Primary standards in the context of measurement and instrumentation refer to the highest level of measurement standards that serve as the ultimate reference for a particular physical quantity. These standards are used to establish and maintain the accuracy and traceability of measurements made in various fields, such as science, industry, and commerce. Primary standards are typically established based on fundamental constants of nature or immutable properties of materials, ensuring their long-term stability and reliability.
Here are a few examples of primary standards for various physical quantities:
Length: The International Prototype of the Kilogram (IPK) was a primary standard for the kilogram, which was defined by a physical object. However, in recent years, the kilogram has been redefined using the Planck constant, linking it to a fundamental constant of nature rather than a physical object. For length, the primary standard is the meter, which is defined based on the speed of light in a vacuum.
Time: The primary standard for time is the International System of Units (SI) second, which is based on the vibration of cesium atoms.
Temperature: The International Temperature Scale of 1990 (ITS-90) is a primary standard for temperature. It is defined using fixed points based on specific thermodynamic properties of materials.
Electric Current: The primary standard for electric current is the ampere, defined in terms of the force between two parallel conductors carrying current.
Amount of Substance: The primary standard for the amount of substance is the mole, defined in terms of the Avogadro constant.
Luminous Intensity: The primary standard for luminous intensity is the candela, defined based on the spectral radiance of a black body.
Subatomic Particles: Certain subatomic particles, like electrons, have fundamental charges that are used as primary standards for electric charge.
It's important to note that primary standards are not practical for everyday measurements due to their complexity and specificity. Instead, secondary standards, such as calibration instruments, are used in laboratories and industries to disseminate the primary standard's accuracy to a broader range of applications.
Traceability is a key concept in measurement, ensuring that measurements made using secondary or working standards can be linked back to primary standards through a chain of comparisons. This traceability helps maintain the consistency, accuracy, and reliability of measurements across different laboratories and industries.