Standards and Reference Materials for Thermal Measurements

摘要

Worldwide, one's lives are governed at all levels by standards of one type or other issued from a range local to national and ultimately international levels. In particular in science and engineering there is the necessity for consensus to be obtained on what, how, why and when necessary reliable information should be made available for use in order to obtain a "level playing field". This is exemplified in the subject of properties of materials and especially measurement of thermal performance in all applications. A uniform approach to measure a property can be developed as a standard but it is then necessary to validate its use to appropriate precision levels. In the present context, the term standard relates to both a conforming document describing the viable method containing details of necessary apparatus, experimental procedure and analysis to provide a reliable value and to one or more reference materials, specimens or artifacts required to verify and define the precision and bias of the method. Until the middle of the twentieth century the measurement of thermal conductivity and allied properties to acceptable precision levels was often considered a "simple" task in theory as shown in the earlier standards. Subsequently it has proved to be quite complex and difficult in practice, especially at temperature and environment levels much above or below ambient. Essentially, this is due to the fact that the property range for a material, composite or system, depending on its type and form and the effects of temperature, environment etc., covers a such broad range of over six orders of magnitude thereby requiring use of a number of widely different methods of measurement all requiring verification using standards. The paper discusses developments in both subjects with specific emphasis on the particular driving forces due to developing applications and specific needs. Differences in the type and use of available standards are described with specific examples, together with illustrations of their correct use and on occasion misuse or abuse. A short review of current availability of materials is listed together with suggestions of requirements for new methods and appropriate candidate reference materials.