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international standard of units. The system has been adopted worldwide. Other unit systems are
commonly used in the consumer market and so deserve mention. These other unit systems are not
standards and are treated as conversions from SI. Examples of these include the inch-pound (I-P)
unit system found in the United States and the gravitational mks (meter-kilogram-second or metric)
unit system common to much of the world.
Primary standards are necessary because the value assigned to a unit is actually arbitrary. For
example, over 4500 years ago the Egyptian cubit was used as a standard of length and based on the
length from outstretched fingertips to the elbow. It was later codified with a master of marble, a stick
about 52 cm in length, on which scratches were etched to define subunits of length. This standard
served well for centuries!
So whether today’s standard unit of length, the meter, is the length of a king’s forearm or the
distance light travels in a fraction of a second really only depends on how we want to define it. To
avoid confusion, units are defined by international agreement through the use of primary standards.
Once agreed upon, a primary standard forms the exact definition of the unit until it is changed by
some later agreement. Important features sought in any standard should include global availability,
continued reliability, and stability with minimal sensitivity to external environmental sources. Next
we examine some basic dimensions and the primary standards that form the definition of the units
that describe them (9).
Base Dimensions and Their Units
Mass
The dimension of mass is defined by the kilogram. Originally, the unit of the kilogram was defined
by the mass of one liter of water at room temperature. But today an equivalent yet more consistent
definition defines the kilogram exactly as the mass of a particular platinum-iridium cylindrical bar
that is maintained under very specific conditions at the International Bureau of Weights and
Measures located in Sevres, France. This particular bar (consisting of 90% platinum and 10%
iridium by mass) forms the primary standard for the kilogram. It remains today as the only basic unit
still defined in terms of a material object.
In the United States, the I-P unit system (also referred to as the U.S. customary units) remains
widely used. In the I-P system, mass is defined by the pound-mass, lb m , which is derived directly and
exactly from the definition of the kilogram:
1 lb m ¼ 0:4535924 kg
ð1:13Þ
Equivalent standards for the kilogram and other standards units are maintained by the U.S.
National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, and other
national labs around the globe. NIST claims that their mass standard is accurate to an uncertainty of
within 1 mg in 27,200 kg.
Time and Frequency
The dimension of time is defined by the unit of a second. One second (s) is defined as the time
elapsed during 9,192,631,770 periods of the radiation emitted between two excitation levels of the
fundamental state of cesium-133 (10). Despite this seemingly unusual definition, this primary
standard can be reliably reproduced at suitably equipped laboratories throughout the world to an
uncertainty of within two parts in 10 trillion.
24 Chapter 1 Basic Concepts of Measurement Methods
15:40:35 Page 24
international standard of units. The system has been adopted worldwide. Other unit systems are
commonly used in the consumer market and so deserve mention. These other unit systems are not
standards and are treated as conversions from SI. Examples of these include the inch-pound (I-P)
unit system found in the United States and the gravitational mks (meter-kilogram-second or metric)
unit system common to much of the world.
Primary standards are necessary because the value assigned to a unit is actually arbitrary. For
example, over 4500 years ago the Egyptian cubit was used as a standard of length and based on the
length from outstretched fingertips to the elbow. It was later codified with a master of marble, a stick
about 52 cm in length, on which scratches were etched to define subunits of length. This standard
served well for centuries!
So whether today’s standard unit of length, the meter, is the length of a king’s forearm or the
distance light travels in a fraction of a second really only depends on how we want to define it. To
avoid confusion, units are defined by international agreement through the use of primary standards.
Once agreed upon, a primary standard forms the exact definition of the unit until it is changed by
some later agreement. Important features sought in any standard should include global availability,
continued reliability, and stability with minimal sensitivity to external environmental sources. Next
we examine some basic dimensions and the primary standards that form the definition of the units
that describe them (9).
Base Dimensions and Their Units
Mass
The dimension of mass is defined by the kilogram. Originally, the unit of the kilogram was defined
by the mass of one liter of water at room temperature. But today an equivalent yet more consistent
definition defines the kilogram exactly as the mass of a particular platinum-iridium cylindrical bar
that is maintained under very specific conditions at the International Bureau of Weights and
Measures located in Sevres, France. This particular bar (consisting of 90% platinum and 10%
iridium by mass) forms the primary standard for the kilogram. It remains today as the only basic unit
still defined in terms of a material object.
In the United States, the I-P unit system (also referred to as the U.S. customary units) remains
widely used. In the I-P system, mass is defined by the pound-mass, lb m , which is derived directly and
exactly from the definition of the kilogram:
1 lb m ¼ 0:4535924 kg
ð1:13Þ
Equivalent standards for the kilogram and other standards units are maintained by the U.S.
National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, and other
national labs around the globe. NIST claims that their mass standard is accurate to an uncertainty of
within 1 mg in 27,200 kg.
Time and Frequency
The dimension of time is defined by the unit of a second. One second (s) is defined as the time
elapsed during 9,192,631,770 periods of the radiation emitted between two excitation levels of the
fundamental state of cesium-133 (10). Despite this seemingly unusual definition, this primary
standard can be reliably reproduced at suitably equipped laboratories throughout the world to an
uncertainty of within two parts in 10 trillion.
24 Chapter 1 Basic Concepts of Measurement Methods