Thermometric points, standard

If it were desired to have ice, water, and steam coexist, then there is no degree of freedom left The system must be maintained at the point T - 273.16 K and P - 4.58 torr. For this reason, the triple point of water serves as a convenient thermometric reference standard (see Section 1.2). 

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. 

However, it is clear that slight variations in vessel shape, etched markings, or external pressure can lead to disagreements as to which thermometer gives the true temperature. Moreover, the reference points chosen to standardize the readings between different thermometers could be subject to disagreements (see Sidebar 2.4), as could the choice of thermometric fluid (e.g., Hg vs. water, each of which has different values of aP in different temperature ranges). Under these circumstances, the choice of the true temperature scale may become subject to non-scientific influences. We therefore seek a universal standard that avoids such arbitrary choices. 

ACIDIMETRY. An analytical method for determining the quantity of arid in a given sample by titration against a standard solution of a base, or, more broadly, a method of analysis by titration where the end point is recognized by a change in pH (hydrogen ion concentration). See also Analysis (Chemical) pH (Hydrogen Ion Concentration) Titration (Potentiometric) and Titration (Thermometric). 

When the temperature of pure ice is gradually raised under the ordinary atmospheric pressure, melting always commences sharply at a certain invariable temperature, which remains constant until fusion is complete. On account of the ease with which this constant temperature can be attained it has been chosen as the standard zero for the Celsius (Centigrade) and Reaumur thermometric scales. The melting-point is slightly affected by pressure, eaeh increase of one atmosphere lowering the transition temperature of ice to water by approximately 0-0075. 

Since the property X of different thermometric substances, or the different thermometric properties of a given substance, do not vary in an identical manner with temperature, various thermometers, all of which have been standardized at 0 and 100° C, may indicate different temperatures when in thermal equilibrium with the same body at an intermediate point. A mercury thermometer and a toluene thermometer, for example, which agree at 0 and 100° C, would differ by several degrees in the vicinity of 60° C. Even mercury thermometers in tubes made of various types of glass indicate slightly different temperatures. ... 

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-... 

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. 

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. 

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