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Mechanical - Metrology and Measurements - Concepts of Measurement

Generalized Measurement System

   Posted On :  23.09.2016 12:05 pm

A measuring system exists to provide information about the physical value of some variable being measured.

GENERALIZED MEASUREMENT SYSTEM

 

 

 

A measuring system exists to provide information about the physical value of some variable being measured. In simple cases, the system can consist of only a single unit that gives an output reading or signal according to the magnitude of the unknown variable applied to it.  However, in more complex measurement situations, a measuring system consists of several separate elements as shown in Figure1.1.



 

Units

 

Table 1.1 Physical Quantities and its unit

 


 

Standards

 

 

 

The term standard is used to denote universally accepted specifications for devices. Components or processes which ensure conformity and interchangeability throughout a particular industry. A standard provides a reference for assigning a numerical value to a measured quantity. Each basic measurable quantity has associated with it an ultimate standard. Working standards, those used in conjunction with the various measurement making instruments.

 

 

The national institute of standards and technology (NIST) formerly called National Bureau of Standards (NBS), it was established by an act of congress in 1901, and the need for such body had been noted by the founders of the constitution. In order to maintain accuracy, standards in a vast industrial complex must be traceable to a single source, which may be national standards.

 

 

The following is the generalization of echelons of standards in the national measurement system.

 

1. Calibration standards

 

2. Metrology standards

 

3. National standards

 

 

 

1.     Calibration standards: Working standards of industrial or governmental laboratories.

 

2.     Metrology standards: Reference standards of industrial or Governmental laboratories.

 

National standards: It includes prototype and natural phenomenon of SI (Systems International), the world wide system of weight and measures standards. Application of precise measurement has increased so much, that a single national laboratory to perform directly all the calibrations and standardization required by a large country with high technical development. It has led to the establishment of a considerable number of standardizing laboratories in industry and in various other areas. A standard provides a reference or datum for assigning a numerical value to a measured quantity.

 

 

Classification of Standards

 

To maintain accuracy and interchangeability it is necessary that Standards to be traceable to a single source, usually the National Standards of the country, which are further linked to International Standards. The accuracy of National Standards is transferred to working standards through a chain of intermediate standards in a manner given below.

 

•National                 Standards

 

•National Reference Standards •WorkingStandards

 

 

•Plant             Laboratory   Reference   Standards

 

•Plant             Laboratory   Working   Standards

 

•Shop         Floor   Standards

 

Evidently, there is degradation of accuracy in passing from the defining standards to the shop floor standards. The accuracy of particular standard depends on a combination of the number of times it has been compared with a standard in a higher echelon, the frequency of such comparisons, the care with which it was done, and the stability of the particular standards itself.

 

Accuracy of Measurements

 

The purpose of measurement is to determine the true dimensions of a part. But no measurement can be made absolutely accurate. There is always some error. The amount of error depends upon the following factors:

 

             The   accuracy   andinstrumentdesign   of   the   measur

             The   skill   of   the   operator

 

             Method   adopted   for   measurement

 

             Temperature   variations

 

             Elastic   deformation   of   the   part   or   in

 

Thus, the true dimension of the part cannot be determined but can only by approximate. The agreement of the measured value with the true value of the measured quantity is called accuracy. If the measurement of dimensions of a part approximates very closely to the true value of that dimension, it is said to be accurate. Thus the term accuracy denotes the closeness of the measured value with the true value. The difference between the measured value and the true value is the error of measurement. The lesser the error, more is the accuracy.

 

Precision

 

The terms precision and accuracy are used in connection with the performance of the instrument. Precision is the repeatability of the measuring process. It refers to the group of measurements for the same characteristics taken under identical conditions. It indicates to what extent the identically performed measurements agree with each other. If the instrument is not precise it will give different (widely varying) results for the same dimension when measured again and again. The set of observations will scatter about the

 

mean. The scatter of these measurements is designated as σ, the sta used as an index of precision. The less the scattering more precise is the instrument.

 

Thus,          lower,   the   value   of   σ,   the   more   prec

 

 

Accuracy

 

Accuracy is the degree to which the measured value of the quality characteristic agrees with the true value. The difference between the true value and the measured value is known as error of measurement. It is practically difficult to measure exactly the true value and therefore a set of observations is made whose mean value is taken as the true value of the quality measured.

 

 

Distinction between Precision and Accuracy

 


Accuracy is very often confused with precision though much different. The distinction between the precision and accuracy will become clear by the following example. Several measurements are made on a component by different types of instruments (A, B and C respectively) and the results are plotted. In any set of measurements, the individual measurements are scattered about the mean, and the precision signifies how well the various measurements performed by same instrument on the same quality characteristic agree with each other. The difference between the mean of set of readings on the same quality characteristic and the true value is called as error. Less the error more accurate is the instrument. Figure shows that the instrument A is precise since the results of number of measurements are close to the average value. However, there is a large difference (error) between the true value and the average value hence it is not accurate. The readings taken by the instruments are scattered much from the average value and hence it is not precise but accurate as there is a small difference between the average value and true value.

 

Factors affecting the accuracy of the Measuring System

 

The basic components of an accuracy evaluation are the five elements of a measuring system such as:

 

             Factors   affecting   the   calibration   sta

 

             Factors   affecting   the   work   piece.

 

             Factorsngthe inherentaffecticharacteristics of the instrument.

 

             Factors   affecting   the   person,   who   car

 

             Factors   affecting   the   environment.

 

1.     Factors affecting the Standard: It may be affected by:

 

-Coefficient of thermal expansion

 

-Calibration interval

 

-Stability with time

 

-Elastic properties

 

-Geometric compatibility

 

 

 

2.                 Factors affecting the Work piece: These are: -Cleanliness

 

-Surface finish, waviness, scratch, surface defects etc., -Hidden geometry

 

-Elastic properties,-adequate datum on the work piece -Arrangement of supporting work piece

 

-Thermal equalization etc.

 

 

 

 

 

3.                 Factors affecting the inherent characteristics of Instrument: -Adequate amplification for accuracy objective

 

-Scale error

 

-Effect of friction, backlash, hysteresis, zero drift error

 

-Deformation in handling or use, when heavy work pieces are measured -Calibration errors

 

-Mechanical parts (slides, guide ways or moving elements) -Repeatability and readability

 

-Contact geometry for both work piece and standard.

 

 

4.     Factors affecting person:

-Training, skill

 

-Sense of precision appreciation

 

-Ability to select measuring instruments and standards -Sensible appreciation of measuring cost

 

-Attitude towards personal accuracy achievements

 

-Planning measurement techniques for minimum cost, consistent with precision requirements etc.

 

 

5.  Factors affecting Environment:

 

-Temperature, humidity etc.

 

-Clean surrounding and minimum vibration enhance precision -Adequate illumination

 

-Temperature equalization between standard, work piece, andinstrument -Thermal expansion effects due to heat radiation from lights

 

-Heating elements, sunlight and people

 

-Manual handling may also introduce thermal expansion.

Higher accuracy can be achieved only if, ail the sources of error due to the above five elements in the measuring system are analyzed and steps taken to eliminate them. The above analysis of five basic metrology elements can be composed into the acronym SWIPE, for convenient reference where,

S –STANDARD            W –WORKPIECE I –INSTRUMENT

 

P –PERSON                  E –ENVIRONMENT


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