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CHAPTER 2
PRESSURE MEASUREMENT
PRESSURE DEFINITIONPRESSURE DEFINITION
FORCEPRESSURE is force exerted per unit area of surface.
FORCEP = F/A
P = pressure
F = force
A = surface area exposed to the force
AREAPressure Units:
Pound / Square Inch = P / SI = PSIPound / Square Inch = P / SI = PSI
Newton / Square Meter = N / m2
100 000 N / m2 = 1 Bar100,000 N / m = 1 Bar
14.5 psi = 1 Bar
Inches of mercury “inch Hg” “mm Hg”Inches of mercury inch. Hg , mm.Hg
TYPES OF PRESSURETYPES OF PRESSURE
The pressure at
point X can be
expressed as:
o 10” of mercury
absolute
o 20” of mercury o 20 of mercury
vacuum
20 i h f o -20 inches of
mercury gauge
PRESSURE SENSING ELEMENTS
A basic instrument system consists of three elements:
1. SENSOR or INPUT DEVICE
2 SIGNAL PROCESSOR2. SIGNAL PROCESSOR
3. RECEIVER or OUTPUT DEVICE
The block diagram of a sensor is shown below.
Pressure sensors either convert the pressure into
h i l t i t l t i l t t mechanical movement or into an electrical output.
Complete gauges not only sense the pressure but indicate
them on a dial or scale.
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1) Bourdon TubeBourdon tube is a flattened metal tube has an oval cross-section with one end of the tube connected to the process pressure. The other end is sealed and connected to the pointer or transmitter mechanism. When a pressure difference exists between the inside and outside, the tube tends to straighten out and the end moves.
C-type bourdon
o Range as low as 0 - 15 psi up to 0-1500
, g
psi.
o They are simple and accurate, but they are bulky and get damaged with over-ranging.
Helical bourdon
o Range as low as 0 - 200 psi up to 0 - 6000 ipsi.
o Heavy-duty helical bourdons can sometimes tolerate as high as ten times h ithe maximum range pressure.
Spiral bourdon
o Range as low as 0 10 psi up to 0 100 000
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o Range as low as 0 -10 psi up to 0-100,000 psi.
16/03/2006Designed by Rao KV
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2) Bellows sensor A bellows sensor is an axially
flexible cylindrical enclosure flexible, cylindrical enclosure
with folded sides. When
pressure is applied through an pressure is applied through an
opening, the closed end
extends axially.extends axially.
A bellows sensor can
t l h accurately measure much
lower pressures than a
bourdon tubebourdon tube.
Bellows elements can measure
absolute pressure, gauge
pressure, vacuum, or
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differential pressure.
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3. Diaphragm sensoro Diaphragm is usually metallic and comes in two different
fi ti i l d l configurations; single and capsular.
o The single diaphragm is either flat or with concentric corrugations.
Th l di h i t f t di h ld d t th t o The capsular diaphragm consists of two diaphragms welded together at their perimeters.
o Evacuated capsules are used for absolute pressure reference and single p p gdiaphragms for very sensitive measurements.
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ELECTRONIC PRESSURE SENSORS
This table shows the ranges of pressures and vacuums that various
sensor types are capable of detectingsensor types are capable of detecting.
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4. Strain-Gauge Pressure Sensor
o Strain-gauge pressure sensors are o Strain gauge pressure sensors are used in most types of electronic pressure transmitters. When metallic conductors or semiconductors are subjected to mechanical strain, there is a change in resistance This there is a change in resistance. This resistance is then electrically converted into a 4-20 mA signal proportional to the pressure.
o There are many different designs of strain-gauge pressure sensors. The most common designs use a metallic di h t i l t th diaphragm to isolate the process fluid and exert a force on a force bar.
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bar.
o Most of the strain elements in current use are semiconductor type.
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Strain-Gage Based Pressure Cell
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5. Capacitive Pressure Sensor
o Capacitive pressure sensors are also used in electronic pressure
transmitters. With these devices the change in capacitance
resulting from the movement of an elastic element is
proportional to the pressure applied to the elastic element.
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o The elastic element is usually a stainless steel diaphragm. As shown, a
high-frequency oscillator is controlled by the sensing element. high frequency oscillator is controlled by the sensing element.
Changes in pressure deflect the diaphragm and the resultant change in
capacitance changes the oscillator frequency. p g q y
o The variation in oscillator frequency is converted to a 4-20 mA signal,
proportional to the pressureproportional to the pressure.
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PRACTICAL CONSIDERATIONS
C t i li ti ill b d th Certain applications will be so severe and the
pressure sensor will not remain functional for
reasonable amount of time.
The following protection devices can be used to
protect the pressure sensor.p p
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Di h l d t
1) Diaphragm Seals
o Diaphragm seals are used to
isolate the pressure sensor from
the process fluid.
o This is done when the fluid is
toxic, corrosive, dirty, solidifies at
ambient temperature, or is ambient temperature, or is
extremely cold and may freeze
the instrumentthe instrument..
o The diaphragm seal is a thin,
flexible disk, which separates the
pressure sensor from the process
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media.
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2) Siphons
• Siphons are generally used to isolate a hot-
process media from the pressure sensor. p p
• The siphon is a metal, tubular device shaped
l h b f ll d h h hin a loop. It can either be filled with a high-
boiling-point liquid or process condensate
which acts as a barrier to the heat contained
in the hot gases or steam.
• The path the hot vapor takes to the pressure
sensor is relatively long and narrow with a sensor is relatively long and narrow with a
lot of surface area for cooling siphons.
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3) Throttling Devices
o Throttling devices are commonly used to dampen high-frequency
pressure fluctuations by putting a restriction in the inlet to the
pressure sensor.
a) Throttling Screws
o Throttling screws are the simplest
means of providing a restriction.
o Throttling screws have several o Throttling screws have several
orifice sizes.
Th i t d i t t d o They are inserted into a tapped
hole in the base (socket) of the
pressure sensor to provide a flow
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pressure sensor to provide a flow
restriction
o Pressure Snubbers are very common for
b) Pressure Snubbers
o Pressure Snubbers are very common for
reducing pressure fluctuations and filtering
the media. Snubbers have a porous the media. Snubbers have a porous
element, which restricts the velocity and
filters the fluid.
o The pulsation dampener is also sometimes
called a pressure Snubbers but does not called a pressure Snubbers, but does not
have a filtering element.
o The most common design has a bar-stock
fitting. As the pressure pulse comes
through the dampener, the piston is forced
up and restricts the flow from the large
h b b l i th tl t f th
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chamber by closing the outlet of the
chamber.
c) Pressure-Limiting Valves
o Pressure-limiting valves protect the pressure
sensor from overpressure by blocking the
process fluid at a preset limit.
o There are several designs of pressure-o There are several designs of pressure
limiting valves. One has the fluid coming in
the inlet passing through a filter and out to the inlet, passing through a filter, and out to
the pressure sensor.
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PRESSURE MEASUREMENT DEVICES
1 MECHANICAL DEVICES1. MECHANICAL DEVICES
In mechanical pressure measurement devices the measuring
elements (sensors) can never separate it from the rest of the
system (signal processor and receiver/output device) as in many y ( g p / p ) y
cases they are all integral (e.g. a mechanical pressure gauge
incorporates all of these elements). incorporates all of these elements).
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PRESSURE GAUGE
Precautions
Tubes for gauges to be o Tubes for gauges to be
used on Acetylene must
b d f t l be made of steel.
o Associated Fittings for use
on oxygen must be kept
entirely free of oil.y
o Gauges used on
Hydrogen plants must be
gold plated.
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Gauge errors
oA gauge with a zero error will always read high or low by a oA gauge with a zero error will always read high or low by a constant amount.
A gauge with a span error has an internal magnification error oA gauge with a span error has an internal magnification error, where the error increases as long as the measured pressure increases.c eases
oA gauge with a linearity error may read correctly at 0 and 100% but will not follow a linear path between these points. 100% but will not follow a linear path between these points.
PRESSURE TRANSMITTER
a) Pneumatic transmitter
Supply: 20 Psi OR 1.4 Bar
Output: 3 to 15 Psi OR 0.2 to 1.0 Bar
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b) Electronic transmitter
Supply: 24 V DC
Output: 4 to 20 mA
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1. Pressure is applied to the sensor.
2. A change in pressure is measured by a change in the sensor
outputoutput.
3. The sensor signal is conditioned for various parameters.
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4. The conditioned signal is converted to an appropriate analogue
output (i.e. 4 – 20 mA)
c) Smart Transmitter
HHC (Hand Held
o Smart Transmitter can be programmed with a HHC (Hand
(Communicator).
o Smart Transmitter can be programmed with a HHC (Hand
Held Communicator).
o They come with addressable and communication facility.
o Transmitter can be re-ranged and zero checked without
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o Transmitter can be re ranged and zero checked without
disconnecting from the line.
Electronic Transmitter Wiring Connections
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PRESSURE SWITCHES o An electric pressure switch senses pressure and opens or l l t i l it h closes an electrical switch
element at a set pressure to signal another electrical device.
o Most pressure switches trip at a pressure above atmospheric, and are called gauge pressure or g g psimply pressure switches.
o Switches can also be manufactured to trip at pressure manufactured to trip at pressure referenced to a complete vacuum and this is called absolute pressure switches absolute pressure switches.
o Pressure switches are set to trip at a certain point with rising or f lli falling pressure.
o The difference in the trip point and the set point is called dead
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and the set point is called dead band or reset or switch differential.
Electrical Switch Contacts
The electrical switch is
usually:y
single-pole double-
throwthrow
or
double-pole double-
throw.
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PRESSURE SWITCHESSwitch Terminology
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PRESSURE REGULATOR
o Pneumatic regulators are used o Pneumatic regulators are used
with every pneumatic
Instrument Instrument.
o Regulator maintains a steady
outlet pressure irrespective of
variation in the upstream
pressure.
o Regulator comes with a filter to o Regulator comes with a filter to
remove wet the dust.
o A drain valve remove any
condensation of air.
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