ENGINEERING MEASUREMENTS 2

Information about ENGINEERING MEASUREMENTS 2

Published on August 11, 2014

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ENGINEERING MEASUREMENTS : ENGINEERING MEASUREMENTS BY Dr. A. SHIBL Mechanical Measurements: Mechanical Measurements Act of measurement—the quantitative comparison between a predefined standard and a measurand to produce a measured result Measurand : physical parameter or variable to be measured Standard: basis for comparison of quantitative value to measurand. Standards organizations: Standards organizations SASO— Saudi Arabian Standards organization ISO—International Organization for Standardization Others—ASME, NFPA, ASTM, etc. Reliability of Measurements: Reliability of Measurements Measurements must be reliable to be useful Incorrect information is more damaging than no information There is no perfect measurement Accuracy of measurements Precision of measurements Uncertainty of measurements Do not accept data without questioning the source and uncertainty of the measurements Fundamentals Methods of Measurements: Fundamentals Methods of Measurements There are two basic methods of measurement: Direct compariso n: with a primary or secondary standard Indirect compariso n—conversion of measurand input into an analogous form which can be processed and presented as known function of input - A transducer is required to convert the measurand into another form Sensors: Sensors Use of a mercury thermometer to measure temperature Use of a radar signal to measure velocity Use of a strain gage to measure the strain in a material Transducers frequently convert mechanical measurements into electrical responses (voltage, amperage or resistance) Generalized Measurement System: Generalized Measurement System Sensor or transducer stage to detect measurand and Convert input to a form suitable for processing e.g. : - Temp. to voltage - Force to distance Signal conditioning stage to modify the transduced signal e.g. : Amplification, Attenuation, Filtering, Encoding Terminating readout stage to present desired output (Analog or Digital form) Generalized Measurement System: Generalized Measurement System Types of Input Signals: Types of Input Signals Static Dynamic (Time dependence) - Steady periodic, complex periodic - Nonperiodic: nearly periodic or transient - Single pulse. - Random Analog or digital: - Analog; continuous signal, - Digital; distinct values, step changes. Calibration: Calibration Calibration involves the determination of the relationship between the input and output of a measurement system Eliminate Bias error The proving of a measurement system’s capability to quantify the input accurately Calibration is accomplished by applying known magnitudes of the input and observing the measurement system output The indirect measuring system must be calibrated. CALIBRATION: CALIBRATION Once a measurement device is selected, it must be calibrated Calibration –Comparison of instrument’s reading to a calibration standard Calibration standard created from a measurement Inherent error Basic issue is how do we know that what we record has any relation to what we wish to measure? Calibration using Primary or/and Secondary Standards: Calibration using Primary or/and Secondary Standards Known input signal and find the output. - To establish the correct output scale. - To find instrument reliability. - To eliminate bias error (systematic error) For linear relation o/p ∝ I/p needs single point calibration. For non-linear relation needs multi-point calibrations. Static calibration – vs – Dynamic calibration Primary Standards For Comparison and Calibration : Primary Standards For Comparison and Calibration SI System: Meter – Kg -- Sec.– Kelvin – volt - Mole – Ampere – Radian LENGTH (meter): Distance traveled by light in vacuum during 1/299792458 of a sec. MASS (Kg.): International prototype (alloy of platinum and iridium) kept near Paris. TIME (Sec.): Duration of 9192631770 periods of the radiation emitted between two excitation levels of Cesium-133 TEMPERATURE (Kelvin): K = o C + 273 Dimensional Analysis: Dimensional Analysis Data presented in dimensionless form. Reducing N o of experimental variables. N o of variables - N o of dims.= N o of π groups Use pi method or by inspection Basic dimensions: M L T θ (kg,m,sec, o k) Saving(time&$) ( 10 tests –vs- 10 4 tests for F= f n (L,V, ρ , μ ) ) Force coef. F/ ρ v 2 L 2 = f n (Reynolds number ρ vL/ μ ) Helping in exp. Planning, insight, and similitude. Uncertainty of Measurements: Uncertainty of Measurements Measurement error = Measured result - T rue value The true value of a measurand is Unknown ( Error is unknown ) The potential value of error can be estimated (uncertainty) Two types of error: - Systematic errors (bias) and Random errors ( Statistics to estimate random errors) SOURCE OF ERRORS: SOURCE OF ERRORS BIAS AND RANDOM ERRORS: BIAS AND RANDOM ERRORS Measurement errors : Measurement errors Bias and Random Errors: Bias and Random Errors Resistive Displacement Sensor : Resistive Displacement Sensor Capacitive Displacement Sensor C= Capacitance, εo &εr =Permittivity of air and Dielectric : Capacitive Displacement Sensor C= Capacitance, ε o & ε r =Permittivity of air and Dielectric Linear Variable differential Transformer ( LVDT ): Linear Variable differential Transformer ( LVDT ) Linear Variable differential Transformer ( LVDT ): Linear Variable differential Transformer ( LVDT ) Primary coil voltage: V S sin(ωt) Secondary coil induced emf: V 1 =k 1 sin(ωt+ϕ) and V 2 =k 2 sin(ωt+ϕ) k 1 and k 2 proportional to the position of the coil When the coil is in the central position, k 1 =k 2 V OUT = V 1 -V 2 = 0 When the coil is is displaced , k 1 ≠ k 2 V OUT =(k 1 -k 2 )sin(ωt+ϕ) Wheatstone Bridge: Wheatstone Bridge Strain Gage [Gage Factor = (∆R/R)/(∆L/L) & Young’s Modulus = (P/A) / (∆L/L) ] : Strain Gage [ Gage Factor = (∆R/R)/(∆L/L) & Young’s Modulus = (P/A) / (∆L/L) ] Viscosity Measurements: Viscosity Measurements Fluid Viscosity: Fluid Viscosity Flow Rate Measurements: Flow Rate Measurements Pitot Tube Traverse Points: Pitot Tube Traverse Points Flow Instrumentation: Flow Instrumentation Orifice, venturi tube, flow tube, flow nozzles. Pitot tubes, elbow-tap meters, target meters. Rotameter and Nutating disk Obstruction Flow Meter: Obstruction Flow Meter  Miscellaneous Flow Meters:  Miscellaneous Flow Meters Turbine, vortex shedding flow meters. Mass meters include Coriolis and thermal types.  Hot-Wire Anemometer: Electrically heated, fine platinum wire immersed in flow  Wire is cooled as flow is increased Measure either change in wire resistance or heating current to determine flow Electromagnetic Flow meter:Electromotive force induced in fluid as it flows through magnetic field and measured with electrodes which is proportional to flow rate Ultrasonic Flow equipment: Uses Doppler frequency shift of ultrasonic signals reflected off discontinuities in fluid Laser Doppler Anemometer which employ Doppler effect and Hetrodyning of two signals Flow Meters: Flow Meters Vortex magnetic Turbine Coriolis mass flow meter Flow velocity measurement: Flow velocity measurement Rotameter: Rotameter MEASUREMENT STAGES: MEASUREMENT STAGES Primary Sensing (Strain gage, thermometer) Retrieves energy from the measured system Produces some form of output Variable conversion Changes data from one physical form to another Elongation to resistance, temperature to volume change Variable manipulation Performs mathematical operation on data Amplifier, filter MEASUREMENT STAGES: MEASUREMENT STAGES Data transmission Gets data between measurement elements Wire, speedometer cable, satellite downlink system Data storage/playback Stores data for later retrieval Hard drive, RAM Data presentation Indicators, alarms, analog recording, digital recording Optical Pyrometer: Optical Pyrometer Thermocouple : Thermocouple Thermocouples in Series and in Parallel: Thermocouples in Series and in Parallel THERMOCOUPLE TIME CONSTANT : THERMOCOUPLE TIME CONSTANT The conservation of energy: m c p dT / dt = h A (T o – T) m : mass of thermocouple junction, C p : specific heat of thermocouple junction h : heat transfer coefficient , A : surface area of thermocouple T : junction temperature , T o : environs temperature θ =T – T o / T i - T o T i = initial measurement junction temperature, then the solution is θ = e (-t / τ ) where we have defined the time constant for this process as τ = m c p /h A Hot Wire: Hot Wire King’s Law: King’s Law Laser Doppler Anemometer: Laser Doppler Anemometer Strain Gage: Strain Gage Periodic Wave and its Spectrum: Periodic Wave and its Spectrum Time Domain & Freq. Domain: Time Domain & Freq. Domain frequency spectrum examples: frequency spectrum examples Square and Hanning window functions: Square and Hanning window functions Periodic Signals: Periodic Signals Sine Wave Digitising: Sine Wave Digitising Periodic Wave and its Spectrum: Periodic Wave and its Spectrum Square Wave and its Spectrum: Square Wave and its Spectrum Analog and Digital Signals: Analog and Digital Signals Analog RC Filtering: Analog RC Filtering Bias (systematic) and Random (precise) Errors: Bias (systematic) and Random (precise) Errors Errors in Measuring a Variable: Errors in Measuring a Variable Propagation of Errors: Propagation of Errors Combination of Errors: Combination of Errors Dimensional Analysis: Dimensional Analysis Data presented in dimensionless form. Reducing N o of experimental variables. N o of variables - N o of dims.= N o of π groups Use pi method or by inspection Basic dimensions: M L T θ (kg,m,sec, o k) Saving(time&$) ( 10 tests –vs- 10 4 tests for F= f n (L,V, ρ , μ ) ) Force coef. F/ ρ v 2 L 2 = f n (Reynolds number ρ vL/ μ ) Helping in exp. Planning, insight, and similitude. Application of Mech. Measurements: Application of Mech. Measurements Monitoring and operation of process. Control of a process (accurate control f n measurement acc.) Experimentation: - Testing and performance operation - Verification of properties or theory - Information needed for analysis e.g. Checking or evaluation of: Oil viscosity variation with temp. Pump performance curve piping head loss Lift and drag of new airfoil shape…….etc. Objectives of Mechanical Measurements: Objectives of Mechanical Measurements Measurement of physical variables: Force vector (N) , Velocity vector (m/sec.) , T ( o C) , P (Pascal) , Frequency (Hz=cycle/sec).. Measurement of Mechanical Parameters: Re= ρ vd/ μ , Mach No.= v/c, P D =0.5 ρ V 2 Accurate and Reliable Measurements: Real value – vs – Measured value Calibration using Primary or/and Secondary Standards: Calibration using Primary or/and Secondary Standards Known input signal and find the output. - To establish the correct output scale. - To find instrument reliability. - To eliminate bias error (systematic error) For linear relation o/p ∝ I/p needs single point calibration. For non-linear relation needs multi-point calibrations. Static calibration – vs – Dynamic calibration Primary Standards For Comparison and Calibration : Primary Standards For Comparison and Calibration SI System: Meter – Kg -- Sec.– Kelvin – volt - Mole – Ampere – Radian LENGTH (meter): Distance traveled by light in vacuum during 1/299792458 of a sec. MASS (Kg.): International prototype (alloy of platinum and iridium) kept near Paris. TIME (Sec.): Duration of 9192631770 periods of the radiation emitted between two excitation levels of Cesium-133 TEMPERATURE (Kelvin): K = o C + 273 Measuring System Stages : Measuring System Stages FLOWMETER SELECTION: FLOWMETER SELECTION Flowmeter   element Recommended   Service Range Pressure loss Typical   Accuracy,  % L (Dia.) Cost Orifice Clean, dirty  liquids; some  slurries 4 to 1 Medium ±2 to ±4 of full scale 10 to 30 Low Wedge Slurries and Viscous liquids 3 to 1 Low tomedium ±0.5 to ±2 of full scale 10 to 30 High Venturi tube Clean, dirty and viscous liquids;   4 to 1 Low ±1 of full scale 5 to 20 Medium Flow nozzle Clean and dirty liquids 4 to 1 Medium ±1 to ±2 of full scale 10 to 30 Medium Pitot tube Clean liquids 3 to 1 Very low ±3 to ±5 of full scale 20 to 30 Low Elbow meter Clean, dirty  liquids; some slurries 3 to 1 Very low ±5 to ±10 of full scale 30 Low Target meter Clean, dirty viscous liquids; 10 to 1 Medium ±1 to ±5  of full scale 10 to 30 Medium Variable area Clean, dirty viscous liquids 10 to 1 Medium ±1 to ±10 of full scale None Low Positive Displacement Clean, viscous  liquids 10 to 1 High ±0.5 of rate None Medium Turbine Clean, viscous liquids 20 to 1 High ±0.25 of rate 5 to 10 High Vortex CLean, dirty liquids 10 to 1 Medium ±1 of rate 10 to 20 High Electromagnetic Clean, dirty viscous conductive liquids& slurries 40 to 1 None ±0.5 of rate 5 High Ultrasonic (Doppler) Dirty, viscous liquids and slurries 10 to 1 None ±5 of full scale 5 to 30 High Ultrasonic(Travel Time) Clean, viscous liquids 20 to 1 None ±1 to ±5  of full scale 5 to 30 High Mass  (Coriolis) Clean, dirty viscous liquids; some slurries 10 to 1 Low ±0.4 of rate None High Mass (Thermal) Clean, dirty viscous liquids; some slurries 10 to 1 Low ±1 of full scale None High Weir (V-notch) Clean, dirty liquids 100 to 1 Very low ±2 to ±5 of full scale None Medium Flume  (Parshall) Clean, dirty liquids 50 to 1 Very low ±2 to ±5 of full scale None Medium UNCERTAINTY IN PLANING: UNCERTAINTY IN PLANING During the design of the experiment Identify all possible sources of error: Experiment set up: facility effects, environmental effects, human , ….. Measurement system: velocity, temperature,... Estimate possible severity of each source Discuss with advisor. For those that are considered “important”, identify strategies. Experimental design and/or test protocols (e.g. repeat tests) Plan for quantitative analysis of reduced data Quantitative analysis relies on math model of the system Often good for measurement systems: pitot probe, strain gauge,... UNCERTAINTY STAGES: UNCERTAINTY STAGES During the experiment Execute experiment with replications Record notes in lab notebook Check for mistakes and Bias errors During data reduction Calculate error bars for measurements Check for outlier points During data interpretation/reporting Consider errors when interpreting data 1 st order &N th order Assure findings are beyond uncertainty of experiment Display error bars in way that aids in understanding findings Dynamic Performance : Dynamic Performance Sampling and Aliasing error: Sampling and Aliasing error Resolution of an A/D Converter: Resolution of an A/D Converter Experimental Design and Analysis: Experimental Design and Analysis Simple Comparative Experiment. One Factor: t-Test (2-levels or treatments) PowerPoint Presentation: F Tests Least Significant Difference: Least Significant Difference Factorial Design: Factorial Design

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