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. 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 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. 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 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. sites.google.com/site/ncpdhbkhn 11 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. sites.google.com/site/ncpdhbkhn 17 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. sites.google.com/site/ncpdhbkhn 18 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. 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. 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 sites.google.com/site/ncpdhbkhn 21 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. 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 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 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. /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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