International Practical Temperature

Some of tfie physical piopeities of tungsten ate given in Table 3 fuithei property data ate available (12—14). For thermodynamic values. References 5,15, and 16 should be consulted. Two values are given for the melting point. The value of 3660 K was selected as a secondary reference for the 1968 international practical temperature scale. However, since 1961, the four values that have been reported ranged from 3680 to 3695 and averaged 3688 K. 

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). 

High purity platinum wire is used in resistance thermometers because the temperature coefficient of resistance of pure platinum is linear over a wide temperature range. The platinum resistance thermometer is the recognized instrument for the interpolation of the international practical temperature scale from—259.35 to 630.74°C. Whereas such precision measurements require very high purity platinum, for most routine industrial measurements lower purity metal can be tolerated. Conventional wire-wound devices are quite fragile and this disadvantage has led to the introduction of printed resistance thermometers, which are cheap to produce and much more durable. They can be used as an inexpensive replacement for thermocouple applications in intermediate temperature applications. 

The International Practical Temperature Scale of 1968 (IPTS-68) is currently the internationally accepted method of measuring temperature reproducibly. A standard platinum resistance thermometer is the transfer medium that is used over most of the range of practical thermometry. 

The Kelvin scale is thus defined in terms of an ideal reversible heat engine. At first such a scale does not appear to be practical, because all natural processes are irreversible. In a few cases, particularly at very low temperatures, a reversible process can be approximated and a temperature actually measured. However, in most cases this method of measuring temperatures is extremely inconvenient. Fortunately, as is proved in Section 3.7, the Kelvin scale is identical to the ideal gas temperature scale. In actual practice we use the International Practical Temperature Scale, which is defined to be as identical as possible to the ideal gas scale. Thus, the thermodynamic scale, the ideal gas scale, and the International Practical Temperature Scale are all consistent scales. Henceforth, we use the symbol T for each of these three scales and reserve the symbol 9 for any other thermodynamic scale. 

The International Practical Temperature Scale is the basis of most present-day temperature measurements. The scale was established by an international commission in 1948 with a text revision in 1960. A third revision of the scale was formally adopted in 1990 and is reproduced in Table 3.158. Reproducible temperature points have been established by physical constants of readily available materials define the scale. 

Primary Temperature Points Defined by the International Practical Temperature Scale (IPTS-90)... 

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). 

If it is assumed that this volume change divides equally among the 100°C, then the volume change per degree is 0.000433 cm3 °C l. When this assumption is not valid, the thermometer gives readings in disagreement with the International Practical Temperature Scale. 

International Practical Temperature Scale, 5-6 Irreversibility, 40-41, 554-555 and entropy changes, 155-157, 554 Isentropic process, 153-155, 187-189, 223-231, 235-240... 

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. 

Example 1.2 Table 1.3 lists the specific volumes of water, mercury, hydrogen at l(atm), and hydrogen at lOO(atm) for a number of temperatures on the International Practical Temperature Scale. Assume that each substance is the fluid in a thermometer, calibrated at the ice and steam points as suggested at the beginning of this section. To determine how good these thermometers are, calculate what each reads at the true temperatures for which data are given. 

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. 

There have been three international temperature scales The International Temperature Scale of 1927, The International Temperature Scale of 1948 which was renamed the International Practical Temperature Scale of 1948 (IPTS-48) in 1960, ° and The International Practical Temperature Scale of 1968 (IPTS-68).22... 

C. R. Barber, The International Practical Temperature Scale of 1968, Adopted by the Comite International des Poids et Mesures, Metrologia 5, 35 (1969). 

T. B. Douglas, Conversion of Existing Calorimetrically Determined Thermodynamic Properties to the Basis of the International Practical Temperature Scale of 1968, J. Res. Natl. Bur. Stand. (U.S.) Sect. A 73, 451 (1969). (Formulas for converting H, Cp, S, and G from one temperature scale to another. Differences between the two scales are tabulated from 90 to 10 000 K.)... 

The low temperature heat capacity and entropy of copper have been well established by the critical review of Furukawa et al. ( ). Their recommended smoothed values are adopted with minor corrections for a change in the relative atomic mass from 63.54 to 63.546 (2) and for a change to the International Practical Temperature Scale of 1968 (3). These corrections tncrease the entropy at 298.15 K from 7.923 to 7.928 cal K" mol" and the enthalpy difference, H (298.15 K) - H (0 K), from 1.1962 to 1.1967 kcal mol". The values recommended by CODATA (4) are those of Furukawa (1 ). 

The normal boiling point of sulfur at 1 atm, 717.824 K, is a secondary standard on the International Practical Temperature Scale of 1968. The vapor composition at this temperature is a mixture of several sulfur species, the predominant species being Sg(g), S, (g), and Sg(g). In our reference state for sulfur, we have arbitrarily chosen 0.5 S2(g) to be the gas phase species. 

Temperature values to be used for the currently valid thermodynamic formulas (EOS-80) of seawater (Fofonoff and Millard, 1983 Mamayev et al., 1991) need to be expressed in the International Practical Temperature Scale of 1968 (IPTS-68). Practically measured temperatures are assumed to be in IPTS-68 before January 1, 1990, and in the International Temperature Scale of 1990 (ITS-90) afterward (Mamayev et al., 1991). Temperatures required for the thermodynamic potential functions for water, ice, and seawater need to be expressed in ITS-90, see Section 20.2.1. 

INTERNATIONAL PRACTICAL TEMPERATURE SCALE OF 1968 (AMENDED EDITION OF 197S)... 

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