Thermometric fixed points

The definition of reference thermometric fixed point is an equilibrium state of a definite substance the realization of a fixed point must depend only on the composition and on the substance . Hence boiling points, for example, cannot be considered fixed points, since they depend on pressure. Only triple points fulfil this definition as can be deduced from the Gibbs rule for pure substances ... 

In addition to these defining and secondary temperature standards, a thermocouple wire (SRM 733, a silver-28 at.% gold alloy) has been certified, which serves to compare manufactured wire to standard reference thermocouple tables between 4 and 273 K. SRM 767, a superconductive thermometric fixed-point device, provides temperature calibration in the range 0.5 to 7.2 K This device incorporates five high-purity elements (lead, indium, aluminum, zinc, and cadmium) in long, thin cylinders whose superconductive transition temperatures are certified to be reproducible within 1 mK. 

Work is in progress on modifying the existing superconductive thermometric fixed-point device to extend the range to both lower and higher temperatures. Another thermocouple wire (SRM 1967, platinum) will soon be available. This platinum wire, referred to in the thermometry literature as Pt-67, will take the place of Pt-27, which was the standard referred to until 1973. 

The idea that certain physical states could reproduce always the same temperature rises in the second half of seventeenth century (Hooke, 1664 Renaldini, 1694, see e.g. ref. [8]). Intuitions of this idea can be also found in Aristotele and Galeno. Nowadays, the importance of the control of the thermometric calibration is underestimated and the use of reference fixed points is usually limited to metrological laboratories. 

Even if nowadays, the MCT may be considered a primary thermometer only on a narrow temperature range, it is considered the best dissemination standard in the millikelvin range [56-59], In fact, the 3He melting pressure is a good thermometric property because of its sensitivity over three decades of temperature with a resolution A T/T up to 10 5 [56], The good repeatability, the insensitivity to magnetic fields up to 0.5 T [60] and the presence of temperature-fixed points allow for the control of possible shifts in the calibration curve of the pressure transducer. The usefulness of these fixed points is evident, considering that the ITS-90 is based just on the definition of fixed points. 

If linear interpolation is applied in this way then two thermometers using different thermometric properties brought into contact with the same heat reservoir will give identical readings only at the fixed points. 

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. 

The establishment of the International Temperature Scale has required that the thermodynamic temperatures of the fixed points be determined with as much accuracy as possible. For this purpose a device was needed that measures essentially the thermodynamic temperature and does not depend on any particular thermometric substance. On the other... 

The third stage in the development of the modern thermometer began in 1665 with the development of the first thermometric standard scale. In that year Robert Boyle, Robert Hooke, and Christian Huygens suggested independently that thermometers could be calibrated effectively from a single fixed point. Degrees would represent a standard expansion or contraction fraction of the volume of the thermometric substance measured at the fixed point. Boyle set the fixed point at the freezing tempera-... 

Some investigators, Martine included, advocated the use of a thermometric scale based upon two fixed points. This had several great advantages. Any suitable liquid could then be used as... 

Why is the triple point of water (ice-liquid-vapor) a better fixed point for establishing a thermometric scale than either the melting point of ice or the boiling point of water ... 

The values of the constants in the Uiermometric function are determined with reference to fixed thermometnc points whose temperatures are arbitrarily assumed. The fixed thermometric points most frequently employed are the ice point, steam point and triple point of water. 

Gallium is used as a fixed thermometric standard to calibrate thermistor probes of electronic thermometers used in clinical chemistry and experimental laboratory analyses (Scansetti 1992). This standard is appropriate because (i) the melting point of gallium (29.78 °C) falls within the region of critical importance to laboratory biological determinations (25-37°C) and (ii) the melt can be maintained accurately and constantly for several hours (Mangum 1977). 

Equation (1-4) is used for the mercury or alcohol thermometer, as well as for gas thermometers containing or He. It is found experimentally that the niimerical value of the temperature, except the values at the ice and steam points which are fixed, depends upon the particular system and thermometric property employed. Because there is less variation among gas thermometers, a gas is usually chosen as the standard thermometric substance. 

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