PDT 1103 METROLOGY CHAPTER 2 Language and System

PDT 110/3 METROLOGY CHAPTER 2 Language and System of Measurement

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.

OVERVIEW v As often happens today, language obscures the meaning of the questions we ask and the responses we receive. v For example, if an automobile engine is bored for larger pistons and tested for acceleration time, the 0 to 96 kph (60 mph) time might be cut by 2 seconds. Then, in production, when someone asks how "accurately" the new pistons must fit for the same level of performance, how should we answer? v One person says, "For a good fit. " Another says, "The maximum clearance cannot exceed 0. 050 mm (0. 002 in. ) or be less than 0. 025 mm (0. 001 in. ). " Still another exclaims "Right on!" v One of these answers probably contain the information we need, but the right answer depends on the meaning of the word accuracy in the question.

Figure 2. 1 How big? How far apart?

Figure 2. 2 What is the size? A rectangle 20 x 20 mm A A rectangle 20 x 30 mm 20 20 20 B 30 R 2. 5 C 20 D 19 30 20 20

Do you know? ? 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

METROLOGY TERMS Dimensional Measurement Principal e. g: i) length Secondary e. g: i) angle ii) curvature • Dimensional measurement are used daily in designing building and operating the objects that is surround us for communicating about past, present and future object • Using this measurements, we can describe surface finish, flatness and angular relationship among features

METROLOGY TERMS (cont’) • Linear measurement express the distance separating two points ( from reference point to measured point) Measurement Unit of length Multiplier i) Cardinal ii) Fractional iii) Decimal • Unit of length is easy to understand as long as everyone agrees on a standard for the unit of length. • Multiplier can be cardinal, decimal or exponential numbers. See figure 2. 3.

Figure 2. 3 Exponential multipliers are not only more compact, they are much easier to use in calculations. They are widely used in engineering and science.

When to Inspect/Measure? • Traditionally, measurements have been made after the part has been produced, an approach known as post-process inspection or post-process measurement. • Today, measurement are being made while the part is produced on the machine, an approach known as in-process, online or real-time measurement.

THE ACT OF MEASUREMENT • • The act of measurement is a comparison of the standard of length or the distance to be reproduced an unknown feature. Two method to compare unknown lengths to the standards I. Interchange method II. Displacement method See figure 2. 4

Interchange method • Measurement by comparison. • Compares both ends of the unknown feature to both ends of the standard at the same time. • Instruments called comparators has been created based on the interchange method of measurement.

Displacement Method • Measurement by translation or transfer. • Involves the separate examination of each end of the feature. • As we go from one point to the other, we displace something. • The relationship of the distance displaced to the standard constitutes the measurement.

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

Example 1 • In-process measurement: Caliper types • Interchange Method

Example 2 • In-process measurement: Friction roller types • Interchange method

Example 3 • In-process measurement: Go and no go gauge • Displacement method

Definition: ACCURACY, PRECISION AND RELIABILITY • Accuracy is the degree of closeness between a measured quantity value and true quantity value of a measurand. • The concept of accuracy is not a quantity and is not given a numerical quantity value. A measurement is said to be more accurate when it offers a smaller measurement error. • Accuracy is sometimes understood as closeness of agreement between measured quantity values that are being attributed to the measurand.

Definition: ACCURACY, PRECISION AND RELIABILITY…cont. • Precision, also called reproducibility or repeatability, the degree to which further measurements or calculations show the same or similar results. • Precision is usually expressed numerically by measures of imprecision, such as standard deviation, variance, or coefficient of variation under the specified conditions of measurement. • Reliability is quality of a measurement indicating the degree to which the measure is consistent, that is, repeated measurements would give the same result. Ref: International vocabulary of metrology

Target analogy

Figure 2. 6 Which of these targets represents Accurate shooting? Precise shooting? Reliable shooting?

Explanation from Figure 2. 6 • Compare shooter A and shooter B. – Shooter B more precisely but less accurately than A. • Compare shooter A, B and C. – C’s shot more accurate than A and B. • Compare shooter C, D, and E. – C is not precise as D. – E’s the most precise and most accurate than others.

Figure 2. 7 A change in one variable, such as wind, alters the results as shown. Does this show which shooting was most reliable? The addition of crosswind causes scores to decrease in all cases except E’s, because the reliability.

Precision • General Meaning – Exactness, Degree of exactitude • Measures – Fineness of readings • Specific Meaning – The lower the standard deviation of measurement, the higher the precision

Accuracy • General Meaning – Desirability • Measures – Ratio of correct to incorrect reading • Specific Meaning – The number of measurements within a specified standard as compared with those outside

Reliability • General Meaning – Probability of achieving desired result • Measures – Reliability of correct readings • Specific Meaning – The probability of performing without failure a specific function under given conditions for specified period of time

MEASUREMENT SYSTEMS • A system of measurement is a set of units which can be used to specify anything which can be measured. Measurement System Metric system English system

Metric System • The new name of metric system is ‘Le Système International d’Unitès’ (International system of unit), abbreviated as SI. • A standard system of measurements based on the meter, second, kilogram, and Celsius degrees. . • Figure 2. 8 shows the terms, symbols and abbreviation used in SI.

Figure 2. 8 These are the prefixes used in SI to show magnitude. A centimeter, for example, is one-hundredth of a meter

English System • Standard measurements of English system based on the inch, second, pound, and Fahrenheit degrees.

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 we are trying to accomplish?

PRACTICAL CRITERIA • Every step in the measurement process is potentially a source of error. • To achieve the most precise and reliable measurement system that requires the fewest steps, from instrument selection to the final computation made with your results.

PRACTICAL CRITERIA (cont’) • To determine the best system of measurement, we use three factors. Computational factor Metrological factor Which act of measurement will yield usable result Communicative factor Communication factorwhich system makes it easiest for us to share the measurement with other people Which system yields figures that we can use mathematically

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) – Advantage: 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.

General rules for rounding off 1. When the digit to be dropped less than 5, there is no change in the preceding figures. Ex. The number 56. 748 rounded off to the nearest 0. 1 becomes 56. 7. 2. When the digit to be dropped is greater than 5, the preceding digit is increased by 1. Ex. The number 2. 146 rounded off to the nearest 0. 01 becomes 2. 15. 3. When the digit to be dropped is exactly 5, round off to the nearest even number. • American standard: raise the remaining last digit if it is odd and leave it same if it is even. Ex. The number 21. 45 rounded off to the nearest 0. 1 becomes 21. 4.