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Instruments
Many instruments
have been invented to measure pressure, with different advantages and
disadvantages. Pressure range, sensitivity, dynamic response and cost all vary
by several orders of magnitude from one instrument design to the next. The
oldest type is the liquid column (a vertical tube filled with mercury)
manometer invented by Evangelista Torricelli in 1643. The U-Tube was invented
by Christian Huygens in 1661.
Hydrostatic
Hydrostatic
gauges (such as the mercury column manometer) compare pressure to the
hydrostatic force per unit area at the base of a column of fluid. Hydrostatic
gauge measurements are independent of the type of gas being measured, and can
be designed to have a very linear calibration. They have poor dynamic response.
Piston
Piston-type
gauges counterbalance the pressure of a fluid with a spring (for example
tire-pressure gauges of comparatively low accuracy) or a solid weight, in which
case it is known as a deadweight tester and may be used for calibration of
other gauges.
Liquid column
The difference
in fluid height in a liquid column manometer is proportional to the pressure
difference.
Liquid column
gauges consist of a vertical column of liquid in a tube whose ends are exposed
to different pressures. The column will rise or fall until its weight is in
equilibrium with the pressure differential between the two ends of the tube. A
very simple version is a U-shaped tube half-full of liquid, one side of which
is connected to the region of interest while the reference pressure (which
might be the atmospheric pressure or a vacuum) is applied to the other. The
difference in liquid level represents the applied pressure. The pressure
exerted by a column of fluid of height h and density ρ is given by the
hydrostatic pressure equation, P = hgρ. Therefore the pressure difference
between the applied pressure Pa and the reference pressure P0 in a U-tube
manometer can be found by solving Pa − P0 = hgρ. In other words, the pressure
on either end of the liquid (shown in blue in the figure to the right) must be
balanced (since the liquid is static) and so Pa = P0 + hgρ. If the fluid being
measured is significantly dense, hydrostatic corrections may have to be made
for the height between the moving surface of the manometer working fluid and
the location where the pressure measurement is desired except when measuring
differential pressure of a fluid (for example across an orifice plate or
venturi), in which case the density ρ should be corrected by subtracting the
density of the fluid being measured.[2]
Although any
fluid can be used, mercury is preferred for its high density (13.534 g/cm3) and
low vapour pressure. For low pressure differences well above the vapour
pressure of water, water is commonly used (and "inches of water" is a
common pressure unit). Liquid-column pressure gauges are independent of the
type of gas being measured and have a highly linear calibration. They have poor
dynamic response. When measuring vacuum, the working liquid may evaporate and
contaminate the vacuum if its vapor pressure is too high. When measuring liquid
pressure, a loop filled with gas or a light fluid can isolate the liquids to
prevent them from mixing but this can be unnecessary, for example when mercury
is used as the manometer fluid to measure differential pressure of a fluid such
as water. Simple hydrostatic gauges can measure pressures ranging from a few
Torr (a few 100 Pa) to a few atmospheres. (Approximately 1,000,000 Pa)
A single-limb
liquid-column manometer has a larger reservoir instead of one side of the
U-tube and has a scale beside the narrower column. The column may be inclined
to further amplify the liquid movement. Based on the use and structure
following type of manometers are used[3]
1. Simple Manometer
2. Micro manometer
3. Differential manometer
4. Inverted differential manometer
A McLeod gauge,
drained of mercury
McLeod gauge
A McLeod gauge
isolates a sample of gas and compresses it in a modified mercury manometer
until the pressure is a few mmHg. The gas must be well-behaved during its
compression (it must not condense, for example). The technique is slow and
unsuited to continual monitoring, but is capable of good accuracy.
Useful range:
above 10-4 torr [4] (roughly 10-2 Pa) as high as 10−6 Torr (0.1 mPa),
0.1 mPa is the
lowest direct measurement of pressure that is possible with current technology.
Other vacuum gauges can measure lower pressures, but only indirectly by
measurement of other pressure-controlled properties. These indirect
measurements must be calibrated to SI units via a direct measurement, most
commonly a McLeod gauge.[5]
Aneroid
Aneroid gauges
are based on a metallic pressure sensing element that flexes elastically under
the effect of a pressure difference across the element. "Aneroid"
means "without fluid," and the term originally distinguished these
gauges from the hydrostatic gauges described above. However, aneroid gauges can
be used to measure the pressure of a liquid as well as a gas, and they are not
the only type of gauge that can operate without fluid. For this reason, they
are often called mechanical gauges in modern language. Aneroid gauges are not
dependent on the type of gas being measured, unlike thermal and ionization
gauges, and are less likely to contaminate the system than hydrostatic gauges.
The pressure sensing element may be a Bourdon tube, a diaphragm, a capsule, or
a set of bellows, which will change shape in response to the pressure of the
region in question. The deflection of the pressure sensing element may be read
by a linkage connected to a needle, or it may be read by a secondary
transducer. The most common secondary transducers in modern vacuum gauges
measure a change in capacitance due to the mechanical deflection. Gauges that
rely on a change in capacitances are often referred to as Baratron gauges.
Bourdon
Membrane-type
manometer
The Bourdon
pressure gauge uses the principle
that a flattened tube tends to change to a
more circular cross-section when pressurized. Although this change in
cross-section may be hardly noticeable, and thus involving moderate stresses
within the elastic range of easily workable materials, the strain of the
material of the tube is magnified by forming the tube into a C shape or even a
helix, such that the entire tube tends to straighten out or uncoil,
elastically, as it is pressurized. Eugene Bourdon patented his gauge in France
in 1849, and it was widely adopted because of its superior sensitivity,
linearity, and accuracy; Edward Ashcroft purchased Bourdon's American patent
rights in 1852 and became a major manufacturer of gauges. Also in 1849, Bernard
Schaeffer in Magdeburg, Germany patented a successful diaphragm (see below)
pressure gauge, which, together with the Bourdon gauge, revolutionized pressure
measurement in industry.[6] But in 1875 after Bourdon's patents expired, his
company Schaeffer and Budenberg also manufactured Bourdon tube gauges.
In practice, a
flattened thin-wall, closed-end tube is connected at the hollow end to a fixed
pipe containing the fluid pressure to be measured. As the pressure increases,
the closed end moves in an arc, and this motion is converted into the rotation
of a (segment of a) gear by a connecting link that is usually adjustable. A
small-diameter pinion gear is on the pointer shaft, so the motion is magnified
further by the gear ratio. The positioning of the indicator card behind the
pointer, the initial pointer shaft position, the linkage length and initial
position, all provide means to calibrate the pointer to indicate the desired
range of pressure for variations in the behaviour of the Bourdon tube itself.
Differential pressure can be measured by gauges containing two different
Bourdon tubes, with connecting linkages.
Bourdon tubes
measure gauge pressure, relative to ambient atmospheric pressure, as opposed to
absolute pressure; vacuum is sensed as a reverse motion. Some aneroid
barometers use Bourdon tubes closed at both ends (but most use diaphragms or
capsules, see below). When the measured pressure is rapidly pulsing, such as
when the gauge is near a reprocating pump, an orifice restriction in the
connecting pipe is frequently used to avoid unnecessary wear on the gears and
provide an average reading; when the whole gauge is subject to mechanical
vibration, the entire case including the pointer and indicator card can be
filled with an oil or glycerin. Tapping on the face of the gauge is not
recommended as it will tend to falsify actual readings initially presented by
the gauge.The boudon tube is separate from the face of the gauge and this has
ne effect on the actual reading of pressure. Typical high-quality modern gauges
provide an accuracy of ±2% of span, and a special high-precision gauge can be
as accurate as 0.1% of full scale.[7]
In the following
illustrations the transparent cover face of the pictured combination pressure
and vacuum gauge has been removed and the mechanism removed from the case. This
particular gauge is a combination vacuum and pressure gauge used for automotive
diagnosis:
Indicator side
with card and dial
Mechanical side
with Bourdon tube
• the left side of the face, used for measuring manifold
vacuum, is calibrated in centimetres of mercury on its inner scale and inches
of mercury on its outer scale.
• the right portion of the face is used to measure fuel pump
pressure and is calibrated in fractions of 1 kgf/cm² on its inner scale and pounds
per square inch on its outer scale.
Mechanical
details
Mechanical
details
Stationary
parts:
• A: Receiver block. This joins the inlet pipe to the fixed
end of the Bourdon tube (1) and secures the chassis plate (B). The two holes
receive screws that secure the case.
• B: Chassis plate. The face card is attached to this. It
contains bearing holes for the axles.
• C: Secondary chassis plate. It supports the outer ends of
the axles.
• D: Posts to join and space the two chassis plates.
Moving Parts:
1. Stationary end of Bourdon tube. This
communicates with the inlet pipe through the receiver block.
2. Moving end of Bourdon tube. This end is
sealed.
3. Pivot and pivot pin.
4. Link joining pivot pin to lever (5) with
pins to allow joint rotation.
5. Lever. This an extension of the sector
gear (7).
6. Sector gear axle pin.
7. Sector gear.
8. Indicator needle axle. This has a spur
gear that engages the sector gear (7) and extends through the face to drive the
indicator needle. Due to the short distance between the lever arm link boss and
the pivot pin and the difference between the effective radius of the sector
gear and that of the spur gear, any motion of the Bourdon tube is greatly
amplified. A small motion of the tube results in a large motion of the
indicator needle.
9. Hair spring to preload the gear train to
eliminate gear lash and hysteresis.
Diaphragm
A pile of
pressure capsules with corrugated diaphragms in an aneroid barograph.
A second type of
aneroid gauge uses the deflection of a flexible membrane that separates regions
of different pressure.
The amount of deflection is repeatable for known
pressures so the pressure can be determined by using calibration. The
deformation of a thin diaphragm is dependent on the difference in pressure between
its two faces. The reference face can be open to atmosphere to measure gauge
pressure, open to a second port to measure differential pressure, or can be
sealed against a vacuum or other fixed reference pressure to measure absolute
pressure. The deformation can be measured using mechanical, optical or
capacitive techniques. Ceramic and metallic diaphragms are used.
Useful range:
above 10-2 Torr [8] (roughly 1 Pa)
For absolute
measurements, welded pressure capsules with diaphragms on either side are often
used.
Shape:
• Flat
• corrugated
• flattened tube
• capsule
Bellows
In gauges
intended to sense small pressures or pressure differences, or require that an
absolute pressure be measured, the gear train and needle may be driven by an
enclosed and sealed bellows chamber, called an aneroid, which means
"without liquid". (Early barometers used a column of liquid such as
water or the liquid metal mercury suspended by a vacuum.) This bellows
configuration is used in aneroid barometers (barometers with an indicating
needle and dial card), altimeters, altitude recording barographs, and the
altitude telemetry instruments used in weather balloon radiosondes. These
devices use the sealed chamber as a reference pressure and are driven by the
external pressure. Other sensitive aircraft instruments such as air speed
indicators and rate of climb indicators (variometers) have connections both to
the internal part of the aneroid chamber and to an external enclosing chamber.
Pressure Gauges
and switches
Pressure
gauges and switches are among the most often used instruments in a plant. But
because of their great numbers, attention to maintenance--and reliability--can
be compromised. As a consequence, it is not uncommon in older plants to see
many gauges and switches out of service. This is unfortunate because, if a
plant is operated with a failed pressure switch, the safety of the plant may be
compromised. Conversely, if a plant can operate safely while a gauge is
defective, it shows that the gauge was not needed in the first place.
Therefore, one goal of good process instrumentation design is to install fewer
but more useful and more reliable pressure gauges and switches.
One way to reduce the number of gauges in a
plant is to stop installing them on the basis of habit (such as placing a
pressure gauge on the discharge of every pump). Instead, review the need for
each device individually. During the review one should ask: "What will I
do with the reading of this gauge?" and install one only if there is a
logical answer to the question. If a gauge only indicates that a pump is
running, it is not needed, since one can hear and see that. If the gauge
indicates the pressure (or pressure drop) in the process, that information is
valuable only if one can do something about it (like cleaning a filter);
otherwise it is useless. If one approaches the specification of pressure gauges
with this mentality, the number of gauges used will be reduced. If a plant uses
fewer, better gauges, reliability will increase.
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