CHAPTER 2 LANGUAGE AND SYSTEMS OF

MEASUREMENT

DPT 312METROLOGY

OVERVIEW

“ a child reportedly asked President Lincoln how long his legs were. The President answered, ‘ long enough to reach from my body to ground’. “

A humorous response, but the clever use of language does not provide an accurate response to the child’s question.

Figure 2.1

How big?

How far apart?

Figure 2.2

What is the size?

B20

30

D 20

20

R2.5

20

A 20

C 20

30

19

A human hair is approximately 0.003 inch thick

A pieces of paper is approximately 0.003 inch thick

A grain of salt is approximately 0.004 inch in size

Do you know??

METROLOGY TERMS Dimensional measurement are used daily in designing

building and operating the objects that is surround us for communicating about past, present and future object

The principal dimensional measurement is length(length – the number of units of measurement that separates two points)

Secondary measurement are angle and curvature Linear measurement – the distance separating two points

that has direction (reference pt to measured pt) Measurement

* A dimension is an intended size, whereas length is the measured size

Unit of length

multiplier

cardinal

fractional

decimal

Exponential multipliers are not only more compact, they are much easier to use in

calculations. They are widely used in engineering and science.

Figure 2.3

The Act of Measurement

The act of measurement is a comparison of the standard of length or the distance to be reproduced and an unknown feature

Two method to compare unknown lengths to the standards

I. Interchange method (measurement by comparison)

II. Displacement method (measurement by translation or transfer)

Figure 2.4: All measurement consist of the comparison of the unknown with a known. The

method for comparison vary but fall into one of two groups: interchange or displacement

ACCURACY, PRECISION AND RELIABILITY

Accuracy, precision and reliability are used interchangeably

But in precision measurement for manufacturing, the use of the wrong term affects the choice of measuring instrument and the efforts of the quality control team

The incorrect use of one of these terms could mean failure to meet quality control standards, excess production, excess materials use and more

Figure 2.6 Which of these targets represents

Accurate shooting? Precise shooting?

Reliable shooting?

Figure 2.7 A change in one variable, such as wind, alters the results as shown. Does this show which

shooting was most reliable?

ACCURACY

Accuracy is a comparison : the desired result is compared with the actual measurement.

“ quality of conformity” Answer the question good enough? Indicates whether the object

complies with a standard, is too big or small, or is in or out of tolerance.

PRECISION Precision reports the dispersement of

results or degree of repeatability within the manufacturing and measurement systems.

“quality of refinement” Precision answer the question how much? It provides a fineness for the range of

sizes allowable for one part or many parts, or it defines the fineness to which an instrument can read

RELIABILITY

Reliability shows the relationship between the predicted results and actual result and whether or not we actually can predict what will happen

Reliability can refer to the manufacturing or inspection process or to the part’s expected future performance

Figure 2.8 Reducing the target size by one half shows that the accuracy requirement may dictate the

precision requirement

ORIGIN OF THE METRIC SYSTEM

1790, French Republican established the meter: the distance between the north pole and the equator passing through Paris, divided into ten million parts

1799, metal standard, the Meter of the Archives was made and adopted as official standard

1870, the first series of international conference to establish the metric system worldwide was held (48 delegates representing 25 countries)

1889, a general conference in Paris approved the work of the committee. ( the one new standard most nearly equal to the Meter of the Archives was selected as the International Prototype Meter

1960, the metric system was revised worldwide, and the system was renamed to distinguish it from the other metric system. (Le Systeme International d’Unites- SI)

Figure 2.9 These are the prefixes used in SI to show magnitude. A centimeter, for example, is one-hundredth of a

meter

THE INTERNATIONAL INCH

The Mendenhall Act defined the U.S inch as 25.4000508mm

But the British Imperial Standard Inch was 25.399978mm

1922 - the British revised the standard to 29.399956mm

1951 - Canada revised their in to exactly 25.4mm

1959 - all three governments (U.S, Britain, Canada) agreed that one “international” inch would equal 25.4mm.

FUNDAMENTAL CRITERIA

In order to evaluate possible measurement systems, we must understand as much as possible about each system and be unbiased as possible

Natural of the

system

e.g Length of a forearm The distance from pole to equator

Economic consideratio

n

In the past – machines, tools, and manuals were replaced only when the old equipment was obsoleteToday – manufacturers are competing in global economy- manufacturers must recognize international demands and adapt manufacturing process to the accepted standards of the global economy

Either / or reasoning

As long as metrologists have a thorough understanding of conversion methods and their implications to precision and quality assurance, there is no reason why we cannot accommodate the use of both systems of measurement

THE BEST SYSTEM

The best system of measurement depends on what is being measured

What use the measurement has, whether scientific, commercial or cultural

The audience must understand the results of measurement process.

Use measurement system that helps other people understand the goals that what we are trying to accomplish

PRACTICAL CRITERIA

Metrological factor Computational factor

Communicative factor

Which act of measurement will yield usable result

Which system yields figures that we can use mathematically

Communication factor- which system makes it

easiest for us to share the measurement with other

people

Figure 2.11 When the two system are critically analyzed, neither is all good nor all bad

Rounding off- Numerical Values

When we round off, we eliminate unnecessary figures in any computation

However, you must know both the correct method of rounding off and the number of significant figures needed in order to round off properly.

Consistency is one of the most component of reliable measurement (use same method of measurement)

*then, even we make errors, the errors have a chance to cancel each other out or to be caught more easily)

The reason for rounding off is to eliminate the meaningless digits

Figure 2.12 In this group of five values the rounded up column yielded high values, the rounded down, low values. The more items involved, the greater the total error

and average error.

CONCLUSION

To understand the language of manufacturers, you need to understand how big 0.01 inch and 0.001 inch are

To understand standards, reverence surfaces and surface finishes, you need to understand how big 0.000001 inch is

Quiz

1. What is principal dimensional measurement? length2. What is secondary dimensional measurement? Angle & curve3. Name two methods to compare unknown length to the

standard. Displacement & interchange4. When measurement are repeatedly taken on the same unit,

the extent to which an instrument replicates its measurements is precision

5. How many 0.001 inches in one inch? 10006. If 1mm= 0.0394 inches, 700 mm equal to how many inches?

27.581. The ability of two or more inspectors to obtain consistent

results repeatedly when measuring the same set of parts and the same measuring instrument called precision

2. Round off 25.7853901 to two significant digits 25.793. Le Systeme International d Unites is abbreviated for SI