Page 127 - Mechanical Engineers' Handbook (Volume 2)

Mechanical Engineers’ Handbook: Instrumentation, Systems, Controls, and MEMS, Volume 2, Third Edition.

Edited by Myer Kutz

Copyright  2006 by John Wiley & Sons, Inc.

CHAPTER 4
MEASUREMENTS
E. L. Hixson and E. A. Ripperger
University of Texas
Austin, Texas
1 STANDARDS AND ACCURACY 116 3.1 Internal Estimates 124
1.1 Standards 116 3.2 Use of Normal Distribution
1.2 Accuracy and Precision 117 to Calculate Probable Error
1.3 Sensitivity and Resolution 118 in X 125
1.4 Linearity 118 3.3 External Estimates 126
2 IMPEDANCE CONCEPTS 119 REFERENCES 129
3 ERROR ANALYSIS 123
1 STANDARDS AND ACCURACY

1.1 Standards
Measurement is the process by which a quantitative comparison is made between a standard
and a measurand. The measurand is the particular quantity of interest—the thing that is to
be quantified. The standard of comparison is of the same character as the measurand, and
so far as mechanical engineering is concerned the standards are defined by law and main-
tained by the National Institute of Standards and Technology (NIST, formerly known as the
National Bureau of Standards). The four independent standards which have been defined are
1
length, time, mass, and temperature. All other standards are derived from these four. Before
1960 the standard for length was the international prototype meter, kept at Sevres, France.
In 1960 the meter was redefined as 1,650,763.73 wavelengths of krypton light. Then in 1983,
at the Seventeenth General Conference on Weights and Measures, a new standard was
adopted: A meter is the distance traveled in a vacuum by light in 1/299,792,458 seconds. 2
However, there is a copy of the international prototype meter, known as the national pro-
totype meter, kept by NIST. Below that level there are several bars known as national ref-
erence standards and below that there are the working standards. Interlaboratory standards
in factories and laboratories are sent to NIST for comparison with the working standards.
These interlaboratory standards are the ones usually available to engineers.
Standards for the other three basic quantities have also been adopted by NIST, and
accurate measuring devices for those quantities should be calibrated against those standards.
The standard mass is a cylinder of platinum–iridium, the international kilogram, also
kept at Sevres, France. It is the only one of the basic standards that is still established by a
prototype. In the United States the basic unit of mass is the basic prototype kilogram No.
20. Working copies of this standard are used to determine the accuracy of interlaboratory
standards. Force is not one of the fundamental quantities, but in the United States the standard
unit of force is the pound, defined as the gravitational attraction for a certain platinum mass
at sea level and 45 latitude.
Absolute time, or the time when some event occurred in history, is not of much interest
to engineers. Engineers are more likely to need to measure time intervals, that is, the time

116