In the science of measurement, a standard is an object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity. Standards are the fundamental reference for a system of weights and measures, against which all other measuring devices are compared. Historical standards for length, volume, and mass were defined by many different authorities, which resulted in confusion and inaccuracy of measurements. Modern measurements are defined in relationship to internationally-standardized reference objects, which are used under carefully controlled laboratory conditions to define the units of length, mass, electrical potential, and other physical quantities.
Primary measurement standards may be used strictly in measurement laboratories. Less precisely controlled working standards are used for calibration of industrial measurement equipment. Primary standards that define units may be inconvenient for everyday use, so working standards represent the primary definition in a form that is easier to use. For example, the definition of the "metre" is based a laboratory experiment combining the speed of light and the duration of a second, but a machine shop will have working standard gauge blocks that are used for checking its measuring instruments.
Initially many units of measure were defined in terms of unique artifacts which were the legal basis of units of measure. A continuing trend in metrology is to eliminate as many as possible of the artifact standards and instead define practical units of measure in terms of fundamental physical constants, as demonstrated by standardized technique. One example is the unit of electrical potential, the volt. Formerly it was defined in terms of standard cell electrochemical batteries, which limited the stability and precision of the definition. Currently the volt is defined in terms of the output of a Josephson junction, which bears a direct relationship to fundamental physical constants. One advantage of elimination of artifact standards is that inter-comparison of artifacts is no longer required. Another advantage would be that the loss or damage of the artifact standards would not disrupt the system of measures.
Examples of primary reference standards
In contrast, the reference standard for the meter is no longer defined by a physical object. In 1983, the standard meter was redefined as the distance light travels in a vacuum during 1/299,792,458 of a second.
Secondary reference standards
Secondary reference standards are very close approximations of primary reference standards. For example, national measuring laboratories such as the US's National Institute of Standards and Technology (NIST) may house stainless steel balls of one kilogram which are used to set standards for manufacturing supermarket measuring scales.
Working standards and certified reference materials used in industry have a traceable relationship to the secondary and primary standards.
- Phillip Ostwald,Jairo Muñoz, Manufacturing Processes and Systems (9th Edition)John Wiley & Sons, 1997 ISBN 978-0-471-04741-4 page 616