Bài giảng Sensors and analytical devices - Part C: Some Basic Measurement Methods (Phần 4) - Nguyễn Công Phương

Nguyễn Công Phương Sensors and Analytical Devices Some Basic Measurement Methods, Flow 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 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 4 Introduction (1) • Flow measurement is to quantify the rate of flow of materials. • It is quite a common requirement in the process industries. • The materials measured may be in a solid, liquid, or gaseous state. • Solid state: flow can only be qualified as the mass flow rate. • Mass flow rate: the mass of material that flows in one unit of time. • Liquid or gaseous state: flow can be qualified as either the mass flow rate or the volume flow rate. • Volume flow rate: the volume of material that flows in one unit of time. • Note: mass is invariant, whereas volume is a variable quantity. sites.google.com/site/ncpdhbkhn 5 Introduction (2) • A particular complication in the measurement of flow rate of liquids & gases flowing in pipes: the need to consider whether the flow is laminar or turbulent. • Laminar flow: – A motion of the fluid being in a direction parallel to the sides of the pipe. – It occurs in straight lengths of pipe when the fluid is flowing at a low velocity. – Note that even laminar flow is not uniform across the cross section of the pipe: the velocity is greatest at the center of the pipe, & decreases to zero immediately next to the wall of the pipe. • Turbulent flow: – A complex pattern of flow that is not in a uniform direction. – Occurs in nonstraight sections of pipe & also occurs in straight sections when the fluid velocity exceeds a critical value. – Difficult to measure. • The usual practice is to restrict flow measurement to places where the flow is laminar, or at least approximately laminar – Measure the flow in the center of a long, straight length of pipe if the flow velocity is below the critical value for turbulent flow. – If the mean fluid velocityLaminar is high, it is often possibleTurbulent to find somewhere within the flow path where a larger diameter pipe exists & therefore the flow velocity is lower. sites.google.com/site/ncpdhbkhn 6 Flow Measurement 1. Introduction 2. Mass Flow Rate a) Conveyor-Based Methods b) Coriolis Flowmeter c) Thermal Mass Flow Measurement 3. Volume Flow Rate 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 7 Conveyor-Based Methods ConveyorSafetyDevices.html • Appropriate for measuring the flow of solids in the form of powders or small granular particles. • A conveyor is a very suitable means of transporting materials in this form. • Transporting materials on a conveyor allows the mass flow rate to be calculated in terms of the mass of material: Q = Mv/L – Q: the mass flow rate – M: the mass of material distributed over a length, L, of the conveyor – v: the velocity of the conveyor sites.google.com/site/ncpdhbkhn 8 Coriolis Flowmeter • Dominates the mass flowmeter market. • Consists either of a pair of parallel vibrating tube or as a single vibrating tube that is formed into a configuration that has two parallel sections. • Tubes are commonly made of stainless steel. ow_meter#Operating_principle_of_ • The two vibrating tubes deflect according to a_coriolis_flow_meter the mass flow rate of the measured fluid that is flowing inside: d = kfR – d: the net deflection – k: a constant – f: the frequency of the tube vibration – R: the mass flow rate • Excellent accuracy: ±0.2% • Low maintenance requirements • Expensive • Failure may occur after a period of use because of mechanical fatigue in the tube. • Tubes are also subject to corrosion. sites.google.com/site/ncpdhbkhn 9 Thermal Mass Flow Measurement • Used primarily to measure the flow rate of gases. • The principle of operation is to direct the flowing material past a heated element. • The mass flow rate is inferred in two ways: – by measuring the temperature rise in the flowing material, or – by measuring the heater power required to achieve a constant set temperature in the flowing material. • Inaccuracy: ±2% • Can measure very small flow rates: –9 10 liters per minute. ArticleID=277 sites.google.com/site/ncpdhbkhn 10 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate a) Differential Pressure (Obstruction-Type) Meters b) Variable Area Flowmeters (Rotameters) c) Positive Displacement Flowmeters d) Turbine Meters e) Electromagnetic Flowmeters f) Vortex-Shedding Flowmeters g) Ultrasonic Flowmeters h) Other Types of Flowmeters for Measuring Volume Flow Rate i) Open Channel Flowmeters 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 11 Differential Pressure (Obstruction-Type) Meters (1) • Involve the insertion of some device into a fluid-carrying pipe. • This device causes an obstruction & creates a pressure difference on either side of the device. • Devices: orifice place, Venturi tube, flow nozzle, & Dall flow tube. • When such a restriction is placed in a pipe, the velocity of the fluid through the restriction increases & the pressure decreases. • The volume flow rate is proportional to the square root of the pressure difference across the obstruction. • The normal procedure is to use a differential pressure transducer, which is commonly a diaphragm-type device. • The pitot static tube: – Another device that measures flow by creating a pressure difference within a fluid-carrying pipe. – However, there is negligible obstruction of flow in the pipe. – The pitot tube is a very thin tube that obstructs only a small part of the flowing fluid & thus measures at a single point across the cross section of the pipe. sites.google.com/site/ncpdhbkhn 12 Differential Pressure (Obstruction-Type) Meters (2) engineering.com/restriction-orifice-ro- flow-control-instrument.html Orifice plate Segmental Orifice Plate sites.google.com/site/ncpdhbkhn 13 Differential Pressure (Obstruction-Type) Meters (3) • The orifice plate is a metal disc with a hole on it. • Inserted into the pipe carrying the flowing fluid. • Simple, inexpensive, available in a wide range of sizes. • Account for almost 50% of the instruments used in industry for measuring volume flow rate. • Problems: – Inaccuracy: at least ±2% – Permanent pressure loss: 50 to 90% of (P1 – P2) – A gradual change in the sharp edges of the hole: it wears away – Particles tend to stick behind the hole. • To minimize pressure loss: Venturi, flow nozzle, & Dall flow tube. Segmental Orifice Plate sites.google.com/site/ncpdhbkhn 14 Differential Pressure (Obstruction-Type) Meters (4) venturi-and-orifice-103728/ Venturi sites.google.com/site/ncpdhbkhn 15 Differential Pressure (Obstruction-Type) Meters (5) • Has a precision-engineered tube of a special shape. • Inaccuracy: ±1% • Permanent pressure loss: 10 – 15% of (P1 – P2) • The most expensive of all the obstruction devices. sites.google.com/site/ncpdhbkhn 16 Differential Pressure (Obstruction-Type) Meters (6) venturi-and-orifice-103728/ Flow nozzle sites.google.com/site/ncpdhbkhn 17 Differential Pressure (Obstruction-Type) Meters (7) • Of simple construction  less expensive than either a Venturi or a Dall flow tube. • Pressure loss: 35 – 50% of (P1 – P2). sites.google.com/site/ncpdhbkhn 18 Differential Pressure (Obstruction-Type) Meters (8) measurement.html Dall flow tube sites.google.com/site/ncpdhbkhn 19 Differential Pressure (Obstruction-Type) Meters (9) • Consists of 2 conical reducers inserted into a fluid-carrying pipe. • Has a very similar internal shape to the Venturi, except that it lacks a throat. • Much easier to manufacture  cheaper than Venturi. • Inaccuracy: ±1.5% • Its shorter length makes the engineering task of inserting it into the flow line easier. • Pressure loss: 5% of (P1 – P2). sites.google.com/site/ncpdhbkhn 20 Differential Pressure (Obstruction-Type) Meters (10) NotesTheoryAndEquations/Theory_Gas_Pitot_Tube.htm Pitot static tube sites.google.com/site/ncpdhbkhn 21 Differential Pressure (Obstruction-Type) Meters (11) • Used mainly for making temporary measurements of flow. • Also used in some instances for permanent flow monitoring. • Measures the local velocity offlow at a particular point within a pipe. • May be very useful where there is a requirement to measure local flow rates across the cross section of a pipe in the case of nonuniform flow. • Inexpensive, & very simple to install. • Inaccuracy: ±5%. sites.google.com/site/ncpdhbkhn 22 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate a) Differential Pressure (Obstruction-Type) Meters b) Variable Area Flowmeters (Rotameters) c) Positive Displacement Flowmeters d) Turbine Meters e) Electromagnetic Flowmeters f) Vortex-Shedding Flowmeters g) Ultrasonic Flowmeters h) Other Types of Flowmeters for Measuring Volume Flow Rate i) Open Channel Flowmeters 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 23 Variable Area Flowmeters • A.k.a. rotameters. • Account for about 20% of all flowmeters sold. • Consist of a tapered glass tube containing a float that takes up a stable position where its submerged weight is balanced by the up thrust due to the differential pressure across it. • The position of the float is to measure the flow rate. • Reliable, inexpensive. • Inaccuracy: ±5%. sites.google.com/site/ncpdhbkhn 24 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate a) Differential Pressure (Obstruction-Type) Meters b) Variable Area Flowmeters (Rotameters) c) Positive Displacement Flowmeters d) Turbine Meters e) Electromagnetic Flowmeters f) Vortex-Shedding Flowmeters g) Ultrasonic Flowmeters h) Other Types of Flowmeters for Measuring Volume Flow Rate i) Open Channel Flowmeters 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 25 Positive Displacement Flowmeters • Account for nearly 10% of the total number of flowmeters used in industry. • Used in large numbers for metering dosmetic gas & water consumption. • Inaccuracy: from ±2% (the least expensive) to ±0.5%. • The hight quality instruments are used extensively within the oil industry. • All use mechanical divisions to displace discrete volumes of fluid successively. • Low friction, low maintenance, & long life. • A small pressure loss. • Types: rotary piston meter, nutating disk meter, oval gear meter. sites.google.com/site/ncpdhbkhn 26 Rotary Piston Meter sites.google.com/site/ncpdhbkhn 27 Nutating Disk Meter sites.google.com/site/ncpdhbkhn 28 Oval Gear Meter sites.google.com/site/ncpdhbkhn 29 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate a) Differential Pressure (Obstruction-Type) Meters b) Variable Area Flowmeters (Rotameters) c) Positive Displacement Flowmeters d) Turbine Meters e) Electromagnetic Flowmeters f) Vortex-Shedding Flowmeters g) Ultrasonic Flowmeters h) Other Types of Flowmeters for Measuring Volume Flow Rate i) Open Channel Flowmeters 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 30 Turbine Meters • Consist of a multibladed wheel mounted in a pipe along an axis parallel to the direction of fluid flow in the pipe. • The flow of fluid past the wheel causes it to rotate at a rate proportional to the volume flow rate of the fluid. • The turbine blades are made of a ferromagnetic material. • When the turbine wheel moves past an external coil, it induces a voltage pulse in the coil. turbine-flow-meters/ • Inaccuracy: ±0.2%. • Less rugged & reliable than flow restriction-type instruments. • Bearing wear is a particular problem. sites.google.com/site/ncpdhbkhn 31 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate a) Differential Pressure (Obstruction-Type) Meters b) Variable Area Flowmeters (Rotameters) c) Positive Displacement Flowmeters d) Turbine Meters e) Electromagnetic Flowmeters f) Vortex-Shedding Flowmeters g) Ultrasonic Flowmeters h) Other Types of Flowmeters for Measuring Volume Flow Rate i) Open Channel Flowmeters 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 32 Electromagnetic Flowmeters (1) electromagnetic-flow-meters-grounding/ • A.k.a. magnetic flowmeters. • Are limited to measuring the volume flow rate of electrically conductive fluids. • Consist of a stainless-steel cylindrical tube fitted with an insulating liner, which carries the measured fluid. • A magnetic field is created in the tube by magnetic field coils placed either side of the tube. • The voltage induced in the fluid is measured by two electrodes inserted into opposite sides of the tube. • The end of these electrodes are usually flush with the inner surface of the cylinder. • By Faraday’s law of electromagnetic induction: U = BDv sites.google.com/site/ncpdhbkhn 33 Electromagnetic Flowmeters (2) • Inaccuracy: ±1% • Expensive: – The electrodes (titanium, tantalum, etc.), – Running costs (electricity consumption), – The need for careful calibration of each instrument individually during manufacture. • Account for 15% of the new flowmeters sold. • Problem: the insulating lining is subject to damage when abrasive fluids are being handled. • Battery-powered versions are now available commercially. sites.google.com/site/ncpdhbkhn 34 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate a) Differential Pressure (Obstruction-Type) Meters b) Variable Area Flowmeters (Rotameters) c) Positive Displacement Flowmeters d) Turbine Meters e) Electromagnetic Flowmeters f) Vortex-Shedding Flowmeters g) Ultrasonic Flowmeters h) Other Types of Flowmeters for Measuring Volume Flow Rate i) Open Channel Flowmeters 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 35 Vortex-Shedding Flowmeters 230/vortex-shedding-flow-meters • Used as an alternative to traditional differential pressure meters in many applications. • Based on the natural phenomenon of vortex shedding, created by placing an unstreamlined obstacle (bluff body) in a fluid-carrying pipe. • The shedding frequency of these alternately shed vortices is proportional to the fluid velocity past the body. • Various thermal, magnetic, ultrasonic, & capacitive vortex detection techniques are employed in different instruments. • Have no moving parts, operate over a wide flow range, have low power consumption, require little maintenance, low cost. • Can measure both liquid & gas flows. • Inaccuracy: ±1%. • Problem: susceptible to pipe vibrations. sites.google.com/site/ncpdhbkhn 36 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate a) Differential Pressure (Obstruction-Type) Meters b) Variable Area Flowmeters (Rotameters) c) Positive Displacement Flowmeters d) Turbine Meters e) Electromagnetic Flowmeters f) Vortex-Shedding Flowmeters g) Ultrasonic Flowmeters h) Other Types of Flowmeters for Measuring Volume Flow Rate i) Open Channel Flowmeters 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 37 Ultrasonic Flowmeters • A noninvasive method. • Particularly useful for measuring the flow of corrosive fluids & slurries. • Advantages: – High reliability & low maintenance requirements. – Can be clamped externally onto existing pipe work instead of being inserted as an integral part of the flow line  low cost & safety – Any contamination of the fluid being measured (e.g., food & drugs) is avoided. • Still less common than differential pressure or electromagnetic flowmeters. • Types: Doppler shift & transit time. sites.google.com/site/ncpdhbkhn 38 Doppler Shift Ultrasonic Flowmeter (1) • A fundamental requirement of these instruments is the presence of scattering elements within the flowing fluid. • These elements deflect the ultrasonic energy output from the transmitter such that it enters the receiver. • Elements: solid particles, gas bubles. • These elements cause a frequency shift between transmitted & reflected ultrasonic energy. sites.google.com/site/ncpdhbkhn 39 Doppler Shift Ultrasonic Flowmeter (2) • Consists essentially of an ultrasonic transmitter – receiver pair clamped onto the outside wall of a fluid-carrying vessel. • Ultrasonic energy consists of a train of short bursts of sinusoidal waveforms at frequency between 0.5 & 20 MHz. • The flow velocity: v = f(ftransmitted wave , freceived wave , vsound in fluid) • The electronics involved is relatively simple  inexpensive. • Inaccuracy: – Depends on the flow profile; the constancty of pipe wall thickness; the number, size, & spatial distribution of scatterers; the accuracy of determined velocity of sound in the fluid. – Can approach ±10% • Often used merely as flow indicators rather than for accurate quantification. • For versions that are fitted inside the flow rate: – Overcome the problem of variable pipe thickness – Inaccuracy: ±0.5% sites.google.com/site/ncpdhbkhn 40 Transit Time Ultrasonic Flowmeter (1) ignstandards/sensors/flowm eters/flowmeter_ustt.cfm • An instrument designed for measuring the volume flow rate in clean liquids or gases. • Consists of a pair of ultrasonic transducers mounted along an axis aligned at angle θ with respect to the fluid flow axis. • Each transducer consists of a transmitter-receiver pair. • The transmitter emits ultrasonic energy that travels across to the receiver on the opposite side of the pipe. sites.google.com/site/ncpdhbkhn 41 Transit Time Ultrasonic Flowmeter (2) ignstandards/sensors/flowm eters/flowmeter_ustt.cfm • Fluid flowing in the pipe causes a time difference between the transit times of beams traveling upstream & downstream. • Measurement of this difference allows the flow velocity to be calculated. • The typical magnitude of this time difference is 100 ns in a total transit time of 100 μs. • High-precision electronics are required to measure the difference. sites.google.com/site/ncpdhbkhn 42 Transit Time Ultrasonic Flowmeter (3) ignstandards/sensors/flowm eters/flowmeter_ustt.cfm • 3 ways to measure the time shift: – Direct measurement (speed of sound is required) – Conversion to a phase change (speed of sound is required) – Conversion to a frequency change (speed of sound is NOT required) • Transit time flowmeter is of more general use than Doppler shift flowmeters. • Inaccuracy: ±0.5%. • More expensive than a Doppler shift flowmeter. sites.google.com/site/ncpdhbkhn 43 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate a) Differential Pressure (Obstruction-Type) Meters b) Variable Area Flowmeters (Rotameters) c) Positive Displacement Flowmeters d) Turbine Meters e) Electromagnetic Flowmeters f) Vortex-Shedding Flowmeters g) Ultrasonic Flowmeters h) Other Types of Flowmeters for Measuring Volume Flow Rate i. Gate-Type Meter ii. Jet Meter iii. Laser Doppler Flowmeter iv. Thermal Anemometers i) Open Channel Flowmeters 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 44 Gate-Type Meter, Air Vane Meter watch?v=Xkyfjo7spFc • Measures deflection of the flap. • Used to measure airflow within automotive fuel – injection systems. sites.google.com/site/ncpdhbkhn 45 Gate-Type Meter, Target Meter Target_Type_Flow_Meter-- 4686355_4687565.html • Consists of a circular, disc-shaped flap in the pipe. • Fluid flow rate is inferred from the force exerted on the disc measured by strain gauges bonded to it. • Very useful for measuring the flow of dilute slurries. • Rather expensive. sites.google.com/site/ncpdhbkhn 46 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate a) Differential Pressure (Obstruction-Type) Meters b) Variable Area Flowmeters (Rotameters) c) Positive Displacement Flowmeters d) Turbine Meters e) Electromagnetic Flowmeters f) Vortex-Shedding Flowmeters g) Ultrasonic Flowmeters h) Other Types of Flowmeters for Measuring Volume Flow Rate i. Gate-Type Meter ii. Jet Meter iii. Laser Doppler Flowmeter iv. Thermal Anemometers i) Open Channel Flowmeters 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 47 Jet Meter sites.google.com/site/ncpdhbkhn 48 Laser Doppler Flowmeter (1) ignstandards/sensors/laser_ doppler/laser_doppler_flow _theory.cfm • Gives direct measurements of flow velocity for liquids containing suspended particles flowing in a pipe. • Light from a laser is focused by an optical system to a point in the flow. • The movement of particles causes a Doppler shift of the scattered light & produces a signal in a photo detector that is related to the fluid velocity. • Range: from 10 μm/s to 105 m/s. sites.google.com/site/ncpdhbkhn 49 Laser Doppler Flowmeter (2) • Sufficient particles for satisfactory operation are normally present naturally in most liquid & gaseous fluids  the introduction of artificial particles is rarely needed. • Advantages: – Measures flow velocity directly, rather than inferring it from a pressure difference. – Causes no interruption in the flow. – The instrument can be made very small  it can measure velocity in confined areas. • Disadvantage: it measures local flow velocity in the vicinity of the focal point of the light beam  can lead to large errrors if the flow profile is not uniform. • Main application: measuring blood flow in medical applications. sites.google.com/site/ncpdhbkhn 50 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate a) Differential Pressure (Obstruction-Type) Meters b) Variable Area Flowmeters (Rotameters) c) Positive Displacement Flowmeters d) Turbine Meters e) Electromagnetic Flowmeters f) Vortex-Shedding Flowmeters g) Ultrasonic Flowmeters h) Other Types of Flowmeters for Measuring Volume Flow Rate i. Gate-Type Meter ii. Jet Meter iii. Laser Doppler Flowmeter iv. Thermal Anemometers i) Open Channel Flowmeters 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 51 Thermal Anemometers /alnor-velometer-thermal- anemometers.htm • To measure the volume flow rate of gases in pipes. • Consists of a piece of thin (typical diameter 5μm), electrically heated wire (usually tungsten, platinum-iridium alloy) inserted into the gas flow. • The flowing gas has a cooling effecton the wire, which reduces its resistance. • Measurement of resistance change allows the volume flow rate of the gas to be calculated. • Advantages: – Very fast speed of response  an ideal instrument in conditions where the flow velocity is changing. – Insensitive to the direction of gas flow  very useful device in conditions of turbulent flow. • Disadvantage: not robust because of the very small diameter of the wire. sites.google.com/site/ncpdhbkhn 52 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate a) Differential Pressure (Obstruction-Type) Meters b) Variable Area Flowmeters (Rotameters) c) Positive Displacement Flowmeters d) Turbine Meters e) Electromagnetic Flowmeters f) Vortex-Shedding Flowmeters g) Ultrasonic Flowmeters h) Other Types of Flowmeters for Measuring Volume Flow Rate i) Open Channel Flowmeters 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 53 Open Channel Flowmeters • Measure the flow of liquids in open channels & are particularly relevant to measuring the flow of water in rivers as part of environmental management schemes. • 2 methods: – Build a weir on constant width across the flow & measure the velocity of flow & the height of liquid immediately before the weir. – If the flow is conductive, 2 or more electromagnetic meters 360PrimDev.html can be placed side by side to measure the flow velocity. sites.google.com/site/ncpdhbkhn 54 Flow Measurement 1. Introduction 2. Mass Flow Rate 3. Volume Flow Rate 4. Choice between Flowmeters sites.google.com/site/ncpdhbkhn 55 Choice between Flowmeters • The number of relevant factors to be considered when specifying a flowmeter for a particular application is very large: – Temperature, pressure, density, viscosity, chemical properties & abrasiveness, whether it contains particles, whether it is a liquid or gas, etc. – Accuracy, range, acceptable pressure drop, output signal characteristics, reliability, service life. – Purchase cost, installation difficulties, maintenance requirements, service life. • Where only a visual indication of flow rate is needed, the variable are meter is popular. • Where a flow measurement in the form of an electrical signal si required, the choice is very large. • The orifice plate is used extremely commonly, accounting for almost 50% of instruments currently in use in industry. • Other forms of differential pressure meters & electromagnetic flowmeters are used in large numbers. sites.google.com/site/ncpdhbkhn 56