Page 128 - Mechanical Engineers' Handbook (Volume 2)

1 Standards and Accuracy 117

between two events. The basic unit for time measurements is the second. At one time the
second was defined as 1/86,400 of the average period of rotation of the earth on its axis,
but that is not a practical standard. The period varies and the earth is slowing up. Conse-
quently a new standard based on the oscillations associated with a certain transition within
the cesium atom was defined and adopted. That standard, the cesium clock, has now been
superceded by the cesium fountain atomic clock as the primary time and frequency standard
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of the United States. Although this cesium ‘‘clock’’ is the basic frequency standard, it is
not generally usable by mechanical engineers. Secondary standards such as tuning forks,
crystals, electronic oscillators, and so on are used, but from time to time access to time
standards of a higher order of accuracy may be required. To help meet these requirements,
NIST broadcasts 24 hours per day, 7 days per week time and frequency information from
radio stations WWV, WWVB, and WWVL located in Fort Collins, Colorado, and WWVH
located in Hawaii. Other nations also broadcast timing signals. For details on the time signal
broadcasts, potential users should consult NIST. 4
Temperature is one of four fundamental quantities in the international measuring system.
Temperature is fundamentally different in nature from length, time, and mass. It is an inten-
sive quantity, whereas the others are extensive. Join together two bodies that have the same
temperature and you will have a larger body at that same temperature. If you join two bodies
which have a certain mass, you will have one body of twice the mass of the original body.
Two bodies are said to be at the same temperature if they are in thermal equilibrium. The
international practical temperature scale, adopted in 1990 (ITS-90) by the International Com-
mittee on Weights and Measurement is the one now in effect and the one with which en-
gineers are primarily concerned. In this system the kelvin (K) is the basic unit of temperature.
It is 1/273.16 of the temperature at the triple point of water, the temperature at which the
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solid, liquid, and vapor phases of water exist in equilibrium. Degrees Celsius ( C) is related
to degrees kelvin by the equation
t T 273.15

where t degrees Celsius
T degrees kelvin

1.2 Accuracy and Precision
In measurement practice four terms are frequently used to describe an instrument. They are
accuracy, precision, sensitivity, and linearity. Accuracy, as applied to an instrument, is the
closeness with which a reading approaches the true value. Since there is some error in every
reading, the ‘‘true value’’ is never known. In the discussion of error analysis which follows,
methods of estimating the ‘‘closeness’’ with which the determination of a measured value
approaches the true value will be presented. Precision is the degree to which readings agree
among themselves. If the same value is measured many times and all the measurements
agree very closely, the instrument is said to have a high degree of precision. It may not,
however, be a very accurate instrument. Accurate calibration is necessary for accurate mea-
surement. Measuring instruments must, for accuracy, be from time to time compared to a
standard. These will usually be laboratory or company standards which are in turn compared
from time to time with a working standard at NIST. This chain can be thought of as the
pedigree of the instrument, and the calibration of the instrument is said to be traceable to
NIST.