Chapter 1 Metrology Basics C 505 1 Understand

Chapter 1 Metrology Basics �C 505. 1 Understand basics of metrology, standards and calculate least count of measuring instruments.

Selection of instrument �Range of instrument �Scale spacing �Sensitivity �Scale division value �Accuracy of instrument �Repeatability �Effect of environmental condition �Errors generated during measurement �Cost of instrument �Life of instrument & need of calibration

Precautions & care while handling instrument �Avoid mishandling �Surfaces not be touched by hand �Wash hand properly apply petroleum jelly before handling instrument

Standards & Comparators

Line Standards �Line standard is the standard in which distance is measured between two lines. �CharacteristicsØ It is Quick and easy Ø Parallax error can be generated Ø They are not much accurate e. g. steel rule.

End Standard �End standard- When distance is measured between two surfaces �Characteristics Ø Accurate up to 0. 005 mm Ø They are costly, difficult to use. Ø The surfaces are to be protected. e. g. Slip gauges, end bars.

Wavelength Standard �Material standards are subjected to destroy day by day Concepts leads to generate wavelength standard �Simple light rays are used as source of wavelength standard. �Cadmium 11, krypton-86 and mercury 198 can be sources of light wavelengths �It was decided that krypton-86 in a hot cathode discharge lamp at 68 K, generates orange radiations and they can be used as ultimate wavelength standard. 1 meter=1650763. 73×wavelength Kr-86

Difference between line, end and wavelength standard

Slip Gauges Johnasson gauges Reactangular blocks of steel. Reference standards

Slip Gauge Accessories �Measuring Jaws �Scribing and center point �Holders and base

Wringing �Wringing –the property of measuring faces of a gauge block of adhering by sliding or pressing hr gauge against the measuring faces of other gauge block or the reference faces of datum surfaces, �It occurs due to molecular adhesion between a liquid film and mating surfaces

Wringing �First keep two slip on one another and slide them �Press the faces into contact , perpendicular to each other and give a small twisting motion.

Sets of slip gauges M 45

M 87

Comparators �Works on relative measurement �Definition-Is a device Ø Pick up small variation in dimension Ø Magnifies it Ø Displays it by indicating devices

Requirements of good comparator �Robust design and construction �Linear scale �High magnification �Versatility �Indicator should be constant in it return to zero �No effect of environmental conditions �Measuring pressure-low & constant �Sensitive As per requirement �Reliable �Easy to handle & compact �Cheaper and easily available in market

Use of comparators �In mass production �As laboratory standard �For inspection of newly purchased gauges �In selective assembly of parts �As a working gauge & inspection

Classification of comparators �Mechanical Comparator �Pneumatic comparator �Optical comparator �Electrical Comparator �Electronics Comparator �Combined comparator

Dial Gauge Indicator Working principle. Conversion of linear movement into angular movement using rack and pinion. Gear train for magnification and scale pointer as indicating device

Construction and working of dial gauge indicator R=Rack, A=Pinion S= spring A, B, C, D, E, F=Gears P=plunger G=Guide way Po= pointer Sc=Scale H=Support with hair coil spring

Construction and working �When plunger P moves up with rack R, gear A rotates as shown. Linear movement is converted into angular �As A rotates, B also rotates which is in mesh with C, C is on the same shaft as that of D. �When D rotates pinion E rotates To the center of E Po is attached. �Pointer Po shows reading on the scale.

Sigma Comparator �Principle�Two rectangular blocks out of which one is fixed and other movable are assembled as shown �The force is applied from the top side it will give angular deflection of θ

�Plunger is mounted on a slit washer to move up and down for frictionless movements �A knief edge pivots on the groove, whose lower edge is on the moving block �Y-arm is attached to the movable block as shown Y arm is wounded on the small drum of radius R with the phoshorous bronze taps.

Working of sigma comparator �When plunger moves in downward direction knief edge also moves in the same �Knief edge will give force on movable block this will tend to deflect it by a small angle θ �Y arm causes change in position by rotating drum of small radius ‘r’ The pointer is attached to the centre of drum which will rotate and shows some reading on the scale �The magnification of a comparator depends upon L, l, x and r �Magnification=L*l/x*r

Pneumatic comparator Working principle. Back pressure gauge �Air flows with a constant pressure through two orifices o 1 and o 2 �P 1 is upstream pressure of the first orifice and P 2 is the pressure between two orifice. �P 1 +P 2 when O 2 is blocked. Also it tends to zero as O 2 is blocked �Also it tends to zero as O 2 is increased upto ∞ �P 2 depends upon L

Pneumatic comparator

Construction & working of Pneumatic comparator � Constuction � Uses high pressure air from compressor � High pressure air pipe is connected to gauging head � High pressure air is passing through air filter � Manometer is used to measure backpressure generated in the system � Orifice is used in pipeline between dip tube and manometer for avoiding backpressure to be discharged in compressor line.

Working �Water is filled in tank and dip tube is inserted upto level H �High pressure and excess air may bubble out in water tank �Air flows through control orifice to gauging head �Because of restrictions given by gauging head back pressure is exerted on the air

�Back pressure generated is shown by manometer i. e. by height difference ∂h �The manometer height can be calibrated by using standard gauging heads and jobs

Electrical Comparator �Principle�It works on mutual inductance �When core is inserted in a magnetic fields of primary windings, the voltage induced in secondary is proportional to the core position.

LVDT

Construction And working �Electrical principle and mechanical movements of plunger which is joined to core �Transformer consisting of three coils wounded on insulated bobin Working Itf the core is centralized in the middle position of two secondary windings, the voltage induced in secondary winding gives net output 0

Relative advantages and disadvantages Comparator Advantages Mechanical Electrical Pneumatic Disadvantages 1. Cheaper 2. No need of electricity 3. Linear scale 1. More moving parts 2. More friction 3. Less accuracy 4. Robustn and compact 4. Sensitive to vibration 5. Easy to handle 5. Range is limited 6. portable 6. Parallax error is possible 1. Small moving parts 1. Needs Electric supply 2. Very high magnification 3. High range 4. Not sensitive to vibration 2. Variation in supply affects accuracy 3. Heating coil may cause errors 4. expensive 1. 2. 3. 4. 5. 1. 2. 3. 4. No contact with job Small moving parts More accuracy High magnification Instrument can be used placed at remote from job Auxiliary equipment needed Scale not uniform Not portable Different gauging head required