Đo áp suất: Bài giảng Sensors và analytical devices

Nguyễn Công Phương

Sensors and Analytical Devices

Some Basic Measurement Methods, Pressure Measurement

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

Introduction 1. 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.

sites.google.com/site/ncpdhbkhn 5

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

sites.google.com/site/ncpdhbkhn 6

Pressure Measurement

Introduction 1. 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.

sites.google.com/site/ncpdhbkhn 8

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.

sites.google.com/site/ncpdhbkhn 9

Pressure Measurement

Introduction 1. 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.

http://www.ni.com

• A.k.a. Baratron gauge. • Inaccuracy: ±0.2.

sites.google.com/site/ncpdhbkhn 11

Pressure Measurement

Introduction 1. 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 in

Light out

sites.google.com/site/ncpdhbkhn 13

Pressure Measurement

Introduction 1. 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

Introduction 1. 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)

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/

sites.google.com/site/ncpdhbkhn 17

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.

sites.google.com/site/ncpdhbkhn 18

Pressure Measurement

Fiber – Optic Pressure Sensors

Introduction 1. 2. Diaphragms 3. Capacitive Pressure Sensor 4. 5. Bellows 6. Bourdon Tube 7. Manometers

a) U-Tube Manometer b) Well-Type Manometer c)

Electronic Pressure Gauges

Inclined Manometer 8. Resonant Wire Devices 9. 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

• 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  gauge pressure. If side B is sealed & evacuated  absolute pressure. If 2 unknown p1 & p2 are applied  differential pressure.

• Liquid:

P h g

– Water: inexpensive, used extensively, but not used when measuring the pressure of fluids that react with or dissolve in water





h g

p 1

p 2

– Aniline, carbon tetrachloride, bromoform, mercury, etc.

http://www.yourdictionary.com/manometer

sites.google.com/site/ncpdhbkhn 20

Well-Type Manometer

• A.k.a. cistern/reservoir

manometer.

• One-half of the tube is

made very large so that it forms a well.

• The change in the level of the well as the measured pressure varies is negligible.

• Gauge pressure: p = hρ

http://4mechtech.blogspot.com/2011/ 12/well-type-manometer.html

sites.google.com/site/ncpdhbkhn 21

Inclined Manometer

http://www.asianvalves.net/instruments/Manometer.html

sites.google.com/site/ncpdhbkhn 22

Pressure Measurement

Introduction 1. 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.

http://automationwiki.com/index.php?title =Resonant_Wire_Pressure_Transducer

sites.google.com/site/ncpdhbkhn 24

Pressure Measurement

Introduction 1. 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

Introduction Diaphragms Capacitive Pressure Sensor Fiber – Optic Pressure Sensors Bellows Bourdon Tube

Resonant Wire Devices Electronic Pressure Gauges

1. 2. 3. 4. 5. 6. 7. Manometers 8. 9. 10. Special Measurement Devices for Low Pressures

Thermocouple Gauge Thermistor Gauge Pirani Gauge

a) b) c) 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.

sites.google.com/site/ncpdhbkhn 28

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

Introduction Diaphragms Capacitive Pressure Sensor Fiber – Optic Pressure Sensors Bellows Bourdon Tube

Resonant Wire Devices Electronic Pressure Gauges

1. 2. 3. 4. 5. 6. 7. Manometers 8. 9. 10. Special Measurement Devices for Low Pressures

Thermocouple Gauge Thermistor Gauge Pirani Gauge

a) b) c) 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





P unknown

Ah V

http://www.efunda.com/designstandards/sensors/mcleod/mcleod_intro.cfm

sites.google.com/site/ncpdhbkhn 32

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. • Range: 10–10 to 1 mbar.

http://www.lesker.com/newweb/gauges /gauges_technicalnotes_1.cfm

sites.google.com/site/ncpdhbkhn 33

Pressure Measurement

Introduction 1. 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

Introduction 1. 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