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5.8 High-Pressure Sensors

B. G. LIPTK (1969, 1982, 1995, 2003)

Types and Ranges: A. Optical (Section 5.7), up to 60,000 PSIG (4338 bars)

B. Piezoelectric (Section 5.7), up to 100,000 PSIG (6896 bars)

C. Magnetic (Section 5.7), up to 100,000 PSIG (6896 bars)

D. Dead-weight testers, up to 100,000 PSIG (6896 bars)

E. Helical Bourdon (Section 5.4), up to 100,000 PSIG (6896 bars)

F. Manganin cells, up to 400,000 PSIG (27,586 bars) or more

G. Strain gauge (Section 5.7), up to 200,000 PSIG (13,793 bars)

H. Bulk modulus cells, up to 200,000 PSIG (13,793 bars)

I. Button type pressure repeater, up to 10,000 PSIG (6896 bars)

Inaccuracy: For dead-weight testers, 0.1% of span or better; for strain gauges from about 0.1%

of span to 0.25% of full scale, for Manganin cells from 0.1 to 0.5% of full scale; for

pressure repeaters 0.5 to 1% full scale, for helical bourdon tubes 1% of span; for

bulk modulus cells from 1 to 2% of full span

Costs: For types A, B, C, and G, see Section 5.7; for type E, see Section 5.4. Most transducers

are from $300 to $500. The simplest dead-weight gauges with moderate ranges and

0.1% inaccuracy cost around $1200 to $1500; the average portable pressure/vacuum

calibrator costs around $5000; the most sophisticated 0.03% hydraulic calibrator units

cost about $18,000.

Partial List of Suppliers: 3D Instruments LLD (D) (www.3dinstruments.com)ABB Automation Technology (E) (www.abb.com)

Ametek Inc. (D, E) (www.ametekusg.com)

Ametek Drexelbrook (G) (www.drexelbrook.com)

Barber Colman Industrial (G) (www.barber-colman.com)

Barksdale (G) (www.barksdale.com)

Barton Instrument (G) (www.barton-instruments.com)

Cosa Instrument (D) (www.cosa-instrument.com)

DH Instruments (D) (www.dhinstruments.com)

Dresser Instrument (A, D, E, G) (www.dresserinstruments.com)

Druck Inc. (B, G) (www.pressure.com)

Dwyer Instruments (G) (www.dwyer-inst.com)

Entran Devices Inc. (G) (www.entran.com)

Fisher Controls Int., a Div. of Emerson Process Management (E)(www.emersonprocess.com)

Foxboro-Invensys (E, F) (www.foxboro.com)

Helicoid Instruments Div. of Bristol Babcock (E) (www.bristolbabcock.com)

Honeywell Inc. (E) (www.honeywell.com)

Kistler-Morse (G)

Marsh Instrument Co. (E) (www.marshbellofram.com)

Marshalltown Instruments Inc. (E) (www.marshbellofram.com)

Mensor Corp. (B, E, quartz helix) (www.e-pressure.com)

Mid-West Instrument (E) (www.midwestinstrument.com)

MKS Instruments (D) (www.mksinst.com)

Morehouse Instrument (D)

Moeller Instrument Co. (E) (www.moellerinstrument.com)

Moore Products, now part of Siemens Inc. (E) (www.sea.siemens.com)

PI High

Flow Sheet Symbol

2003 by Bla Liptk

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5.8 High-Pressure Sensors 763

Noshok Inc. (E) (www.noshok.com)

OCI Instruments Inc. (E) (www.ociinstruments.com)

Palmer Instruments Inc. (E) (www.palmerinstruments.com)

Perma-Cal Corp. (E) (www.perma-cal.com)

Reotemp Instrument (D) (www.reotemp.com)

Rosemount Inc., a Div. of Emerson Process Management (E)

(www.emersonprocess.com)

Ruska Instrument (D) (www.ruska.com)Scanivalve Corp. (G) (www.scanivalve.com)

Senso-Metrics Inc. (G) (www.senso-metrics.com)

Sensotec (G) (www.senso-metrics.com)

Smar International (D) (www.smar.com)

H.O. Trerice Co. (E) (www.hotrerice.com)

Vaisala Inc. (D) (www.vaisala.com)

Validyne Engineering Corp. (E) (www.validyne.com)

Viatran Corp. (G) (www.viatran.com)

Wallace & Tiernan (D) (www.wallace-tiernan.com)

Wallace & Tiernan Inc. (E) (www.usfwt.com)

Weiss Instruments Inc. (E) (www.weissinstruments.com)

Weksler Instruments Corp. (E) (www.dresserinstruments.com)

Wika Instrument Corp. (E) (www.wika.com)

Yokogawa Corp. of America (E) (www.yca.com)

The term high pressure is relative, because in an average

plant the pressure of 1,000 PSIG (69 bars) is usually consid-ered to be high, while in synthetic diamond manufacturing

100,000 PSIG is viewed as normal. For the purposes of this

section, we will define high-pressure instruments as devices

that are capable of measuring pressures in excess of 10,000

to 20,000 PSIG (700 to 1,400 bars). Some of these detectorshave already been discussed in Section 5.4 (helical Bourdons)

and in Section 5.7 (strain gauge, optical, piezoelectric, and

magnetic types). Therefore, in this section the emphasis will

be on the description of dead-weight piston gauges, bulkmodulus, and Manganin cells.

INTRODUCTION

High pressure can be measured by:

1. Dead-weight testers

2. Pressure repeaters

3. Elastic deformation gauges, such as helical bourdon

tubes, strain gauges, or bulk modulus cells

4. Detecting the change in electrical resistance in mate-

rials like Manganin

One might group these sensors by other characteristics, such as:

1. Mechanical, such as pressure repeaters, helical bour-

don tubes, or dead weight testers

2. Electronic, like the strain gauge devices

3. Very high pressure detectors, as the bulk modulus andthe Manganin cells.

The only primary high-pressure detector is the dead

weight sensor, which is also a rather slow measuring device.

The sensors that detect elastic deformation follow Hokes

Law but not with absolute accuracy and all have at least 0.1%

hysteresis. The Manganin gauge was first described by theNobel prize winning physicist Bridgman

1who recommended

it as a secondary gauge.

MECHANICAL HIGH PRESSURE SENSORS

Dead-Weight Piston Gauges

As illustrated in Figure 5.8a, these are piston gauges in whichthe test pressure is balanced against a known weight that is

applied to a known piston area. The test pressure is applied

by the secondary piston. The principal purpose of these

free-piston gauges is as a primary standard to calibrate other

pressure sensors. The National Bureau of Standards (NBS)has been using these devices for many years.

Piston gauges, or dead-weight testers, are normally pro-

vided with a number of interchangeable piston assemblies

and NBS-certified weights. They can be used to calibrate at

pressure levels as low as 5 PSIG (35 kPa) or as high as

FIG. 5.8a

Dead-weight piston tester.

Dead

Weight

Primary PistonGauge

under TestCylinder

Screw

Secondary

Piston

2003 by Bla Liptk

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764 Pressure Measurement

100,000 PSIG (690 MPa). The range has been extended to

even greater pressures, but research on piston and cylinder

material and their treatment to withstand loads is a limitation.

Assuming that one wants to generate a pressure of 100,000

PSIG while keeping the dead weight under 1000 lb (450 kg),

it is necessary to reduce the piston area to 0.01 in2

(6.4 mm2).

This means that a 0.1 in. (2.5 mm) diameter piston will have

to support a 1000 lb weight, while also being rotated.

The accuracy of dead-weight piston testers has improved

over the years. For higher pressure services, the main

improvement resulted from controlling the piston-cylinder

clearance by pressurizing the outside surface of the cylinder.

Thus, the piston-cylinder clearance is kept constant, resulting

in a slow rate of fall for the piston unaffected by pressure

level. The laboratory piston gauges are standardized by NBS,

calibrating the associated weights and measuring the piston

diameter. NBS has found these dead-weight testers to be inac-

curate to 1.5 parts in 10,000 of the measured pressure at values

greater than 40,000 PSIG (280 MPa) and to 5 parts in 100,000

at lower pressures. The inaccuracy of industrial dead weight

testers is better than 0.1% of span.

The free-piston gauge is limited to its principal purpose,

a primary standard for calibrating other pressure sensors,

because it is slow in response and is not practical for direct

industrial installation.

The utility of the high-accuracy piston gauges is being

extended to the lower pressure ranges by the titling-type, air-

lubricated designs. With such design, pressures (and pressure

differentials) in the millimeter of mercury range have been

detected to one part in 100,000 full-scale error.

Button-Type Pressure Repeater

This instrument (Figure 5.8b) is discussed in more detail in

Section 5.12. It has been developed for extruder monitoring

and control in the plastics and synthetic fiber industries. It

can repeat the process pressures within an error of 0.5 to 1%,

and it can operate up to 10,000 PSIG (69 MPa) and at tem-

peratures up to 800F (430C).

Helical Bourdon

The detailed features of this instrument (Figure 5.8c) are

discussed in Section 5.4. The helical elements used in thisinstrument are available with spans up to 0 to 80,000 PSIG

(0 to 550 MPa) and can detect pressures with an error of

about 1% of span.

BULK MODULUS CELLS

These cells, shown in Figure 5.8d, are comprised of a hollow

cylindrical steel probe closed at the inner end, and a stem that

projects beyond the outer end of the probe. When subjected

to process pressures, the active part of the probe contracts

isotropically, causing its tip to be displaced to the right. As a

result, the stem moves outward, increasing the distance it

projects beyond the outer end. The stem motion can be detected

by electromagnetic pickup, capacitance pickup, or the use of

mechanical displacement transmitters (pneumatic or electronic).

The unit is available with ranges of 050,000 to 0200,000

PSIG (0350 to 01,400 MPa), and its inaccuracy is 1 to 2%

of full scale. Its advantages, when compared with other high-

pressure sensors, include its relatively fast response, its

remote-reading characteristic, and its design that is absolutely

FIG. 5.8b

Button-diaphragm-type pressure repeater.

FIG. 5.8c

Helical Bourdon-type pressure sensor.

FIG. 5.8d

Bulk modulus cell.

Output

Air Signal (P2) Balancing

Diaphragm

A2

A1

P1P2 =

P1 A1 = P2 A2

Air Supply Vent

Force Bar

Regulating

Valve

SensingDiaphragm

ThereforeA1A2

P1

200(P1)

Process

Pressure

Moving

Tip

Pressure Stem

Probe Cell

Body

Packing

2003 by Bla Liptk

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5.8 High-Pressure Sensors 765

safe because the probe is not subject to fatigue. The hyster-

esis and temperature sensitivity of the bulk modulus cell are

similar to those of other elastic element pressure sensors.

PRESSURE-SENSITIVE WIRES

The electric resistance of wires can be changed by applying

linear strain or by applying hydrostatic pressure to the surface

of a helically wound coil mounted on a core. This second

approach is utilized in the operation of the Manganin or gold-chromium wire type pressure sensors. These materials have

been selected because their electric resistance changes very

little with temperature variations, while it does change appre-

ciably with changes in the applied process pressure.

When a small coil of Manganin wire is subjected to high-

process pressures, the coil resistance changes linearly withpressure. The pressure-resistance relationship for Manganin

is substantial, positive, and linear, and therefore can bedetected by a bridge. Manganin is relatively insensitive to

temperature variations.

These cells can be obtained with ranges from 0 to 50,000

PSIG (0 to 3,450 bars) to 0 to 425,000 PSIG (0 to 29,300bars), and their inaccuracy is between

1/10 and

1/2% of full

scale.

The main disadvantage of this cell is its delicate nature.

Both the gauge coils and the coil protection bellows can be

easily damaged by rapid changes in pressure or liquid viscosity.

The pressure-resistance relationship of other materials,such as platinum, gold-chromium, or lead, have some of the

same desirable features as Manganin, and they too have beenused as elements in pressure-resistance cells.

CHANGE-OF-STATE DETECTION

One other method for high-pressure sensing is to determine

the pressure at which change-of-state occurs in various mate-rials and then to apply that as a standard. Some of the change-

of-state points have already been determined. For example, it

has been established that the melting point of mercury at 0C

is 109,765 30 PSIG (757 0.2 MPa). Similarly, the first

polymorphic transition point of bismuth has been found to bebetween 365,000 and 370,000 PSIG (2519 and 2553 MPa).

DYNAMIC SENSORS

The interest in dynamic pressure measurement to detect blast

pressures, rapid chemical reactions, combustion pressures of

rocket propellants, and so on has increased in recent years.

Several electronic transducers have been developed for usewith elastic elements. Because these devices were covered

in Section 5.7, only a brief listing will be given here.

Electronic transducers for dynamic pressure detection

include the piezoelectric transducers; the bonded and

unbonded strain gauge elements; and the variable reluctance,

differential transformer, and electrical capacitance types.

Strain gauges bonded to diaphragm or bellows elements

have given good performance in measuring blast pressures.

In connection with underwater explosions and noises, piezo-

electric crystals have been successfully used. These units are

directionally sensitive to force, necessitating a seal interposed

between the element and the process and converting pressure

to force for optimum response.

Reference

1. Bridgman, P.W., Physics of High Pressure, London: G. Bell & Sons,

Ltd., New York: MacMillan, 1952.

Bibliography

Babichev, G.G., Kozlovskiy, S.I., Romanov, V.A., and Sharan, N.N., Pres-

sure Transducers with Frequency Output on the Base of Strain-

Sensitive Unijunction Transistors, Paper 2.31, 1st IEEE Interna-

tional Conference on Sensors (IEEE Sensors 2002), Orlando, FL,

June 2002.

Bailey, S.J., Pressure Sensors and Transmitters Affected by Technological

Change, Control Engineering, January 1984.

von Beckerath, A., Eberlein, A., Julien, H., Kerstein, P., and Kreutzer, J.,

WIKA Handbook on Pressure and Temperature Measurement, U.S. ed.,

Lawrenceville, GA: Wika Instrument Corp., 1998.

Bourdon Pressure Gauges, Measurements and Control, December 1991.

Buckon, L., Considerations in Selecting a Pressure Calibration Device,

Paper #910449, Instrumentation, Systems, and Automation Society,

1991.

Budenberg, G.F., Dead Weight Pressure Measurement, Instruments and

Control Systems, February 1971.

Comber, J. and Hockman, P., Pressure Monitoring: Whats Happening?

Instruments and Control Systems, April 1980.

Demorest, W.J., Pressure Measurement, Chemical Engineering, September

30, 1985.

Hall, J., Monitoring Pressure with Newer Technologies, Instruments and

Control Systems, April 1979.

Hughes, T.A., Pressure Measurement, EMC series, downloadable PDF,

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Park, NC, 2002.

Instrumentation, Systems, and Automation Society, Industrial Measure-

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Park, NC, 2002.

Instrumentation, Systems, and Automation Society, Pressure: Indicators

and Transmitters, CD-ROM, Research Triangle Park, NC, 2002.

Johnson, D., Pressure Sensing: Its Everywhere, Control Engineering,

April 2001.

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mentation Technology, August 1968.

Lewis, J.D., Pressure Sensing: A Practical Primer, InTech, December

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Marrano, S.J., How to Choose and Apply Pressure Transmitters, Control,

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Merritt, R., Keeping Up With Pressure Sensors,Instruments and Control

Systems, April 1982.

2003 by Bla Liptk

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