Analytical review of the temperature fixed points

Standards of physical quantities are used for reproduction and storage of physical units as well as for transfer its size by appropriate measuring instrument. The temperature is an intense value and is determined only indirectly depending on certain physical quantities of temperature, for example, resistance, thermoelectric power and so on. This situation has led to the fact that there are several temperature scales, each of which has its unit of temperature measure. Therefore, the international temperature scale was proposed for the international standardization of temperature measurement. The basis of international temperature scale is temperature defining points based on well-reproducible phase transitions corresponding to the triple point (the temperature equilibrium between solid, liquid and gaseous phases) or the melting point or crystallization (temperature equilibrium of liquid and solid phases), mainly of pure metals. The cleanness of metal should be 99,999%. The international temperature scale was improved for several times. The interpolation methods and temperature reference points has changed, but the principle remains the same - the basis of the scale is a set of phase transitions of pure substances and interpolation means graded at these points. Currently the world uses the international temperature scale ITS-90 which was adopted at the XVIII General Conference on Weights and Measures. The major changes in scale compared to the previous related to the expansion of the range scale changing the temperature defining points, introduction of new interpolation devices and new methods of constructing interpolation dependencies for platinum resistance thermometers. Temperature scale ITS-90 is based on 17 basic temperature defining points. This leads to the fact that the scale has sufficiently large temperature intervals between temperature defining points some of which exceed 240 K. To reduce the temperature intervals between the temperature defining points ITS-90, the secondary temperature defining points are used. The boiling point, melting point and crystallization point of pure substances, eutectic alloys and compounds are used with the implementation of secondary temperature defining points. Uncertainty of reproduction the temperature by secondary temperature reference points is less than 1 mK. Taking into consideration the above we can conclude that the eutectic alloys are used as raw material of secondary defining points with prospects of using in ITS-90. Eutectic alloy consists of two or more substances the crystallization process of which occurs simultaneously. The temperature crystallization of eutectic alloy is lower than the temperature crystallization of each of the materials used in the alloy. Its value depends on the percentage composition of components and on the uniformity of eutectic alloy. The article presents the classification of eutectic alloys according to the composition of the substance. The composition of some existing eutectic alloys and their melting point is also presented. The use of eutectic alloys as a working material of temperature defining point has several advantages: the emergence of new eutectic materials and their use in the future will significantly reduce the temperature intervals between temperature defining points ITS-90; eutectic temperature defining points can be used as a mobile, it allows you to calibrate the sensors directly on objects. However, the eutectic temperature defining points have a number of disadvantages compared with the main temperature defining points ITS-90: uncertainty of reproduction of temperature phase transition by eutectic temperature defining points is more than by the basic temperature defining points; the working material of eutectic temperature defining points with eventually usage becomes heterogeneous that leads to changes in the shape and temperature plateau phase transition; the percentage of eutectic alloy which is used in temperatures defining points must be strictly followed. The use of existing eutectic alloys allow in some cases to reduce the temperature intervals between temperature reference points more than 4 times. The emergence of new eutectic alloys will allow filling the temperature intervals by eutectic temperature reference points. Therefore, further research is planned to investigate the dependence of change shape and temperature plateau phase transition of eutectic alloy from cyclical heating and cooling of temperature defining point, as well as development of methods and means of homogenizing the eutectic material of temperature defining point.