Transcript L20_Pressure

MECH 373
Instrumentation and Measurements
Lecture 20
Measuring Pressure and Temperature
(Chapter 9)
• Measuring Pressure
• Measuring Temperature
Lecture 20
Lecture Notes on MECH 373 – Instrumentation and Measurements
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Measuring Acceleration and Vibration
Accelerometers using Piezoelectric Sensing Elements
• An accelerometer using a piezoelectric material as the sensing element is shown
below:
• It consists of a housing, a mass called the seismic mass, and a piezoelectric
sensing element, which typically uses the longitudinal piezoelectric effect.
• An initial force between the mass and sensor is obtained with a preloading
spring sleeve.
Lecture Notes on MECH 373 – Instrumentation and Measurements
Lecture 20
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Measuring Acceleration and Vibration
• As the housing for the accelerometer is subject to an acceleration, the
force exerted by the mass on the quartz crystal is altered.
• This generates a charge on the crystal, which can be sensed with a
charge amplifier.
• Piezoelectric accelerometers are available in many ranges up to ±1000g,
where g is the acceleration due to gravity.
• Quartz crystal accelerometers can have very high values of natural
frequency up to 125 kHz.
• This allows them to measure frequencies as high as 25 kHz.
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Measuring Acceleration and Vibration
Vibrometer
• An instrument that is used to measure the ground motion in earthquakes
and sometimes to measure vibration in machines is called the vibrometer.
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Measuring Acceleration and Vibration
• Although the basic components are the same as the piezoelectric or
strain-gage accelerometers, the mode of operation is different.
• In the vibrometer, the spring is quite soft and as the housing moves, the
mass remains approximately stationary. The relative motion, y, is large
and sensed with a potentiometer.
• These devices are used to measure vibrations with frequencies that are
high relative to the natural frequency of the spring-mass system, which is
often less than 1 Hz.
• The vibrometer effectively measures the displacement of the base rather
than the acceleration.
• Thus, these devices are most sensitive to vibrations with moderate
frequencies and fairly large displacement amplitudes.
• High frequency vibrations usually have small values of displacement
amplitude and are better measured with accelerometers.
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Measuring Force
Load Cell
• “Load cell” is a term used to describe a transducer that generates a voltage signal
as a result of an applied force, usually along a particular direction.
• Virtually any simple metal structure deforms when subjected to a force, and as
long as the resulting stresses are below the material yield stress, the deflection (δ)
and resulting strain (ε) are linear functions of the applied force:
F  C1
F  C2 
where C’s are constants determined from analysis or calibration.
• The most common force-measuring devices are strain-gage load cells. They are
often constructed of a metal and have a shape such that the range of forces to be
measured results in a measurable output voltage over the desired operating range.
• Figure 8.34(a) shows a cantilever beam instrumented with four strain gages, two
on the top and two on the bottom, to measure normal or bending stresses. These
four gages form the Wheatstone bridge and offer effective temperature
compensation. The output of the bridge is four times the output of an individual
gage.
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An unbalanced Wheatstone Bridge
Rx is the STRAIN GAUGE,
generally VAB ≠ 0
VAB
Rx changes due to strain VAB changes
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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Measuring Force
• Figure 8.34(b) shows a hollow-cylinder load cell. It also uses four strain gages and
is also temperature compensated. As the cylinder is compressed, it becomes slightly
shorter, while the diameter becomes slightly larger. As a result, two gages measure
the axial compression. The other two, mounted transversely, measure the tensile
diametral strain. Since the transverse strain is only Poisson's ratio times the axial
strain, the output is less than four times the output of a single axial strain gage. For
a Poisson's ratio of 0.3, the output will be about 2.6 times the output of a single
axial strain gage.
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Measuring Force
• Due to their simple design, any range can be readily manufactured.
Commercial load cells are available with ranges from ounces up to several
hundred thousand pounds.
• Unlike accelerometers, it is not useful to specify the frequency response of
commercial load cells because the mass and flexibility of the instrumented
system control the dynamic response.
• Furthermore, an installed load cell will add flexibility to the system and
also affect the dynamic response.
• If the flexibility of strain-gage load cells is too high, load cells using
piezoelectric sensors, which are much stiffer, are commercially available.
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Measurement Systems
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Measuring Pressure
• Pressure is measured in three different forms: absolute pressure,
gage pressure, and differential pressure.
• Absolute pressure is that used in thermodynamics to determine the
state of a substance.
• Gage pressure is the pressure relative to the local ambient air
pressure.
• Differential pressure is simply the difference in pressure at two
points in a system.
• The relationship
pressure forms is
p between
 p different
p
abs
gage
ambient
Pressure Transducers
Lecture 20
• A very common and inexpensive device used to measure fluid
pressure is the diaphragm strain-gage pressure transducer, shown in
next slide.
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Lecture Notes on MECH 373 – Instrumentation and Measurements
Measuring Pressure
• The test pressure is applied to one side of the diaphragm, a reference
to the other side. A deflection of the diaphragm is sensed with strain
gages.
• The reference pressure can be atmospheric, so the transducer measures
gage pressure.
• The reference side could be sealed and evacuated so that the
transducer measures absolute pressure.
• Both sides could be connected to different test pressures so that the
measurement is of differential pressure.
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Measuring Pressure
• In the past, the diaphragm was made of metal and foil strain gages
were used. Recently, it has become common to make the diaphragm of a
semiconductor material (usually silicon) with semiconductor strain
gages formed into the diaphragm. This is a less expensive construction
technique, and since semiconductor gages have high gage factors, the
sensitivity is improved.
• Normally, the Wheatstone-bridge signal conditioner is built into the
transducer (all branches of the bridge are active gages), and the strain
gages are connected to give temperature compensation.
• Most strain-gage pressure transducers produce a DC output in the
millivolt range, but some include internal amplifiers and have outputs in
the range 0 to 5 or 0 to 10 V. The higher voltage output units are less
susceptible to environmental electrical noise.
• Pressures can also be sensed with LVDT (linear variable differential
transformer) devices like the one shown below:
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Measuring Pressure
• This diagram shows an arrangement with a flexible chamber called a
capsule and an LVDT to sense the displacement. This design is more
expensive than those using strain-gage sensors but may be more durable
in an application requiring a long lifetime.
• The output is a DC voltage with a range on the order of 0-5 or 0-10 V.
In the process industries, the voltage output usually be converted to a 4
to 20 mA current for signal transmission.
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Measuring Pressure
• Capacitive sensors are sometimes used in pressure transducers.
• When one or more fixed metal plates are placed directly above or
below a metallic diaphragm, a capacitor is created that forms an effective
secondary element. Displacement of the diagram changes the average
gap separation, which varies the capacitance developed between the two
plates.
• Capacitive pressure transducers, like those shown below, are
particularly useful for very low pressures (as low as 0.1 Pa) since captive
sensors can detect extremely small deflections.
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Measuring Pressure
• Transducers used for high-frequency pressure measurements usually
use a piezoelectric sensing element. A piezoelectric transducer is shown
below:
• These transducers generally use transverse-effect piezoelectric sensing
elements. The piezoelectric material is very stiff, and the transducers
have a high natural frequency in many applications. If the diaphragm (or
other displacing element) is very flexible, the natural frequency will be
low and the transducer output will be misleading for high-frequency
pressure measurements.
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Measuring Pressure
• The diaphragm is of the flush-mounted type. When the transducer is
installed, it comes into direct contact with the fluid in the pipe or
chamber. There are two reasons for this:
- If a cavity were included as in the other transducers, it might
significantly alter the measurand due to loading.
- The natural frequency would be reduced and the ability to respond
to transients would be impaired.
• Piezoelectric pressure transducers can have natural frequencies up to
150 kHz and are usable up to about 30 kHz.
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Measuring Pressure (Summary)
􀂋 Strain gage types
􀂋 Capacitive
very low pressures
􀂋 Piezoelectric
high-frequency pressure
􀂋 LVDT
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