736

5.5 Diaphragm or Capsule-Type Sensors

B. G. LIPTÁK

(1969, 1982, 1995),

REVIEWED

BY

J. WELCH

(1995)

J. E. JAMISON

(2003)

Design Pressure:

Up to atmospheric with evaluated motion balance capsule; up to 50 PSIG (344 kPa)or more with evacuated force balance capsule; up to 200 PSIG (1.4 MPa) withatmospheric reference motion balance capsule; up to 1500 PSIG (10 MPa) withatmospheric reference force balance capsule.

Design Temperature:

Phosphor bronze (

−

50 to 250

°

F, or

−

46 to 120

°

C). Ni-Span C (

−

50 to 300

°

F, or

−

46to 149

°

C), 316 stainless steel (

−

400 to 600

°

F, or

−

240 to 316

°

C), Inconel (

−

300 to1000

°

F, or

−

184 to 538

°

C). The other components besides the diaphragm elementcan limit the operating to 250

°

F (120

°

C) or less.

Materials of Construction:

Buna-N, nylon, Inconel, Ni-Span C, phosphor bronze, 316 stainless steel, berylliumcopper, Monel, brass, titanium, tantalum, Hastelloy, nickel, duranickel, Teflon, Kel-F,polytetrafluoroethylene, CrNi, Ni-Cr-Co alloy.

Range:

Absolute pressure ranges from 0–5 mmHg to 0–50 PSIA (0–0.7 to 0–350 kPa); gaugepressure ranges from 0–0.5 in. H

2

O to 0–200 PSIG (0–0.12 kPa to 0–1.4 MPa).

Inaccuracy:

0.1 to 1% of span. Standard indicators, recorders, and switches are the least accurate;intelligent transmitters are the most accurate.

Costs:

$200 to $500 for indicators and switches, $500 to $1200 for direct recorders, and$800 to $2000 for direct controllers and transmitters.

Partial List of Suppliers:

ABB Automation Technology (formerly Kent-Taylor) (www.abb.com)Ametek Inc. U.S. Gauge Div. (www.ametekusg.com)Bailey, a Unit of ABB (www.abb.com)Barton Instruments Systems (www.barton-instruments.com)Dresser Industries, Instrument Div. (www.dresserinstruments.com)Dwyer Instruments Inc. (www.dwyer-inst.com)Fischer & Porter, a Unit of ABB (www.abb.com)Fisher Controls International, a Div. of Emerson Process Management (www.emersonprocess.com)

The Foxboro Co. (www.foxboro.com)Honeywell Inc. (www.honeywell.com)Mid-West Instrument (www.midwestinstrument.com)Moeller Instrument Co. (www.moellerinstrument.com)Moore Products, now part of Siemens Inc. (www.sea.siemens.com)Rosemount Inc. (www.rosemount.com)Validyne Engineering Corp. (www.validyne.com)Weksler Instrument Corp. (www.dresserinstruments.com)WIKA Instrument Corp. (www.wika.com)Yokogawa Corp. of America (www.yca.com)

PI

PT

Flow Sheet Symbol

© 2003 by Béla Lipták

5.5 Diaphragm or Capsule-Type Sensors

737

INTRODUCTION

This section is devoted to the description of diaphragm- andcapsule-type pressure sensing elements and to the force andmotion balance devices that utilize them. As shown in Figure 5.5a,the variety of such pressure detector designs is wide andvaried.

These diaphragm-type pressure sensors are also dis-cussed under differential pressure detectors (Section 5.6) andunder electronic pressure sensors (Section 5.7) because thestrain gauge, capacitance, potentiometric, resonant wire,piezoelectric, inductive, reluctive, and optical transducers canall be provided with diaphragm elements. Similarly, some ofthe high-pressure sensors (Section 5.8), pressure repeaters(Section 5.12), and vacuum sensors (Section 5.14) can alsobe provided with diaphragms as their sensing elements.

The full range deflection of a single diaphragm is usuallylimited to about 0.002 in. (0.05 mm), and the amount of deflec-tion varies with the 4th power of the diameter of the diaphragm.Therefore, for the same amount of pressure change, the dia-phragm deflection increases 16-fold if the diameter is doubled.One method of increasing the total deflection is to weld severaldiaphragms together into capsules.

DIAPHRAGM ELEMENTS

Pressure sensors that depend on the deflection of a diaphragmhave been in use for over a century. In the last few decades,the elastic hysteresis, friction, and drift effects have been

reduced to approximately

±

0.1% of span in the high-qualitydesigns. This has been achieved mostly by the use of micro-processor technology in smart transmitters. In traditional dia-phragms the hysteresis error amounted to about 0.25 to 0.5%of full scale and nonlinearity could range from 0.1 to 2% offull scale.

What the microprocessor contributed in improving theperformance of diaphragm-type sensors was its ability torecall from its memory the appropriate correction factors forthe different values of diaphragm deflections. In other words,the microprocessor does not eliminate the nonlinearity ofdiaphragms, but it does memorize the amount of nonlinearityand electronically corrects for it.

Materials and Configurations

Diaphragm materials with good elastic qualities, such asberyllium copper, and with very low temperature coefficientsof elasticity, such as Ni-Span C, are used. Inconel and stain-less steel are used when extreme operating temperatures ora corrosive process requires them. Quartz diaphragms areused when minimum hysteresis and drift are desired.

The diaphragm is a flexible disc, either flat or with con-centric corrugations, that is made from sheet metal of precisedimensions. The pressure deflection characteristics of bothflat and corrugated diaphragms have been well investigated.The available corrugated profile types include sinusoidal,saw-tooth, or trapezoidal (see Figure 5.5b). The type of sealand peripheral clamping (fixing) used are dependent on thecharacteristics of the diaphragm.

FIG. 5.5a

The variety of diaphragm- and capsule-type pressure and differential pressure detectors and transmitters is very great. (Courtesy of Foxboro-Invensys.)

© 2003 by Béla Lipták

738

Pressure Measurement

Some instruments use the diaphragm as the pressuresensor; others use it as a component in a capsular element.Figure 5.5b shows a single corrugated diaphragm and alsosome capsule designs. The capsules consist of two diaphragmswelded together at their peripheries. Two basic types ofcapsules are illustrated: the convex and the nested. Evacuatedcapsules are used for absolute pressure detection, and singlediaphragms are used for highly sensitive measurements. Thesensitivity of a capsule increases in proportion to its diameter,which in the conventional designs varies from 1 to 6 in. (25.4to 152.4 mm).

Multiple capsule elements can be built from either convexor nested capsules as shown on Figure 5.5d. These elementsare useful in increasing the output motion resulting from apressure change.

SENSOR CONFIGURATIONS

In this section, four basic designs of diaphragm pressuresensors will be discussed. They will be distinguished by thepressure reference used (full vacuum or atmospheric) andby the method of balancing the forces generated by theprocess pressure into force or motion balance designs.When atmospheric reference is used, the reading is calledgauge pressure; when the reference is full vacuum (actuallyabout 0.001 mmHg), the measurement is called absolutepressure. When differential pressure is measured there is noreference, as the two measurements are only subtracted.

Motion balance units are capable of driving local, directreading indicators but are subject to hysteresis, friction, anddead-band errors. Force balance designs are transmittingdevices with high accuracy but without direct indicationcapability.

Absolute Pressure Sensors

Motion Balance

Figure 5.5e illustrates a motion balancetype absolute pressure detector. The capsular element is fullyevacuated and changes its length as a function of the processpressure in its housing. Because its length depends on thedifference between internal and external pressures andbecause the internals have been fully evacuated, the element’slength is a measure of the absolute pressure acting on theoutside of the capsule. The capsular element is sealed insidethe pressure-tight housing. The meter shaft transmits the cap-sule motion to the readout device or controller through abellows seal. This seal protects against air leakage undervacuum and is fairly frictionless.

The capsule and the bellows seal are normally availablein bronze, stainless steel, and nickel materials, or they can besilver or gold plated. Because the evacuated diaphragm ele-ment would collapse as the pressure on its outside increases,the capsules have been designed to bottom on each other whenthey are exposed to pressures above the range of the instrument.

FIG. 5.5b

Common convolution cross-sections. (Courtesy of WIKA InstrumentCorporation.)

FIG. 5.5c

Standard diaphragm elements.

Sinusoidal Corrugation

Corrugation with Overpressure Protection

Sawtooth Corrugation

Corrugated SingleDiaphragm

Motion Motion

Convex DiaphragmCapsule

Nested DiaphragmCapsule

FIG. 5.5d

Multiple capsule element.

Free End

Spacers

Fixed End

© 2003 by Béla Lipták

5.5 Diaphragm or Capsule-Type Sensors

739

Therefore, the unit can be exposed to pressures up to atmo-spheric without damage to the capsule element.

The ranges available with this instrument can be as lowas 0 to 5 mmHg (0 to 0.7 kPa) or as wide as 0 to 760 mmHg(0 to 100 kPa). Inaccuracy can be better than

±

1% of fullscale for the wide range elements, and for the narrow rangecapsules, it is a fraction of a millimeter of mercury.

Force Balance Design

The force balance detector illustratedin Figure 5.5f is utilized in pressure transmitters, but not indirect indicator, because there is no motion available to drivea pointer. If local indication is desired, an output gauge can beinstalled on the transmitter. On the smart electronic transmit-ters this gauge is an integral part of the detector (Figure 5.6d),while on the old pneumatic transmitters, a pressure gaugedetects the output air pressure. For a detailed sketch of apneumatic force balance transmitter see Figure 5.6e.

The main sensing element in a force balance pressuretransmitter is the capsule. The pressure being detected is

applied to the left side of the diaphragm in the capsule, whilethe space on the other side of the diaphragm is fully evacuated,providing a zero absolute pressure reference (Figure 5.5f). Theforce felt by the force bar is related to the difference betweenfull vacuum on the right side of the diaphragm and the processpressure on the other side or to the absolute pressure of thesystem measured. Due to the force balance nature of the unit,the force bar is constantly balanced; therefore, the sensingdiaphragm does not move as long as the pressure detected iswithin the range of the instrument. If the range of the capsuleis exceeded, the diaphragm moves slightly to the right whereit can rest on and be supported by a backup plate with match-ing convolutions. This feature provides the transmitter’s highover-range protection.

Transmitter Ranges and Materials

The standard material ofconstruction for these diaphragms is stainless steel. Table 5.5glists some of the important features of this transmitter family.The transmitters are distinguished from one another by thediameter of the capsule. The transmitter ranges are adjustablein the field between the minimum and maximum values listed.Special transmitters are available in all-Monel construction orwith Hastelloy body and tantalum capsule.

Atmospheric Reference

Motion Balance Design

In this pressure detector, shown inFigure 5.5h, the process pressure inside the capsule is bal-anced against the spring action of the element. The outsidesurface of the diaphragm assembly is exposed to atmosphericpressure, which is then the pressure reference for this instru-ment. This unit is available either as a direct readout recorderor as a transmitter for remote readout. Due to the straight-line expansion of the capsule, the force-to-friction ratio isreasonably high. The diaphragm area is selected to producea substantially greater force than required to operate themovement.

FIG. 5.5e

Motion balance absolute pressure sensor.

FIG. 5.5f

Force balance absolute pressure detector.

Fixed End

EvacuatedMultipleCapsuleElement

MeterShaft

ProcessConnection

Bellows“Seal”

To Force BalanceMechanism

(For Details See Fig. 5.6e)

Force BarFulcrum& Seal

Back-UpPlate

ProcessConnection

Diaphragm

EvacuatedSpace

TABLE 5.5g

Features of Force Balance-Type Absolute Pressure Instruments

Features

Transmitter Range

Low Medium High

Capsule diameter: 5 in. (127 mm) in (89 mm) 2 in. (51 mm)

Minimum range: 0 to 10 mmHg 0 to 40 mmHg 0 to 375 mmHg

Maximum range: 0 to 40 mmHg 0 to 400 mmHg 0 to 1520 mmHg

Overpressureprotection up to:

50 PSIG(3.5 bars)

100 PSIG(7 bars)

150 PSIG(10.5 bars)

Inaccuracy (% span):* 1% to 1% to 1%

Operating temperature:

−

40 to 250

°

F(

−

40 to 121

°

C)

−

40 to 250

°

F(

−

40 to 121

°

C)

−

40 to 250

°

F(

−

40 to 121

°

C)

*“Intelligent” transmitters can reduce this error to 0.25% of span withanalog and to 0.1% with digital signal transmission.

3 12

12

12

© 2003 by Béla Lipták

740

Pressure Measurement

The diaphragm capsules can be either convex or nested(see Figure 5.5c). The nested capsule can be exposed tohigher overpressures than the convex ones, but all units aredesigned to withstand at least 100% overpressure over theirfull range. Figure 5.5i shows the nested capsule made of aconvex and a concave diaphragm with the process pressureacting on the outside of the capsule rather than on the insideas in Figure 5.5e. This is the reason why high overpressurescan be tolerated with the nested capsule. The range of thiselement is a function of the materials of construction, capsulediameter, and the particular design.

Materials of Construction and Spans

Some of the frequen-tly used diaphragm materials include: Cu-Ni-Mn (60% copper,20% nickel, 20% manganese), phosphor bronze, Ni-Span C(constant modulus nickel alloy, 42% nickel, 2.4% titanium,5.4% chrome, 50% iron), 316 stainless steel, and Inconel.The available capsule diameters vary from 2 to 5 in. (50.8 to127.0 mm), with the larger diameters being sensitive enoughto detect the lower pressures.

The nested capsule-type elements illustrated in Figure 5.5ican detect both gauge and vacuum pressures. Gauge pressurescompress the space between the nested diaphragms, whilevacuums open it up. When used to detect vacuum, the instrument

is referred to as a “compound” gauge. Because the vacuum ismeasured against an atmospheric reference and because baro-metric pressure can change substantially, this is not an accuratemethod of detecting vacuums.

The minimum span available with these units is 0 to 3in. H

2

O (0 to 0.7 kPa) either below or above atmospheric,and the maximum span can be as high as 180 PSI (1200 kPa).This can mean, for example, a range of 20 to 200 PSIG (138to 1400 kPa) or a full vacuum to 165 PSIG (1100 kPa)compound range. This range of pressure is covered by avariety of capsular elements, each having a minimum andmaximum span.

For orientation purposes only, a few of the standard cap-sule spans are given in Table 5.5j. The zero adjustment onthe instrument allows the range to be shifted to above orbelow atmospheric, and the span adjustment on the unitallows for narrowing or widening the range within the min-imum and maximum span limits noted. The inaccuracy ofthese units is normally

±

1% of span or better.

Slack Diaphragms

Figure 5.5k illustrates the slack-dia-phragm type motion balance pressure sensor with atmo-spheric pressure reference. In this unit the process pressureis balanced against either the spring action of the diaphragmor against a calibrated spring, as illustrated in Figure 5.5k.

This device is the most sensitive pressure detector in thefamily of elastic element sensors, and as such, it is capableof measuring near-atmospheric draft pressures. If the housingencloses both sides of the diaphragm, the unit can act as adifferential pressure detector. The diaphragms are availablein metal, such as stainless steel, or can be made of elastomers,

FIG. 5.5h

Motion balance pressure sensor with atmospheric reference.

FIG. 5.5i

Nested diaphragm sensor.

OverrangeProtector

ProcessConnection

ProcessConnection

TABLE 5.5j

Features of Motion Balance-Type Absolute Pressure Instruments

Capsule Minimum Span Maximum Span

2 in. diameter Cu-Ni-Mn 20 in. H

2

O (5 kPa) 5 PSI (34.4 kPa)

3 in. diameter Cu-Ni-Mn 8 in. H

2

O (2 kPa) 40 in. H

2

O (10 kPa)

Ni-Span C—large diameter 3 in. H

2

O (0.75 kPa) 5 PSI (34.4 kPa)

Ni-Span C—small diameter 5 PSI (34.4 kPa) 15 PSI (103 kPa)

Ni-Span C—nested capsule 12 PSI (82.7 kPa) 180 PSI (1.24 MPa)

FIG. 5.5k

Slack-diaphragm detector.

ProcessConnection

© 2003 by Béla Lipták

5.5 Diaphragm or Capsule-Type Sensors

741

such as Buna-N or nylon. The slack-diaphragm elements canactuate local, direct readout devices or transmitters for remotereadout. Although the pressures measured are small, theforce-to-friction ratio is still satisfactory due to the largediaphragm areas in these designs.

The available spans range from 0–0.5 to 0–120 in. H

2

O(0–0.12 to 0–30 kPa). These spans can be either on thepositive or vacuum pressure side or can cover compoundranges. Inaccuracy is in the range of

±

1 to

±

2% of span, andthe slack-diaphragm elements can withstand overpressurebetween 5 and 25 PSIG (35 and 173 kPa), depending on theparticular design.

Force Balance Designs

The force balance differential pres-sure detectors (discussed in Section 5.6) can also be used asgauge pressure sensors if the low-pressure side of the differ-ential pressure (d/p) cell is left open to atmosphere. In suchinstallations, the output signal corresponds to the processpressure, which is connected to the high-pressure side of thed/p capsule.

Figures 5.6d, 5.6e, and 5.6f illustrate the design and oper-ating components of these instruments. The minimum spanavailable with these units is 0 to 2 in. H

2

O (0 to 0.5 kPa) andthe maximum span is 0 to 30 PSIG (0 to 2000 kPa). Onlythree capsules are required to cover this wide range. Theminimum and maximum spans and other features of thestandard d/p cells are listed in Table 5.5l.

Features and Construction Choices

These capsules can bebuilt into the conventional d/p cell body shown in Figure 5.6aor into the flat and extended diaphragm arrangements illus-trated in Figure 5.6f. In the flat diaphragm design, the low-range capsule requires a 6 in. (150 mm) flange, while themedium- and high-range capsules can be accommodated by3 in. (75 mm) flanges. The extended diaphragm version isavailable with medium- or high-range capsules only andrequires a 4 in. (100 mm) mounting flange.

These transmitters have inaccuracies between

±

1

/

2 and

±

1% of span in their standard configurations and 0.1% ofspan in their microprocessor-based intelligent versions. Theycan withstand operating temperatures of

−

40 to 400

°

F (

−

40to 200

°

C). The maximum static pressure that they can beexposed to varies from 225 PSIG (1.6 MPa) for the 6 in.

diameter flanged unit to 6000 PSIG (41 MPa) for the high-pressure version of the standard unit shown in Figure 5.6a.

The parts exposed to the process fluid can be made of alarge variety of materials, including carbon and stainlesssteel, Monel, Duranickel, Hastelloy, tantalum, and nickel, orprovided with coatings such as Kel-F or Teflon.

PRESSURE REPEATERS

Pressure repeaters are force balance devices capable of gen-erating an air output signal of the same pressure as that ofthe process. These units are useful for isolating the processand thereby preventing corrosion, the possibility of pluggingin the sensing line, and other hazards. Because these unitsare discussed in Section 5.12, only a summary of their fea-tures is given here.

The flanged repeaters shown in Figures 5.12c and 5.12dare available in 2, 3, and 4 in. sizes (50, 75, and 100 mm),with either flat or extended diaphragm construction. Theextended design is used when the main consideration forinstalling a repeater is protection against plugging. The avail-able materials for wetted parts include stainless steel, Hastel-loy, nickel, Teflon, Kel-F, polyvinyl chloride, Penton, andothers.

Available spans vary from 0–1 to 0–100 PSIG (0–6.9 kPato 0–0.7 MPa), and the repeater inaccuracy can be as goodas

±

1

/

4% or 0.1 in. H

2

O (2.5 mm H

2

O) per PSIG repeated.When positive pressures are to be repeated, the air (or

other gas) supply pressure has to be 125% of the highestpressure to be repeated. On vacuum service, the repeaterinaccuracy of

±

2 mmHg (

±

0.3 kPa) normally limits use toabsolute pressures above 40 mmHg (5.2 kPa).

The repeaters can work either on a vacuum referencesource (see Figure 5.12d), in which case the reference vac-uum always has to be lower than the lowest absolute pressureto be repeated, or on atmospheric pressure reference with asuppression spring. If the spring is set at 16.5 PSI (114 kPa),the repeater output will be zero-shifted by this amount. Whenthe process is at atmospheric pressure, the output is 16.5PSIG (114 kPa), and when the process is at full vacuum, theoutput is 1.5 PSIG (10 kPa).

The operating temperature and static pressure limitationsof these units are similar to those of the force balance d/p cells.

TABLE 5.5l

d/p Cell Capsule Capabilities

Low Range Medium Range High Range

Minimum span—in. H

2

OMinimum span—kPa

0–20–0.5

0–250–6.2

0–30 PSID0–210

Maximum span—in. H

2

OMaximum span—kPa

0–1500–37.5

0–10000–250

0–3000 PSID0–207 bars

Maximum zero suppressionMaximum zero elevation

Maximum spanMinimum span

Calibrated span

© 2003 by Béla Lipták

742

Pressure Measurement

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© 2003 by Béla Lipták