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PROCESS INSTRUMENTATIONMEASUREMENT AND CONTROL
Pressure Measurement
Pressure Transducers
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SECTION 2.PRESSURE TRANSDUCERS
4.2.1 Introduction
Whilst there are a wide range of pressure measuring
devices on the market, they may be divided into
main groups: mechanical and electromechanical
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4.2.2 MECHANICAL SENSORS
Mechanical pressure measuring elements
include:
Manometer
Dead weight tester Bourdon tubes
Bellow elements
Diaphragm element
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4.2.1 MANOMETERS
In any column of liquid (Figure 4.2.1),the head
pressure p is given by:
P gh
Where :P head in pressure
density
h height
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In its simplest form the manometer is a Utube about
half with liquid (commonly water, mercury or alcohol).
With both ends of the tube open, the liquid is at same
height in each leg (Figure 4.2.2(a)).
When positive pressure P is applied to one leg, the
liquid is forced down in that leg and up in the
other(Figure 4.2.2(b)).
The height h, indicates the difference in applied
pressure P and the atmospheric pressure P.
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Since the pressure in both tubes must balance:
P P + gh
h (P -P)/g
For water and mercury the conversion intoPascals is:
Water: Pa = mmHO x 9,80665
Mercury: Pa= mmHg x133,332
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Alternatively, when a vacuum is applied to one
leg, the liquid in that leg and falls in the other
(Figure 4.2.2(c)).
This time the difference in height h indicates the
amount of vacuum. Because the difference in
height the two columns is always a true
indication of the pressure, regardless of variation
in the internal diameter of the tubing, the U-tube
manometer is a primary standard.
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A disadvantage of the U-tube manometer is that
reading must be taken at two different places.
This shortcoming is overcome in the well-type
manometer (Figure 4.2.3)where the
well(reservoir) is sufficiently large that thechange of level in the reservoir is negligible.
Alternatively, the change in reservoir liquid level
may be compensated for on the scale.
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To increase readability and sensitivity further
the well-type manometer indicating tube can be
inclined.(Figure 4.2.4) to produce a greater linear
movement along the tube for a given pressure
difference. Because the inclined manometer isfrequently used for determining the over- fired
draft in boiler uptakes and it often is called a
Draft Gauge.
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4.2.2.2 DEAD WEIGHT TESTER
Suitable for measuring pressure in the ranges
from about 3 to kPs (gas) up to 1 GPa
(hydraulic), the dead-weight tester is used
essentially as a primary pressure calibration
standard and provides accuracies of down to
0.02% of reading.
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The dead-weight tester is based on measuring the
force acting on known area. As illustrated in
Figure 4.2.5, hydraulic fluid, contained within a
pressure cylinder, acts on a position, having a
known cross-section area, which support a knownweight.
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The operation, the screw press is operated until the
pressure within the system is sufficient to raise the
position, with its associated weights, off its stop. At thispoint, ignoring frictional losses, the pressure acting on
the piston is given by:
P force /cross-sectional area
P m.g /A
Where:
P pressure
g acceleration due to gravityA cross sectional area
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Two points should be noted. In most system, the apparatus is
surrounded by atmospheric pressure and the calibrated
pressure is thus gauge pressure. Some system are mounted
within an evacuated chamber in order to derive absolute
pressure.
A second point is that, as indicated above, performance will be
affected by the acceleration due to gravity. Thus gives rise to a
variation of about to 0.5% around the globe. Consequently, it isimportant that local value of the acceleration due to gravity is
known and corrected for.
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4.2.2.3 BOURDON TUBE
Figure 4.2.3 illustrates the Cshaped or CBourdon
tube.
The tube is commonly manufactured of phosphor
bronze having a flattened cross-sectional area and is
sealed at one end.
When pressure is applied to the open end of the tube it
will tend to straighten and the relatively small travel
of the end of tube is amplified by means of a link to
drive a pointer through a drive segment and toothed
gear.
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Although the movement of the tip of the tube is non-
linear this can be compensated for in the link and rearmechanism.
The link also usually incorporates a bi-metallic
element for temperature compensation.(Figure 4.2.7)
Bourdon tubes are available to cover the range from
0-30 k Pa up to 0-50 M Pa
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The Bourdon tube can be fabricated from a
variety of materials including phosphor
Bronze beryllium copper, 4130 alloy steel, 316
and 403 stainless, Monel, and titanium
With the choice determining both the range and
corrosion resistance.
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The C- Bourdon tube usually has a relatively
short arc of around 250and provides a typical
accuracy of 2%. It is used for local pressure
indication connected directly to process vessels
and lines. Inexpensive, C-Bourdon tubeinstruments feature a wide operating range, good
sensitive and fast response.
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The main problem with C- Bourdon tube is their
susceptibility to damage due to shock or vibration
although this can be overcome by filling the
instrument case with a damping fluid such as
glycerin or silicon oil. Apart from reducingresonance-induced fracturing of the measuring
element, the liquid filling prevents aggressive or
corrosive gases from entering the instrument and
prevents condensation from forming.
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For the lower measurement range use is often made of the
spiral Bourdon tube (Figure 4.2.8) in which the tube makes
several turns to increase the effective angular length and
thus increase the movement of the free end for a givenpressure input. Because the need for further mechanical
amplification is reduced, the tube end is mechanically linked
direct to the pointer.
This eliminates the necessity for the toothed quadrant- withthe consequent reduction backlash and friction errors.
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Whilst the CBourdon is generally suitable for pressure up toabout 1 Mpa,
In any bourdon tube element, the higher the pressure required,
the thicker the wall of the tubing. Thus whilst spiral tube low
pressure elements may have only two or three coils, high pressureelements with their thicker wall, may require up to 20 coil in
order not maintain their sensitivity. Generally the spiral tube is
suitable for pressure up to 30 Mpa.
A variation on the spiral tube is the helix tube where the tube is
wound longitudinally to provide ranges up to 50 Mpa. The mail disadvantage of both spiral and helix elements is that
very expensive.
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4.2.2.4 BELLOW ELEMENTS
The bellow measuring device is made up of a
series of thin-walled cylindrical elements that
from the bellows arrangement and is used where
a large degree of travel is required in a restricted
space. Bellows element are also used for lowerpressure ranges and for ranges that cross from
vacuum into positive gauge pressure.
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The bellows measuring device(Figure4.2.9)is made of series of thin-walled
cylindrical element that from the bellow
arrangement and used where degree oftravel is required in a restricted space.
Bellows elements are also used for lower
pressure ranges and for ranges thatcross from vacuum positive gauge
pressure.
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The principle of operation is based on the fact when a
pressure is applied to the bellows element, its length
changes.
In the arrangement shown in Figure 4.2.9, the springloaded bellows elements is enclosed within a pressure
container that to the process pressure source.
When pressure is applied the bellows compress against
the opposing pressure source. When pressure is applied
the bellows compress against the opposing force of the
spring- with the vertical movement transmitted,
through a suitable linkage, to pointer or actuating
device.
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Because pressure is exerted over a large area,
the bellows element produces a considerable force
per unit change in pressure. As a result, bellows
elements are used in the range from 0-500 Pa up
to 100 k Pa.Bellows elements are often used to actuate an
on/off switch for in the air conditioning industry.
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4.2.5 DIAPHRAGM ELEMENTS
In the simple diaphragm a flexible disc, which
can either be flat or have concentric corrugation,
is fabricated from sheet metal to exacting high to
tolerance dimensions. In the instrument shown
in Figure 4.2.10, the diaphragm is usedindependently as a pressure sensor. Applied
pressure deflects the diaphragm which move a
push rod.
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Due to their and position ,diaphragms display
high mechanical resistance and are less shock-
sensitive. Compared to Bourdon tubes, the travel
of a diaphragm is very small and thus both
quality and tolerances must meet very exactingstandards.
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The diaphragm is also the basic component of a
capsular element comprising two diaphragms joinedtogether by crimping or fusion-welding. In Figure
4.2.11, the orientation of the corrugation of two
diaphragms is oppose and again a push rod is used to
actuate a toothed drive segments, gear and pointer.
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4.2.3 ELECTRICAL DISPLACEMENT SENSORS
In modern process control system, pressure
measurement is normally carried out using a
different pressure transducer. The role of such a
pressure transducer is to measure the differential
pressure and convert it to an electrical signalthat can be transmitted from the field to the
control room or the pressure controlling system.
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As illustrated in figure 4.2.12, most industrial
differential cell make use of isolation diaphragms
that isolate the transmitter. Movement of the
isolation diaphragms is transmitted via the
isolating fluid(e.g. silicon fluid) to the measurediaphragm whose deflection is a measure of the
differential pressure.
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Deflection of the measuring diaphragm is extremelysmall(of the order of a few millimeters)and measure is
normally carried out by one of five basic methods:
1. Inductance
2. Strain gauge3. Capacitance
4. Piezoresistive
5. Piezoelectric
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4.2.3.1 INDUCTANCE
The inductivebased sensor(often referred to as a
linear differential transformer)(Figure 4.2.13) senses the displacement of magnetic core
mounted on the measuring diaphragm.
This is shown the displacement of a magnetic core
mounted on the measuring diaphragm. This is showschematically in Figure 4.2.14.
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Displacement of the magnetic core changes the
coupling between the primary and the two
secondarys.
With no differential pressure, the measuring
diaphragm is not deflected and the voltageinduced in the measuring coil is equal and
opposite with a net output of zero.
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When the measuring diaphragm is defected the
output voltage magnitude and phase of each
secondary will vary in direct proportional to the
pressure applied to the movable element.
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4.2.3.2 STRAIN GAUGES
In the simplest form, adhesive is used to bond
four metal strain gauge elements directly to the
diaphragm(Figure 4.2.15). In most instances, the
strain gauges elements are arranged to form the
four arms of a Wheatstone bridge.
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Thus , a change in pressure produces mechanical
deformation which result in a change of electrical
resistance that is proportional to the change in
pressure. The greater the pressure applied to the
diaphragm, the more it will deflect.
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The bonded foil strain gauge is the most reliable
and durable of four technologies and can be used
in ultra high pressure application(0-600 kPa
through to 700 MPa). Its durability makes it
suitable for application that experience pressurecycling, shock, and vibration.
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Another major advantage of this technology is
that foil strain gauge can be matched and bonded
with extreme accuracy, these is no need to
include any temperature compensating devices
within the transmitter.
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The mail limitation of this technology is its poor
performance at below 500 k Pa. If the diaphragm
is too thin, the strain gauges begin to interfere
with the diaphragms motion. Increased
sensitivity can be obtained using a metallic thinsensor where the strain gauge is vapor deposited
or spattered onto the diaphragm sensing
element.
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4.2.3 CAPACITANCE
The variable capacitance transmitter(Figure
4.2.16) is the most widely used method of
measuring differential pressure. The upstream
and downstream pressure are applied to isolation
diaphragms on the high and low pressure sides,and are transmitted to the sensing
diaphragm(movable electrode)- usually through
a fail oil. Movement of the sensing diaphragm
changes its distance from the fixed plate
electrodes, resulting in a change in
capacitance.
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Capacitance based transmitters are simple,reliable , accurate, small, in size and weight, and
remain stable over a wide temperature range.
The main advantage of the capacitive transmitter
is that it is extremely sensitive to small changesin pressure- down to 250 Pa pressure.
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4.2.3.4 PIEZORESISTIVE
Similar in operation to the strain gauge , thepiezoresistive element employs four nearly
identical piezio-resistive diffused the surface of a
thin circular wafer of N- type silicon. The
diaphragm is formed by chemically etching acircular cavity- with the un etched portion
forming a rigid boundary and surface. The
mechanical strength of silicon generally impose
an upper limit of around about 3 MPa.
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Through not as rugged as a foil strain gage,piezoresistive gauges are generally more
sensitive than metallic thick film device and thus
produce a measurable signal at lower strain.
However, silicon piezoresistive sensingtechnology is not suited for application that
experience extreme pressure cycles, shocks, or
vibration, due to the weakness of the silicon
piezoresistors. Further, the upper temperature
limit for diffused silicon strain gauge based
transmitters runs between 125 and 200C.
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Ceramic piezoresistive sensing technology usesconductive link deposition on the reference side of
a ceramic sensing diaphragm. Like silicon
piezoresistive, this technology provides a strong,
sensitive output signal in lower pressure ranges.Ceramic piezoresistive technology is slightly
more rugged than silicon piezoresistive and is
used in pressure ranges of 0- 100 kPa to 0 -
10MPa. Further, the ceramic wetted face may be
used in corrosive fluid application
P
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4.2.3.5 PIEZOELECTRIC
When a mechanical stretching or compressing forceis applied to an asymmetrical crystalline
material, such as barium titanite or
quartz(Figure 4.2.17), equal and opposite
electrical changes appear across it . themagnitude of the charges appear across it. The
magnitude of the charges depends on the
dimension of the quartz crystal and the
magnitude of the applied force.
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Pressure transducers based on this phenomenonproduce an out that is proportional to a change in
applied pressure and do not thus respond to
static conditions. Available in a wide range of
dynamic pressure from to 200 kPa to 100 MPawith an accuracy down to 0.075%, piezoelectric
pressure transducer have a very fast response(