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Unit 1 Concept of Measurement

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Syllabus General concept Generalized measurement system-Units and standards-measuring instruments- sensitivity, readability, range of accuracy, precision-static and dynamic response repeatability- systematic and random errors- correction, calibration, interchangeability

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Definition Metrology is the name given to the science of pure measurement. Engineering Metrology is restricted to measurements of length & angle Measurement is defined as the process of numerical evaluation of a dimension or the process of comparison with standard measuring instruments

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Why measure things? Check quality? Check tolerances? Allow statistical process control (SPC)?

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Need of Measurement Establish standard Interchange ability Customer Satisfaction Validate the design Physical parameter into meaningful number True dimension Evaluate the Performance

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Methods of Measurement Direct method Indirect method Comparative method Coincidence method Contact method Deflection method Complementary method

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Direct method Measurements are directly obtained Ex: Vernier Caliper, Scales

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Indirect method Obtained by measuring other quantities Ex : Weight = Length x Breadth x Height x Density

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Comparative Method Its compared with other known value Ex: Comparators

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Coincidence method Measurements coincide with certain lines and signals Fundamental method Measuring a quantity directly in related with the definition of that quantity Contact method Sensor/Measuring tip touch the surface area

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Complementary method The value of quantity to be measured is combined with known value of the same quantity Ex:Volume determination by liquid displacement

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Deflection method The value to be measured is directly indicated by a deflection of pointer Ex: Pressure Measurement

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GENERALIZED MEASURING SYSTEM

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Common elements of system Primary sensing element Variable conversion element Variable manipulation element Data transmission element Data processing element Data presentation element

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Primary sensing element Variable conversion element Variable manipulation element Data transmission element Data processing element Data presentation element Temperature Observer

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Units and standards

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SI: fundamental Units Physical QuantityUnit NameSymbol lengthmeterm masskilogramkg timeseconds electric currentampereA temperatureKelvinK amount of substancemolemol luminous intensitycandelacd

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SI: Derived Units Physical QuantityUnit NameSymbol areasquare meterm2m2 volumecubic meterm3m3 speed meter per second m/s acceleration meter per second squared m/s 2 weight, forcenewtonN pressurepascalPa energy, workjouleJ

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Supplementary units Physical QuantityUnit NameSymbol Plane angleRadian rad Solid angleSteradian sr

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Standards International standards Primary standards Secondary standards Working standards

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International International Organization of Legal Metrology, Paris International Bureau of Weights and Measures at Sevres, France India National Physical Laboratory Dr. K.S. Krishnan Marg New Delhi - 110012 India Phone: 91-11-45609212 Fax: 91-11-45609310 Email: root@nplindia.org or root@nplindia.ernet.inroot@nplindia.orgroot@nplindia.ernet.in

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Measuring Instruments Deflection and null type instruments Analog and digital instruments Active and passive instruments Automatic and manually operated instruments Contacting and non contacting instruments Absolute and secondary instruments Intelligent instruments.

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DEFLECTION AND NULL TYPE Physical effect generated by the measuring quantity Equivalent opposing effect to nullify the physical effect caused by the quantity

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ANALOG AND DIGITAL INSTRUMENTS Physical variables of interest in the form of continuous or stepless variations Physical variables are represented by digital quantities

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ACTIVE AND PASSIVE INSTRUMENTS Instruments are those that require some source of auxiliary power The energy requirements of the instruments are met entirely from the input signal

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Automatic and manually operated Manually operated requires the service of human operator Automated doesn't requires human operator

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Contacting And Non Contacting Instruments A contacting with measuring medium Measure the desired input even though they are not in close contact with the measuring medium

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Absolute and Secondary Instruments These instruments give the value of the electrical quantity in terms of absolute quantities Deflection of the instruments can read directly

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Intelligent instruments Microprocessors are incorporated with measuring instruments

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Help topics http://www.tresnainstrument.com/education. html http://www.tresnainstrument.com/education. html

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Characteristics of Measuring Instrument Sensitivity Readability Range of accuracy Precision

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Definition Sensitivity- Sensitivity is defined as the ratio of the magnitude of response (output signal) to the magnitude of the quantity being measured (input signal) Readability- Readability is defined as the closeness with which the scale of the analog instrument can be read

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Definition Range of accuracy- Accuracy of a measuring system is defined as the closeness of the instrument output to the true value of the measured quantity Precision- Precision is defined as the ability of the instrument to reproduce a certain set of readings within a given accuracy

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Sensitivity If the calibration curve is liner, as shown, the sensitivity of the instrument is the slope of the calibration curve. If the calibration curve is not linear as shown, then the sensitivity varies with the input.

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Sensitivity This is the relationship between a change in the output reading for a given change of the input. (This relationship may be linear or non-linear.) Sensitivity is often known as scale factor or instrument magnification and an instrument with a large sensitivity (scale factor) will indicate a large movement of the indicator for a small input change.

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Load Cell Force, F Output, V o Output, V o (V) Input, F i (kN) Slope = 5 V/kN K Input, F (kN) Output, V o (V) Sensitivity, K = 5 V/kN Block Diagram:

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Example (1) A 0.01 /A meter with 5 A fsd, R m = /A x A = 0.01 x 5 = 0.05 V max across the Meter will be = 5 A x 0.05 = 0.25 V for fsd. (2) A 0.1 /A meter with 5 A fsd,will drop 2.5 V (i.e., it is 10 times less sensitive), which may bias the results

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Readability Readability is defined as the ease with which readings may be taken with an instrument. Readability difficulties may often occur due to parallax errors when an observer is noting the position of a pointer on a calibrated scale

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Readability What is the value ?

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Accuracy Accuracy = the extent to which a measured value agrees with a true value The difference between the measured value & the true value is known as Error of measurement Accuracy is the quality of conformity

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Example: Accuracy Who is more accurate when measuring a book that has a true length of 17.0 cm? A : 17.0 cm, 16.0 cm, 18.0 cm, 15.0 cm B :: 15.5 cm, 15.0 cm, 15.2 cm, 15.3 cm

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Precision The precision of a measurement depends on the instrument used to measure it. For example, how long is this block?

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How big is the beetle? Measure between the head and the tail! Between 1.5 and 1.6 in Measured length: 1.54 in The 1 and 5 are known with certainty The last digit (4) is estimated between the two nearest fine division marks.

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Example: Precision Who is more precise when measuring the same 17.0 cm book? A: 17.0 cm, 16.0 cm, 18.0 cm, 15.0 cm B :: 15.5 cm, 15.0 cm, 15.2 cm, 15.3 cm

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Accuracy vs. Precision High Accuracy High Precision Low Accuracy

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Three targets with three arrows each to shoot. The person hit the bull's-eye? Both accurate and precise Precise but not accurate Neither accurate nor precise How do they compare? Can you define accuracy vs. precision?

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Uncertainty The word uncertainty casts a doubt about the exactness of the measurement results True value = Estimated value + Uncertainty

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Why Is There Uncertainty? Measurements are performed with instruments, and no instrument can read to an infinite number of decimal places Which of the instruments below has the greatest uncertainty in measurement?

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Reading a Meterstick. l 2.... I.... I 3....I.... I 4.. cm First digit (known)= 2 2.?? cm Second digit (known)= 0.7 2.7? cm Third digit (estimated) between 0.05- 0.08 cm Length reported=2.77 cm or2.76 cm or2.78 cm

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Known + Estimated Digits In 2.77 cm Known digits 2 and 7 are 100% certain Known digits 2 and 7 are 100% certain The third digit 7 is estimated (uncertain) The third digit 7 is estimated (uncertain) In the reported length, all three digits (2.77 cm) are significant including the estimated one In the reported length, all three digits (2.77 cm) are significant including the estimated one

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Performance of Instruments All instrumentation systems are characterized by the system characteristics or system response There are two basic characteristics of Measuring instruments, they are Static character Dynamic character

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Static Characteristics The instruments, which are used to measure the quantities which are slowly varying with time or mostly constant, i.e., do not vary with time, is called static characteristics.

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STATIC CHARACTERISTICS OF AN INSTRUMENTS Accuracy Precision Sensitivity Resolution Threshold Drift Error Repeatability Reproducibility Dead zone Backlash True value Hysteresis Linearity Range or Span Bias Tolerance Stability

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Resolution This is defined as the smallest input increment change that gives some small but definite numerical change in the output.

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Threshold This minimum value of input below which no output can be appeared is known as threshold of the instrument. input Output

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Drift Drift or Zero drift is variation in the output of an instrument which is not caused by any change in the input; it is commonly caused by internal temperature changes and component instability. Sensitivity drift defines the amount by which instruments sensitivity varies as ambient conditions change.

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input Output zero drift input Output sensitivity drift input Output sensitivity drift zero drift

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Error The deviation of the true value from the desired value is called Error Repeatability It is the closeness value of same output for same input under same operating condition Reproducibility - It is the closeness value of same output for same input under same operating condition over a period of time

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Range The Range is the total range of values which an instrument is capable of measuring.

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Hysteresis This is the algebraic difference between the average errors at corresponding points of measurement when approached from opposite directions, i.e. increasing as opposed to decreasing values of the input. Actual/ Input Value Measured Value Ideal Hysteresis is caused by energy storage/ dissipation in the system.

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Zero stability The ability of the instrument to return to zero reading after the measured has returned to zero

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Dead band This is the range of different input values over which there is no change in output value.

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Linearity- The ability to reproduce the input characteristics symmetrically and linearly

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Backlash Lost motion or free play of mechanical elements are known as backlash True value The errorless value of measured variable is known as true value Bias The Constant Error Tolerance- Maximum Allowable error in Measurement

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Dynamic Characteristics The set of criteria defined for the instruments, which are changes rapidly with time, is called dynamic characteristics.

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Dynamic Characteristics Steady state periodic Transient Speed of response Measuring lag Fidelity Dynamic error

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Steady state periodic Magnitude has a definite repeating time cycle Transient Magnitude whose output does not have definite repeating time cycle Speed of response- System responds to changes in the measured quantity

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Measuring lag Retardation type :Begins immediately after the change in measured quantity Time delay lag : Begins after a dead time after the application of the input Fidelity- The degree to which a measurement system indicates changes in the measured quantity without error Dynamic error- Difference between the true value of the quantity changing with time & the value indicated by the measurement system

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Errors in Instruments Error = True value Measured value or Error = Measured value - True value

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Types of Errors Error of Measurement Instrumental error Error of observation Based on nature of errors Based on control

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Error of Measurement Systematic error -Predictable way in accordance due to conditions change Random error - Unpredictable manner Parasitic error - Incorrect execution of measurement

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Instrumental error Error of a physical measure Error of a measuring mechanism Error of indication of a measuring instrument Error due to temperature Error due to friction Error due to inertia

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Error of observation Reading error Parallax error Interpolation error

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Nature of Errors Systematic error Random error

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Based on control Controllable errors Calibration errors Environmental (Ambient /Atmospheric Condition) Errors Stylus pressure errors Avoidable errors Non - Controllable errors

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Correction Correction is defined as a value which is added algebraically to the uncorrected result of the measurement to compensate to an assumed systematic error. Ex : Vernier Caliper, Micrometer

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Calibration Calibration is the process of determining and adjusting an instruments accuracy to make sure its accuracy is with in manufacturing specifications.

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Interchangeability A part which can be substituted for the component manufactured to the small shape and dimensions is known a interchangeable part. The operation of substituting the part for similar manufactured components of the shape and dimensions is known as interchangeability.

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Compiled byN.SRILAKSHMANAN Assistant Professor Department of Mechanical Engineering SNSCE