International Practical Temperature Scale fixed points

Approximately every twenty years, the international temperature scale is updated to incorporate the most recent measurements of the equilibrium thermodynamic temperature of the fixed points, to revise the interpolation equations, or to change the specifications of the interpolating measuring devices. The latest of these scales is the international temperature scale of 1990 (ITS-90). It supersedes the earlier international practical temperature scale of 1968 (IPTS-68), along with an interim scale (EPT-76). These temperature scales replaced earlier versions (ITS-48 and ITS-27). 

In 1954 the General Conference wanted to redefine the temperature scale using various primary points in addition to the two points of freezing and boiling water. The triple point of water (at 273.16 K) proved easy to obtain and very accurate (one part in a million). In 1960 the triple point of water and five other fixed points were accepted for an International Practical Temperature Scale. This scale was superseded in 1968 by the International Practical Temperature Scale (IPTS 1968), which added eight more fixed points. The current scale is shown in Table 2.29. 

The establishing, or fixing, of points for temperature scales is done so that anyone, anywhere can replicate a specific temperature to create or verify a thermometer. The specific temperature points become (in effect) the International Prototypes for heat. The General Conference of Weights and Measures accepted the new International Practical Temperature Scale of 1968 (IPTS 1968) with 13 fixed points (see Table 2.29). The new (IPTS 1968) scale was a revision from the IPTS 1948 (which had been amended in 1960). 

In practice it is the International Practical Temperature Scale of1968 (IPTS-68) which is used for calibration of scientific and industrial instruments-t This scale has been so chosen that temperatures measured on it closely approximate ideal-gas temperatures the differences are within the limits of present accuracy of measurement. The IPTS-68 is based on assigned values of temperature for a number of reproducible equilibrium states (defining fixed points) and on standard instruments calibrated at these temperatures. Interpolation between the fixed-point temperatures is provided by formulas that establish the relation between readings of the standard instruments and values of the international practical temperature. The defining fixed points are specified phase-equilibrium states of pure substances, t a given in Table 1.2. 

Table 10.7 The Fixed Points of IPTS-68, the International Practical Temperature Scale of 1968 ...

The 1968 International Practical Temperature Scale (IPTS-68) relies on seven fixed points (Table 10.7). Above 1064.43°C, IPTS-68 uses Planck s equation for blackbody radiation, Eq. (5.7.4). 

The International Practical Temperature Scale of 1968 (IPTS-68) has been replaced by the International Temperature Scale of 1990 (ITS-90). The ITS-90 scale is basically arbitraiy in its definition but is intended to approximate closely the thermodynamic temperature scale. It is based on assigned values of the temperatures of a number of defining fixed points and on interpolation formulas for standard instruments (practical thermometers) that have been cahbrated at those fixed points. The fixed points of ITS-90 are given in Table 1. 

Primary standards are those developed and maintained by national standards laboratories such as the National Bureau of Standards. These laboratories develop, maintain, and disseminate standards, such as the International Practical Temperature Scale. The IPTS-68 is disseminated to the users through secondary standards such as calibrated thermometers, fixed point references, and so on (see Table II). Some of these thermometers are calibrated directly against the defining fixed points and others are calibrated over the range of need by a comparison calibration against a standard interpolating thermometer. This ensures that the basis for temperature measurement, the IPTS-68, is the same everywhere throughout the world. 

The International Practical Temperature Scale consists of a number of equilibrium states which have been assigned temperatures on the Celsius scale, to facilitate the measurement of temperatures everywhere. Since the number of degrees between the triple point of water and any other equilibrium state, such as the freezing point of molten zinc, is a matter of experimental determination, the temperature values assigned to the chosen fixed points are changed from time to time as techniques improve. 

The temperature at which a particular event occurs, or the temperature range over which a reaction happens, are often characteristic of the nature and history of a sample, and sometimes of the methods used to study it. Sharp transitions, such as the melting of pure materials, may be used to calibrate equipment and as the fixed points of thermometry and of the International Practical Temperature Scale (IPTS). 

An mtemationaUy agreed upon secondary, empirical temperature scale is described in Fig. A.8.1 It is called the International Temperature Scale of 1990, in short, the ITS 1990 (Tjo in kelvin, K tgo in degrees Celsius, °C). It makes use of the higher precision possible with secondary thermometers. The given fixed points have been agreed upon internationally to achieve as close a match to the kelvin scale as possible. The values for the fixed points are to be corrected when better absolute measurements become available. Earher international temperature scales were accepted in 1927 (ITS 27), 1948 (ITS and IPTS 48), and 1968 (IPTS) (IPTS is the abbreviation for International Practical Temperature Scale , the second version of the 1948 scale and the 1968 scale). 

Table III.l. Defining fixed points of the International Practical Temperature Scale...

Well-designed low-pressure gas thermometers can be used to determine (really approximate) the thermodynamic temperature. However, from a practical standpoint, where precision and simplicity in the implementation and transfer are the major considerations, secondary thermometers were chosen as the defining standard thermometers for a practical temperature scale. This scale was defined by the use of fixed reference points whose thermodynamic temperatures were determined from gas thermometer measurements. The International Committee of Weights and Measures (Comite International des Poids et Mesures, CIPM) is responsible for developing and maintaining the scale. 

ITS-90 is the latest in a series of international temperature scales. Each represents an improvement over an earlier scale as more reliable values for the fixed points are established. It has been the practice to update the ITS temperature scale approximately every twenty years. Earlier scales include (ITS-27), (ITS-48), (IPTS-68), and a provisional scale in 1976. 

In 1968, an international agreement was reached about the definition of an official (practical) scale of temperature for T> 14 K. This temperature scale IPTS-68, corrected in 1975 [11], was defined by reference fixed points given by transitions of pure substances. To extend the low-temperature range of IPTS-68, the EPT 76 [12-13] gave nine reference temperatures defined by phase transition of pure substances in particular the superconductive transition (between 0.5 and 9K) of five pure metals was introduced. Moreover,... 

Different empirical temperature scales will naturally differ from each other except at the respective fixed thermometnc points, Even different scales of the same type (say different Centigrade scales) will differ at all temperatures, except the steam point and ice point, depending on the fortuitous properties of the system chosen as a thermometer. It is, therefore, necessary to remove these differences and to obtain a more universal scale. This has been achieved in two ways. The practical way of achieving uniformity is to lay down detailed rules concerning the thermometer (actually different thermometers depending on the range of temperatures to be measured). Such rules have been agreed on internationally and... 

Practical difficulties arise in making very precise determinations of temperature on the thermodynamic scale the precision of the more refined thermometric techniques considerably exceeds the accuracy with which the experimental thermometer scale may be related to the thermodynamic scale. For this reason, a scale known as the International Temperature Scale has been devised, with several fixed points and with interpolation formulas based on practical thermometers (e.g., the platinum resistance thermometer between 13.803 K and 1234.93 K). This scale is intended to correspond as closely as possible to the thermodynamic scale but to permit more precision in the measurement of temperatures. Further details about this scale are given in Chapter XVII. 

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