5.8 Pressure Measurement - High Pressure Sensors 4p

7/30/2019 5.8 Pressure Measurement - High Pressure Sensors 4p

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High-Pressure enso o r's '

Types and Ranges:

Inaccuracy:

costs:

A. Optical (Section 5.7), up to 60,000 PSIG (4338 bars)

B. P iezoelec tric (Section 5.7), up to 10 0,000 PSIG (6 896 bars)C. Magnetic (Section 5.7), up to 100,000 PSIG (6896 bars)D. D ead-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 moreG. Strain gauge (Section 5.7), up to 200,000 PSIG (13,793 bars)H. Bulk mod ulus cells, up to 200,000 PSIG (13,793 bars)I. Button type pressure repeater, up to 10,000 PSIG (6896 bars)

Flow Sheet Symbol

For dead-weight testers, 0.1% of span or better; for strain gauges from ab out 0.1%of span to 0.25% of full scale, for Manganin cells from 0.1 to 0.5% of full scale; forpressure repeaters 0.5 to 1% full scale, for helical bourdon tubes 1% of span; forbulk modulus cells from 1 to 2% of full span

For types A, B, C , and G , see Section 5.7; for type E, see Section 5.4. M ost transducersare from $300 to $500. Th e simplest dead-weight gauges with moderate ranges and0.1% inaccuracy cost around $1200 to $1500, the average portable pressure/vacuumcalibrator costs around $5000; the most sophisticated 0.03% hydraulic calibrator unitscost about $18,000.

Partial List of Suppliers: 3D Instruments LLD (D) (www.3dinstruments.com)

ABB Automation Technology (E) (www.abb.com)Arnetek Inc. (D, E) (www.ametekusg.com)

Ame tek Drexelbrook (G) (www.drexe1brook.com)Barber Colman Industrial (G) (www.barber-colrnan.com)

Barksdale (G) (www.barksda1e~om)Barton Instrument (G) (www.barton-instruments.com)Cosa Instrument @) (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)

Maph Instrument Co. (E) (www.marshbellofram.com)Marshalltown Instruments Inc. (E ) (www .marshbellofram.com)

Mensor C orp. ( B, E, quartz helix) (www.e-pressure.com)Mid-West Instrument (E) (ww w.rnidwestinstrument.com)MKS Instruments (D) (www.rnksinst.com)Morehouse Instrument (D)Moeller Instrument Co. (E) (www.moellerinstrument.com)Moore Products, now part of Siemens Inc. (E) (w ww.sea.siemens.com)



I , . - - -Noshok Inc. iE) (www.no~hck.cern)

OCI Instruni;~,ts Inc. (E) jwww.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)

Sca iv a lv e C o rp . (G ) www.scanivalve.com)

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

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

Smar International @) (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 (69bars) is usually consid-

ered to be high, while in synthetic diamond manufacturing

180,000 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 detectors

have 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, bulk

modulus, and Manganin cells.

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 and

the Manganin cells.

The only primary high-pressure detector is the dead

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

hysteresis. The Manganin gauge was first described by the

Nobel prize winning physicist ~ r i d ~ r n a n 'ho 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 which

the 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

Weight=-l

weight sensor, which is also a rather slow measuring device. UG,5,8a

The sensors that detect elastic deformation follow Hoke's Dead-weight piston tester



764 Pressure Me asu~~m-17:

100,000 PSIG (690 MPiij. Thc rani? 53s exte~2edo

even greater pressures, t u i ~,esl;arch n v ls to ~n_&cvlinder

material and their treatment t- r i h t m d oads is 2 limitation.

Assuming that one wac:s t generate a rressure of 100,000

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

it is necessary to reduce the p is t~~nrea to 0.01 in2 (6.4 mm2).

This means that a 0.1 in. (2.5 rza) diameter piston wil: have

to support a 1000 lb weieht, --:hi15 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 sressure

level. The laboratory piston gauges are standardized 3y NBS,

calibrating the associated weights md measuring the piston

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

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

greater than 40,000 PSIG (280MPa) 2nd to 5 parts in 100,000

at lower pressures. The inaccaracy of industrial dead weight

testers is better thanM.%

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-- Pressure Repeater

This instrument (Figure 5.8b) is discussed in more detail inSection 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 800°F (430°C).

Output

Air Signal (P2) Balancing

Sensing '! 1Diaphragm

k A i - 4

Therefore

m,a84Button-diaphragm-type pressure repeater:

Process-ressure

ving

Tip

m S.&

Helical Bou rdon-type pressure sensor:

Pressure

The detailed features of this instrument (Figure 5 . 8 ~ ) re

discussed in Section 5.4. The helical elements used in this

instrument 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.

Cell packingBody

FA6.5Jd

Bulk modulus cell.

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

by electromagnetic pickup, capacitance pickup, or the use of

These cells, shown in Figure 5.8d, are comprised of a hollow mechanical displacement transmitten (pneumatic or electronic).

cylindrical steel probe closed at the inner end, and a stem that The unit is available with ranges of 0-50,000 to 0-200,000

projects beyond the outer end of the probe. When subjected PSIG (0-350 to 0-1,400 MPa), and its inaccuracy is f to 2%

to process pressures, the active part of the probe contracts of full scale. Its advantages, when compared with other high-

isotropically, causing its tip to be displaced to the right. As a pressure sensors, include its relatively fast response, its

result, the stem moves outward, increasing the distance it remote-reading characteristic, and its design that is absolutely



safe kcau se 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-SENSmVE WIRES

The electric resistance of wires can be changed by applyinglinear 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 with

pressure. The pressure-resistance relationship for Manganin

'is substantial, positive, and linear, and therefore can be

detected 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 29,300

bars), and their inaccuracy is between *'/lo andf12 % 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 been

used as elements in pressure-resistance cells.

CHANGE-OF-STATE DETECllON

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 O°C

is 109,765 +30 PSIG (757 M.2 MPa). Similarly, the first

polymorphic transition point of bismuth has been found to be

between 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 use

with elastic elements. Because these devices were covered

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

Electronic txanscfucers f ~ rdywm.ic piesxre dete

include -the piezoi~ectric ransducers; the bonded and

unbonded strain gsuge elements; and the variable t~:luctance,

differential tra~zformes, ~i dlectrical capacitance types.

Strain gauges bonded to diaphragm or bellows elements

have given good performance in measuring blast precqures.

In connection with anderwater explosions noises, piezo-

its arelectric crystals hwe been successfully used. These UP:

directionally sensithe to force, necessitating a seal interposed

between the element and the process and convex ting pressure

to force for optimum response.

1. Bridgman, P.W., Physics of High Pressure, ' .ond -n: G. Bell & Sons,

Ltd., New York: MacMillan, 1952.

Bibliography

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