Nội dung Text: Lecture Sensors and analytical devices: Pressure measurement - Nguyễn Công Phương
Nguyễn Công Phương
Sensors and Analytical Devices
Some Basic Measurement Methods,
Pressure Measurement
Contents
A. Introduction
B. Sensors Characteristics
C. Some Basic Measurement Methods
D. Measurement Systems
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Some Basic Measurement Methods
I. Sensor Technologies
II. Temperature Measurement
III.Pressure Measurement
IV.Flow Measurement
V. Level Measurement
VI.Mass, Force, and Torque Measurement
VII.Translational Motion, Vibration, and Shock
Measurement
VIII.Rotational Motion Transducers
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Introduction (1)
• Very commonly in most industrial process control systems, & the
next-most measured process parameter after temperature.
• Absolute pressure (a/abs):
– The difference between the pressure of the fluid & the absolute zero of
pressure.
– Made for such purposes as aircraft altitude measuremen (altimeters) &
when quantifying atmospheric pressure.
• Gauge pressure (g):
– The difference between the pressure of a fluid & atmospheric pressure.
– Made by instruments such as those measuring the pressure in vehicle
tires.
Absolute pr. = Gauge pr. + Atmospheric pr.
• Difference pressure (d):
– The difference between two absolute pressure values.
– For some purposes in industrial process, especially as part of some
fluid flow rate-measuring devices.
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Introduction (2)
• Pressure = Force/Area
• Unit:
– SI: Pascal (Pa) = Newtons per square meter (N/m2)
– 1bar = 10,000Pa
– US & Canada: pounds per square inch (psi)
– Sometimes: milimeter of mercury or milimeter of water
(particularly for blood pressure)
– For low pressure: torr; 1torr = 133.3Pa
• To avoid ambiguity:
– (a) or (abs) for absolute pressure: 2.50Pa (a)
– (g) for gauge pressure: 10bar (g)
– (d) for differential: 5bar (d)
– US & Canada: psia, psig, psid
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Diaphragms (1)
Translational movement
Unknown pressure
• Applied pressure causes displacement of the diaphragm &
this movement is measured by a displacement transducer.
• Different versions of diaphragm sensors can measure both
absolute pressure (up to 50bar) & gauge pressure (up to
2000bar) according to whether the space on on side of the
diaphram is, perspectively, evacuated or open to the
atmosphere
• A diaphragm can also measure differential pressure (up to
2.5bar) by applying the 2 pressures to the 2 sides of the
diaphragm.
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Diaphragms (2)
• The diaphragm can be plastic, metal alloy,
stainless steel, or ceramic.
• Plastic diaphragms are the least expensive, but
metal diaphragms give better accuracy.
• Stainless steel is normally used in high
temperature or corrosive environments.
• Ceramic diaphragms are resistant even to
strong acids & alkalis.
• The typical magnitude of diaphragm
displacement is 0.1mm.
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Capacitive Pressure Sensor
• Simply a diaphragm-type
device.
• Diaphragm displacement
is determined by
measuring the
capacitance change
between the diaphragm &
a metal plate that is close
to it. http://www.ni.com
• A.k.a. Baratron gauge.
• Inaccuracy: ±0.2.
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Bellows
• Typically fabricated as a seamless tube of either metal
or metal alloy.
• Pressure changes within the bellows produce
translational motion of the end of the bellows.
• This motion can be measured by capacitive or inductive
transducers or potentiometer.
• Can measure either absolute pressure (up to 2.5bar) or
gauge pressure (up to 150bar).
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Bourdon Tube (1)
http://www.omega
.com/literature/tra
nsactions/volume3
/pressure.html
• Relative inexpensive & used commonly for
measuring the gauge pressure of both gaseous &
liquid fluids.
• Consists of a specially shaped piece of oval-
section, flexible, metal tube that is fixed atd one
end & free to move at the other end.
• When pressure is applied at the open, fixed end
of the tube, the oval cross section becomes more
circular there is displacement of the free end
of the tube. http://www.coleandswallow.com/bourdon-tube/
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Bourdon Tube (2)
http://www.omega
.com/literature/tra
nsactions/volume3
/pressure.html
• The displacement of the free end is measured by some
form of displacement transducers.
• The maximum possible deflection of the free end of the
tube is proportional to the angle subtended by the arc
through which the tube is bent.
• For a C-type tube, the maximum value for this arc is
less than 360o.
• Spiral & helical tubes are used if greater measurement
sensitivity & resolution are required.
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Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
a) U-Tube Manometer
b) Well-Type Manometer
c) Inclined Manometer
8. Resonant Wire Devices
9. Electronic Pressure Gauges
10. Special Measurement Devices for Low Pressures
11. High – Pressure Measurement
12. Differential Pressure-Measuring Devices
13. Selection of Pressure Sensors
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U-Tube Manometer
A B
• The most common form of manometer.
• ρ: the specific gravity of the fluid.
• If an unknown is applied to side A, &
side B is open to the atmosphere h
gauge pressure.
• If side B is sealed & evacuated
absolute pressure.
• If 2 unknown p1 & p2 are applied
differential pressure.
http://www.yourdictionary.com/manometer
• Liquid:
– Water: inexpensive, used extensively,
but not used when measuring the P h g
pressure of fluids that react with or
dissolve in water
– Aniline, carbon tetrachloride, p1 p2 h g
bromoform, mercury, etc.
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