Metrology & Measurement

Definitions of Metrology

Field 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 feilds

a. Establishing units of Measurement & their standards.

b. Measurements, methods, execution & estimation of their accuracy.

c. Measuring instruments, Properties examined with respect to their intended purpose.

d. Observers capabilities with reference to making measurements.

e. Design, manufacturing & testing of gauges of all kinds.

Types of Metrology

1. Scientific Metrology: It deals with the organization & development of measurement

standards & with their maintenance at the highest level.2. 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.

3. 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 Metrology

Fundamental 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 Inspection

i. To ensure components & parts conform to the established standards.

ii. To meet the interchangeability of manufacturer.iii. To provide the means of finding the problem

areas.iv. To produce the parts having acceptable quality

level.v. To judge the possibility of rework of defective

parts and re-engineer the process.vi. To purchase good quality of raw materials, tools

& equipment.

Metrological Terminologies

Accuracy: 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

Accuracy: It is the degree to which the measured value of the

quality characteristics agree with the true value. It is referred to the absence of bias to the conformity

of results to the true value of quality characteristics being measured.

The measured value is the is the sum of the quantity measured & the error of the instrument.

Standard deviation of measured value is:

Principal Aspects of Measurement

Precision: 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 Measurements

1. Direct 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.2. 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.3. Absolute Method:

Also called fundamental method & is based on the measurement of the base quantities used to define a particular quantity.

4. Comparison Method: The value of quantity to be measured is compared with a known

value of a same or related quantity to it. E.g. dial indicators & other comparators.

5. Substitution Method: The quantity is measured by direct comparison on an indicating

device by replacing the measurable quantity with another which produces the same effect on the indicating device.

6. Coincidence Method: There is a very small difference b/w the value of the quantity to

be measure & the reference. It is also called differential method of measurement.

7. Transposition Method: In this method the value of the quantity measured is first

balanced by an initial known value P of the same quantity. Then the value of the quantity measured is put in place of that

known value & is balanced again by another known value Q. Finally the value of the quantity is to be measured by

8. 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.9. 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.10. 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 Selection

i. On the basis of Function:a. Length Measuring Instrumentsb. Angle Measuring Instrumentsc. Surface Roughness Measuring Instrumentsd. Shape Checking Instruments

ii. On the basis of Accuracy:a. Most Accurate Instrumentsb. Moderate Accurate Instrumentsc. Below Moderate Accurate Instruments

iii. On the basis of Precision:a. Precision Measuring instrumentsb. Non Precision Measuring Instruments

Factors Affecting Accuracy of Measuring Instruments

1. Standards of Calibration for Setting Accuracy

2. Work piece control during Measurement

3. Inherent Characteristics of measuring instruments

4. Inspector (Human Factor)

5. Environmental Conditions

Errors in Measurement

Error in Measurement=Measured value-True ValueError may be Absolute or Relative.1. Absolute Error: It is divided into two types:a. True absolute Error:

It is defined as the algebraic difference b/w the result of measurement & the conventional true value of the quantity Measured.

b. 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

2. Relative Error: It is the quotient of the absolute error to the true/

actual value (may be true or arithmetic mean of a series of measurements)

Relative Error = Relative Error = dx/x

Percentile Error (Ep) = Relative Error * (100)

Errors in Measurement

3. Static 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:a. Reading Error:

Errors when the line of sight is not perpendicular to the measuring scale.

Error = X tan θb. 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 Measurement

c. Alignment error: This occurs when the checking of an instrument is not correctly

aligned with the direction of the desired measurement.

= D(1 – cos θ) To avoid alignment error Abbe’s Principle has to be followed which

states that: “the axis or line of measurement should coincide with the axis of the measuring instrument or the line of the measuring scale”

Errors in Measurement

d. Environmental Error: The error arising from the effect of the surroundings like pressure,

temperature, humidity, magnetic & electric fields etc. It can be controlled by controlling the atmospheric factors.

4. Loading error: If the datum surface is not flat or if foreign matters like dirt, chips etc.

get entrapped b/w the datum & work piece then there will be Loading error.

Also poor contact b/w instrument & work piece can cause this. To avoid such errors an instrument with wide area of contact should

not be used.5. Dynamic error:

It is caused by time variation in the Measurand. It is the result of incapability of the system to respond reliably to time varying measurement.

Inertia, damping & friction are causes of dynamic error.

Errors in Measurement

Analysis of accumulation of error by the statistical method categorizes as controllable & random errors

a. Controllable Error: These are controllable in both magnitude & sense. They are repetitive in nature & are of similar forms These are also called systematic errors. They include the

following errors:i. Calibration Error:

Caused due to the variation in the calibrated scale from its normal indicating value.

ii. Stylus pressure error: The too small or too large pressure applied on a work piece

while measuring causes stylus pressure.iii. Avoidable Error:

These errors occur due to parallax & non alignment of the work piece.

Errors in Measurement

b. Random Error: These errors are accidental, non consistent in nature. As they occur randomly they cannot be eliminated

since no definite cause can be located. Small variation in the setting standards & the work

piece can cause such errors.

Units of Measurements (Base)

Units of Measurements (Derived)

Units of Measurements (SI Prefixes)