UNITS AND MEASUREMENTS

Metrology.Metrology is defined as the Science of pure measurement. But in engineering purposes, it is restricted to measurements of length and angles and other qualities which are expressed in linear or angular terms.

Units and Standards

Units of Measurement:C.G.S. System of UnitsCentimeter Gram Second system of unit M.K.S. System of Units:Meter kilogram second system of unitsInternational System (SI) of Units:the meter (m), kilogram (k), second (s), and ampere (A) of the MKSA system and, in addition, the Kelvin (K) and the candela (cd) as the units of temperature and luminous

Precision Degree of repetitiveness. If an instrument is not precise it will give different results for the same dimension for the repeated readings.Accuracy The maximum amount by which the result differ from true value(ie) Closeness to true value

Terminology in instrumentation

Calibration is the process of establishing the relationship between a measuring device and the units of measure. This is done by comparing a devise or the output of an instrument to a standard having known measurement characteristics. SensitivityIt is ratio between output signal to input signal

Readability is a measure of an instrument's ability to display incremental changes in its output value.True size Theoretical size of a dimension which is free from errors.Actual size size obtained through measurement with permissible error

Repeatability is the variation in measurements taken by a single person or instrument on the same item and under the same conditions. A measurement may be said to be repeatable when this variation is smaller than some agreed limit. Reproducibility is one of the main principles of the scientific method, and refers to the ability of a test or experiment to be accurately reproduced, or replicated, by someone else working independently.

Methods of measurement.1.Direct Method2.Indirect Method3.Comparison Method4.Coincidence Method. Classification of measuring instruments.1.Angle measuring instruments2.Length measuring instruments3.Instruments for surface finish4.Instruments for deviations.

Measurement MethodsDirect method. compare the quantity directly with the primary or secondary standard.

2. Indirect method. *Measurements and Sources of Errors*

Measurements and Sources of Errors

Measurement Methods (Cont.)

3. Comparison method: the comparison of an unknown quantity to a known quantity called a standard using Dial Indicator.

4. Coincidence method.

Sources of errorControllable Errors- Calibration Errors ,ambient Conditions , Stylus pressure, avoidable errors Random ErrorsThese occur randomly and the specific causes of such errors cannot be determined, but likely sources of this type of error are small variations in the position of setting standards and workpiece, slight displacement of lever joints in the measuring joints in the measuring instrument,

Parallax Error :On most dials the indicating finger or pointer lies in a plane parallel to the scale but displaced a small distance away to allow free movement of the pointer. It is then essential to observe the pointer along a line normal to the scale otherwise a reading error will occur.

Definition of Standards: A standard is defined as something that is set up and established by an authority as rule of the measure of quantity, weight, extent, value or quality.For example, a meter is a standard established by an international organization for measurement of length. Industry, commerce, international trade in modern civilization would be impossible without a good system of standards.

Role of Standards: The role of standards is to achieve uniform, consistent and repeatable measurements throughout the world. Today our entire industrial economy is based on the interchangeability of parts the method of manufacture. To achieve this, a measuring system adequate to define the features to the accuracy required & the standards of sufficient accuracy to support the measuring system are necessary.

STANDARDS OF LENGTH

In practice, the accurate measurement must be made by comparison with a standard of known dimension and such a standard is called Primary Standard.The first accurate standard was made in England and was known as Imperial Standard yard which was followed by International Prototype meter made in France. Since these two standards of length were made of metal alloys they are called material length standards.

LENGTH / LINE STANDARDSWhen the length being measured is expressed as the distance between two lines, then it is called Line Standard. Examples: Measuring scales, Imperial standard yard, International prototype meter, etc.

Characteristics of Line Standards: 1. Scales can be accurately engraved but it is difficult to take the full advantage of this accuracy. Ex: A steel rule can be read to about 0.2 mm of true dimension.2. A scale is quick and easy to use over a wide range of measurements.3. The wear on the leading ends results in under sizing4. A scale does not possess a built in datum which would allow easy scale alignment with the axis of measurement, this again results in under sizing.5. Scales are subjected to parallax effect, which is a source of both positive & negative reading errors6. Scales are not convenient for close tolerance length measurements except in conjunction with microscopes.

END STANDARDSWhen the length being measured is expressed as the distance between two parallel faces, then it is called End standard.End standards can be made to a very high degree of accuracy. Ex: Slip gauges, Gap gauges, Ends of micrometer anvils, etc.

Characteristics of End Standards:1. End standards are highly accurate and are well suited for measurements of close tolerances as small as 0.0005 mm.2. They are time consuming in use and prove only one dimension at a time.3. End standards are subjected to wear on their measuring faces.4. End standards have a built in datum, because their measuring faces are flat & parallel and can be positively located on a datum surface.5. They are not subjected to the parallax effect since their use depends on feel.6. Groups of blocks may be wrung together to build up any length. But faulty wringing leads to damage.7. The accuracy of both end & line standards are affected by temperature change.

Light (Optical) wave Length Standard:

The wavelength of a monochromatic light which is invariable unit of length provides an acceptable standard. Krypton 86 atom is selected for wavelength standard of length. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquefied air. The high power and relative ease of operation of krypton discharge tubes caused (from 1960 to 1983) the official meter to be defined in terms of one orange-red spectral line of krypton-86. Error in measurement being 1 part in 100 millions.1 metre = 1650763.73 wavelengths of Kr 86;1 yard = 1509458.3 wavelengths of Kr 86.

Advantages of using wave length standards:1. Length does not change.2. It can be easily reproduced easily if destroyed.3. This primary unit is easily accessible to any physical laboratories.4. No wear and tear5. Wavelength standard can be reproduced consistently at any time and at any place.

Linear measuring instruments

Outside caliper.Inside caliper.Vernier caliperScrew gaugevernier height gaugevernier depth gaugeDial gauges

SLIP GAUGES OR GAUGE BLOCKS (JOHANSSON GAUGES)

Slip gauges are rectangular blocks of steel having cross section of 30 mm face length & 10 mm face width.

Slip gauges are blocks of high-carbon-steel that have been hardened and stabilized by heat treatment. They are ground and lapped to size to very high standards of accuracy and surface finish. A gauge block (also known Johansson gauge, slip gauge, or Jo block) is a precision length measuring standard consisting of a ground and lapped metal or ceramic block. Slip gauges were invented in 1896 by Swedish machinist Carl Edward Johansson.

Gauge or Jo Blocks ISO 3650Come in sets to 150 mm with 88 blocksMinimum step 1 mMake up any dimension by wringing blocks togetherSets come in several grades, typical for shop use is grade AS-1 with thinner blocks good to sub-micronsSets include two wear blocks to be mounted to either end of stack to protect the measurement blocks from wear

Gauge blocks

Slip gauges

Slip gauges are rectangular blocks of high grade steel with close tolerance.It ensure the resistance to wear.They are than stabilized by heating and cooling successively in stages so that hardening stresses are removed.

Slip gaugesAfter being hardened they are carefullyfinished by high grade lapping to a high degree finish, flatness and accuracy.Its having a truly flat surface for accurate reading.The cross section of these gauge is 9 x 30 mm for size upto 10 mm and 9 x 35 mm for larger size.

Uses of slip gaugesSlip gauges are used forTo get the high accuracyFor checking the accuracy of vernier calliper, micrometer and such other measuring instrument.For measure the angle of work piece.

Wringing of slip gauge

SLIDETWIST

Slip Gauges Direct precise measurement, where the accuracy of the work piece demands it.For checking accuracy of venire calipers, micro metes, and such other measuring instruments.Setting up a comparator to specific dimension.For measuring angle of work piece and also for angular setting in conjunction with a sine bar.The distances of plugs, spigots, etc. on fixture are often best measured with the slip gauges or end bars for large dimensions.To check gap between parallel locations such as in gap gauges or between two mating parts.Slip gauges are rectangular blocks of high grade steel with exceptionally close tolerances. These blocks are suitably hardened through out to ensure maximum resistance to wear. They are then stabilized by heating and cooling successively in stages so that hardening stresses are removed.

Dial indicators Test indicators short rangehigh sensitivity used as null devicesLong range needle makes many revolutions has counter dialUsed for rough centering, thickness measurementMounted in a stand either stationary while part moves or indicator moves along a straightedge with part stillGenerally looking for minimum indicator movementEnd result given as TIR Total Indicator Reading or RunoutAlso attached to bore & depth gagesDigital & dial versions availableCan be fitted with various tips

Figure 45.6 Dial indicator: front view shows dial and graduated face; back view shows rear of instrument with cover plate removed (photo courtesy of Federal Products Co.).Dial Indicator

Mechanical Gauges: Dial IndicatorsMechanical gages are designed to mechanically magnify the deviation to permit observation Most common instrument in this category is the dial indicator, which converts and amplifies the linear movement of a contact pointer into rotation of a dialThe dial is graduated in small units such as 0.01 mm or 0.001 inchApplications: measuring straightness, flatness, parallelism, squareness, roundness, and runout

Figure 45.7 Dial indicator setup to measure runout; as part is rotated about its center, variations in outside surface relative to center are indicated on the dial.Dial Indicator to Measure Runout

MicrometerMicrometer works on principle of Screw and nut.When the screw is turn through the nut through one revolution it advance by one pitch distance.Least count of micrometerThe Micrometer has a screw of 0.5 mm pitch with a thimble graduated in 50 division to provide a direct reading of Pitch/n = 0.5/50 = 0.01 mmMicrometer total reading = Main scale reading + Reading on the thimble * L.C

Outside mircometers ISO 3611

Outside mircometers ISO 3611Most common are 25 mm capacity available to 500 mm Accurate to .01 mm with resolution to .002 mm on better gradesUse a micrometer stand with larger sizes if practicalCommonly used for measuring diameters use ball tips for plane surfacesRatchet gives uniform squeeze between anvils Digital versions help avoid reading errors

Depth Micrometer

Angular measurements

1. Vernier bevel Protractor2. Tool room microscope3. Sine bar and dial gauge4. Auto Collimator5. Taper measuring machine6. Roller, Slip gauge, and micrometer.

Angular MeasurementCircles are divided into 360 equal parts, each being a degree. Each of these degrees can be evenly divided into 60 equal parts. These parts are called minutes. These minutes can be evenly divided into 60 equal parts. These parts are called seconds.

Angular Measure Tools

ProtractorWhole degree increments

Multi-Use GaugePre-set positions for 45 and 90 degrees, 59 degree drill point angle, and whole degree increments.

Multi-Use GaugePre-set position for 90 degrees.

Multi-Use GaugePre-set position for 45 degrees.

Multi-Use GageMeasuring 59 degree drill point angle.

Protractor HeadWhole degree increments

ProtractorBuilt-in Spirit Level

ProtractorAngular Measure with Protractor Head

Precision angles to within 5' (0.083)It Consists of :-Base Vernier scaleProtractor dialSliding bladeDial clamp nutUniversal Bevel Protractor

Vernier ProtractorUsed to measure obtuse angle (90-180)Acute-angle attachment fastened to protractor to measure angles less than 90Main scale divided into two arcs of 180Scale divided into 12 spaces on each side of 0If zero on vernier scale coincides with line on main: reading in degrees

Reading a Vernier ProtractorNote number of whole degrees between zero on main scale and zero on vernier scaleProceeding in same direction, note which vernier line coincides with main scale line50 Multiply number by 5' and add to degrees on protractor dial4 x 5'= 20' Reading = 50 20'

Sine BarsUsed when accuracy of angle must be checked to less than 5 minutesConsists of steel bar with two cylinders of equal diameter fastened near endsCenters of cylinders exactly 90 to edgeDistance between centers usually 5 or 10 inches and 100 or 200 millimeters.Made of stabilized tool hardened steel

Sine Bar

Sine Bars

Surface finish measurementSurface finish refers to the quality finish or roughness over the surface. Surface texture :Repetitive or random deviations form the normal surface which form the pattern of the surface. Surface texture include roughness, waveness, lay and flows.. Primary texture :This refers to the roughness of a surface, as opposed to its waviness (secondary texture)

Methods of measuring surface finish

. 1) Surface Inspection (or) comparison method 2. Direct Instrumenta) Touch Inspection b) Visual Inspection c) Scratch Inspectiond) Microscopic Inspectione) Surface photographf) Micro - Interferometerg) Wallace surface Dynamometerh) Reflected light Intensity

Roughness measurementMaximum Peak to Valley. Height of Roughness.Root Mean Square Value (R.M.S. Value)..Centre Line Average Method (C.L.A. Value)

Surface finish measuring instrumentsProfilometer.The Tomlinson Surface MeterTaylor-Hobson Talysurf.

UNIT IV

TEMPERATUREMEASUREMENTS

CLASSIFICATION OF TEMPERATUREMEASURING EQUIPMENTS

Bimetallic Thermometers:

Principle Involved : These use the principles of metallic expansion when temperature changes. A bimetallic strip is shown in figure which is straight initially. When temperature changes, its shape also changes into an arc.

BIMETALIC THERMOMETER USEThe displacement of the free end can be converted into an electric signal through use of secondary transducers like variable resistance, inductance and capacitance transducers. Figure shows a strip of bimetal in the form of a spiral. The curvature of the strip varies with temperature. This causes the pointer to deflect. A scale is provided which has been calibrated to show the temperature directly. This kind of spiral is mostly used in devices measuring ambient temperature and air-conditioning thermostats. Advantages of Bimetallic Thermometers1. Simple2. Inexpensive3. Accuracy of 0.5% to 2%

RESISTANCE THERMOMETERSBasic principle of resistance thermometers?When an electric conductor is subjected to temperature change the resistance of the conductor changes. This change in resistance of the conductor becomes a measure of the change in temperature when calibrated.

Thermocouples

Principles Involved : When heat is applied to the junction of two dissimilar metals, an e.m.f. is generated. (Figure)

Thermistors:

Thermistor is a temperature sensitive variable resistor made of a ceramic like semiconducting material. They are made of metal oxides and their mixtures like oxides of cobalt, copper, nickel, etc. Unlike metals, thermistors respond negatively to temperature. They behave as resistors with a high negative temperature coefficient of resistance. Typically, for each 1 C rise in temperature, the resistance of a thermistor decreases by about 5%. This high sensitivity to temperature changes makes the thermistor useful in precision temperature measurements. The resistance of thermistors vary from 0.5 to 0.75M . Variation of resistivity with temperature is shown in figure.

UNIT III

FLOW MEASUREMENT

FLOW METERSFlow meter measures the actual flow rate. TYPES OF FLOWMETERSVENTURIMETERPITOT TUBEFLOW NOZZLEORIFICE PLATE

VENTURIMETERUSES1. Low head loss about 10% of differential pressure head. 2. High co-efficient of discharge.3. Capable of measuring high flow rates in pipes having very large diameter. 4. Characteristics are well established so they are extensively used in process and other industries.

VENTURI PRINCIPLEThis is just like an orifice meter. It has three distinct parts, namely convergent cone, throat and divergent cone. A manometer measures the pressure difference between two sections as shown in figure. Let a1 -Area at the inlet (1-1)A2-Area at the section (2-2)x-Pressure head difference Cd-Discharge coefficient

, Q =

Orifice METER

Let a1 Area at section I-Ia0 Area of orifice Cd Discharge coefficient Then, Flow rate

ROTO METERSRotameter:A rotameter is a variable area type flow meter. It consists of a vertical tapered tube with a float which is free to move within the tube. The fluid goes from the bottom to the top. When no fluid flows, the float rests at the bottom of the tube. The float is made of such a diameter that it completely blocks the inlet. When flow starts in the pipeline and fluid reaches the float, the buoyant effect of fluid makes the float lighter. The float passage remains closed until the pressure of the flowing material plus the buoyance effect exceeds the downward pressure due to the float weight. Thus, depending on flow, the float assumes a position. Thus the float gives the reading of flow rate.

Pitot Tube

Principle: Transformation of kinetic energy of a liquid into potential energy in the form of a static head.Figure shows a pitot tube installed in a pipeline where it acts like a probe. The tube consists of two concentric tubes, the inner tube with its open ends faces the liquid.

Pitot tube principleouter tube has a closed end and has four to eight holes in its wall. The pressure in the outer tube is the static pressure in the line. Total pressure is sum of static pressure and the pressure due to the impact of fluid.

IfP-Pressure at inlet (Stagnation pressure)Ps-Static pressure -Density, then

Velocity v = from which flow rate is determined.

UNIT V

FORCE MEASUREMENT

FORCE MEASUREMENTForce. The mechanical quantity which changes or tends to change the motion or shape of a body to which it is applied is called force. .Force measureing equipmentsload cellsLoad cells are devices used for force measurement through indirect methods.

Force measuring equipmentsScale and balance a. Equal arm balanceb. Unequal arm balancec. Pendulum scale

2. Elastic force meter Proving ring

3. Load cella. Strain gauge load cell b. Hydraulic load cell c. Pneumatic load cell

Torque measuring equipmentsMechanical torsion meterOptical torsion meterElectrical torsion meterStrain gauge torsion meter

Types of strain gauges.

Unbonded strain gauge Bonded strain gaugeFine wire strain gauge Metal foil strain gauge Piezo-resistive strain gauge

PROVING RING Use of proving Rings Proving rings are steel rings used for calibration of material testing machines in situations where, due to their bulkness, dead weight standards cannot be used. P ring is a circular ring of rectangular section and may support tensile or comprehensive force across its diameter. the change in radius in the direction of force, is given by where d is the outer diameter of the ring and K is stiffness. Deflection of the ring is measured using a precision micrometer. To get precise measurements, one edge of the micrometer is mounted on a vibrating reed which is plucked to obtain a vibratory motion. The micrometer contact is then moved forward until a noticeable damping of the vibration is observed.

LOAD CELLS Use of Load Cell Force transducers intended for weighing purposes are called load cells. Instead of using total deflection as a measure of load, strain gauge load cells measure load in terms of unit strains. A load cell utilizes an elastic member as the primary transducer and strain gauges as secondary transducer. Figure shows one such load cell arrangement.

DYNAMO METERSMechanical Dynamometer:These come under the absorption type. An example for this kind is prony brake. In Prony brake, mechanical energy is converted into heat through dry friction between the wooden brake blocks and the flywheel (pulley) of the machine. One block carries a lever arm. An arrangement is provided to tighten the rope which is connected to the arm. Rope is tightened so as to increase ht frictional resistance between the blocks and the pulley. If F Load applied and Power dissipated r - Lever arm N Speed of flywheel (rpm)Torque T = F.rThe capacity of Prony brake is limited because:Due to wear of wooden blocks, friction coefficient varies. So, unsuitable for large powers when used for long periods. To limit temperature rise, cooling is to be ensured.

D.C. Dynamometer

D.C. dynamometer is usable as an absorption as well as transmission dynamometer. So, it finds its use in I.C. Engines, steam turbines and pumps. A d.c. dynamometer is basically a d.c. motor with a provision to run it as a d.c. generator where the input mechanical energy, after conversion to electrical energy, can either be dissipated through a resistance grid or recovered for use. When used as an absorption dynamometer it acts as d.c. generator. (figure) Cradling in trunnion bearings permits the determination of reaction torque.

Eddy CURRENT DYNAMOMETERCurrent or Inductor Dynamometers:This is an example for absorption type dynamometers. Principle: When a conducting material moves through a magnetic flux field, voltage is generated, which causes current to flow. If the conductor is a wire forming a part of a complete circuit will be caused to flow through that circuit, and with some form of commutating device a form of a.c. or d.c. generator may result. An eddy current dynamometer is shown in figure. It consists of a metal disc or wheel which is rotated in the flux of a magnetic field. The field if produced by field elements or coils excited by an external source and attached to the dynamometer housing which is mounted in trunnion bearings. As the disc turns, eddy currents are generated. Its reaction with the magnetic field tends to rotate the complete housing in the trunnion bearings. Water cooling is employed.

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