Lecture 2:
Pressure Measurements
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Pressure
• Pressure is the force per unit area that a liquid or gas
exerts on its surroundings, such as the force or
pressure of the atmosphere on the surface of the
Earth, and the force that liquids exert on the bottom
and walls of a container.
• Pressure is not only an important parameter for
process control, but also as an indirect measurement
for other parameters.
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Pressure Measurement
Pressure is most commonly expressed in
– pounds per square inch (psi) in English units; or
– Pascals (Pa) in metric units, which is the force in Newtons
per square meter (N/m2).
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Absolute, differential, and gauge pressures
1. Total vacuum is zero pressure or lack of pressure, as would be experienced in outer
space, and is very difficult to achieve in practice.
2. Atmospheric pressure is the pressure on the Earth’s surface, due to the weight of the
gases in the Earth’s atmosphere (14.7 psi or 101.36 kPa absolute). The pressure
decreases above sea level.
3. Absolute pressure is the pressure measured with respect to a vacuum, and is
expressed in psia or kPa(a).
4. Gauge pressure is the pressure measured with respect to atmospheric pressure, and is
normally expressed in psig or kPa(g). Note the use of a and g when referencing the
pressure to absolute and gauge.
5. Differential pressure is the pressure measured with respect to another pressure, and
is expressed as the difference between the two values. This represents two points in a
pressure or flow system, and is referred to as the “delta p,” or ∆p.
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Other Definitions
• Static pressure is the pressure of a fluid or gas that is
stationary or not in motion.
• Dynamic pressure is the pressure exerted by a fluid
or gas when it impacts on a surface or an object due
to its motion or flow.
• Impact pressure (total pressure) is the sum of the
static and dynamic pressures on a surface or object.
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Pressure Gauges
• Many of the devices used to monitor fluid pressure in
industrial processes involve the monitoring of the elastic
deformation of diaphragms, bellows and tubes. The following
are some common examples of such sensors.
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Diaphragms
• Diaphragms consist of a thin layer or
film of a material supported on a rigid
frame.
• The movement of the center of a
circular diaphragm as a result of a
pressure difference between its two
sides is the basis of a pressure gauge.
• The displacement will be proportional
to the value of pressure difference.
• The movement of the center of a
diaphragm can be monitored by some
form of displacement sensor.
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Diaphragms
• Pressure can be applied to one side of the film for gauge
sensing, with the other inlet port being left open to the
atmosphere.
• Pressures can be applied to both sides of the film for
differential sensing.
• Absolute pressure sensing can be achieved by having a partial
vacuum on one side of the diaphragm.
• A wide range of materials can be used for the sensing film:
from rubber to plastic for low pressures, silicon for medium
pressures, and stainless steel for high pressures.
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Diaphragms
• When a pressure is applied to the diaphragm, the film distorts
or becomes slightly spherical, and can be sensed using a strain
gauge, piezoelectric, or changes in capacitance techniques.
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Diaphragms
• Sensing the position of the diaphragm using capacitance and
ac bridge is very accurate, and excellent linear correlation
between pressure and output signal amplitude can be
obtained.
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A Piezoelectric pressure sensor
• When certain crystals, such
as quartz and tourmaline,
are stretched or
compressed, electrical
charges appear on their
surfaces. This effect is called
piezo-electricity.
• As a result of the diaphragm
movement a potential
difference appears across it.
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A Piezoelectric pressure sensor
• The voltage produced by the crystal is proportional to
Pressure exerted on the crystal surface. This voltage can be
amplified, and hence the device is used as a pressure sensor.
• This sensor does not require any voltage supply.
• However, if the pressure keeps the diaphragm at a particular
displacement, the resulting electrical charge is not maintained
but leaks away. Therefore, this sensor is not suitable for static
pressure measurements but is suitable for fast changing
pressure.
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Bellows
• Bellows are a corrugated tube or a tube with convolutions.
• When pressure is applied to the bellows, it elongates by stretching
the convolutions, rather than the end diaphragms.
• The bellows is the most sensitive of the mechanical devices for lowpressure measurements (i.e., 0.5 to 210 kPa).
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Bellows
• Bellows devices can be used for absolute, gauge, and differential pressure
measurements.
• Differential measurements can be made by mechanically connecting two
bellows to be opposing each other when pressure is applied to them, as
shown in below.
• When pressures P1 and P2 are applied to the bellows, a differential scale
reading is obtained. P2 could be atmospheric pressure for gauge
measurements.
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Using LVDT with bellows
The LVDT can be used to convert the linear motion of a bellows into an
electrical signal. The bellows converts the differential pressure between P1
and P2 into linear motion, which changes the position of the core in the LVDT.
The device can be used as a gauge sensor when P2 is open to the
atmosphere.
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Bourdon tube
• The Bourdon tube is C-shaped tube with an almost rectangular
or elliptical cross-section made from e.g. stainless steel or
phosphor bronze.
• When the pressure inside the tube increases the closed end of
the C opens out, thus the displacement of the closed end
becomes a measure of the pressure.
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Bourdon tube
• The operating principle is that the outer edge of the cross
section has a larger surface than the inner portion. When
pressure is applied to the Bourdon tube, the outer edge has a
proportionally larger total force applied because of its larger
surface area, and hence the free end of the tube moves
outward.
• This movement can be mechanically coupled to a pointer,
which will indicate pressure on a scale, or it can be coupled to
a potentiometer, which will give a resistance value
proportional to pressure as an electrical signal.
• The Bourdon tube is reliable, inexpensive, and one of the most
common general-purpose pressure gauges.
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The bourdon tube pressure sensor. The bourdon tube (C-shaped portion)
expands with pressure, turning the dial (below the bezel, not seen) through
a leverage arm and gear mechanism.
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Bourdon tube
Bourdon tubes can also be shaped into helical or spiral shapes to
increase their measurement range.
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