International Temperature Scale of 1990
Introduction
The International Temperature Scale of 1990 (ITS-90) is a calibration standard for temperature measurements, established by the International Committee for Weights and Measures (CIPM). It serves as a practical approximation of the thermodynamic temperature scale, which is based on the principles of thermodynamics and the properties of ideal gases. ITS-90 is widely used in scientific research, industrial applications, and metrology to ensure consistency and accuracy in temperature measurements across the globe.
Historical Background
The development of temperature scales has been a crucial aspect of scientific progress. Before ITS-90, the International Practical Temperature Scale of 1968 (IPTS-68) was in use. However, discrepancies between IPTS-68 and thermodynamic temperatures prompted the need for a more accurate and reliable scale. ITS-90 was introduced to address these issues, providing a more precise alignment with the thermodynamic temperature scale and improving the reproducibility of temperature measurements.
Definition and Implementation
ITS-90 is defined by a series of fixed points and interpolation formulas. These fixed points are based on the phase transitions of pure substances, such as the melting point of gallium and the triple point of water. The scale covers a wide range of temperatures, from 0.65 K to 1357.77 K, and is divided into different regions, each with specific interpolation methods.
Fixed Points
The fixed points of ITS-90 are critical for its implementation. They include the triple point of hydrogen, the boiling point of neon, and the freezing point of silver, among others. These points are chosen for their reproducibility and stability, allowing for precise calibration of thermometers.
Interpolation Methods
Different temperature ranges within ITS-90 use specific interpolation methods. For temperatures below 24.5561 K, vapor pressure thermometry is used. Between 13.8033 K and 1234.93 K, resistance thermometry is employed, utilizing platinum resistance thermometers. Above 1234.93 K, radiation thermometry is applied, based on Planck's law of blackbody radiation.
Applications
ITS-90 is utilized in various fields, including meteorology, materials science, and engineering. Its precise calibration standards are essential for experiments requiring accurate temperature control, such as those in cryogenics and high-temperature metallurgy. Industries that rely on temperature-sensitive processes, like semiconductor manufacturing and chemical production, also depend on ITS-90 for quality assurance.
Advantages and Limitations
ITS-90 offers several advantages over previous scales, including improved accuracy and consistency with the thermodynamic temperature scale. However, it is not without limitations. The scale's reliance on specific fixed points and interpolation methods can introduce uncertainties, particularly at extreme temperatures. Additionally, the maintenance of ITS-90 requires specialized equipment and expertise, which may not be accessible in all laboratories.
Future Developments
The field of temperature measurement continues to evolve, with ongoing research aimed at further refining temperature scales. Advances in quantum thermometry and the potential redefinition of the Kelvin based on fundamental constants are areas of active investigation. These developments may lead to a new international temperature scale that surpasses the accuracy and applicability of ITS-90.