Transcript AUTOMATION & ROBOTICS LECTURE#05 PRESSURE SENSORS By: Engr. Irfan Ahmed Halepoto Assistant Professor PRESSURE SENSOR TRANSDUCERS • Pressure is an expression of the force required.

AUTOMATION & ROBOTICS
LECTURE#05
PRESSURE SENSORS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor
PRESSURE SENSOR TRANSDUCERS
• Pressure is an expression of the force required to stop
a fluid from expanding, and is usually stated in terms of
force per unit area
– (pounds per square inch-psi)
• Pressure sensor acts as a transducer; it generates a
signal as a function of the pressure imposed.
• A pressure sensor measures pressure, typically of
gases or liquids.
• Pressure sensors are used for control, monitoring and
measurement of variables such as fluid/gas flow,
speed, water level, and altitude.
Pressure sensors - Units
• The basic SI unit of pressure is the Pascal , which is N/m2.
• One common unit used in vacuum is Torr, which is defined
as 1 mm-Hg, or 1/760 of an atmosphere.
 Torr (Torricelli): pressure exerted by a 1mm of mercury
(at 0C and normal atmospheric pressure)
 Atmosphere Pressure: force per unit area exerted
against a surface by the weight of air above that surface.
• 1 atm = 101.325 kPa = 760 torr
• 1 atm= 14.696 psi
• 1 psi=6,894.76Pa
• Another commonly used unit is bar, which is 100K pascal.
– Bar : atmospheric pressure on Earth at sea level.
Pressure sensors Units: Relationship
pound-force
per square
inch (psi)
Unit
Pascal
(Pa)
bar
atmosphere
Torr
(atm)
1 Pa
1
10−5
9.8692×10−6
7.5006×10−3 145.04×10−6
1 bar
100,000
1
0.98692
750.06
14.504
1 atm
101,325
1.01325
1
760
14.696
1 Torr
133.322
1.3332×10−3 1.3158×10−3
1
19.337×10−3
1 psi
6,894.76
68.948×10−3 68.046×10−3
51.715
1
Pressure Measurement Scales
• Five basic scales used to measure pressure:
– Atmosphere pressure
– Gauge pressure
– Absolute pressure
– Differential pressure
– Vacuum pressure
Pressure Measurement......................Formulization
Pressure is quoted as being Absolute or Gauge
1.
Some Fluid = Some Pressure = Some absolute pressure
2.
No Fluid = No Pressure = Zero absolute pressure
Whereas
1.
Fluid Pressure+ Atmospheric Pressure= Gauge Pressure
2.
No Fluid + Atmospheric Pressure = Zero Gauge Pressure
Which follows
Gauge Pressure – Atmospheric Pressure = Pressure due to fluid itself =
Absolute fluid pressure
Atmosphere pressure
• Atmospheric pressure is the air pressure
exerted upon the earth.
• It is approximately 14.7 psi at sea level and
decreases as elevation increases.
• For measuring a low vacuum below one
atmosphere, the atmospheric pressure is
often taken as “0”, and the unit in-Hg (inch
of Hg) is often used to indicate the vacuum
(1 atm = 29.92 in-Hg) without the negative
sign.
• Compound vacuum pressure gauge can
measure pressures both above and below
one atmosphere.
compound vacuumpressure gauge
Gauge Pressure Scale
• Gauge pressure scales use atmospheric
pressure as a reference point and
extends in the positive direction.
• If the sensing element is exposed to the
atmosphere, it registers zero pressure.
• Gauge pressure is zero referenced
against ambient air pressure, so it is
equal to absolute pressure minus
atmospheric pressure.
– Negative signs are usually omitted.
• units of measurement are recorded as
psig (pounds per square inch, gage).
• Examples: Tire pressure, blood pressure
Absolute Pressure Scale
• Absolute pressure is always used to
measure high vacuums, where Torr and bar
are the most commonly used units.
• Absolute pressure is referenced to absolute
zero (no fluid), or the complete lack of
pressure.
• Absolute pressures are always indicated by
positive numbers.
• If the sensing element is exposed to the
atmosphere, it will register 14.7 psia
(pounds per square inch, absolute).
• Absolute pressure is zero referenced
against a perfect vacuum, so it is equal to
gauge pressure + atmospheric pressure.
• Examples: atmospheric pressures, vacuum
pressures, altimeter pressures (Altitude).
Differential Pressure Scale
• Differential pressure is used to express
the difference between two measured
pressures.
• It is determined by subtracting the
lower reading from the higher reading.
• Differential pressures are commonly
used in industrial process systems.
Vacuum Pressure Scale
• The vacuum scale ranges from absolute
zero pressure to atmospheric pressure
(as a reference point).
• The absolute zero pressure point, which
is also called total vacuum, represents a
total lack of pressure
• Most common vacuum scales use
inches of mercury (in Hg) to express the
value
• A vacuum gage will read zero when
measuring atmospheric pressure and
29.92 in Hg when measuring a complete
vacuum.
Comparison of Pressure Scales
Pressure Measurement Methods
• Pressure control is an important process in many
industrial applications requiring accurate measurement
and control
• Instruments are often classed by whether they make
direct or inferred measurements
• Both electronic and non-electronic instruments are
used in pressure measurement.
• As pressure changed ,the flexible element moved, and
this motion was used to rotate a pointer in front of dial.
• Two broad categories of pressure measurement.
– Mechanical Pressure Transducers
– Electrical Pressure Transducers
Mechanical Pressure Transducers
• Mechanical (Elastic) Pressure Transducers utlizes
flexible element as a sensor.
• Pressure sensor measured in form of
1.
2.
3.
4.
5.
6.
Liquid Column Gauges
Manometer
Bourdon tube pressure gauge
Diaphragm (capsule) pressure transducers
Bellows
Spring and Piston.
Liquid Column Gauges
• Measuring pressure is possible
by monitoring the height of a
liquid in a column.
• These gauges are very accurate
and may be used as calibration
tools for other instruments
• A barometer is an example of a
liquid column gauge.
– A barometer is a instrument
used in meteorology to
measure
atmospheric
pressure by using water, air,
or mercury.
Manometer
• The most common liquid column
device to measure pressure is the
manometer.
• A common simple manometer consists
of a U shaped tube of glass filled with
some liquid.
– Typically the liquid is mercury
because of its high density
• Each column is exposed to a different
pressure source.
• Read the rise of liquid in one column
and the drop in the other, and add them
together.
Types of Manometers
Bourdon Tube Pressure Gauge
• Most common device around us is the pressure gauage
which utilizes a bourdon tube as its sensing elements.
• The tube serves as the primary detector-transducer,
changing pressure into near linear displacement
• A bourdon tube is a curved, hollow tube with the process
pressure applied to the fluid in the tube.
• Pressure in the tube causes, tube to deform or uncoil.
Bourdon tubes are generally are of three types.
1. C-type
2. Helical type
3. Spiral type
Bourdon Tube Pressure Gauge: Types
Bourdon Tube Construction Mechanism
• Bourdon tube is a hollow tube with an elliptical cross section.
• When a pressure difference exists between the inside and outside, the
tube tends to straighten out and the end moves.
• Movement is usually coupled to a needle on a dial to make a complete
gauge.
• It can also be connected to a secondary device such as an air nozzle to
control air pressure or to a suitable transducer to convert it into an
electric signal.
– This type can be used for measuring pressure difference.
– Air pressure of Vehicle Tire
Essentials of a Bourdon-tube pressure gage
1.Pressure causes the section
of the flattened tube tends
toward a more circular form
2.This causes the free end A to
move outward
3.The resulting motion is
transmitted by link B to sector
gear C
4.Then to pinion D and cause
the indicator hand to move
over the scale
Diaphragm pressure transducers
• Diaphragm is constructed of two
flexible disks, and when a
pressure is applied to one face
of the diaphragm, the position of
the disk face changes due to
deformation.
• Position can be related to
pressure.
• Diaphragm expands when very
small pressures are applied.
• Movement is transmitted to a
pointer on a dial through a fine
mechanical linkage
• Diaphragm are mostly made of
Rubber.
Diaphragm pressure transducers: Construction
Diaphragm Pressure Gauge
•
They are used to measure
gauge pressures over very
low ranges.
• Two types of diaphragm
pressure gauges are:
1. Metallic diaphragms gauge
(brass or bronze)
2. Slack diaphragms gauge
(Rubber)
Bellows
• Bellows are used to measure
low pressures.
• Bellows is a closed vessel with
sides that can expand and
contract, like an accordion.
• The position of the bellows
without pressure can be
determined by the bellows itself
or a spring.
• The pressure is applied to the
face of the bellows, and its
deformation and its position
depend upon the pressure.
Bellows Constructional Elements
• Bellows elastic elements are made of
brass, phosphor bronze, stainless steel,
beryllium-copper, or other metal suitable
for the intended service of the gauge.
• Motion of the element (bellows) is
transmitted by suitable linkage and
gears to a dial pointer.
• Most bellows gauges are spring-loaded,
as spring opposes the bellows and thus
prevents full expansion of the bellows.
– Thus by limiting the expansion of the
bellows, protects and prolongs the
bellows life.
• Because of the elasticity in both the
bellows and the spring in a springloaded bellows element, the relationship
between the applied pressure and
bellows movement is linear.
Spring & Piston Type
• The pressure acts directly on the
piston and compresses the spring.
• The position of the piston is directly
related to the pressure.
• A window in the outer case allows
the pressure to be indicated.
• The piston movement may be
connected to a secondary device to
convert movement into an electrical
signal.
• This type is usually used in
hydraulics where the ability to
withstand shock, vibration and
sudden pressure changes is needed
(shock proof gauge).
Electric Pressure Transducers..... Why???
• Transmission requirements for remote display as
electric signal transmission can be through cable or
cordless.
• Electric signals give quicker responses and high
accuracy in digital measurements.
• The linearity property of the electric signal produced to
pressure applied favors simplicity.
• They can be used for extreme pressure applications,
i.e. high vacuum and pressure measurements.
• EPTs are immune to hysteresis, shock and
mechanical vibrations.
Electric Pressure Transducers: Block Diagram
1. Pressure sensing element such as a bellow , a
diaphragm or a bourdon tube
2. Primary conversion element e.g. resistance or voltage
3. Secondary conversion element
Electrical pressure transducers: Elements
Electrical pressure transducers consists of three elements.
• Mechanical Pressure Transducers to Pneumatic Pressure
Transmitters to Electric Pressure Transmitters
ELECTRICAL PRESSURE TRANSDUCERS
• The conversion of pressure into an electrical signal is
achieved by the physical deformation of strain gages
which are bonded into the diaphragm of the pressure
transducer and wired into a Wheatstone bridge
configuration.
• Pressure applied to the pressure transducer produces
a deflection of the diaphragm which introduces strain
to the gages.
• The strain will produce an electrical resistance change
proportional to the pressure.
Electric Pressure Transducers: Classification
• There are various ways of converting the mechanical
movement into an electric signal.
• Following are types that directly produce an electric
signal.
 Strain gauge pressure transducers
 Capacitive pressure transducers
 Piezoelectric pressure transducers
 Piezoresistive pressure transducers
 Potentiometer pressure transducers
 Resonant wire pressure transducers
Strain Gauge Pressure Transducer
• Strain gauge is a device used to measure
the strain of an object.
• Mechanism: Pressure--Mechanical Motion-Strain Gage
• Strain gauge pressure transducers are
used for narrow pressure span and for
differential pressure measurments.
• Strain gauges are typically constructed of
piezoelectric semiconductor material
• Pressure transducer contains a diaphragm
which is deformed by the pressure which
can cause a strain gauge to stretch or
compress.
• This deformation of the strain gauge
causes the variation in length and cross
sectional area due to which its resistance
changes.
• Strain gauges are used as part of a
Wheatstone Bridge to measure pressure.
Strain Gauge Pressure Transducer:Strcuture
• Strain gauge is a passive type resistance pressure
transducer whose electrical resistance changes when it is
stretched or compressed .
• The wire filament is attached to a structure under strain and
the resistance in the strained wire is measured
Strain Gauge Pressure Transducer:Construction & Working
Capacitive Pressure Transducer
• Mechanism: Pressure-- Diaphragm Motion-- Capacitance
• The deflection of the diaphragm constitutes a capacitor in which
the distance between the plates is pressure sensitive.
• Capacitive Pressure Transducer are use in low vacuum pressure
applications.
C=ε0 εr A/d
•
•
•
•
•
•
•
Where,
C= capacitance of a capacitor in farad
A = area of each plate in m2
d = distance between two plates in m
εr= dielectric constant
ε0 = 8.854*10^-12 farad/m2
Thus, capacitance can be varied by changing distance between
the plates, area of the plate or value of the dielectric medium
between the plates.
• Any change in these factors cause change in capacitance.
Capacitive Pressure Transducer:Construction
Capacitive pressure transducer includes:
• a pair of electrically insulative elastic
diaphragms disposed adjacent to each
other and bonded together in a spaced
apart relationship to form a sealed
cavity,
• a conductive layer applied to the
inside surface of each of the
diaphragms
• a small absolute pressure provided in
the cavity.
– This small absolute pressure cavity
essentially reduces the effects of
the
negative
temperature
coefficient of the modulus of
elasticity of the diaphragms.
In capacitive transducers, pressure is utilized to vary any of the above mentioned
factors which will cause change in capacitance and that is a measureable by any
suitable electric bridge circuit and is proportional to the pressure.
Capacitive Pressure Transducer:Configuration
• The sensing diaphragm and capacitor form a differential variable
separation capacitor.
• When the two input pressures are equal the diaphragm is
positioned centrally and the capacitance are equal.
• A difference in the two input pressure causes displacement of the
sensing diaphragm and is sensed as a difference between the two
capacitances
Piezoelectric Pressure Transducer
• When pressure, force or acceleration is applied to a quartz
crystal, a charge is developed across the crystal that is
proportional to the force applied.
• When pressure is applied to a crystal, it is elastically
deformed, which results in a flow of electric charge, that
indicate the pressure as applied to the crystal
• These sensors cannot detect static pressures, but are used
to measure rapidly changing pressures resulting from blasts,
explosions, pressure pulsations (in rocket motors, engines,
compressors) or other sources of shock or vibration.
• Output of Piezoelectric pressure sensors is expressed in
"relative" pressure units (such as psir instead of psig).
• The maximum range of such sensors is 5,000 or 10,000
psir.
• Desirable features of includes rugged construction, small
size, high speed, and self-generated signal.
• On the other hand, they are sensitive to temperature
variations and require special cabling and amplification
Piezoelectric Pressure Transducer:Construction
Piezoresistive Pressure Transducer
• Piezoresistive pressure sensors operate
based on the resistivity dependence of
silicon under stress
• Like the Strain gauge, Piezoresistive
sensor consists of a diaphragm onto
which four pairs of silicon resistors are
bonded.
• Unlike the construction of a strain gage
sensor, here the diaphragm itself is made
of silicon and the resistors are diffused
into the silicon during the manufacturing
process
• Diaphragm is completed by bonding the
diaphragm to an unprocessed wafer of
silicon.
• Silicon diaphragm is shielded from direct
contact with the process materials by a
fluid-filled protective diaphragm made of
stainless steel or some other alloy that
meets the corrosion requirements of the
service.
• Piezoresistive pressure sensors can be
used from about 3 psi to a maximum of
about 14,000 psi (21 KPa to 100 MPa).
Potentiometer Pressure Transducer
• The device consists of a precision potentiometer whose wiper
alarm is mechanically linked to bourdon tube or bellow.
• The movement of wiper alarm across the potentiometer
converts the mechanically detected sensor deflection into a
resistance measurement using a Wheatstone bridge circuit.
Resonant Wire Pressure Transducer
• Used for low differential pressure applications
• A wire is griped by a static member at one end and by
the sensing diaphragm at the other.
• An oscillatory circuit causes the wire to oscillate at its
resonant frequency.
• A change in process pressure changes the wire
tension which in turn changes the resonant frequency
of the wire.
• A digital counter circuit detects the shift.
Resonant Wire Pressure Transducer
Where and How have EPTs failed?
• EPTs require a constant supply of electricity for them
to function. They do not come with built-in power
supply.
• High performance comes at a cost.
• Installation of auxiliary display modules and electrical
circuitry increases capital cost.
• Physical properties, like temperature, which can affect
electrical constants may affect the consistency of
EPTs.
• For this reason, temperature compensation is always
required with EPTs.
• Some electrical phenomena, like piezoelectric energy,
have limited applicability. This limits their use in
industry.
• Electricity exposes personnel to potential hazards.
Summary of Pressure Sensor Characteristics