thepressure through the oil to the (internal) chipsdiaphragm.

To minimise the influence of the thermal ex-pansion of the transmission fluid on the pres-sure measurement, the sensor design must beoptimised in such a way that the free internalvolume for the given contour of the stainless-steel diaphragm is minimal. Among otherthings, special displacement bodies are used

for this purpose.A header is normally used for mounting andelectrical connection of the sensor chip. It hasintegrated glass-to-metal seals for the elec-trical connection between the inner and outerchambers and can be hermetically welded tothe case. The sensor element, glued to therear side of the header, is connected to the pinsusing bond wires (Fig. 11) and transmits theelectrical signals from the sensor element tothe connected electronics in the external cham-ber of the sensor. A ventilation tube, whichleads to the rear side of the sensor diaphragm,

Electricalconnection

Fig. 11:Bonding of thesilicon chip and theheader


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Sensor principles by comparison 19

is located in the centre of the header. If thechamber behind the sensor element is evacu-ated and the ventilation tube is closed, it is

possible to use such a piezo-resistive sensor tomeasure absolute pressure, since the vacuumof the hollow space serves as a pressure refer-ence. In sensors designed for gauge pressuremeasurement, the ventilation tube remainsopen and ensures continuous venting to therear side of the diaphragm, so that the meas-

urement is always performed relative to thelocal atmospheric pressure. The venting is real-ised either through the outer case or via aventilated cable to the outside. This ventilationtube must be carefully protected against con-tamination, especially moisture ingress, sincethe sensor is very susceptible to this and mayeven become inoperative.

Sensor principles by comparison

There is no ideal sensor principle since each ofthem has certain advantages and disadvantages(Table 2). The sensor type that is most suitablefor an application is primarily determined by

Measurementof absolute orgauge pressure

Requirement Sensor principle

Metal

thin-film

sensor

Ceramic

thick-film

sensor

Piezo-resistive

sensor

Measurement of the

absolute pressure

Very low pressureranges

Very high pressure

ranges

Shock and vibration

resistance

Long-term stability

Requirement fulfilled Requirement partly fulfilled Requirement not fulfilled

Table 2:Sensor principles bycomparison


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the demands of the application. It is not onlythe basic sensor technology that is key for thesuitability of the sensor, but above all the prac-

ticalities of its implementation. Depending onthe application, the sensor principles describedmay indeed make the implementation eithereasier or more difficult.The material in contact with the pressure me-dium (wetted parts) and its suitability for cer-tain media are of fundamental importance.

Thus, one of the disadvantages of the ceramicthick-film sensor in comparison with the metalthin-film sensor is that it requires additionalsealing between the non-metallic diaphragmmaterial and the case. This almost alwaysprevents universal applicability.The product ranges of sensor manufacturersare usually tailored and optimised to differentapplications dependent upon such consider-ations. Only universal instruments allow theusers themselves to select the suitable sensorprinciple. The leading manufacturers offer pro-ficient support for this purpose.

Selection of asuitable sensor


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21

Pressure measuring

instrumentsThis chapter presents the most common typesof electronic pressure measuring instrumentsand gives an overview of their design inrespect of a long service life. Subsequently,functional safety under environmental influ-ences will be addressed, and how it can be en-

sured through product testing.

Instrument types at a glance

Common instrument types are pressure trans-mitters, level probes, pressure switches andprocess transmitters. Basically, these elec-

tronic pressure measuring instruments consistof a pressure connection, a pressure sensor,electronics, an electrical connection and thecase (Fig. 12).In addition to those mentioned above, there arealso simpler instrument types known as pres-sure sensor modules; often consisting of no

Electrical connection

Electronics

Pressure sensor

Pressure connection Environmental conditions

Pressure

Outputsignal

Fig. 12:Structure of a

pressure measuringinstrument


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22 Pressure measuring instruments

more than a pressure sensor and simple mech-anical and electrical interfaces. These typesare particularly suitable for complete integra-

tion into users systems.

Pressure transmitterA pressure transmitter (Fig. 13) has standard-ised interfaces, both on the process side and onthe electrical output signal side, and convertsthe physical pressure value to a standard in-

dustrial signal. The pressure connection is usedto lead the pressure directly onto the sensor. Ithas a (standardised) thread and an integratedsealing system to enable easy connection of

Standardinstrument andfunctionality

Fig. 13:Pressure transmitter

the pressure transmitter simply by screwing it

in at the relevant measuring point. A suitablecase protects the sensor and the electronicsagainst environmental influences. The elec-tronics transform a weak sensor signal into astandardised and temperature-compensatedsignal; e.g. the common industrial signal of4 20 mA. The output signal is transmittedvia a (standardised) plug or cable for sub-sequent signal evaluation.


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Instrument types at a glance 23

Level probeThe level probe (Fig. 14), sometimes also re-ferred to as a submersible transmitter, is a spe-

cial type of pressure transmitter used for levelmeasurements in tanks, wells, shafts and boreholes. For this purpose the level probe meas-ures the hydrostatic pressure at the bottom ofthe vessel or well. Particularly important is thechoice of material for the case and cables,and also the seals at connection points, due to

complete and permanent submersion into themedium. Venting of the sensor system, re-quired for the gauge pressure measurement, isachieved via a ventilation tube passed throughthe cable.

Pressure switchIn many applications electronic pressureswitches replace the mechanical pressureswitches that used to be very common, sincethey offer, as a result of their design principle,additional functions such as digital display, ad-

justable switch points and considerably higherreliability. They are most frequently used inmachine building.

An electronic pressure switch is based on anelectronic pressure transmitter and thereforeoffers the entire functionality of a transmitter.With the integrated electronic switch, whichcan close or open an electrical circuit, it isable to perform simple control tasks. Theswitch point and the reset point can be set in-

dividually.By default, a pressure switch only outputsbinary signals such as switch point or resetpoint reached or not reached but it doesnot output how far the measured pressure isfrom the switch or reset point. That is whymany pressure switches have a display and ad-ditionally an analogue output signal. The setparameters and measured pressure can be read

Fig. 14:Level probe


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Fig. 15:Pressure switch withdisplay

off the display. In addition, the measured pres-sure can be transmitted by the analogue outputsignal to a downstream control unit. Thus, thiswidely adopted type of electronic pressureswitch includes a switch, a pressure transmitter

and a digital indicator all in one instrument(Fig. 15).

Process transmitterThe process transmitter (Fig. 16) is a pressuretransmitter with a pressure range that can beset within a predefined pressure range (turn-

down). It is mainly used in process en-gineering, since in this application area it isnecessary to adjust every single measuringpoint to a multitude of specific requirementsthat must be individually set by the operatoron site. The process transmitters have a veryhigh measurement accuracy within the entirepressure range. In addition, the pressurerange, the zero point and further parameters

User-configurable


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Instrument types at a glance 25

can usually be set individually. For this

purpose many process transmitters haveboth digital display and additional operatingelements and extensive operating softwaredirectly within the instrument.

Pressure transducerProviders of pressure transducers usually offer

a multitude of sensor modules that can bedirectly matched to the requirements of theuser. They have, for example, a user-specificpressure connection and/or a user-specificelectric interface (Fig. 17). Only very fewmanufacturers of electronic pressure measure-ment technology even offer the so-calledbare pressure sensor as a module. For these,the users must develop their own design solu-

Application-

specific features

Fig. 16:Process transmitterwith display


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Fig. 17:Pressure transducers

tions in order to get the pressure to the sensorand evaluate the sensor signal.For pressure transducers it is generally thecase that their correct function must be ensured

by the users design-related measures. There-fore, this option is usually only suitable formass-produced equipment.

Instrument qualification andreliability

A whole series of examinations is required forelectronic pressure measuring instruments to bequalified for a particular application. Of funda-mental importance here is the required reliabil-ity with respect to the service life of the instru-ment under the expected operating conditions.The (mean) service life is the mean time to fail-

ure (MTTF). It mainly depends on the operat-ing conditions. As a result of the operating con-ditions, the failure probability of the individualcomponents of an electronic pressure measur-ing instrument can vary considerably.

Pressure connectionPressure connections are standardised to agreat extent, easy to dimension and easy to

Mean time to

failure: MTTF


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Instrument qualification and reliability 27

handle. For pressure values up to 1000 bar,most are considered to be failure-proof, i.e.they offer practically unlimited service life. At

most for seals, in particular seals made of or-ganic materials, certain ageing effects are to beexpected. As long as the pressure medium iscompatible with the material and the operatingtemperature range is not exceeded, almost noserious problems occur. Detailed informationon media resistance is given in the relevant

technical literature and manufacturers specifi-cations.

Sensor systemWhen assessing the service life of the sensorsystem, a differentiation must be made be-tween the different sensor principles. Sincethese sensor types are exposed to completelydifferent loadings and the materials or ma-terial combinations used respond completelydifferently, a highly differentiated approachis absolutely essential.

Metal thin-film sensorsThe classic metal thin-film sensor represents

a clearly-defined system. The main body, in-cluding the weld seam, is usually over-dimen-sioned in order to ensure permanent stableconditions at the diaphragm and thus a longservice life. Dimensioning of the diaphragmgeometry and positioning of the strain gaugesare optimised using the Finite Elements

Method (FEM). This helps to achieve a lineardeflection of the diaphragm under pressure inthe case of radial and tangential tension overa large load range, which enables accuratemeasurement of the pressure values. Since thematerials used are mostly ductile steels orspecial alloys, the FEM simulations can alsoensure that the deflection of the diaphragmmaterial across the entire pressure range re-

Resistanceto media

FEM simulation


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mains far below the yield point (Fig. 18).Thus, local overloads with correspondingplastic deformation are avoided.

Mechanic

alstress

Fig. 18:FEM simulationof the equivalentstress intensity on thedeflected diaphragmof a metal thin-filmsensor

The fatigue life can be determined using stand-ard procedures such as fatigue testing, its re-sults are represented by the S-N curve (Whlercurve). The known and trusted manufacturersconsider 108 load cycles to be a safe designcriteria. Particularly for new developments,geometrical variations or material replace-

ments, despite this high value, manufacturerswill not do without the validation of theirdesign through empirical data based on fatiguetests conducted over weeks, or even months,on test benches. One of the reasons for this isthat, besides mechanical stress distribution,manufacturing procedures such as heat treat-

ment of steel and forming processes, as wellas production-related surface defects, for ex-ample striations, may also have significantinfluences.

Ceramic thick-film sensorsThe main body of the ceramic thick-film sen-sor is also overdimensioned. However, twomaterial-related differences must be consid-

Fatigue life test


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ered: on the one hand, the ceramic of themain body, unlike steel, does not suffer fa-tigue (ageing) so long as it is neither over-

loaded nor suffers additional stress due tomechanical or thermal shock. However, im-perfections such as slight impurities or micro-scopic mechanical defects on the surface mayresult in dramatic changes to the burst pres-sure and must therefore be monitored care-fully during the manufacturing process. On

the other hand, the ceramic main bodies re-quire carefully dimensioned mounting orseating and an additional seal in the transitionto the pressure port. Usually the ageing of thisseal, under the influence of load and tempera-ture changes and under the influence of theapplications pressure media, represents thelimiting factor. Therefore, there is often noother choice than to determine the servicelife and thus the suitability individuallythrough load cycle tests, especially under theinfluence of the medium and ambient tem-peratures.

Piezo-resistive sensors

To some degree, the same applies for thepiezo-resistive sensors as for the ceramic thick-film sensors. While the sensor material itself isalmost unaffected by fatigue, the rest of thesensor system must be designed carefully andevaluated for potential risks using, for ex-ample, failure mode and effects analysis

(FMEA). This applies both to the design andconstruction of the diaphragm seal (consistingof a diaphragm, capsule housing with dia-phragm bed and pressure port as well as a dis-placement body) and the design and bondingmethods used for the header and support forthe piezo chip. Load cycle tests, in particularfor high-pressure ranges, are also absolutelyessential for piezo-resistive sensors. However,

Load cycle testsunder tempera-ture influence

FMEA

Specific loadcycle tests


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the systems are so complex that individualtests are usually required.

ElectronicsThe dimensioning guidelines common instandard industrial electronics also apply tothe circuitry and electronic components usedin electronic pressure measurement technol-ogy. Of course, attention must be paid to thecorrelation between the number of compon-

ents used and the number of required solder-ing points as well as the strong correlation be-tween the service life of electronic compon-ents and the temperature. The approvedstandard methods can be used for the calcula-tion of service life. Since the MTTF valuesusually obtained in this way can be several

Fig. 19:Machine toperform the Highly

Accelerated Life Test(HALT)


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hundreds of years, they cannot be verified ex-perimentally, so accelerated ageing methodsmust be used (Fig. 19).

Instrument testsTo ensure the functionality of electronic pres-sure measuring instruments under all environ-mental conditions, the research laboratories ofmajor manufacturers regularly perform longseries of different tests. Some testing, e.g. that

for electromagnetic compatibility (EMC), isstipulated by law. The instruments cannot beplaced on the market if they have not passedthese tests. Other tests are carried out to meetparticular market requirements and/or to spe-cific operating conditions. If standards or dir-ectives for certain tests exist, then they areimplemented. If the standards do not provideadequate test procedures, market-specific andapplication-oriented tests are often developed.For market-specific test procedures the appli-cation conditions are simulated as accuratelyas possible. The test objects are often not onlyexposed to one test, but must pass a wholeseries of tests. They are exposed, for example,

first to strong vibrations (Fig. 20) and must

EMC tests

Application-specific tests

Fig. 20:

Vibration test


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then resist severe shocks. Alternatively theywill be successively heated, cooled, immersedin water, exposed to salt spray and daubed

with solvents or lubricants and additives. Aninstrument will have passed the test series onlyif it measures with its original accuracy bothduring and after the tests. This type of ex-tended product testing is usually known asapplication-specific standard testing proced-ure. Since the test contents and procedures

are specified by each manufacturer individu-ally, they must generally be requested by theend-user and evaluated accordingly.

Environmental influences andspecial requirements

Temperature influenceSince temperature influences many propertiesof a material, it also affects the proper oper-ation of measuring instruments. Very high orvery low temperatures can damage or evendestroy parts of the measuring instrument. Inparticular, plastic parts and sealing materials

age much faster under the influence of con-stant high or low temperatures. For example, ifthe temperature is too low, they lose their elas-ticity.To ensure proper function of the pressuremeasuring instruments, some manufacturersspecify temperature ranges in their data sheets

for the pressure medium, ambient conditionsand during storage. Other manufacturers de-fine an operating temperature range which in-cludes both the medium and ambient tempera-ture range. The measuring instrument will notbe damaged provided these specifications areadhered to. The data specified in the datasheets regarding the measuring accuracy (seepage 58 ff.), on the other hand, are only valid

Manufacturerspecifications


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Environmental influences and special requirements 33

for the temperature-compensated range whichis significantly smaller and will also be speci-fied in the data sheets.

Compatibility with the pressure mediumThe pressure media are as many and diverse asthe applications of pressure measurement tech-nology. In pneumatics it is mostly air mixedwith residues of compressor oil and condensedwater; in level measurement it is mostly fuel,

oils or chemicals. In hydraulics the pressure ofthe hydraulic oil must be measured; in re-frigeration technology, the pressure of the re-frigerant must be measured.All physical and chemical characteristics of thepressure medium must be considered when se-lecting the material and other properties ofthose parts of the pressure measuring instru-ment in contact with the pressure medium. Spe-cial attention must be paid to the fact that thediaphragms are only a few microns thick. Ma-terial abrasion due to corrosion cannot be ac-cepted; not only because it would erode the dia-phragm, but also since the measurement char-acteristics would change continuously. Due to

the small material thickness there is a risk ofpressure medium diffusing through the dia-phragm and reacting with the materials behindit, for example filling media and adhesives.To prevent chemical reactions initiated by ag-gressive media, measures such as having aflush stainless-steel diaphragm with a highly-

resistant coating made of special plastic,ceramic materials or noble metals are oftentaken. As an alternative, the wetted parts canbe made of titanium or other special materialssuch as alloys based on nickel, molybdenumor cobalt.The reactivity of the pressure medium is, how-ever, just one aspect from a whole range. If,for example, the water used as a pressure me-

Diaphragm

Pressureconnection


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dium does not drain completely and subse-quently freezes, it may damage the internalsensor diaphragm as a result of expansion.

Lime deposits can also clog the pressure port.Some media, such as those with high viscosityor high solids content, require a pressure con-nection without a pressure port. For this pur-pose a flush variant of the sensor diaphragm isused (see page 52 f.).

Protection against soiling and waterThe electronic components and electrical con-nections must be protected against the ingressof any foreign objects or water in order to en-sure they continue to operate. The IP ratingsdefined in the DIN EN 60529 standard specifywhat level of protection is provided by an elec-trical or electronic instrument at room temper-ature against contact with, and intrusion of,foreign objects (first digit) as well as againstingress of water (second digit). A higher IP rat-ing does not automatically imply an improve-ment in protection. For example, IP67 (totaldust ingress protection, protection against tem-porary immersion) does not necessarily cover

IP65 (total dust ingress protection, spray waterprotected), since the load due to spray watercan be significantly higher than the load dur-ing temporary immersion. For the IP68 rating(total dust ingress protection, protectionagainst permanent submersion), the manufac-turer must always specify additionally the

duration and depth of immersion. These con-ditions are not specified in the standard.Sealing problems can also be caused throughtemperature variations. Therefore, some manu-facturers utilise different testing procedures toverify that their measuring instruments remainfunctional and measure within the specifiedaccuracy limits even after temperature vari-ations.

IP rating


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The use of pressure measuring instrumentsoutdoors places especially high demands onthem. A combination of high ambient humidity

and low temperature can lead to condensationor even icing. Large cyclic climatic fluctu-ations can lead to the accumulation of waterwithin the instrument if the instrument is notsealed (pumping effect).Intensive moisture accumulation (continuouscondensation) on the measuring instrument,

and partially inside it, occurs regularly if theambient humidity is high and the temperatureof the pressure medium is much lower than theambient temperature. In this case, a specialcase design is needed, which can only berealised for certain instruments optimised forsuch operating conditions.

Mechanical load capacityIn many applications the pressure measuringinstruments are sometimes exposed to signifi-cant shock and vibration loadings. Vibrationloads are oscillating mechanical loads oflonger duration. In contrast, shock is consid-ered as an impulse wave which abates quickly

compared to vibration. Strong vibrations, forexample, have an effect when using pressuremeasuring instruments on test benches andengines. Shocks occur, for example, duringmobile use in a vehicle driving on a roughroad, or during stationary application in ma-chines with high accelerations during oper-

ation, such as solid forming presses or dropforges.For the pressure measuring instrument to beused safely in applications with strong vibra-tions and/or shocks, it must withstand theseloads. The vibration resistance of industrialpressure transmitters is usually in the range of10 to 20 times the acceleration due to gravity(10 g to 20 g). Nowadays, the shock resistance

Case design

Typical vibra-tion and shockresistance


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of industrial pressure transmitters is at severalhundred g.

Electromagnetic radiationEvery electrically operated device can poten-tially emit electromagnetic radiation. However,since an electronic circuit can also be influ-enced by electromagnetic radiation, such in-struments can also influence (interfere with)each other. The requirements for electromag-

netic compatibility (EMC) cover both inter-ference emission and immunity.EMC problems frequently occur if many elec-tronic devices are located within a small space.With increasing automation this is also thecase in many applications of electronic pres-sure measurement technology. EMC problemsoccur more and more frequently, because ofthe increasing operating frequency and electri-cal power of electronic devices, plants or sys-tems.In the European Union (EU) protection re-quirements are stipulated by the EMC direct-ive and its implementations in national laws,which refer to the corresponding harmonised

standards. Mandatory limit values for the in-terference immunity and emitted interferenceare specified in the standards. Only instru-ments developed and manufactured in accord-ance with these standards may be labelled withthe CE mark and placed on the Single Euro-pean Market.

However, for the reasons mentioned above, incertain applications the end-users will placemuch higher demands on the electromagneticcompatibility and, in particular, on the interfer-ence immunity, in order to ensure safe oper-ation even under unfavourable conditions.These are summarised in factory standards orspecial specifications and must be individuallychecked for a particular prototype.

Interferenceemission andimmunity

Legalrequirements

Increasedrequirements


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Explosion protectionFor electronic measuring instruments used inhazardous areas it is necessary to ensure

through technical measures that, in accordancewith the classification of the hazardous area,no ignition source can have an effect. Thereare several technical approaches to achieve ex-plosion protection for an electrical instrument.The corresponding design concepts are re-ferred to as explosion protection types. In elec-

tronic measurement technology the most fre-quently used is the concept of limitation of theignition energy referred to as intrinsic safety(abbreviation i). For this, the current and volt-age of the electrical power supply are limitedin such a way that neither the minimum igni-tion energy nor the ignition temperature of anexplosive mixture are ever reached. Anotherexplosion protection type is enclosing themeasuring instrument in a flameproof en-closure (abbreviation d), where all componentsthat are likely to cause ignition are installedwithin an enclosure that can withstand theinternal explosion pressure. The escapingignition energy is reduced by means of gaps

between the enclosure parts to the extent thatno ignition or external transmission of it ispossible.The operator of a plant or equipment is gener-ally responsible for compliance with the re-quirements for the equipment and facilities.Requirements for equipment that can present

an ignition hazard have been harmonisedacross Europe. They are listed in the ATEXproduct directive, 94/9/EC. The directive de-scribes the conformity assessment procedurefor electrical and non-electrical instrumentsused in hazardous areas. The manufacturer canor must obtain an EC-type examination certifi-cate in accordance with the conformity assess-ment procedure and mark it correspondingly

Types of explo-

sion protection

Fig. 21:Symbol for explosion

protection valid forEurope


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on the instrument (Fig. 21). Within the scopeof the quality assurance system, the manufac-turer bears the responsibility of ensuring that

every single instrument is manufactured inaccordance with this EC-type examinationcertificate.


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39

Standard applications

and requirementsElectronic pressure measuring instrumentstake on a multitude of tasks in the industrialenvironment. Among other things they assistin the extraction of clean potable water fromwells or desalination plants, in the safe controlof the landing flaps of aircraft, in the econom-ical operation of air conditioning and refriger-ation plants, in the production of high-per-formance materials, in the chemical industry,in environmentally-friendly power generationwithin fuel cells and in the efficient control ofheat pumps. They ensure the safe operation ofcranes and elevators, trouble-free operation ofmachine-tools and automated machinery, en-vironmentally sound combustion in engines andthe stable and energy-saving running of powerunits and drives.Despite this diversity, the application of elec-tronic pressure measurement technology cangenerally be assigned to one of three areas: to

the monitoring of critical system pressure, tothe control of pressure or to the indirect meas-urement of process values. The following de-scription of standard applications from allthree areas gives an overview of the demandsplaced on electronic pressure measuring in-struments.

Critical value monitoring

In applications within the field of critical valuemonitoring, the pressure measuring instrumenthas the task of reporting that a certain criticalpressure level has been exceeded or has notbeen achieved. For pure monitoring, pressureswitches are most suitable. A pressure trans-

Trend-settingapplications

Three fieldsof application


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40 Standard applications and requirements

ducer, in addition, enables the continuousmeasurement of the system pressure.For instance, leak detection in systems with

elevated pressure: if there is a leak in a system,the system pressure drops. As soon as the pres-sure drops below the specified critical value,the electronic pressure switch or pressuretransmitter reports this. To detect the leaks assoon as possible, very high measurement ac-curacy is usually required.

Another example is the monitoring of the de-gree of clogging of filters (Fig. 22). With theincreasing degree of clogging, the pressure

Leak detection

Fig. 22:Filter monitoring

conditions upstream and downstream of thefilter also change. If an electronic pressuremeasuring instrument is installed upstream ordownstream of the filter, it can report cloggingof the filter or indicate the optimum time forfilter replacement.


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Pressure control 41

Pressure control

In the case of pressure control using an elec-

tronic pressure measuring instrument, a differ-entiation must be made between the control ofa constant pressure or the control of a pressureprofile.

Control of constant pressure

When supplying media via pumps it is often

advisable to keep the delivery pressure con-stant. This can be achieved with an electronicpressure measuring instrument and an elec-tronic controller. The pressure measuring in-strument sends the measured pressure value tothe controller. The controller checks whetherand to what extent the current pressure (actualvalue) deviates from the nominal pressure(nominal value) and reports this to the pumpcontroller. Depending on the pressure devi-ation, the controller adjusts the drive power insuch a way that the actual pressure value oncemore approximates the nominal pressure value.This offers not only efficient control of theprocess, but also enables energy-efficient op-

eration since the drive power of the pump iscontinually adjusted to the actual demand.

Control of a defined pressure profile

An electronic pressure measuring instrumentand an electronic controller can also be used toensure operation corresponding to a defined

pressure profile, its monitoring and, if neces-sary, recording. A typical example is autofret-tage, during which the pipes are pressurised toa defined multiple of their permitted operatingpressure. This intentional overpressure leads toa partial plasticity and thus to an intentionalcompression of the pipeline material, thusallowing the pipelines to withstand pressurespikes better. In this application the pressure

Autofrettage


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profile must be controlled accurately and theachievement of the defined pressure valuesmust be reliably documented. Since very high

pressure values (of up to several thousand bar)must be measured repeatedly with constantaccuracy, especially high demands are placedon the pressure measuring instruments used insuch applications.

Indirect measurement of processvalues

Indirect force measurement

According to equation 1 (see page 6) it is pos-sible to determine the force generating thepressure by measuring this pressure, providedthe geometry is known. An example is givenin figure 23 which shows lifting hydraulicswith two movable pistons, each with differentsurface areas in contact with the hydraulic oil.If the smaller piston moves downwards with a

Liftinghydraulics

F1

F2

Fig. 23:Hydraulic principle


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Indirect measurement of process values 43

force F1, the larger piston pushes upwards witha greater force F2 since the pressure in theliquid remains constant.

One of the most typical pressure measurementtasks in hydraulic systems is overload moni-toring on lifting gear, clamping devices ortools. If, for example, a crane lifts a load, thepressure required to generate the counteractingforce in the hydraulic liquid increases. If themaximum permitted load is exceeded, the

pressure will also consequently exceed the setupper limit value. In this way it is possible todetect the load torque limit on the basis of themeasured pressure in the hydraulic fluid.Many hydraulic applications are present in mo-bile hydraulics (Fig. 24), for example, in con-struction machinery, agricultural vehicles, lift-ing platforms or forklifts. Pressure measuringinstruments used in such applications must of-ten withstand very high operational shock andvibration loads; they must also have especiallyhigh electromagnetic interference immunity.

Load torquemonitoring

Fig. 24:Pressure measuringinstruments in the

mobile hydraulicsindustry must besuitable even forharsh operatingconditions.


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Furthermore, they must withstand extreme cli-matic conditions during outdoor operation.Since such machines often need cleaning using

high-pressure steam cleaners, they must remainleak-tight from all sides, even under high jetpressures. In addition, they must be resistantnot only to hydraulic oil, but also against manyother media, such as dust, mud and fuel.Especially high demands are placed on thecontrol of a hydraulic press via indirect force

measurement of the hydraulics. A predefinedforce profile must be maintained for everypressing cycle. An electronic pressure meas-uring instrument can be used to monitor andcontrol this profile.

Indirect level measurementThe hydrostatic pressure under a static liquidcolumn increases proportionally with theheight of the column. Thus, for example, thepressure in a water tank becomes 100 mbarhigher, compared to the effective atmosphericpressure on the water surface, with everymetre of water depth (Fig. 25).

Control of ahydraulic press

5

0.5

Hydrostatic pressure in bar

Waterdepthinm

0

10

1.0

Fig. 25:Functional correla-

tion between waterpressure and depth


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Electronic pressure measuring instruments areused for indirect level measurement if the levelof the tank must be monitored; for example to

avoid completely emptying the tank or if it isnecessary to continuously monitor the con-sumption of the tank contents. Depending onthe application, either a level probe is sub-mersed into the tank or a pressure measuringinstrument is attached to the bottom of the ex-terior of the tank and exposed to pressure of

the tank contents through an opening in thebottom of the tank (Fig. 26). If the tank is notvented, or if it is under higher pressure, it isnecessary to measure the pressure prevailingon the surface of the liquid in the tank and totake this into account when determining thehydrostatic pressure. This can be carried out intwo ways: either by using two independentpressure measuring instruments and then

Fig. 26:Level measuringoptions at a tank


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generating the pressure differential in thedownstream control unit, or by using specialdifferential pressure measuring instruments

with two process connections designed for thisapplication.(Re-)Filling of the tank can also be carried outautomatically by means of an electronic pres-sure switch. For this purpose it is necessary todefine two states tank is full and tank isempty and switch on/off the supply pump

using the switch contacts depending on the re-ported state. Continuous level control using anintegrated analogue output or the digital indi-cation on the electronic pressure switch are anadditional benefit.Electronic pressure measuring instruments forlevel measurement are characterised aboveall by their resistance to the pressure mediumand their mostly relatively small pressurerange. A further requirement for level probes(due to their continuous submersion) is that themedium must not enter neither the cable northe probe itself, even at submersion depths ofseveral hundreds of metres. In explosion-pro-tected applications, forexample, in bore holes

for oil and gas exploration or in refineries andchemical industry plants, the measuring instru-ment must above all correspond to the requiredexplosion protection type. For use in wells,shafts and bore holes, the design must be asslim as possible and there are high demandsregarding the robustness of the (mostly very

long) cable.

Indirect temperature measurementIn air conditioning and refrigeration plants orheat pumps, pressure measurements are usedfor the indirect measurement and control oftemperature. For example, they ensure thatfood on the refrigerated shelf or freezer re-mains cool.

Automatedfilling of a tank

Level probesfor special

applications


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In the evaporator of a refrigeration circuit (Fig.27), the cold, liquid refrigerant absorbs theheat from the surroundings needing cooling.During the evaporation stage, it absorbs add-

itional thermal energy from the surroundings the evaporation enthalpy. This phase transitioncan be controlled very accurately by means oftargeted depressurisation of the refrigerantunder pressure in the expansion valve. Thecooling effect obtained can be controlled veryaccurately using the measured and controlled

pressure. The evaporated and heated refriger-ant is compressed again through a compressorwhich makes its temperature and pressure riseagain. With pressure transmitters, it is possibleto determine the pressure in the refrigerant cir-cuit exactly, and to control the expansion valveand the compressor systematically. The meas-ured pressure also allows conclusions to bedrawn on the phase state of the refrigerant.

Compressor

Expansion valve

Heat Heat

Evaporator

Pressuremeasuring

point

Pressuremeasuringpoint

Condenser

and

ventilator

Fig. 27:Refrigeration circuit


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Since liquid refrigerant can damage the com-pressor, it is necessary to ensure that it is stillgaseous prior to compression. In this instance,

pressure measurement also takes on an import-ant safety function. As soon as the compressedand hot refrigerant is in the compressor, itstarts releasing thermal energy into the en-vironment and thus becomes liquid again. Inlarge refrigeration systems a ventilator speedsup the condensation. If the pressure, and in-

directly the temperature, are measured in thecondenser, the ventilator power can be ad-justed exactly to the corresponding require-ments. This demand-oriented ventilator controlleads to significant energy savings. The use ofpressure transmitters in the refrigeration circuitallows both better control of the process andsignificant energy savings.The measuring instruments used should be, onthe one hand, resistant against all common re-frigerants and, on the other hand, they mustmeasure with high accuracy despite the ex-traordinary temperature conditions. Upstreamof the compressor the temperature may reach40C and downstream of the compressor up

to +100C. This accuracy is needed in order toenable very accurate control of depressurisa-tion of the refrigerant in the evaporator. How-ever, in the future, the use of new refrigerantscould lead to much higher demands related tothe operating temperatures and the pressurerange.

Safety function

Process controland energysaving


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49

Criteria for

the instrumentselection

Except for special designs and models whichare specifically for particular applications,pressure measuring instruments are generallyavailable in many variants, which differ fromeach other with regard to their pressure range,pressure connection, electrical connection,output signal and measuring accuracy in par-ticular. The selection of a pressure measuringinstrument suitable for a specific applicationis therefore a complex process. This chapterprovides an overview of the most important

specifications for pressure measuring instru-ments.

Pressure range

The pressure range specified in the data sheetof a pressure measuring instrument defines the

limits within which the pressure can be meas-ured or monitored. Essential for the specifica-tion of the pressure range are the lower andupper limits of the pressure range (Fig. 28) andwhether it is absolute or gauge pressure. Theaccuracy data specified in the data sheet ap-plies within the pressure range.

Pressure ranges specified in the data sheetwhich are under and over the limits of thepressure range are referred to as overpressureranges. Pressures within the overpressurerange will not cause any permanent damage tothe sensor; however, the measuring error limitsspecified in the data sheet may be exceeded.Only pressure values above the overpressurelimit, i.e. known as the destructive range, can

Adjacentpressure ranges


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50 Criteria for the instrument selection

lead to irreversible damage of the measuringinstrument. It does not matter whether thispressure is present constantly or only for ashort period of time. Once the specified burstpressure has been exceeded, the complete de-struction of the parts exposed to the pressure

and the sudden release of the pressure mediumcan be expected. Therefore, these operatingconditions must always be avoided throughcareful design.Special attention is required in the eventof pressure spikes in the case of dynamic pres-sure elements. They are caused, for example,

by the switching on and off of a pump, theconnection or disconnection of a hydraulicsystem and, in particular, by the openingand closing of the fast-acting valves in fluidflows. These pressure surges can reach amultiple of the operating pressure. This effectsometimes occurs in households if a tap isturned off quickly. It is known, technically,as water hammer. The pressure wave de-

Pressure spikes

Lower limit

Overpressurerange

Pressurerange

Destructiverange

Upper limit

Overpressurerange

Burstpressure

Pressure

Overpressure limit

Signal

Fig. 28:Measuring range,overpressure rangesand destructiverange


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Pressure range 51

veloped propagates through the entire systemand leads to extremely high loads, and oftento the overload of the sensors. Pressure spikes

in the destructive range can even cause thesensor element to burst. Therefore, they rep-resent a safety hazard and must always beconsidered when designing the plant. Com-mon ways to reduce pressure spikes are to usethrottles in the pressure port and EDM drill-ings. Such restrictions prevent the uninhibited

propagation of a pressure wave by reflectingmuch of it.Extremely high pressure spikes can be causedby cavitation and the micro-diesel effect.Cavitation is generally described as the for-mation and implosive dissolution of hollowspaces in liquids due to pressure variations.The resulting short-term pressure and tem-perature peaks can even lead to material re-moval on metallic components. If, due to cavi-tation, small bubbles consisting of a combust-ible air-hydrocarbon mixture are formed,these can burn due to local spontaneous self-ignition during pressure increase this isknown as the micro-diesel effect. If no special

measures are taken, the pressure wave result-ing from a micro-explosion can cause seriouspressure spikes in the hydraulic system and,as a consequence, lead to the destruction ofcomponents. Due to the design-based and thedesired sensitivity of the pressure sensors, it isnecessary either to effectively prevent these

effects or to ensure the sensors are suitably pro-tected from the impacts of these effects. Thoseelectronic pressure measuring instruments de-signed specifically for hazardous applicationshave protective mechanisms built-in, e.g. thepreviously mentioned EDM drillings, spe-cially designed throttle elements or special-ised baffle and deflector plates within thepressure port.

Protectionagainst cavita-tion and micro-diesel effect


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Pressure connection

The pressure connection, also frequently re-

ferred to as the process connection, is used tochannel the pressure medium to the sensor. Al-most all pressure connections have a standardthread and can therefore be screwed in at themeasuring point without problems.Leading manufacturers often provide a multi-tude of different pressure connections for theirpressure measuring instruments in order tomeet the various requirements of the widestrange of industries and applications, as well asregional and national standards.

Internal and flush diaphragmsThere is a differentiation between pressureconnections with an internal diaphragm and

connections with a flush diaphragm. In pro-

Internaldiaphragm

Pressureconnection

Pressure medium

Transmissionuid

Pressureconnection

Pressure medium

Flushdiaphragm

Internaldiaphragm

Fig. 29:Internal (top)and flush (bottom)

diaphragm


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Pressure connection 53

cess connections with an internal diaphragmthe pressure medium directly contacts thesensor diaphragm through the pressure port

(Fig. 29 top). In process connections with aflush diaphragm the pressure port is itselfclosed flush, using an additional stainless-steeldiaphragm. A transmission fluid transmits thepressure up to the internal sensor diaphragm(Fig. 29 bottom).Pressure connections with internal diaphragms

and a pressure port are easier to handle andcheaper to manufacture than those with a flushdiaphragm. They are primarily used with gas-eous and liquid pressure media. For all pressuremedia that can clog or damage the pressureport (for example crystalline, viscous, aggres-sive, adhesive or abrasive media), use of aflush diaphragm is recommended. Also, if theapplication requires residue-free cleaning ofthe pressure connection, the flush diaphragmshould be preferred to the internal diaphragm.

ThreadIn order to enable the simultaneous screwingin and sealing of the measuring instrument seal

at the measuring point, the pressure connec-tions are usually designed with a thread. Dif-ferent threads are commonly used worldwide(Table 3). Generally, both male and femalethreads are common.

Selection criteria

Threads Short symbol Region/Country

Parallel pipe threads G Western Europe

Self-sealing pipe threads NPT North America

Fine threads UNF North America

Metric threads M Eastern Europe and Russia

Conical Whitworth pipe

threadsR or PT Asia

Table 3:Overview of threads


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SealThe sealing concepts are as diverse as thethreads. Some threads are self-sealing, for ex-

ample taper threads. On the other hand, otherthreads require an additional seal. For thisthere are different application-specific and re-gional solutions. The most common for paral-lel threads are sealing behind the thread (i.e.between the thread and the case) or sealing infront of the thread by means of a metallic

spigot (Fig. 30).

Sealing

Pressureconnection

Sealing

Pressureconnection

Spigot

Fig. 30:Sealing betweenthread and housing(top); sealing withmetal spigot (bottom)

Electrical connection

The electrical connection of an electronicpressure measuring instrument is imple-mented using either a standard plug-in con-nector or using a cable output (Fig. 31). Thenature of the connection has a considerable

Connectoror cable


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Output signals 55

influence on the IP rating of the instrument(see page 34 f.) and often limits the permis-sible ambient temperature range and the re-sistance of the instrument to aggressivemedia or environmental influences (e.g. UVradiation). To ensure the reliability of theelectrical connection in the application, it isnecessary to know exactly the specific instal-

lation conditions and to consider them whenselecting the electrical connection. For plug-in systems, one must above all bear in mindthat the mating plug (selected by the user)and the entire associated cable entry forms anintegral part of the sealing system for the in-strument case.

Output signals

The output signal of an electronic pressuremeasuring instrument is generally an ana-logue voltage or current signal. It is transmit-ted to a control unit connected downstreamof the instrument. However, pressure measur-ing instruments are also available with digital

Reliability

Analogue ordigital

Fig. 31:Various electricalconnections


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outputs. With the exception of switching out-put signals, which are, strictly speaking, al-ready a digital signal, the output signal

should be as proportional as possible to thepressure.For this purpose, the sensor must first of allgenerate a measurable sensor signal propor-tional to the pressure. To achieve this, the re-sistors in the measuring instrument with straingauges on the sensor are wired to a Wheat-

stone measuring bridge. In pressure trans-mitters, process transmitters and pressureswitches with an analogue output signal, lowlevel sensor signals are amplified, filtered andstandardised through the electronic compon-ents. The result is a standard industrial signalwhich is used as an output signal. The mostimportant output signals are described brieflybelow.

Standard analogue output signalThe most common output signal in pressuremeasurement technology is the analogueoutput signal. Commonly used are the currentsignal 4 20 mA and the voltage signals

0 5 V, 0 10 V and 1 5 V. In comparisonto voltage signals, the advantages of the cur-rent signals are a much lower sensitivity toelectromagnetic interference and automaticcompensation of conduction losses by thecurrent loop. The elevated zero point of the4 20 mA current signal and likewise with

the 1 5 V voltage signal also enables cablebreak detection and instrument fault detec-tion.The 4 20 mA output signal is commonlytransmitted using 2-wire technology, whichenables the sensor to source its supply energydirectly from the current loop. The other ana-logue signals require a 3-wire connection thatuses the third lead for the power supply.

Standardindustrialsignals

4 20 mA


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Output signals 57

Ratiometric output signalsThe analogue output signals which are easiestto generate are those which are proportional to

the supply voltage, where the zero point andfinal value represent a constant percentage ofthe sensor supply voltage. Thus, for examplethe 10-90 signal has a zero point which is 10%of the supply voltage and a final value whichis 90%. If the supply voltage decreases by 5%,then the absolute analogue signal also de-

creases by 5%. Thus, the ratio of the outputsignal to the supply voltage remains the same.These sensors are often operated with a(reduced) supply voltage of 5 V. The 10-90signal is then specified in the data sheets as0.5 4.5 V ratiometric. This is the mostcommon ratiometric output signal.

Digital output signalBasically, the transmission of digital outputsignals offers the possibility of communicationwith the pressure measuring instrument via afieldbus system, so operating data and par-ameters can be exchanged. However, bothprocesses are of minor importance in industrial

pressure measurement technology. Therefore,electronic pressure measuring instruments witha connection to CANbus or PROFIBUS-DPplay a minor role in industrial applications atthe moment.Digital communication modulated on an ana-logue output signal (for example using HART

on a standard 4 20 mA signal) is also onlyestablished for pressure measuring instrumentsin certain areas. The reasons for this are aboveall the much higher costs of the pressuremeasuring instrument and the associated per-ipherals, the elaborate integration of the in-struments as a result of additional control soft-ware and the (relatively) low extra benefit.Since, basically, additional configuration of

0.5 4.5 Vratiometric

Bus-compatiblesignal


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the bus for the pressure measuring instrumentis needed, and the diagnosis of a faulty digitalconnection is much more elaborate than for an

analogue connection, for many applicationsthe advantages of a potentially more accuratemeasured value do not outweigh the additionalcosts.

Characteristic curve, accuracy andmeasuring error

The characteristic curve of an instrument re-flects the functional dependency of the outputsignal on the input signal. Ideally, the outputsignal of an electronic pressure measuring in-strument changes with pressure in a linear man-ner. Thus, the ideal characteristic curve is a

straight line. The measured (i.e. the actual)characteristic curve is, however, not an exactstraight line. Even at the start and end point ofthe pressure range the output signal can deviatefrom the corresponding ideal values (Fig. 32).

Actual characteristic

Ideal characteristic

Signa

l

PressureFig. 32:

Ideal and actualcharacteristic curves


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Characteristic curve, accuracy and measuring error 59

The deviation of the actual characteristic curvefrom the ideal one is often referred to as ac-curacy. However, this term is not defined in

any standard. Instead, other values are taken todetermine the measuring errors. The measur-ing errors are usually given as a percentage ofthe span. The span is the difference betweenthe end and start value of the output signal.Thus, for the standard 4 20 mA signal, thespan is 16 mA.

Maximum measuring error

The measuring error includes all relevanterrors at a constant temperature (e.g. refer-ence temperature), such as non-linearity, hys-teresis, zero offset and span error. It can bedetermined directly from the characteristic

curve. If the pressure measuring instrumentis operated at this temperature, then the max-imum measuring error is the maximum errorwith which the pressure can be measured(Fig. 33).

Accuracy

Span

Error atreferencetemperature

Fig. 33:

Measuring error at adefined temperature

Maximum

measuring error


Signa

l

Pressure



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Non-linearityThe measuring error, referred to as non-linear-ity, is defined as the largest possible positive

or negative deviation of the actual characteris-tic curve from the reference straight line. Thereare different methods to determine the refer-ence straight line. The two most common arethe terminal method and the best-fit straightline method (Fig. 34). In the case of the ter-

Determinationof the referenceline


Terminal method

Best-t straight line (BFSL)Signal

Pressure

Fig. 34:Determination ofthe non-linearityaccording toterminal method andbest-fit straight linemethod

minal method, the reference straight line passesthe start and end point of the measured charac-teristic curve. In the case of the best-fit straight

line method, the reference straight line (in datasheets often referred to as BFSL) is positionedin relation to the measured characteristic curvein such a way that the sum of squares of thedeviations is minimal.If one compares both methods with each other,the terminal method usually provides twice aslarge a deviation as the best-fit straight linemethod. A comparison of the non-linearity of

BFSL


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electronic pressure measuring instrumentsfrom different manufacturers is, therefore, onlyrepresentative provided the non-linearity is de-

termined using the same method.The non-linearity is a basic characteristic ofthe sensor system used. If necessary, it can beminimised electronically by the manufacturer.

HysteresisIf the characteristic curve of a measuring in-

strument is recorded with steadily increasingpressure and then with steadily decreasingpressure, it can be observed that the output sig-nals for identical pressures do not match ex-actly. The maximum deviation between the in-creasing and decreasing characteristic curve isreferred to as the hysteresis (Fig. 35).

Electroniclinearisation

Hysteresis

Signal

Pressure

Fig. 35:Hysteresis

The hysteresis depends on the elastic proper-ties of the sensor material and the design prin-ciple of the sensor. It cannot be compensatedthrough any technical measures (e.g. by adjust-ment).

No technicalcompensation


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Zero offset and span errorThe actual zero and end point of the outputsignal can deviate from the ideal zero point

and end point. The zero offset and span errorsare the differences in value between the idealand the actual values of the zero point and endpoint of the output signal. The zero offset and

Two independ-ent errors


Span error

Zero offset

Sig

nal

Pressure


Fig. 36:Zero offset andspan error

the span error must always be considered inde-pendently when assessing the measuring ac-curacy (Fig. 36). In extreme cases, both canprovide the same preceding sign and producethe maximum permitted error value in thesame pressure measuring instrument.

Non-repeatabilityLike other technical systems, electronic pres-sure measuring instruments are also exposedto stochastic influences, i.e. random influ-ences. Therefore, the output signal for thesame pressure values in the case of successivemeasurements is not always exactly the same,even if the measurements are conducted underidentical conditions.


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This measuring error, referred to as non-re-peatability, is given as the greatest deviationduring three successive pressure measurements

under identical conditions and thus is the valueof the difference between the largest and thesmallest measured output signal. Therefore, asmall non-repeatability is a basic requirementof each reliable sensor system with a definedaccuracy.

Temperature errorEvery change in temperature directly influ-ences the measurement-related properties ofthe electronic pressure measuring instrument.Thus, with rising temperature the electrical re-sistance of metals increases and the piezo-resistive resistance of the semiconductors de-creases. Most materials expand when they areheated. This and other effects cause inevitablemeasuring errors as a result of temperaturechanges.To prevent these temperature errors, the manu-facturers of electronic pressure measuring in-struments take a number of measures relatingto both the sensor system and the associated

electronics. Thus, the sensor design (materialsand geometry) is basically optimised to achievea balanced thermal behaviour in order to beable to minimise the non-linearities and discon-tinuous behaviour. Remaining errors, inevitabledue to residual tolerances, are systematic tem-perature errors and can be reduced during the

manufacturing process or by means of suitableon-board digital processing the keyword hereis smart sensor.The compensation of temperature-relatedmeasuring errors during the manufacturingprocess is carried out either directly on thesensor and/or in the associated electronics. Forexample, it is possible to perform laser trim-ming of the measuring bridge. To perform the

Definition

Counter-measures

Compensationduringproduction


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compensation of the entire system, consistingof the sensor and electronics, the accuracy ofthe sensor module (or even of the entire pres-

sure measuring instrument) at different tem-peratures is compared to reference instruments(calibration). If necessary, it is adjusted elec-tronically or, using a specific PCB assembly,via the corresponding compensation resistors.Precision measuring instruments often have anadditional temperature sensor integrated into

the case and related programmed logic thatcompensates the temperature error directlywithin the instrument. This procedure is oftencalled active temperature compensation.In spite of all compensation measures a smalltemperature error will still remain. This erroris specified either as a temperature coefficientor as temperature error range. If the manufac-turer defines a temperature coefficient, a (lin-ear) error is assumed in relation to a referencepoint (e.g. room temperature). At this pointthe temperature error is minimum, and it in-creases with increasing difference from thereference point with the specified coefficient

Activetemperature

compensation

Temperature error band

Error at ambient temperature

Temperatureerror at60C

Error

Temperature20C 60C

Fig. 37:Temperaturecoefficient andtemperature errorband


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in a linear manner (Fig. 37). The sum of thezero temperature error and the span tempera-ture error gives the maximum total tempera-

ture error.If the temperature error is given in the form ofan error band as an alternative, the maximumtemperature error present within the tempera-ture compensated pressure range defines thescope of the error band.

Long-term stabilityBy design the characteristic curve of a pres-sure measuring instrument is not constant dur-ing its entire service life; it can change slightlyover time due to mechanical (pressure change)and, above all, due to thermal influences. Thiscreeping change is referred to as the long-termstability or also as long-term drift.As a rule the long-term stability is determinedby laboratory testing. However, since the test-ing procedures for different manufacturers candiffer significantly, information on long-termstability should not be compared. In addition,simulations always work with reference condi-tions as a basis. The actual long-term stability

under operationa

handbook electronic pressure

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