This document discusses factors to consider when selecting measuring instruments, including sensitivity, hysteresis, range, span, response time, repeatability, accuracy, precision, magnification, stability, resolution, error, drift, reliability and more. It describes types of errors such as static errors, dynamic errors, systematic errors and random errors. Methods to reduce errors from the environment, supports, alignment, dirt, vibrations, wear and other sources are provided. The history of measurement standards from ancient Egypt is briefly mentioned.

1. UNIT I
BASICS OF METROLOGY

2. Factors to be considered in Selection of
Instruments:Sensitivity: It is the ratio of the magnitude of output signal to the magnitude of input signal. It denotes
the smallest change in the measured variable to which the instrument responds.
Sensitivity=(Infinitesimal change of output signal)/(Infinitesimal change of input signal)
If the input-output relation is linear, the sensitivity will be constant for all values of input.
If the instrument is having non-linear static characteristics, the sensitivity of the instrument depends on the value of
the input quantity.
Sensitivity has no unit. It should be as high as possible. To achieve this, the range of an instrument should
not greatly exceed the value to be measured.

3. Hysteresis: All the energy put into the stressed component when loaded is not recovered upon
unloading. Hence, the output of a measurement system will partly depend on its previous input signals and this
is called as hysteresis.
Maximum difference for the same measured quantity between the upscale and downscale readings
during a full traverse in each direction.
Range: It is the minimum and maximum values of a quantity for which an instrument is designed to
measure/ The region between which the instrument is to operate is called range.
Range = Lower Calibration Value – Higher Calibration Value = LC TO HC
Ex: We can say that the range of the instrument(thermometer) is 0̊ C to 100̊ C.
Span: It is the algebraic difference between higher calibration value and lower calibration value.
Span = Hc - Lc
Ex: If the range of an instrument is 100̊ C to 150̊ C, its span is 150̊ C – 100̊ C = 50̊ C

4. Response Time: It is the time which elapses after a sudden change in the measured quantity, until
the instrument gives an indication differing from the true value by an amount less than a given permissible
error.
Speed of response of a measuring instrument is defined as the quickness with which an instrument
responds to a change in input signal.
First order Response of a System
The curve shows the change of indication of an instrument due to sudden change of measured quantity can
take different forms according to the relation between capacitances that have to be filled, inertia elements and
damping elements. When inertia elements are small enough to be negligible, we get first order response
which is due to filling the capacitances in the system through finite channels. The curve of change of
indication with time in that case is an exponential curve.

5. • If the inertia forces are not negligible, we get second order response. there are three
possibilities of response according to the ratio of damping and inertia forces as
follows:
• Overdamped system— where the final indication is approached exponentially from one side.
• Under-damped system—where the pointer approaches the position corresponding to final reading, passes it
and makes a number of oscillations around it before it stops.
• Critically damped system—where the pointer motion is aperiodic but quicker than in the case of
overdamped system.
• In all these cases the response time is determined by the inter-section of one (or two) lines surrounding the
final indication line at a distance equal to the permissible value of dynamic error with the response curve of
the instrument.

6. Repeatability: It is the ability of the measuring instrument to give the same value every time the
measurement of a given quantity is repeated.
It is the closeness between successive measurements of the same quantity with the same instrument by
the same operator over a short span of time, with same value of input under same operating conditions.
Accuracy: The degree of closeness of a measurement compared to the expected value is known as
accuracy.
Precision: A measure of consistency or repeatability of measurement. i.e. successive reading does not
differ. The ability of an instrument to reproduce its readings again and again in the same manner for a
constant input signal.
Magnification: Human limitations or incapability to read instruments places limit on sensitiveness
of instruments. Magnification of the signal from measuring instrument can make it better readable.
Stability: The ability of a measuring instrument to retain its calibration over a long period of time is
called stability. It determines an instruments consistency over time.
Backlash: Maximum distance through which one part of an instrument may be moved without
disturbing the other part.

7. Resolution: Minimum value of input signal required to cause an appreciable change or an
increment in the output is called resolution/ Minimum value that can be measured when the instrument is
gradually increased from non-zero value.
Error: The deviation of the true value from the desired value is called error.
Drift: The variation of change in output for a given input over a period of time is known as drift.
Threshold: Minimum value of input below which no output can be appeared is known as
threshold.
Reliability: Reliability may be explicitly defined as the probability that a system will perform
satisfactory for at least a given period of time when used under stated conditions. The reliability function is
thus same probability expressed as a function of the time period.

8. Difference between accuracy and precision:
S.No. Precision: Accuracy:
1. It is nothing but the repeatability of
the process.
Accuracy is the degree to which the
measured value agrees with the true value
of the measured quantity.
2. Precision is the fineness of the
instrument of the dispersion of the
repeated readings.
Accuracy is the relative between the
observed value and true values.
3. Precision never designates
accuracy.
Accuracy may designate precision.
4. Standard Deviation is the index of
precision. For less value of σ, more
precise is the instrument.
The difference between the measured value
and the true value is the error of the
measurement. If the error is less, then the
accuracy is more.

10. Elements of Metrology:
• Accuracy and cost: Basic objective of metrology should be to provide the accuracy required at the most
economical cost.
• Elements of Accuracy:
Accuracy of measuring system includes elements such as:
1) Calibration Standards
2) Workpiece Standards
3) Measuring Instruments
4) Person or Inspector carrying out the measurement.
5) Environmental influences
The above arrangement and analysis of the five basic metrology elements can be composed into acronym SWIPE:
S=Standard, W=Workpiece, I=Instrument, P=Person and E=Environment.
- Higher accuracy can be achieved only if all the sources of error due to above five elements be analyzed and steps
taken to eliminate them.

11. • Standard: May be affected by ambient influences(thermal expansion), stability with time, elastic
properties, geometric compatibility and position of use.
• Workpiece: Itself may be affected by ambient influences, cleanliness, surface condition, elastic
properties, geometric truth, arrangement of supporting it, etc.,
• Instrument: May be affected by hysteresis, backlash, friction, zero drift error, deformation in handling
or use of heavy workpieces, inadequate amplification, errors in amplification device, calibration errors,
standard errors, correctness of geometrical relationship of workpiece and standard, proper functioning of
contact pressure control, mechanical parts (slides, ways, or moving elements) working efficiently, and
repeatability adequacy etc.
• Personal Errors: Improper training in use and handling, skill, sense of precision and accuracy
appreciation, proper selection of instrument, attitude towards and realisation of personal accuracy
achievements, etc.
• Environment: Affected by temperature ; thermal expansion effects due to heat radiation from light,
heating of components by sunlight and people, temperature equalisation of work, instrument and standard
; surroundings; vibrations ; lighting; pressure gradients (affect optical measuring systems) etc.

12. • The design of measuring systems involves proper analysis of cost-to-accuracy consideration and
the general characteristics of cost and accuracy appears to be as shown.
• It will be clear from the graph that the cost rises exponentially with accuracy. If the measured
quantity relates to a tolerance (i.e. the permissible variation in the measured quantity), the
accuracy objective should be 10% or slightly less of the tolerance.
• In a few cases, because of technological limitations, the accuracy may be 20% of the tolerance ;
because demanding too high accuracy may tend to make the measurement unreliable. In practice,
the desired ratio of accuracy to tolerance is decided by considering the factors such as the cost of
measurement versus quality and the reliability criterion of the product.

13. Errors in Measurement:
• Error is the difference between the measured value (Vm) and the true value (Vt ) of a physical
quantity. Accuracy of a measurement system is measured in terms of error.
• Static Error Es = Vm – Vt
• Error may be positive or negative.
• If the instrument reads higher value than the true value, it is called as positive error.
• If the instrument reads lower value than the true value, it is called as negative error.
• Types of Errors:
• Based on the measurement, Errors may be classified as:
1. Static Errors
2. Dynamic Errors

14. Static Error:
• It results from the physical nature of the various components of the measuring system. It results
from the environmental effect and other external influences on the properties of the apparatus.
• Types of Static Errors:
1. Reading Errors
2. Characteristic Errors
3. Environmental Errors
4. Loading Errors
Reading Error:
It is due to factors such as parallax error, interpolation, optical resolution(readability or output
resolution). When there is error due to parallax, this can be eliminated by use of mirror behind the
readout pointer or indicator. When there is error due to interpolation, it can be eliminated by
increasing the optical resolution by using a magnifier over the scale in the vicinity of the pointer.

15. Characteristic Error:
It is defined as the deviation of the output of the measuring system under constant environmental conditions
from the theoretically predicted performance. If the theoretical output is a straight line, then linearity error,
hysteresis error, repeatability error and resolution errors are part of characteristic errors.
Environmental Errors:
It results from the effect of surrounding temperature, pressure and humidity on measuring system. It also results
from the external influences like magnetic or electric fields, nuclear radiation, vibration or shock, periodic or
random motion, etc., It can be reduced by controlling the atmosphere according to the essential requirements.
Loading Errors:
It results from the change in the measurand itself when it is being measured. It is the difference between the
value of the measurand before and after the measurement system is measured. It is unavoidable and hence the
measuring system should be selected such that its sensing element will minimize the instrument loading error.

16. Dynamic Error:
• It is caused by time variations in the measurand and results from the inability of a measuring system to
respond faithfully to a time-varying measurand. They are generally due to the following factors: i)
Inertia ii) Damping iii) Friction & iv) Other physical constraints in the sensing or readout or display
system.
• Types of Dynamic Errors:
1. Systematic or controllable errors
2. Random errors
Systematic Errors: It is due to experimental mistakes. These are controllable in both their magnitude
and sense. These can be determined and reduced if attempts are made to analyze them.
Types of Systematic Errors:
1. Calibration Errors
2. Ambient Conditions
3. Stylus Pressure
4. Avoidable Errors
5. Experimental Errors

17. • Calibration Errors: Any instrument has to be calibrated before it is put to use. If the instrument is not
calibrated properly, it will show reading with a higher degree of error. They are fixed errors, because they
have been introduced due to improper calibration.
• Ambient Conditions: Variation in the ambient conditions from internationally agreed standard value can
give rise to errors in the measured size of the component. The standard values are barometric pressure
760mm of mercury, and 10 mm of mercury vapour pressure at 20̊ C.
• Stylus Pressure: If any component is measured under a definite stylus pressure both the deformation of the
workpiece surface and deflection of the workpiece shape will occur.
• Avoidable Errors: These include errors due to parallax and the effect of misalignment of the workpiece
center. To measure air temperature by placing a thermometer in sunlight is also a avoidable error.
• Experimental Error: It results when there is variation form the assumed theoretical value.
Random Errors: These type of errors occurs randomly and the specific cases of such errors cannot be
determined, but likely sources of this type of errors are small variations in the position of setting standard and
workpiece, slight displacement of lever joints in measuring instrument, transient fluctuation in friction in the
measuring instrument and operator errors in reading scale and pointer type displays or in reading engraved
scale position.

18. • Other Types of Errors:
• Illegitimate Errors: These errors are due to blunders on the part of the person using the instrument. It may
be due to faulty instrument, faulty adjustment, improper use of instrument and so on.
Types of Illegitimate Errors:
1. Blunder or Mistakes 2. Computational Errors 3. Chaotic Errors
Blunder or Mistakes: Sometimes human beings operating the instrument might outrightedly commit a blunder
in using the instrument or adopting the right procedure.
Computational Errors: The human being performing the calculation might commit a mistake which leads to
this error.
Chaotic Errors: -These errors are due to disturbances such as vibrations, noises, shocks, etc., of sufficient
magnitude tend to affect the test information. It is called chaotic error.
-Sometimes the instrument cannot measure the physical quantity properly and more over
there will be an information lose during signal transmission. It is called transmission error.

19. How to overcome errors:
• Effect of Environment:
Temperature has great influence on accuracy of precision measurements.
 It is essential that gauge blocks and workpieces are handled with insulated forceps and tweezers,
with plastic pads and gloves.
Usually a plastic shield is introduced between the inspector and the machine.
In some interferometers, the machine is entirely enclosed in a transparent plastic box and the
operator manipulates the part with long handles, insulated forceps, etc.,
Prior to measurement parts, gauges, and masters are all stored on heat sink till they reach the
controlled room temperature.
Laboratory should be air conditioned but there should be no direct air currents.
All instruments should be calibrated at the internationally accepted temperature of 20̊ C.

20. • Effect of Support:
In long measuring bars, straight edges, these have been supported as a beam.
Amount of their deflection due to supporting depends on the position of their supports.
From the theory of bending, for a bar of length L, supported equidistant from the center on the
supports by distance l apart, then for no slope at the ends l/L = 0.577 and for minimum deflection
of the beam l/L = 0.554.
First condition is required for supporting standard bars.
Second condition is required in case of straight edges.
• Effect of Alignment:
Abbe’s principle should be followed in measurements to avoid cosine and sine errors.
The axis or line of measurement of the measured part should coincide with the measuring scale or
the axis of measurement of the measuring instrument.
Length measured is in excess by an amount l(1-cosθ)

21. • Errors due to non-alignment of plunger axis and line of measurement:
 Alignment error of 2̊ over 1m introduces an error of mm.
 To ensure correct displacement readings on dial indicator, plunger must be normal to the surface.
• Error due to bent jaws of a Vernier caliper:
 The length l1 between the extremes of the jaws has now become smaller.
• Dirt:
 Can change reading by a fraction of micron.
 Workpieces and masters should be cleaned by chamois or a soft artist’s brush.
 Coated surfaces should be sprayed with a suitable clean solvent.
 Gauges should never be touched with moist fingers.
• Errors due to vibrations:
 Labs can be located away from vibration sources.
 Place or mount gauges on rubber pads.
 Resting a gauge on surface plate also reduces vibration.

22. • Metallurgical Effects:
Materials for gauges should have been properly and naturally seasoned after heat treatment so it
attains stable dimensions.
Amount of surface roughness of gauge should be determined.
Measurements may be disturbed by loosening up of tiny flakes of chrome on stainless steel.
• Errors due to Deflection:
To avoid deflection errors, contact gauging pressure should be as small as possible.
Overhangs should be minimized.
Gauge frame should be made of rigid and adequate cross-section.
Gauge clamps or adjusting devices should be securely tightened.

23. • Errors due to wear in gauges:
Extent of wear hollows in anvils and lack of parallelism can be measured with optical flats.
Spherical contact can be checked by examination with a microscope.
Wear can be minimized by keeping gauges, masters and workpieces clean and away from dirt.
Chrome plated parts have been found to withstand ten times more wear than unplated parts.
• Parallax Error:
Essential to observe the pointer along a line normal to the scale.
• Error due to poor contact:
Gauge with wide areas of contact should not be used on parts with irregular or curved surfaces.

24. History of Standards:
• Ancient Egypt- 3000 years BC, death penalty was inflicted on all those who forgot or neglected their duty to
calibrate the standard unit of length at each full moon night (In order to build temples and pyramids of the
Pharaohs.)
• Cubit- An ancient measurement of length from the elbow to the end of the fingers.
• The first royal cubit was defined as the length of the forearm(from the elbow to the tip of
the extended middle finger) of the ruling Pharaoh, plus the breadth of his hand
(various lengths ranging from 450mm to 670mm)
• The original measurement was transferred to and carved in black granite.
• The workers at the building site were given copies in granite or wood and it was the
responsibility of the architects to maintain them.

25. • In 1528- French Physician J Fernel- Proposed the distance between Paris and Amiens as a general length of
reference.
• In 1661- British architect Sir Christopher Wren- Suggested reference unit should be the
length of the pendulum.
• In 1799 in Paris - the Decimal metric system was created by the deposition of two
platinum standards representing the meter and kilogram – The start of the present
International System of Units. These two standards of length were made of alloys and
Hence referred as material standards.
• Need for establishing standards of length – for determining the agricultural land areas
and for erection of buildings and monuments.

26. 16th Century Feet The distance over the left feet of
sixteen men lined up after they
left church on Sunday morning.
18th Century Yard
Metre
King Henry I, declared that the
yard was the distance from the
tip of the nose to the end of his
thumb, when his arm was
outstretched sideways. This
standard was legalized in 1853
and remained a legal standard
until 1960.
The first metric standard was
developed which was supposed to
be one-ten-millionth of a
quadrant of the earth’s meridian
passing through Paris.

27. 19th Century Upgradation of Metre Standard
Wavelength Standard
In 1872, an International
Commission was setup in Paris to
decide on a more suitable metric
standard and it was finally
established in 1875.
From 1893 onwards, comparison
of the above mentioned standard
with wavelength of light proved a
remarkable stable standard.

28. New Era of Material Standard:
• To avoid confusion in the use of standards of length, an important decision towards a definite
length standard, meter was established in 1790 in France.
• In 19th century, rapid advancement in engineering was due to improved materials available and
more accurate measuring instruments.

29. Types of Standards:
• After realizing the importance & advantage of metric system, most countries adopted meter as the
fundamental unit of linear measurement.
• In recent years, wavelength of monochromatic light, which never changes its characteristics in
any environmental condition is used as an invariable fundamental unit of measurement instead of
the previously developed standards such as meter and yard.
• Definition of Meter: A meter is defined as 1650763.73 wavelengths of the orange radiation in
vacuum of krypton-86 discharge lamp.
• Yard: Yard is defined as 0.9144 meter, which is equivalent to 1509458.35 wavelengths
of the same radiation.
• Three types of measurement standard are:
1. Line Standard
2. End Standard
3. Wavelength Standard

30. LINE STANDARD:
• According to the line standard, which is legally authorized and an act of Parliament, the yard or
meter is defined as the distance between inscribed lines in a bar of metal under certain conditions
of temperature and support. Ex: Measuring scales, Imperial standard yard, International prototype
meter, etc.
• A) The Imperial Standard Yard:
1. Standard served its purpose from 1855 to 1960.
2. It is made up of one inch square cross section Bronze Bar (82% copper, 13% tin, 5% zinc) and
is 38 inches long.
3. A round recess, 1 inch away from the two ends is cut at both ends up to the central or ‘neutral
plane’ of the bar.
4. Further, a small round recess of (1/10) inch in diameter is made below the center.
5. Two gold plugs of (1/10) inch diameter having engravings are inserted into these holes so that
the lines (engravings) are in neutral plane.

32. 6. Yard is defined as the distance between two central transverse lines on the plugs when the
temperature of the bar is constant at 62̊ F and the bar is supported on rollers in a specified manner to
prevent flexure, the distance being taken at the point midway between the two longitudinal lines at
62̊ F for occasional comparison.
7. The purpose of keeping the gold plugs in line with the neutral axis is to ensure that the neutral
axis remains unaffected due to bending, and to protect the gold plugs from accidental damage.
• B) International Standard prototype Meter:
1. Established meter as the Linear measuring Standard in the Year 1875 by the International
Bureau of Weights and Measures.
2. It is defined as the straight line distance, at 0̊ C, between the engraved lines of pure platinum-
iridium alloy (90% platinum & 10% iridium) of 1020 mm total length and having a ‘tresca’
cross section.
3. The graduations are on the upper surface of the web which coincides with the neutral axis of
the section.

34. 4. According to this standard, the length of 1 meter is defined as the straight line distance between
the center portions of the two lines engraved on the polished surface of a pure bar of Platinum-
Iridium alloy of a length of 1000mm between them and having a web cross-section.
5. When used, it is supported at two points by two rollers of at least one cm in diameter
symmetrically situated in the horizontal plane and 589mm apart.

35. END STANDARD:
• Need of an end standard arises as the use of line standards and their copies was difficult at various places in
workshops.
• End standards can be made to a high degree of accuracy by a simple method devised by AJC Brookes in
1920.
• End standards are used for all practical measurements in workshops and general use in precision
engineering in standard laboratories.
• They are in the form of end bars and slip gauges.
• In case of Vernier calipers and micrometers, the job is held between the jaws/anvils of the measuring
instrument and the corresponding reading is noted, while a length bar and slip gauges are used to set the
required length to be used as a reference dimension.

36. • END Bars:
1. Made of steel having cylindrical cross section of 22.2mm
diameter with faces lapped and hardened at the ends are available
in sets of various lengths.
2. End Bars are made from high-carbon chromium steel, ensuring
the faces are hardened to 64 RC (800 HV).
3. Both the ends are threaded, recessed and precision lapped to meet requirements of finish,
flatness, parallelism and gauge length.
4. Available up to 500mm in grades 0,1,2 in an 8-piece set.
5. Length bars can be combined by using an M6 Stud.
6. End bars are usually provided in sets of 9 to 12 pieces in step sizes of 25 mm up to length of 1 m.

37. • Slip Gauges:
1. Invented by Swedish Engineer CE Johnson.
2. Slip Gauges are rectangular blocks of hardened and stabilized high-grade cast steel or ceramic
compound Zirconium Oxide having heat expansion coefficients of 11.5 x 10^-6/K and 9.5 x
10^-6/K respectively and are available with a 9 mm wide, 30 to 35mm long cross section.
Slip Gauge Types:
1. IS 2984-1981, Metric BS-4311:1968, Imperial BS. 888.1950, DIN:861-1988, JIS B 7506-1978.
According to Accuracy:
Grade 00, Grade 0, Grade k, Grade 1, Grade 2.
AA- Master Slip Gauges
A-Reference Gauges
B-Working Gauges

38. 3. Measuring faces of slip gauges are forced and wrung against each other so that the gauges stick
together. This is known as wringing of slip gauges. (This effect is caused partly by molecular
attraction and partly by atmospheric pressure.)
4. To wring two slip gauges together, they are first cleaned and placed together at right angles.
5. Then they are rotated through 90̊ while being pressed together.
Procedure for Wringing of Slip Gauges

39. WAVELENGTH STANDARD:
1. Line and end standards are physical standards and are made up of materials that can change their
size with temperature and other environmental conditions.
2. The International accord 1893 and 1906 determinations of wavelength of the red line of Cadmium,
defined the angstrom which was used as a spectroscopic unit of length, but this was abandoned in
1960.
3. CGPM (Conference Generale des Poids et Mesures) adopted a definition of meter in terms of
wavelength in vacuum of the radiations corresponding to a transition between specified energy
levels of Krypton-86 atom.
4. In 1960, Orange radiation of the isotope krypton-86 used in a hot cathode discharge lamp
maintained at a temperature of 63 k was selected to define meter.
5. The meter was then defined as equal to 1650763.73 wavelengths of the red-orange radiation of
krypton-86 gas. 1 m = 1650763.73 wavelengths. Therefore, 1 yard = 0.9144 m = 0.9144 x
1650763.73 wavelengths = 1509458.3 wavelengths.

40. • In 1975, CGPM redefined the meter as the length of the path travelled by light in vacuum during
a specific fraction of a second.
• The meter is the length of the path travelled by light in vacuum during a time interval of
1
299792458
of a second.

41. Measurement Standards
• Standard is a physical representation of a unit of measurement. A Known accurate measure of physical
quantity is termed as standard.
• These standards are used to determine the values of physical quantities by Comparison method.
• Different standards have been developed for various units including Fundamental as well as derived units.
All these standards are preserved at The International Bureau Of Weight And Measures At Severs, Paris.
• These standards have been classified as follows:
1. International Standards
2. Primary Standards
3. Secondary Standards
4. Working Standards

42. • International Standards:
1. These standards are maintained by the International Bureau of Weights and Measures at Serves, Paris.
2. These standards represent the units of measurement of various physical quantities. It is to be noted
that the highest possible accuracy s maintained here.
3. For the purpose of day to day comparison and calibration, these standards are not available.
4. The main function of the International standards is the calibration and verification of the Primary
Standards.

43. • Primary Standards:
1. The main function of the Primary Standards is the calibration and verification of secondary standards.
2. These standards are maintained at the “National Laboratories or Standard Organisations” at various parts
of the world.
3. In India, “The National Physical Laboratory”, at Delhi maintains these standards.
4. The primary standards are not available for use outside the National Laboratory.
5. These primary standards are absolute standards of high accuracy that can be used as ultimate reference
standards to check, calibrate and certify the secondary standards.
6. The following factors are to be considered while setting primary standards:
a) Material used should highly resist change in dimension due to low/high temperatures.
b) Material characteristics should not get affected due to environmental changes.
c) Machining operations done on the material should yield accuracy.

44. • Secondary Standards:
1. Secondary Standards are basic reference standards used by the measurement and calibration laboratories
in Industries.
2. These standards are maintained by the particular Industry to which they belong. Each Industry has its
own secondary standards.
3. Each Industry periodically sends its secondary standard to the National Standards Laboratory for
calibration and comparison against the primary standard.
4. After comparison and calibration, the National Standards Laboratory returns the secondary standards to
the particular Industry Laboratory with a Certification of measuring accuracy in terms of Primary
Standards.

45. • Working Standards:
1. An accurate and reliable standard that is available with the manufacturer for use by the workers
who carries out the operation in the Industry is called as Working Standards.
2. These standards are used by the worker to check or test the manufactured products.
3. Working standards are to be checked and certified against the primary or the secondary
standards.
4. Ex: Precision Gauge Blocks, Slip Gauge, etc.,