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TRUE VALUE OR IDEAL SCALE
Figure 11

By comparing other objects with kilogram 20, either singly or in combination, we can assign values relative to our accessible scale. A sufficient number of well calibrated standards which can be intercompared, and which may occasionally be compared with our prototype standard, serve to maintain our scale with perhaps a greater precision

Figure 12

A practical measurement method is easy to visualize in the form of a broad outline of the elements of the method such as, the concept of the quantity to be measured, pertinent physical laws, various instruments, standards, the operators, procedures to be used, the environment in which the measurements are to be made, the computations which are to be made, and a means of establishing some parameters of performance. As we briefly review some of these elements, we will find that every mass measurement facility has many things in

common.

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A MEASUREMENT PROCESS

PRODUCES:

I. A USEFUL

MEASURED VALUE

2. AN ESTIMATE OF UNCERTAINTY FOR

THAT VALUE

Figure 19

A measurement process involves the actual physical operation of the specified equipment following the procedures as closely as possible. It is subject to the many variations that can and do occur during the operation. The end result is an estimated best value, which, in order to be useful, must be accompanied by the uncertainty with reference to known performance parameters.

Changes in any one or in a group of elements of the method constitutes, in effect, a different particular method and a different process which will in turn produce a different result and a different uncertainty. Small changes can make the difference between a useful value or a wasted effort.

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Figure 20

Because we must establish the mass of the object in question by measuring the mass difference between it and some known standard, the comparator is a vital element in the process. The inherent characteristic of the comparator is precision - not accuracy. The fundamental question is whether the indicated difference is really a mass difference, or an indication of some other variability. While we may be able to identify large sources of variability, in the limit, we cannot differentiate between instrument precision, variability from extraneous sources, or variability of the standard.

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Figure 26

The first plot indicates a severe rounding off, which may be from several causes. Such a response clearly lacks the appearance of randomness. The second plot at least appears to be random. The third plot, while perhaps appearing to be random, obviously lacks the precision of the second plot. The range of the differences as plotted gives us an idea of the smallest mass difference that can be detected with assurance, and is obviously related to the requirements our measurements must meet. Repeated independent measurements of the same mass difference are essential to the evaluation of the instrument.

Figure 28

One additional requirement, generally beyond the control of the operator, is that of linearity. An instrument. used as a comparator rather than a direct reading device, requires linearity only in the neighborhood of the actual load.

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