1702ME402 MEASUREMENTS AND METROLOGY

1702ME402 – MEASUREMENTSAND METROLOGYUNIT IIMEASUREMENT OF MECHANICALPARAMETERSLecture byDr. J.Jeevamalar, M.E., Ph.D.Associate Professor/ MechanicalE.G.S. Pillay Engineering College,Nagapattinam

SyllabusUNIT II MEASUREMENT OF MECHANICAL PARAMETERS (6 Hrs) Measurement of Force - Principle, analytical balance,platform balance, proving ring. Measurement of Torque - Prony brake, hydraulicdynamometer. Measurement of Power - Linear and Rotational Pressure Measurement - Principle, use of elastic members,Bridgeman gauge, Mcleod gauge, Pirani gauge s,metalresistancethermometer,pyrometers.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

MEASUREMENT OF FORCEForce: Force is a physical quantity fundamental to engineering. Force represents the mechanical quantity which changesor tends to change the relative motion or shape of thebody on which it acts.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

METHODS OF FORCE MEASUREMENT1. Direct Method: This involves a direct comparison with aknown gravitational force on a standard mass.Force Mass x Gravity2. Indirect Method: Force is calculated from acceleration due togravity and the mass of the component.Force Mass x AccelerationDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

DEVICES USED TO MEASURE FORCEI. DIRECT FORCE MEASUREMENT(a) Equal Arm Balance(b) Unequal Arm Balance(c) Pendulum TypeII. INDIRECT FORCE MEASUREMENTa.Accelerometersb.Load Cellsi.ii.iii.c.Strain Gauge Load CellHydraulic Load CellPneumatic Load CellElastic Loaded MembersProving ringDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

I. DIRECT FORCE MEASUREMENT(a) Equal Arm BalanceDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

I. DIRECT FORCE MEASUREMENT(b) Unequal Arm BalanceDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

I. DIRECT FORCE MEASUREMENT(c) Pendulum Type BalanceDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

II. INDIRECT FORCE MEASUREMENTa. AccelerometersA force will make a body to accelerate. By measuring theacceleration, the force may be determined, from the equation F ma.To measure acceleration, accelerometers are used.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

II. INDIRECT FORCE MEASUREMENTb. Load Cellsi. Strain Gauge Load Cell Load cells are extensively usedfor measurement of force. Straingaugetransducerconverts force into electricalsignal. The strain gauge is bonded to abeam or structural member thatdeforms when a force isapplied. Deflection induces a stress thatchanges its resistance. A wheatstone bridge convertsthe resistance change into acalibrated output signal. The sensor's output is afunction of force and distance.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

II. INDIRECT FORCE MEASUREMENTb. Load Cellsi. Strain Gauge Load CellDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

II. INDIRECT FORCE MEASUREMENTb. Load Cellsii. Hydraulic Load CellDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

II. INDIRECT FORCE MEASUREMENTb. Load Cellsiii. Pneumatic Load CellDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

II. INDIRECT FORCE MEASUREMENTc. Elastic Loaded MembersProving Ring Proving Rings can be used for measurement ofboth compressive and tensile forces. It consists of a hollow cylindrical beam of radiusR, thickness t and axial width b. The two ends of the ring are fixed with thestructures between which force is measured. Four strain gages are mounted on the walls ofthe proving ring, two on the inner wall, and twoon the outer wall. When force is applied, gages 2 and 4 willexperience strain –ε (compression), while gages1 and 3 will experience strain ε (tension). The magnitude of the strain is given by theexpression:Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

MEASUREMENT OF TORQUE Force that causes twisting or turning moment. The main purpose of torque measurement is to determinethe mechanical power required or developed by a machine. Torque measurement also helps in obtaining loadinformation necessary for stress or strain analysis. T F x r, N-mDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

INSTRUMENTS FOR MEASURMENT OF TORQUE1.Mechanical Torsion Meter (Stroboscopic Method)2.Optical Torsion Meter3.Electrical Torsion Meter (Using Slotted Discs)4.Strain Gauge Torsion Meter (Strain Gauges / RotatingShafts)Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

1. MECHANICAL TORSION METER(STROBOSCOPIC METHOD)Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

Application: Simple and inexpensive method. Power of shaft also can be calculated (flashing frequency givesinformation about speed).Limitations: Poor accuracy due to small displacement of the pointer. Sensitivity is reduced even due to small variations in speed. Can be used only on shafts rotating at a constant speed.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

2. OPTICAL TORSION METERDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

3. ELECTRICAL TORSION METER(USING SLOTTED DISCS)Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

4. STRAIN GAUGE TORSION METER(STRAIN GAUGES / ROTATING SHAFTS)Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

APPLICATION: Used to measure torque on rotating shaftsADVANTAGES: Fully temperature compensated Sensitivity is highLIMITATIONS: Difficult to connect the bridge circuit to the power source Difficult to connect the display to the bridge circuit.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

MEASUREMENT OF POWER Dynamometer is a machine or a setup which is used for powermeasurement.Types of Dynamometers:1. Absorption type – shaft whose power is to be measured isstopped (engines or electric motors).2. Transmission type – shaft whose power is to be measured isrotating continuously (drives and pully).3. Driving type – shaft whose power is to be measured acts as adriving member (pumps, compressors).Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

1. MECHANICAL DYNAMOMETERS (Absorption type)(PRONY BRAKE DYNAMOMETER) A Prony brake dynamometer is one of the simplest, inexpensive,and the most popular absorption dynamometers. It is a mechanical type of device that depends on dry frictionwherein mechanical energy is converted into heat. Prony brake dynamometer comprises two wooden blocks thatare mounted on either side of the fly wheel in diagrammaticallyopposite directions. The fly wheel is attached to the shaft whose power needs to bedetermined. A lever arm is fixed to one block and the other arm is connectedto an arrangement provided to tighten the rope. Tightening of the rope is performed in order to enhance thefrictional resistance between the blocks and the flywheel.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

PRONY BRAKE DYNANOMETERSDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

2. HYDRUALIC DYNANOMETERS (Absorption type)Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

3. EDDY CURRENT DYNAMOMETERS (Absorption type)(ELECTRIC DYNAMOMETER)Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

4. DC DYNAMOMETERS (Driving type)Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

TEMPERATURE MEASUREMENTS Temperature is a quantity independent of the size of thesystem. Temperature is an indication of intensity of molecularactivity. A condition of a body by virtue of which heat is transferredto or from other bodies A quantity whose difference is proportional to the work froma Carnot engine operating between a hot source and a coldreservoir.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

INSTRUMENTS TO MEASURE TEMPERATURE1. Bimetallic Thermometers2. Fluid Expansion Type3. Pressure Thermometer4. Resistance Thermometer5. Thermistors6. Thermocouples7. Pyrometersa. Total Radiation Pyrometerb. Optical Pyrometer (Disappearing Filament Type)c. Infra Red PyrometerDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

1. BIMETALLIC THERMOMETERSDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

Important properties of material for selection of bimetallic thermometers.(these properties should be high) Coefficient of expansion Modulus of elasticity Elastic limit after cold rolling Electrical conductivity Ductility Metallurgical abilityDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

BIMETALLIC TYPESDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

Applications of Bimetallic strips & Thermometers1.The bimetallic strip is used in control devices.2. The spiral strip is used in air conditioning thermostats.3. The helix strip is used for process application such asrefineries, oil burners, tyre vulcanizes etc.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

Advantages of Bimetallic Thermometer‘s1.They are simple, robust and inexpensive.2.Their accuracy is between 2% to 5% of the scale.3.They can with stand 50% over range in temperatures.4.They can be used where ever a mercury-in-glassthermometer is used.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

2. FLUID EXPANSION THERMOMETERDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

3. PRESSURE THERMOMETERSDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

Advantages These devices cost less. Speed of response and sensitivity are high. These devices are rugged. Direct driving of recording and controlling devices ispossible due to the large output.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

Limitations If long transmitting capillary tubes are used compensationbecomes a must. The filling fluid and the tube are temperature sensitivewhich may introduce errors. Auxiliary power source is required if it is combined withpneumatic or electric transmission systems. The filling fluid might get decomposed introducingcalibration drift.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

4. RESISTANCE THERMOMETER(Resistance Temperature Detectors- RTDS)Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

5. THERMISTORS (THERMAL RESISTORS)Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

Applications1.They are used for measuring varying temperatures.2.Used for temperature compensation in electronic equipment.3.They are used in nine delay circuits.4.They are used to measure thermal conductivity.5.They are used to measure pressure and Flow of liquids.6. Used in precision temperature measurementDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

Advantages1. The cost of the thermistors is low.2. Accuracy is high (measurement up to 0.0I C is attainable).3. For 1 C change in temperature, the resistance changes as far as 6%in certain cases. Can measure high temperatures of the order of800 C to 1100 C.4. Ability to with stand mechanical and electrical stresses.5. Thermistors can be manufactured to very small sizes as theresistivity of them are very high.6. Simple electric circuits can be used to measure change inresistance.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

Limitations1.Thermistors have a non-linear scale-over its range ofoperation.2.The resistance of the thermistor increases when timelapses. This is called as aging effect.3.When current passes through the thermistor, if getsheated. This is called self heating effect.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

6. THERMOCOUPLESMetal AMetal BDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

LAWS OF THERMOCOUPLESThere are three laws of thermocouples namely:1.Law of thermoelectricity or successive or intermediatetemperatures.2.Law of intermediate metals.3.Law of homogenous circuit.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

(I) LAW OF INTERMEDIATE TEMPERATUREDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

(II) LAW OF INTERMEDIATE METALS(III) LAW OF HOMOGENOUS CIRCUIT The application of heat to a single homogenous metal is in itself notcapable of producing or sustaining an electric current.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

7. THERMOPILE(THERMOCOUPLE CONNECTED IN SERIES) A thermopile is an electronic device thatconverts thermal energy into electricalenergy. It is composed of several thermocouplesconnected usually in series or, lesscommonly, in parallel.Dr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

THERMOCOUPLE CONNECTED IN PARALLELDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

8. PYROMETERS Pyrometry is a technique for measuringtemperature without physical contact. It depends upon the relationshipbetween the temperature of hot bodyand eletronicmagnetic radiation emittedby the body.The pyrometers are three types,1. Total radiation pyrometers2. Infra red pyrometers3. Optical radiation pyrometersDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC

a. TOTAL RADIATION PYROMETERSDr. J.Jeevamalar, M.E., Ph.D. / EGSPEC