Yoshiro Yamada (Senior Research Scientist) of the Radiation Thermometry Section, the Temperature and Humidity Division, the Metrology Institute of Japan (Director: Mitsuru Tanaka) of the National Institute of Advanced Industrial Science and Technology (AIST) (President: Hiroyuki Yoshikawa) succeeded in applying to practical use a novel series of high-temperature fixed points developed at AIST, by launching temperature calibration services using five fixed points in the high-temperature range of 1100 to 2500 ℃ in April 2008.  These services enable an improved accuracy in industrial level temperature control, which had been difficult to achieve so far.

The temperature fixed points used in the services are utilizing eutectics of metal-carbon alloys. Since they were first proposed as next-generation high-temperature standards by AIST in 1999,  world's leading standards research institutes have been involved in research of these fixed points.  AIST succeed in applying these fixed points to practical use by launching the world's first new temperature calibration services.

There has long been a wide variety of needs for temperature control over 1100 ℃ in the materials industries, energy industries, etc.  However, high-accuracy control of the thermometer scales used as the base for temperature measurement was difficult.  The launch of the new calibration services will contribute to improving the accuracy of radiation thermometers for non-contact temperature measurements up to 2500 ℃.  In the temperature range between 1100 and 1500 ℃, the improvement is also expected in the accuracy of thermocouple thermometers, which are typical contact thermometers used in this range, and various user needs will be satisfied.

The high-temperature fixed-point technology was presented at HANNOVER MESSE 2008, an international trade fair for industrial technologies held from April 21 to 25, 2008 in Hannover, Germany. 

Temperature-fixed-point apparatus up to 2500 ℃

AIST establishes and maintains various physical and chemical standards as national measurement standards, and supplies calibration services in order to provide the national standards to industries.  With regard to temperature standards, we have performed calibration services, based on the International Temperature Scale of 1990 (ITS-90) that was internationally agreed under la Convention du Mètre, for industrial thermometers over the temperature range from low temperature (24 Kelvin, –249 ℃) to high temperature (2000 ℃).

In the temperature range exceeding 1100 ℃, highly accurate temperature control is required as needs have been increasing for sophisticating steel-manufacturing processes in the materials industry, for ensuring the safety of next-generation nuclear power plants in the energy industry, and so on.  However, the highest temperature defining fixed point in the ITS-90 is the copper point (1084.62 ℃).  Therefore, there has been a demand for a technology that enables improvement in accuracy of thermometers in the high-temperature range above the copper point.

Although fixed points above 1100 ℃ had been studied since the beginning of the 1900s around the world, no practical fixed point had been realized.  In 1999, AIST proposed the novel "metal-carbon eutectic points".  Since then, AIST, as well as other international standards institutes including Bureau International des Poids et Mesures (International Bureau of Weights and Measures) and the National Physical Laboratory in the United Kingdom, have been involved in developing "metal-carbon eutectic points" to utilize them as practical high-temperature fixed points.  A study aiming adoption of the new metal-carbon eutectic points in next-generation international temperature scales is ongoing through an international joint experiment with the above-mentioned research institutes.  In Japan, AIST promotes commercialization of devices for standards, and disseminates calibration techniques through technology transfer to the private sector.

The reason that practical applications of fixed points above 1100 ℃ have never been successfully achieved is that a graphite crucible, which is a container for the pure metals for measuring their melting or freezing points, dissolves at high temperature, resulting in contamination of the pure metals with carbon.  AIST proposed a method to prevent carbon contamination by using a metal-carbon alloy instead of pure metal as fixed-point material to solve the problem: when the proportions of metals and carbon are the same as eutectic compositions, highly reproducible melting plateaux can be obtained for use as temperature fixed points.  For such metal-carbon alloy fixed points, the temperature measurements showed repeatability of less than 0.05 ℃ with a single cell, and reproducibility of less than 0.2 ℃ even among different cells.

With regards to these "metal-carbon eutectic fixed points," AIST has demonstrated the performance of nine fixed points from iron-carbon eutectic point to rhenium-carbon eutectic point.  Among these fixed points, the calibration services in high temperature range use five eutectic fixed points of iron-carbon (1153 ℃), cobalt-carbon (1324 ℃), palladium-carbon (1492 ℃), platinum-carbon (1738 ℃), and rhenium-carbon (2474 ℃), resulting in a temperature calibration service system using fixed points covering the range up to approximately 2500 ℃.

We are currently developing a fixed point around 2750 ℃.  We aim to extend the high-temperature range for the calibration service in the near future.  We also will consider a shift of the current calibration service to a calibration system in the Japan Calibration Service System (JCSS) in line with the Measurement Act.

The ongoing international joint research aims to review international temperature scales at the high-temperature range by 2012.  If it is completed, an improvement in high temperature scale over 2000 ℃ by one order of magnitude will be accomplished on a user basis.