Bài giảng Sensors and analytical devices - Part C: Some Basic Measurement Methods (Phần 3) - Nguyễn Công Phương
Selection of Pressure Sensors
• Usually depends strongly on the intended application.
• Manometers are used commonly when just a visual
indication of pressure level is needed.
• When an electrical form of output is required, the
choice is usually one out of the several types of
diaphragm sensors.
• The resonant wire device is a popular choice for a very
high measurement accuracy.
• For pressure measurement in the vacuum range:
thermocouple gauge, ionization gauge, etc.
• At high pressure (>7000 bar): the manganin wire
sensor
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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
sites.google.com/site/ncpdhbkhn 2
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
sites.google.com/site/ncpdhbkhn 3
Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
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
sites.google.com/site/ncpdhbkhn 4
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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Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
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
sites.google.com/site/ncpdhbkhn 7
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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Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
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
sites.google.com/site/ncpdhbkhn 10
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.
• A.k.a. Baratron gauge.
• Inaccuracy: ±0.2.
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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
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
sites.google.com/site/ncpdhbkhn 12
Fiber – Optic Pressure Sensors
Light Light
in out
sites.google.com/site/ncpdhbkhn 13
Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
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
sites.google.com/site/ncpdhbkhn 14
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).
sites.google.com/site/ncpdhbkhn 15
Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
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
sites.google.com/site/ncpdhbkhn 16
Bourdon Tube (1)
.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.
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Bourdon Tube (2)
.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
sites.google.com/site/ncpdhbkhn 19
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.
• 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 p 2 h g
bromoform, mercury, etc.
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Well-Type Manometer
• A.k.a. cistern/reservoir
manometer.
• One-half of the tube is
made very large so that it
forms a well. h
• The change in the level of
the well as the measured
pressure varies is
negligible. 12/well-type-manometer.html
• Gauge pressure: p = hρ
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Inclined Manometer
sites.google.com/site/ncpdhbkhn 22
Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
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
sites.google.com/site/ncpdhbkhn 23
Resonant Wire Devices
• The wire is positioned in
a magnetic field &
allowed to oscillate.
• The oscillator circuit
results in the oscillation
of wire at its resonant
frequency.
• The variations in
pressure affects the wire
tension, due to which the
resonant frequency of the
wire also gets changed.
• A digital counter circuit
is used to detect the shift. =Resonant_Wire_Pressure_Transducer
sites.google.com/site/ncpdhbkhn 24
Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
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
sites.google.com/site/ncpdhbkhn 25
Electronic Pressure Gauges
• Piezoresistive pressure transducer: this diaphragm-type sensor uses
piezoresistive strain gauges to measure diaphragm displacement.
• Piezoelectric pressure transducer: this diaphragm-type sensor uses a
piezoelectric crystal to measure diaphragm displacement.
• Magnetic pressure transducer: this class of diaphragm-type device measures
diaphragm displacement magnetically using inductive, variable reluctance, or
eddy current sensors.
• Capacitive pressure transducer: this diaphragm-type sensor measures
variation in capacitance between the diaphragm & a fixed metal plate close to
it.
• Fiber-optic pressure sensor: known alternatively as an optical pressure sensor,
this uses a fiber-optic sensor to measure the displacement of either a
diaphragm or a Bourdon tube pressure sensor.
• Potentiometric pressure sensor: this is a device where the translational motion
of a bellow-type pressure sensor is connected to the sliding element of an
electrical potentiometer.
• Resonant pressure transducer: this is a form of resonant wire pressure-
measuring device in which the pressure-induced frequency change is
measured by electronics integrated into the device.
sites.google.com/site/ncpdhbkhn 26
Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
8. Resonant Wire Devices
9. Electronic Pressure Gauges
10. Special Measurement Devices for Low Pressures
a) Thermocouple Gauge
b) Thermistor Gauge
c) Pirani Gauge
d) McLeod Gauge
e) Ionization Gauge
11. High – Pressure Measurement
12. Differential Pressure-Measuring Devices
13. Selection of Pressure Sensors
sites.google.com/site/ncpdhbkhn 27
Thermocouple Gauge
• At low pressure, there is a linear
relationship between pressure &
thermal conductivity.
• Measurement of thermal conductivity
gives an indication of pressure.
• Operation of the guage depends on the
thermal conduction of heat between a
hot wire in the center & the cold outer
surface of a glass tube.
• The temperature measured depends on
the thermal conductivity of the gas in
the tube & hence on its pressure.
• Range: 10–4 mbar up to 1mbar.
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Thermistor Gauge
• Identical in its mode of operation to a
thermocouple gauge.
• Except that a thermistor is used to measure the
temperature of the metal strip rather than a
thermocouple.
sites.google.com/site/ncpdhbkhn 29
Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
8. Resonant Wire Devices
9. Electronic Pressure Gauges
10. Special Measurement Devices for Low Pressures
a) Thermocouple Gauge
b) Thermistor Gauge
c) Pirani Gauge
d) McLeod Gauge
e) Ionization Gauge
11. High – Pressure Measurement
12. Differential Pressure-Measuring Devices
13. Selection of Pressure Sensors
sites.google.com/site/ncpdhbkhn 30
Pirani Gauge
• Similar to a thermocouple gauge, but has 2 identical heated
elements.
• Each heated element consists of 4 coiled tungsten wires connected
in parallel
– One contains the gas at unknown pressure
– The other evacuated to a very low pressure
• Current is passed through the tungsten element, which attains a
certain temperature according to the thermal conductivity of the gas.
• The resistance of the element changes with temperature & causes an
imbalance of the measurement bridge.
sites.google.com/site/ncpdhbkhn 31
McLeod Gauge
Ah2 g
P
unknown V
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Ionization Gauge
• Used only in laboratory conditions.
• 2 forms: hot cathode & cold cathod.
• The filament discharges free electrons.
• There is a current flowing between the
grid & the ion collector.
• This current is proportional to the
number of ions per unit volume.
• This number of ions is proportional to
the gas pressure.
• Cold cathod form: the stream of
electrons is produced by a high voltage
electrical discharge. /gauges_technicalnotes_1.cfm
• Range: 10–10 to 1 mbar.
sites.google.com/site/ncpdhbkhn 33
Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
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
sites.google.com/site/ncpdhbkhn 34
High – Pressure Measurement
• Above 7000 bar.
• Normally carried out electrically by monitoring the change of resistance of
wires of special materials (manganin & gold-chromium alloy).
• A coil of such wire is enclosed in a sealed, kerosene-filled, flexible bellows.
• The unknown pressure is applied to one end of the bellows, which transmit
pressure to the coil.
• The magnitude of the applied pressure is determined by measuring the coil
resistance.
• Range: up to 30,000 bar.
• Inaccuracy: ±0.5%.
sites.google.com/site/ncpdhbkhn 35
Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
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
sites.google.com/site/ncpdhbkhn 36
Differential Pressure-Measuring
Devices
• Have 2 imput ports:
– One unknown pressure is applied to each port,
– Instrument output is the difference between the
two pressures.
• Another way:
– Measure each pressure with a separate instrument,
– & then substract one reading from the other.
• Can be measured by special forms of many of
the pressure-measuring devices discussed
earlier.
sites.google.com/site/ncpdhbkhn 37
Pressure Measurement
1. Introduction
2. Diaphragms
3. Capacitive Pressure Sensor
4. Fiber – Optic Pressure Sensors
5. Bellows
6. Bourdon Tube
7. Manometers
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
sites.google.com/site/ncpdhbkhn 38
Selection of Pressure Sensors
• Usually depends strongly on the intended application.
• Manometers are used commonly when just a visual
indication of pressure level is needed.
• When an electrical form of output is required, the
choice is usually one out of the several types of
diaphragm sensors.
• The resonant wire device is a popular choice for a very
high measurement accuracy.
• For pressure measurement in the vacuum range:
thermocouple gauge, ionization gauge, etc.
• At high pressure (>7000 bar): the manganin wire
sensor.
sites.google.com/site/ncpdhbkhn 39
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