Transcript Document

Instrumentation 2
Pressure
Higher Certificate in Technology
(Manufacturing Technology)
Pressure - Basics
• Many of the processes in the modern world
involve the measurement and control of
pressurized liquid and gas systems
• This monitoring reflects certain performance
criteria that must be controlled to produce the
desirable results of the process and insure its safe
operation
• Boilers, refineries, water systems, and compressed
gas systems are but a few of the many applications
for pressure gauges
Pressure - Basics
• There are many applications for pressure sensors but we
can narrow them down to two major categories:
• Pressure sensing
• This is the direct use of pressure sensors to measure
pressure.
• This is useful in weather instrumentation, aircrafts, cars,
and any other machinery that has pressure functionality
implemented.
• Altitude sensing
• This is useful in aircrafts, rockets, satellites, weather
balloons, and many other applications.
• All these applications make use of the relationship between
changes in pressure relative to the altitude.
Pressure - Basics
• This relationship is governed by the following equation:
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where h = height (m), P = pressure at altitude in Pascals or
Pa, Pref = sea-level pressure in Pascals or Pa
1 Pascal = 1 Pa = 1kg/ms²
This equation is calibrated for an altimeter, up to 36,090
feet (11,000 m).
Outside that range, an error will be introduced which can
be calculated differently for each different pressure sensor.
These error calculations will factor in the error introduced
by the change in temperature as we go up
Pressure - Basics
• Example
• What is the height of the aircraft in meters if the
pressure reading, P, is 90kPa and Pref is 101kPa?
• Solution
h = [(1-(90/101)^0.19026) x 288.15]/0.00198122
h = [(1-0.978299) x 288.15]/0.00198122
h = 6.2528/0.00198122
h = 3156 meters
Pressure - Basics
• Pressure is defined as a force per unit area e.g. kg/m²
• Pressure measurements may be expressed relative to
various zero references
• Absolute pressure of a fluid is referenced against a perfect
vacuum
• Gauge pressure is referenced against ambient air pressure,
so it is equal to absolute pressure minus atmospheric
pressure e.g. gauge pressure = 110 kPa – 101 kPa = 9 kPa
• A standard value of atmospheric pressure has been defined
to be 101.325 Pa, but is variable with altitude and weather
Pressure - Basics
• If the absolute pressure of a fluid stays constant, the gauge
pressure of the same fluid will vary as atmospheric
pressure changes
• Examples of absolute pressure measurements include
barometric pressure, altimeters, and the Manifold Absolute
Pressure (MAP) sensor used in the engine control systems
• Examples of gauge pressure measurements include the tirepressure gauge and sphygmomanometer
• Differential pressure is the difference in pressure between
two points.
• Differential pressure gauges have two inlet ports, each
connected to one of the volumes whose pressure is to be
monitored
Pressure - Basics
• Pressure sensors can be classified in five categories:
• Absolute pressure sensor
• This sensor measures the pressure relative to perfect Vacuum pressure
(0 PSI or no pressure)
• Atmospheric pressure, is about 100kPa (14.7 PSI) at sea level.
Atmospheric pressure is an absolute pressure.
• Gauge pressure sensor
• This sensor is used in different applications because it can be
calibrated to measure the pressure relative to a given atmospheric
pressure at a given location
• An example of gauge pressure would be a tire pressure gauge. When
the tire pressure gauge reads 0 PSI, there is really 14.7 PSI
(atmospheric pressure) in the tire.
Pressure - Basics
• Vacuum pressure sensor
• This sensor is used to measure pressure less than the atmospheric
pressure at a given location.
• Differential pressure sensor
• This sensor measures the difference between two or more pressures
introduced as inputs to the sensing unit
• For example, if we need to know the difference of the pressure of some
fluid going in a pressure boosting unit and the output pressure of that
unit in a way to monitor how much we boosted the fluid pressure; we
use differential sensor.
• Sealed pressure sensor
• This sensor is the same as the Gauge pressure sensor except that it is
previously calibrated by manufacturers to measure pressure relative to
sea level pressure (14.6 PSI).
Pressure - Basics
• Static pressure is uniform in all directions, so pressure
measurements are independent of direction in an immobile
(static) fluid
• Flow, however, applies additional pressure on surfaces
perpendicular to the flow direction
• This directional component of pressure in a moving
(dynamic) fluid is called dynamic pressure
• An instrument facing the flow direction measures the sum
of the static and dynamic pressures; this measurement is
called the total pressure or stagnation pressure
• Since dynamic pressure is referenced to static pressure, it
is neither gauge nor absolute; it is a differential pressure
Pressure - Basics
• Dynamic pressure is used to measure flow rates and
airspeed
• Dynamic pressure can be measured by taking the
differential pressure between instruments parallel and
perpendicular to the flow
• Pitot-static tubes, for example perform this measurement
on airplanes to determine airspeed
• The presence of the measuring instrument inevitably acts
to divert flow and create turbulence, so its shape is critical
to accuracy and the calibration curves are often non-linear
Pressure - Manometer
• The most accurate way to measure low air pressure is to
use a Manometer which is a Hydrostatic gauge
• It 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
• 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
Pressure - Manometer
• The simplest design is a closed-end U-shaped
tube, one side of which is connected to the region
of interest
• Any fluid can be used, but mercury is preferred for
its high density and low vapor pressure
• The units of measurement commonly used are
inches of mercury (in. Hg), using mercury as the
fluid and inches of water (in. w.c.), using water or
oil as the fluid
• Simple hydrostatic gauges can measure pressures
ranging from 100 Pa to above atmospheric
Pressure - Manometer
• Fig. 2.1. In its simplest form the manometer is a U-tube about half
filled with liquid. With both ends of the tube open, the liquid is at the
same height in each leg.
• Fig. 2-2. When positive pressure is applied to one leg, the liquid is
forced down in that leg and up in the other. The difference in height,
"h," which is the sum of the readings above and below zero, indicates
the pressure.
• Fig. 2-3. When a vacuum is applied to one leg, the liquid rises in that
leg and falls in the other. The difference in height, "h," which is the
sum of the readings above and below zero, indicates the amount of
vacuum
Pressure - Manometer
• The difference in fluid height in
a liquid column barometer is
proportional to the pressure
difference
P - Po = pgh
where P = unknown pressure , p
= density liquid, g = gravity, h =
height, Po = atmospheric
pressure
• Density of water for example is
1 kg per 1000cm³
• Density of mercury for example
is 13.6 kg per 1000cm³
Pressure - Manometer
• Example:
• What is the pressure of the gas being measured by a simple
u-tube manometer using the following information:
h = 10cm, Po = 101kPa, p = 1kg/1000cm³, g = 9.81m/s²
• Solution
P – Po = pgh
P – Po = (1000kg/m³)(9.81m/s²)(0.1m)
P – Po = 981Pa = pressure difference between atmospheric
and test pressures
Therefore the pressure of the gas being measured is:
P = 101000Pa – 981Pa = 100019Pa = 100.019kPa if the
column of liquid rose up at the test side
Pressure – Variations in manometer
designs
Pressure – Variations in manometer
designs
Pressure – Variations in manometer
designs
Pressure - Bourdon Tube gauge
• A Bourdon gauge uses a coiled tube which as it expands due to
pressure increase causes a rotation of an arm connected to the tube
• The pressure sensing element is a closed coiled tube connected to the
chamber or pipe in which pressure is to be sensed
• As the gauge pressure increases the tube will tend to uncoil, while a
reduced gauge pressure will cause the tube to coil more tightly
• This motion is transferred through a linkage to a gear train connected
to an indicating needle. The needle is presented in front of a card face
inscribed with the pressure indications associated with particular
needle deflections
• In a barometer, the Bourdon tube is sealed at both ends and the
absolute pressure of the ambient atmosphere is sensed
• Differential Bourdon gauges use two Bourdon tubes and a mechanical
linkage that compares the readings
• Note that a Bourdon gauge can measure liquid pressure as well as gas
pressure
Pressure - Bourdon Tube gauge
• This particular gauge is a
combination vacuum and
pressure gauge used for
automotive diagnosis
• The left side of the face, used
for measuring manifold
vacuum, is calibrated in
centimeters 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
Pressure - Bourdon Tube gauge
Pressure - Bourdon Tube gauge
Calibration
• Calibration occurs just before the final assembly of the gauge to the
protective case and lens
• The assembly consisting of the socket, tube, and movement is
connected to a pressure source with a known "master" gauge
• A "master" gauge is simply a high accuracy gauge of known
calibration
• Adjustments are made in the assembly until the new gauge reflects the
same pressure readings as the master
• Accuracy requirements of 2 percent difference are common, but some
may be 1 percent, .5 percent, or even .25 percent
• Selection of the accuracy range is solely dependant upon how
important the information desired is in relationship to the control and
safety of the process
Pressure - Bourdon Tube gauge
Calibration
• Most manufacturers use a graduated dial featuring a 270 degree sweep
from zero to full range
• These dials can be from less than 1 inch (2.5 centimeters) to 3 feet (.9
meter) in diameter, with the largest typically used for extreme accuracy
• By increasing the dial diameter, the circumference around the
graduation line is made longer, allowing for many finely divided
markings
• These large gauges are usually very fragile and used for master
purposes only
• Masters themselves are inspected for accuracy periodically using dead
weight testers, a very accurate hydraulic apparatus that is traceable to
the National Bureau of Standards in the United States
Pressure - Bourdon Tube gauge
Applications
• The varied applications account for the wide range in
design of the case and lens enclosure
• Some dials are illuminated by the luminescent inks used to
print the graduations or by tiny lamps connected to an
outside electrical source
• Gauges intended for high pressure service usually are of
"dead front" safety design, a case design feature that places
a substantial thickness of case material between the
Bourdon tube and the dial
• This barrier protects the instrument viewer from gauge
fragments should the Bourdon tube rupture due to excess
pressure
• The internal case design directs these high velocity pieces
out the back of the gauge, away from the viewer
Pressure – Dead-Weight Tester
• Pressure transducers can be recalibrated on-line or in a
calibration laboratory
• Laboratory recalibration typically is preferred, but often is
not possible or necessary
• In the laboratory, there usually are two types of calibration
devices: deadweight testers that provide primary, base-line
standards, and "laboratory" or "field" standard calibration
devices that are periodically recalibrated against the
primary
• Of course, these secondary standards are less accurate than
the primary, but they provide a more convenient means of
testing other instruments.
Pressure – Dead-Weight Tester
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A deadweight tester consists of a
pumping piston with a screw that
presses it into the reservoir, a
primary piston that carries the dead
weight, and the gauge or transducer
to be tested
It works by loading the primary
piston (of cross sectional area A),
with the amount of weight (W) that
corresponds to the desired
calibration pressure (P = W/A)
The pumping piston then
pressurizes the whole system by
pressing more fluid into the
reservoir cylinder, until the dead
weight lifts off its support
Pressure – Dead-Weight Tester
• Example
• If the area of the piston used in the Dead weight is 5mm² and the mass
of the moving part (piston and holder) is 0.3kg, what mass is required
to be added to balance a pressure of 5MPa applied to the piston area,
taking g = 9.81m/s ² ?
• Solution
W = weight = mg = (0.3kg + mass weight)(9.81m/s ² )
P = W/A
5000000 Pa = [(0.3kg + mass weight)(9.81m/s ² )]/(0.005m²)
5000000 Pa (0.005m²) = [(0.3kg + mass weight)(9.81m/s ² )]
25000 kgm/s² = 2.943 kgm/s² + [(mass weight)(9.81m/s ² )]
25000 kgm/s² - 2.943 kgm/s² = [(mass weight)(9.81m/s ² )]
[25000 kgm/s² - 2.943 kgm/s²]/9.81m/s² = mass weight
mass weight = 2548.11 kg
Pressure – Dead-Weight Tester
• Today's deadweight testers are more accurate and more
complex than the previous instrument but the essential
operating principles are the same
• In the United States, the National Institute of Standards &
Technology (NIST) provides certified weights and
calibrates laboratory piston gauges by measuring the
diameter of the piston
• Deadweight testers can be used to calibrate at pressure
levels as low as 5 psig (35 kPa) and as high as 100,000
psig (690 MPa)
• Tilting type, air-lubricated designs can detect pressures in
the mm Hg range
• NIST calibrated deadweight testers can be accurate to 5
parts in 100,000 at pressures below 40,000 psig (280 MPa)
Pressure – Dead-Weight Tester
• For an industrial quality deadweight tester, error is
typically 0.1% of span
• A typical secondary standard used for calibrating industrial
pressure transducers contains a precision power supply, an
accurate digital readout, and a high-accuracy resonant
(quartz) pressure sensor
• It is precise enough to be used to calibrate most industrial
pressure transducers, but must be NIST-traceable to be
used as an official calibration standard
• The best accuracy claimed by the manufacturers is
typically 0.05% full scale
Pressure – Semiconductor Pressure
Transducers - Basics
• These sensors are electronic components that provide an
electrical signal and have essentially no moving parts
• Many gauges today already have these sensors mounted
within the case to send information to process control
computers and controllers
• These sensors are intrinsically safe, allowing their use in
flammable or explosive environments
• However, the mechanical gauge does not require the
electrical power source or the computer equipment needed
by the electronic sensor
Pressure – Semiconductor Pressure
Transducers - Basics
• A pressure transmitter is a standardized pressure measurement package
consisting of three basic components: a pressure transducer, its power
supply, and a signal conditioner/retransmitter that converts the
transducer signal into a standardized output
• Pressure transmitters can send the process pressure of interest using an
analog pneumatic (3-15 psig), analog electronic (4-20 mA dc), or
digital electronic signal
• When transducers are directly interfaced with digital data acquisition
systems and are located at some distance from the data acquisition
hardware, high output voltage signals are preferred
• These signals must be protected against both electromagnetic and radio
frequency interference (EMI/RFI) when traveling longer distances
Pressure – Semiconductor Pressure
Transducers - Basics
• Transducer accuracy refers to the degree of conformity of
the measured value to an accepted standard
• It is usually expressed as a percentage of either the full
scale or of the actual reading of the instrument
• In case of percent-full-scale devices, error increases as the
absolute value of the measurement drops
• Repeatability refers to the closeness of agreement among a
number of consecutive measurements of the same variable
• Linearity is a measure of how well the transducer output
increases linearly with increasing pressure
Pressure – Semiconductor Pressure
Transducers – Strain Gauge
• When a strain gage is used to measure the deflection of an elastic
diaphragm or Bourdon tube, it becomes a component in a pressure
transducer
• Strain gage-type pressure transducers are widely used
• Strain-gage transducers are used for narrow-span pressure and for
differential pressure measurements
• Essentially, the strain gage is used to measure the displacement of an
elastic diaphragm due to a difference in pressure across the diaphragm
• These devices can detect gauge pressure if the low pressure port is left
open to the atmosphere or differential pressure if connected to two
process pressures
• If the low pressure side is a sealed vacuum reference, the transmitter
will act as an absolute pressure transmitter
Pressure – Semiconductor Pressure
Transducers – Strain Gauge
• Strain gage transducers are available for
pressure ranges as low as 3 inches of water
to as high as 200,000 psig (1400 MPa)
• Inaccuracy ranges from 0.1% of span to
0.25% of full scale
• Additional error sources can be a 0.25% of
full scale drift over six months and a 0.25%
full scale temperature effect per 1000¡ F.
Pressure – Semiconductor Pressure
Transducers – Strain Gauge
Pressure – Semiconductor Pressure
Transducers – Strain Gauge
Pressure – Semiconductor Pressure
Transducers – Potentiometric
• The potentiometric pressure sensor provides a simple method for
obtaining an electronic output from a mechanical pressure gauge
• The device consists of a precision potentiometer, whose wiper arm is
mechanically linked to a Bourdon or bellows element
• The movement of the wiper arm across the potentiometer converts the
mechanically detected sensor deflection into a resistance measurement,
using a Wheatstone bridge circuit
• The mechanical nature of the linkages connecting the wiper arm to the
Bourdon tube, bellows, or diaphragm element introduces unavoidable
errors into this type of measurement
• Temperature effects cause additional errors because of the differences
in thermal expansion coefficients of the metallic components of the
system
• Errors also will develop due to mechanical wear of the components
and of the contacts
Pressure – Semiconductor Pressure
Transducers – Potentiometric
• Potentiometric transducers can be made extremely small
and installed in very tight quarters, such as inside the
housing of a 4.5-in. dial pressure gauge
• They also provide a strong output that can be read without
additional amplification
• This permits them to be used in low power applications
• They are also inexpensive
• Potentiometric transducers can detect pressures between 5
and 10,000 psig (35 KPa to 70 MPa)
• Their accuracy is between 0.5% and 1% of full scale, not
including drift and the effects of temperature
Pressure – Semiconductor Pressure
Transducers – Potentiometric
Pressure – Semiconductor Pressure
Transducers – Piezoelectric
• 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
• The fundamental difference between these crystal
sensors and static-force devices such as strain
gages is that the electric signal generated by the
crystal decays rapidly
• This characteristic makes these sensors unsuitable
for the measurement of static forces or pressures
but useful for dynamic measurements
Pressure – Semiconductor Pressure
Transducers – Piezoelectric
• When pressure is applied to a crystal, it is elastically
deformed
• This deformation results in a flow of electric charge (which
lasts for a period of a few seconds)
• The resulting electric signal can be measured as an
indication of the pressure which was 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
• Some of these rugged sensors can detect pressure events
having "rise times" on the order of a millionth of a second
Pressure – Semiconductor Pressure
Transducers – Piezoelectric
• The output of such dynamic pressure sensors is often
expressed in "relative" pressure units (such as psir instead
of psig), thereby referencing the measurement to the initial
condition of the crystal
• The maximum range of such sensors is 5,000 or 10,000
psir
• The desirable features of piezoelectric sensors include their
rugged construction, small size, high speed, and selfgenerated signal
• On the other hand, they are sensitive to temperature
variations and require special cabling and amplification
Pressure – Semiconductor Pressure
Transducers – Piezoelectric
Pressure – Semiconductor Pressure
Transducers – Piezoresistive
• Piezoresistive pressure sensors operate based on
the resistivity dependence of silicon under stress
• Similar to a strain gage, a 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
• The diaphragm is completed by bonding the
diaphragm to an unprocessed wafer of silicon
Pressure – Semiconductor Pressure
Transducers – Piezoresistive
• If the sensor is to be used to measure absolute pressure, the bonding
process is performed under vacuum
• If the sensor is to be referenced, the cavity behind the diaphragm is
ported either to the atmosphere or to the reference pressure source
• The 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 are sensitive to changes in temperature
and must be temperature compensated
• Piezoresistive pressure sensors can be used from about 3 psi to a
maximum of about 14,000 psi (21 KPa to 100 MPa).
Pressure – Semiconductor Pressure
Transducers – Optical
• Optical pressure transducers detect the effects of minute
motions due to changes in process pressure and generate a
corresponding electronic output signal
• A light emitting diode (LED) is used as the light source,
and a vane blocks some of the light as it is moved by the
diaphragm
• As the process pressure moves the vane between the
source diode and the measuring diode, the amount of
infrared light received changes
• The optical transducer must compensate for aging of the
LED light source by means of a reference diode, which is
never blocked by the vane
• This reference diode also compensates the signal for buildup of dirt or other coating materials on the optical surfaces
Pressure – Semiconductor Pressure
Transducers – Optical
• The optical pressure transducer is immune to temperature
effects, because the source, measurement and reference
diodes are affected equally by changes in temperature
• Moreover, because the amount of movement required to
make the measurement is very small (under 0.5 mm),
hysteresis and repeatability errors are nearly zero
• Optical pressure transducers do not require much
maintenance
• They have excellent stability and are designed for longduration measurements
• They are available with ranges from 5 psig to 60,000 psig
(35 kPa to 413 MPa) and with 0.1% full scale accuracy
Pressure – Semiconductor Pressure
Transducers – Optical
References
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http://www.omega.com/literature/transactions/volume3/pressure.html
http://www.omega.com/literature/transactions/volume3/pressure2.html
http://www.omega.com/literature/transactions/volume3/pressure3.html
http://en.wikipedia.org/wiki/Pressure_measurement
http://www.efunda.com/formulae/fluids/manometer.cfm
http://www.upscale.utoronto.ca/PVB/Harrison/Manometer/Manometer.html
http://www.dwyer-inst.com/htdocs/pressure/ManometerIntroduction.cfm
http://tpub.com/machines/9c.htm
http://www.answers.com/topic/pressure-gauge
http://www.answers.com/topic/pressure-measurement-1
http://en.wikipedia.org/wiki/Pressure_sensor
http://en.wikipedia.org/wiki/Piezoresistive_effect