Unit 1

Concept of Measurement

GENERAL CONCEPT

Measurement is a complex operation carried out by means of measuring instrument to determine the numerical value of the size which describes the object of measurement.

Act of deriving quantitative information about a object or action by comparison with reference.

Three important element of measurements are

1.Measurand: Physical quantity or property like length, angle being measured.

2.Comparison or comparator:The mean of comparing measurand with some reference

3.Reference: Physical quantity or property to which quantitative comparisons are made.

STANDARD

Example:

Direct measurement using calibrated fixed reference.

•Mechanic has to measure the length of a surface table (measurand).

•First he lays his rule(reference) alongside the table.

•He then carefully align the zero end of his rule with one end of the table; and finally he compares the length of table (measurand) with graduation on his rule (reference) by eye (comparator)

Two important requirement of measurement:

1. The standard used for comparison must be accurate and internationally accepted.

2. The instrument and the process used for the comparison must be provable.

The basic aim of measurement in industries is to check whether a component has been manufactured to the requirement of a specification or not.

Measurement is defined as the process of numerical evaluation of a dimension or the process or comparison, with standard measuring instrument.

Input signal

GENERALIZED MEASURING SYSTEM

Functional elements of an instrument system

Primary sensing element:

First element which receives energy from measured medium and produces an output corresponding to the measurand.This output is then converted into an analog electical signal by transducer.

Variable conversion element:

It converts the output electrical signal of the primary sensing element (voltage,frequency or someother electrical parameters) into a more suitable form without changing the Information content of the input signal.Some instruments it may not be required.

Variable manipulation element:

It is used to manipulate the signal presented to it and preserve original nature of the signal.It amplifies the input signal to the required magnification.An electronic voltage amplifier receives a small voltage as input and produces a greater magnitude voltage as out put.

Data transmission element:It transmits the data from one element to the other, as simple as shaft and gear assy. system or as complicated as a telemetry system which transmit signal from one place to other.

Data processing element: which is used to modify the data before display or recording. To convert the data into useful form.

It may provide correction to the measured physical variable to compensate for zero offset, temperature error etc.

The data presentation element:They finally communicate to human observer for monitoring, control or analysis purpose.

The value of the measured variable my be indicated by an analog indicator (pointer and scale),Digital indicator (ammeter, voltmeter etc.) or by recorder (magnetic taps, cameras etc.).

GENERALIZED MEASURING SYSTEMExample

Temperature measurement of a thermal system

Temperature

Pressure

Capillary tube

Spiral bourdon tube

Linkage and gear

Liquid or gas filled temperature bulb

Pointer and scale

Displacement Magnification

Temperature measurement of a thermal system

UNITS

To specify and perform calculation with physical quantities, the physical quantity must be defined both in kind and magnitude. The standard measuring of each kind of physical quantity is the unit.

Earlier days number of systems of units has been used in various countries. M.K.S and C.G.S systems were used. However for the sake of uniformity of units all over the world S.I has been developed ( System international units ).

FUNDAMENTAL UNITS: independantly chosen and not depend on anyother units. Base units.

Quantity Unit symbol

Length Meter m

Mass Kilogram kg

Time Second s

Temperature Kelvin K

Electric current Ampere A

DERIVED UNITS: expressed in terms fundamental units by defining equation

Mechanical units Electric and magnetic units

Thermal units

Force Power Specific heat

Pressure Energy Latent heat

Weight Electrical resistance Sensible heat

Torque Electrical field strength

Acceleration

velocity

density

Area

Volume

When the length is measured between two lines it is called line standard.Eg. Protype meter and imperial yard

When the distance(length) is measured between two surfaces or faces it is called end standardEg. Slip gauge,gap gauge micrometer

STANDARDS

When the distance(length) is measured between two surfaces or faces it is called end standardEg. Slip gauge,gap gauge micrometer

•Material standards are liable to destruction and their dimensions change due to variations of environmental temperature, pressure,humidity and ageing with time.

•But with the monochromatic light we have the advantage of constant wavelength and since the wavelength is not physical one, it need not be preserved.

•This is reproducible standard of length.

•The error of reproduction can be in the order of 1 part in 100 millions.

•Because of this reason that international standard measures the meter in terms of wave length of krypton 86 (Kr 86).

•1meter = 1650763.73 wave length of Kr86 in vaccume•1 yard = 0.9144m =1509458.25wave length of Kr86 in vaccume

Sub division of standards:

Primary or international Standards: The material is preserved carefully and is and used only at rare intervals for comparison with secondary standard.eg. Protype meter, Imperial yard

Secondary Standards:These are kept at number of places for occasional comparison with tertiary standards when ever required.

Tertiary Standards:Reference purpose in laboratories and work shops. True copy of secondary std, and used for comparison with working std.

Workshop Standard:More frequently in laboratories and work shops. Made of low grade material

Measuring Instruments

Depending upon the accuracy required in measurement various measuring instruments are selected.

Some instrument measure the exact dimension of the component . eg. Vernier caliper and micrometer etc.

Some devices are used to check whether the size of the component is within the specified limits.They are called Limit gauges.eg. Plug gauge, Ring gauge and Snap gauges.

Some instrument compare the size of the work piece with the standard and show deviations.These instruments are known as Comparators.eg. All mechanical,electrical,pneumatic and optical comparators

•The term accuracy denotes the closeness of the measured value with the true value.

•Accuracy is the degree to which the measured value agrees with the true value.

•The difference between the true value and measured value is known as error of measurement.

•The lesser the error, the more is the accuracy

Precision is the repeatability of the measuring instrument.

•It refers to the group of measurements for the same characteistics taken under identical conditions.

•It indicates to what extent the identically performed measurements agrees with each other.

•If the instrument is not precise will give different (widely varying)results for the same dimension when measured again and again.

•The set of observations will scatter about the mean true value.

•The less the scattering more precise the instrument.

Range

Fine and widely spaced graduation lines ordinarily improve the readability.

If the graduation lines are very finely spaced,the scale will be more readable by using microscope, however,with the naked eye the readability will be poor.

It is the susceptibility of a measuring device to have its indications converted into meaningful number

PERFORMANCE OF INSTRUMENT

The knowledge system characteristics or system response of an instrument is essential for selecting the most suitable instrument for specific measuring jobs.

STATIC RESPONSE

If the instrument is required to measure condition not varying with time static characterestics are considered.

Static performance charecteristics are obtained by one form or other of a process called Calibration.

Related terms are Accuracy,Precision,Sensitivity,Repeatability,Reproducibility,Truevalue,Range, Error,Stability,Tolerance,

Threshold-The minimum value of input below which no output can be appeared.

Drift-The variation of change in output for a given input over a period of time.

Backlash-Lost motion or free play of the mechanical elements such as gear,linkage etc.

Hysteresis- All the energy put into the stressed component when loaded is not recovered upon unloading. So, the output of the measurement partially depends on input called hysteresis

DYNAMIC RESPONSE

The parameters to be measured are time varying i.e. dynamic in nature.

Thus the output of the instrument also time varying.

The behaviour of an instrument under such time varying input-output conditions is Called dynamic response.

For a time varying process-variable measurements, the dynamic characterestics of instruments are more important.

The analysis of such dynamic response is called dynamic analysis of the Measurement system.

Repeatability represents the reproducibility of the readings of an instrument when a series of measurements are carried out under fixed conditions of use.

SOURCES OF ERROR

CAUSESCalibration Standards: Thermal Expansion, Calibration interval, Stability with time, Elastic property

Work piece: Cleanliness, Surface finish, Waviness, Scratch, Surface defect, etc.

Inherent characteristics of the Instrument: calibration error,scale error,effect of friction,backlash,deformation in handling or use when heavy work pieces are measured.

Person: training ,skill, sense of precision appreciation,ability to select instrument and stds, attitude towards personal accuracy

Environment:Temperature,humidity,sourrounding vibration, magnetic field if any, heat radiation from light,heating elements, sunlight and people.

TYPES OF ERROR

Parallax error :Occurs when the line of vision is not in line with measuring value.

Error can be reduced by placing a mirror behind the pointer,as this ensures normal reading

Interpolation error: This occurs when the pointer shows some value whch is between any two graduation marks.Depends

Thickness of graduation marks, Spacing of the scale divisionsThickness of the pointer

Error can be reducedUsing a magnifier over the scale orUsing a digital read out system

READING ERROR

CHARACTERISTIC ERROR

Calibration error and repeatability error are the examples of this static type.

STATIC ERROR

Temperature plays an important role.Eg. While using slip gauges, due to handling the slip gauges may acquire human body temperature whereas work piece is at 20 degree C.

To avoid errors of this kind, all metrology laboratories and standard rooms worldwide are maintained at 20 degree C

ENVIRONMENTAL ERROR

DYNAMIC ERROR

I. SYSTEMATIC ERROR

These errors are regularly repetitive in nature and in constant in form.If properly analysed this can be determined and reduced.Hence these are also called controllable errors.

Sources of this type error are:Stylus or contact pressure errorMisalignment error etc.

II. RANDOM ERRORS

These are non–consistent. They occur at random. As specific cause and magnitude of this type is difficult to identify and thus difficult to eleminate this error.

Sources of this error as followsVariation in the position of setting the “standard” and “workpiece”.Displacement of the levers of joints of the measuring instrument.Fluctuations in friction of measuring instuments.Backlash in instruments.

1. Calibration is a pre-measurement process, generally carried out by manufacturers.

2. Constant use of instruments affects their accuracy.

3. The accuracy of the instrument depends upon calibration.

4. If the accuracy is to be maintained the instrument must be checked and recalibrated if necessary.

5. The schedule of such calibration dependes upon the severity of use, environmental conditions, accuracy of measurement required etc.

6. As far as possible calibration should be performed under environmental conditions which are very close to that of actual measurements are carried out.

7. It is carried out by making adjustments such that read out device produces zero output for zero measured input.

8. Calibration is the process checking the dimension and tolerances of a gauge, or the accuracy of a measuring instrument by comparing it to the gauge/instrument that has been certified as a standard of known accuracy.

9. By this we can come to a decision whether this instrument can be used again by calibrating or is it wear out or deteriorated above the limit value.If it is so then it is thrown out or scrapped.

Modern production is based on the concept of interchangeability.

When one component assembles properly with any mating component, both being chosen at random, then this is interchangeable manufature.

It is the uniformity of the size of the components produced which ensures interchangeability.Advantages:

The assembly of mating parts is easier. Since any component picked up from its lot will assemble with any other mating part from an another lot with out additional fitting and machining.

It enhances the production rate.

The standardization of machine parts and manufacturing methods is decided.

It brigs down the assembling cost drastically.

Repair of existing machines or products is simplified because component parts can be easily replaced.

Replacement of worn out parts is easy.

Nominal Size: the size which is used for general identification.Thus the nominal size of a hole and shaft assembly is 60mm, even though the basic size of hole may be 60mm and the basic size of shaft 59.5mm

Basic dimension: worked out purely by design consideration. Indicate by tolerence as to how much variation can be tolerated with out affecting the functioning of assemblyinto which this part will be used.

Basics of fit:A limit and fit system is the system of series of Standard allowances to suit specific range of basic size.

If these standard allowances selected properly and assigned to mating parts ensures Specific Classes of fit.

FitMay be defined as a degree of tightness or looseness between two mating parts to perform a definite function when they are assembled together.

Based on positive,zero and negative values of clearance there are three types.

Clearance fit: Shaft is smaller than hole.so that shft can rotate or slide through with different degrees of freedom.

Tailstock spindle of lathe, feed movement the spindle in drilling machineShaft pulleysGearbox bearingsCrankshaft in their main bearing

Interference fit: Shaft and hole members intended to attach permanently and used as a solidComponent.

Steel tyre on railway car wheelsSteel rings on awooden bullock cart wheelsDrill bush in jig plate

Transition fit: This fit lies midway between Clearance and interference fit.

Parts can be placed wthout difficulty during minor repairs.Parts dis-assembled during operation of a machine –change gears,slip bushing

Tolerance Allowance

It is permissible variation in dimension of a part(either a hole or a shaft).

It is the prescribed difference between the dimension of two mating parts (hole and shaft).

It is difference between higher and lower limits of a dimension of a part

It is the intentional difference between lower limit of hole and higher limit of shaft.

Tolerance is provided on a dimension of a part as it is not possible to make a part to exact specified dimension.

Allowance is to be provided on dimension of a mating parts to obtain desired type of fit.

Difference between Tolerance and Allowance