metrology & measurement

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Meteorology & Measurement

Metrology & Measurement

Definitions of MetrologyField of knowledge concerned with Measurement & includes both theoretical & practical problems related to measurement.Process of making extremely precise measurement.It is the documented control that all equipment is suitably calibrated & maintained in order to perform its function & give reliable results.The science concerned with the establishment, reproduction & transfer of units of measurements & their standards.

Principle Fields of Metrology & Its Related feildsEstablishing units of Measurement & their standards.Measurements, methods, execution & estimation of their accuracy.Measuring instruments, Properties examined with respect to their intended purpose.Observers capabilities with reference to making measurements.Design, manufacturing & testing of gauges of all kinds.

Types of MetrologyScientific Metrology:It deals with the organization & development of measurement standards & with their maintenance at the highest level.Industrial Metrology:It deals with the ensuring of the adequate functioning of measuring instruments used in industry as well as in production & testing Processes.It is necessary to work with quality in industrial activities.Legal Metrology:It is concerned with the accuracy of measurements where these have influence on the transparency of economical transactions, health & safety.Its function is to regulate, advise, supervise & control the manufacturing & calibration of measuring instruments.

Types of MetrologyFundamental Metrology:It may be described as scientific metrology, Supplemented by those parts of legal & industrial metrology that requires scientific competence.It is divided in accordance with the following eleven fields:Mass, Electricity, Length, Time, Frequency, Ionizing radiations, Radioactivity, Photometry, Radiometry, Flow , Acoustics, Amount of Substance & Interdisciplinary metrology.

Need for InspectionTo ensure components & parts conform to the established standards.To meet the interchangeability of manufacturer.To provide the means of finding the problem areas.To produce the parts having acceptable quality level.To judge the possibility of rework of defective parts and re-engineer the process.To purchase good quality of raw materials, tools & equipment.

Metrological TerminologiesAccuracy:The closeness of agreement between a test result & the accepted reference value.Bias:The difference B/w the expectation of the test result & an accepted reference value.Calibration:The set of operations that establish the relationship b/w values indicated by instruments & the corresponding values given by standards, under specified conditions.Confirmation:The set of operations required to ensure that an item of measuring equipment is in a state of compliance with requirements for its intended use.

Correction:It is equal to the assume systematic error.As the systematic error can not be known exactly so correction is subjected to uncertainty.Drift:A slow change of metrological characteristics of a measuring instruments.Error:The indication of measuring instrument output minus the true value of the input quantity.Expectation:The mean value of specified population of measurements.Fiducial error:The error of measuring instrument divide by Fiducial value specified for the instrument.Fiducial value can be the span or upper limit of nominal range of measuring instrument.

Group Standard:A set of standards of chosen value that individually or in combination, provide a series of values of the same kind.Inspection:It involves measurement investigation or testing of one or more characteristics of a product.Magnification:The output signal from a measuring device is to be magnified many times to make it more readable.Measurand:A particular quantity subjected to measurement.Nominal value:An approximate value of a measuring instrument that provide a guide to use it.Precision:The closeness of agreement b/w independent test results obtained under stipulated conditions.Range:The capacity with in which the instrument is capable of measuring.

Readability:It refers to the ease with which the readings of a measuring instrument can be read.Reference value:The value which agreed on reference for comparison.Repeatability conditions:Where independent test results are obtained using same methods, items, place, operator & equipment with short interval of time.Reproducibility:Precession under reproducibility conditions.Reproducibility conditions:Where test results are obtained using same method & items but in different place, operator & equipment.Response time:The time which elapse after sudden change of the measured quantity until the instrument gives an indication.Resolution:The smallest change of the measured quantity which changes the indication of a measuring instrument.

Sensitivity:The smallest change in the value of the measured variable to which the instrument responds.Stability:The ability of measuring instrument to constantly maintain its metrological characteristics with time.Standardization:A process of formulating & applying rules for orderly approach to a specific activity for the benefit & with the cooperation of all the concerned in particular.Testing:A technical investigation to know whether the product fulfills its specified performance.Traceability:Means that a measured result can be related to stated references.Trueness:The closeness of agreement b/w the average value obtained from a large series of test results & an accepted reference value.It is usually expressed in terms if bias.

Uncertainty:It is a parameter associated with the result of a measurement that characterizes the dispersion of the values that could reasonably be attributed to the Measurand.It can also be expressed as an estimate characterizing the range of values with in which the true value of a Measurand lies.When specifying the uncertainty it is necessary to indicate the principle on which the calculation has been made.Verification:An investigation that shows that specified requirements are fulfilled.

Principal Aspects of Measurement

Principal Aspects of MeasurementPrecision:It is the degree of repeatability in the measuring process.It refers to variability of a method when used to make repeated measurements under specific conditions.It is mainly achieved by selecting a correct instrument technology for application.For determining the right level of precision is that the measuring device must be en times more precise than the specified tolerances.

Methods of MeasurementsDirect Method:This is the simplest method of measurement in which the value of the quantity to be measured is obtained directly without any calculations, e.g. measurements by scale, calipers & micrometers.It involves contact or non contact type of inspections.Indirect Method:The value of the quantity to be measured is obtained by measuring other quantities, which are related to required value.E.g. density calculation by measuring mass & volume.Absolute Method:Also called fundamental method & is based on the measurement of the base quantities used to define a particular quantity.

Deflection Method:The value of the quantity to be measured is directly indicated by the deflection of a pointer on a calibrated scale. e.g. dial indicator.Complementary Method:The value of the quantity measured is combined with a known value of the same quantity.E.g. determining volume of solid by liquid displacement.Method of Null Measurement:It is a method of Differential measurement.In this method the difference b/w measured & known value is brought to zero.E.g. measurement by potentiometer.

Measuring Instruments & Their SelectionOn the basis of Function:Length Measuring InstrumentsAngle Measuring InstrumentsSurface Roughness Measuring InstrumentsShape Checking InstrumentsOn the basis of Accuracy:Most Accurate InstrumentsModerate Accurate InstrumentsBelow Moderate Accurate InstrumentsOn the basis of Precision:Precision Measuring instrumentsNon Precision Measuring Instruments

Factors Affecting Accuracy of Measuring InstrumentsStandards of Calibration for Setting Accuracy

Work piece control during Measurement

Inherent Characteristics of measuring instruments

Inspector (Human Factor)

Environmental Conditions

Errors in Measurement Error in Measurement=Measured value-True ValueError may be Absolute or Relative.Absolute Error: It is divided into two types:True absolute Error:It is defined as the algebraic difference b/w the result of measurement & the conventional true value of the quantity Measured.Apparent Absolute Error:It is defined as the algebraic difference b/w the arithmetic mean & one of the results of measurement when a series of measurements are made.

Absolute Error = |Actual Value-Approximate value| If, absolute value = x & approximate value = x+dx, then Absolute Error = dx

Errors in Measurement

Errors in MeasurementStatic Error:These are the result of physical nature of the various components of a measuring system i.e. intrinsic imperfection or limitation instruments.They are further classified as:Reading Error:Errors when the line of sight is not perpendicular to the measuring scale.Error = X tan Characteristic Error:It is the deviation of the system output from the theoretical predicted performance.Linearity, repeatability, hysteresis & resolution error are its types.

Errors in MeasurementAlignment error:This occurs when the checking of an instrument is not correctly aligned with the direction of the desired measurement.

= D(1 cos )To avoid alignment error Abbes Principle has to be f

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