The freezing point of platinum

Abstract

The In tern atio n al Tem perature Scale, which has been in force since 1927* is based on certain values assigned to th e boiling and freezing points of pure substances and on specified m eans of interpolation between, or extrapolation beyond, these points. The highest basic p oint of th e scale is the freezing point of gold, defined as 1063 • 0° C, while for extrapolation from this tempera tu re use is m ade of th e W ien law of radiation, w ith a certain value of the constant C2. Though any tem peratu re above 1063° C is thus completely defined w ithout reference to fu rth er fixed points, determinations of such points are of considerable value. I n particular, th ey serve to indicate the degree of reproducibility of th e scale by th e various users of it, and, when well authenticated, to provide secondary standards for its realization. Of such fixed points th e m ost im p o rtan t has been th e freezing point of palladium (1555° C), b u t th e latest developm ents in furnace technique and refractory m aterials should now enable th e freezing point of platinum to be used with equal, if n o t greater, advantage. The qualities of platinum which render it especially valuable in this connection are as follows : its freedom from oxida tio n ; its high stan d ard of purity, for which a convenient electrical test is available ; its high freezing point (about 1775° C) which approaches the im p o rtan t zone of tem perature covered by th e electric fighting industry These qualities also m ake th e platinum point especially suitable as the basis for a standard of fight, as has been proposed by a num ber of experimenters.I t is w ith th e tw o objects indicated above th a t the National Physical Labes* to ry has undertaken an investigation concerning th e freezing point of plaWpP* th e precise scope of which m ay be defined as follows :- on May 11, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from

Accuracy of Optical Pyrometry in the Range 800 °c to 4000 °c

“…The standard deviation of the mean determination for each observer was about 0.4" C, which iilcluded all calibration errors except the mean effective wavelength error. From the Probine and Bertaud paper this would also amount to a standard deviation of the mean of the three of about 0.7" C, so the total standard deviation uncertainty due to pyrometer errors oilly was 0.8" C. If account is also taken of the published estimates of imperfect blackbody conditioils and possible impurities in the platinum the total uncertainty of the platinum point is raised to 0.9" C. This is supported by the results of Schofield (1934) of the National Physical Laboratory and Hoffman and Tingwaldt (1934) of the Physikalisch-Technische Reichsanstalt, as may be seen from Mean 1769. lo C Estimated standard deviation of each determination 0.9" C Accuracy in the Range 2200" C to 4000" C Tungsten strip lamps cannot be relied on to lceep their lumiilance temperature calibrations if run much above 2200" C (corresponding to a true temperature some 260" C higher), so estimates of accuracy are not easy to confirm above this temperature. In Table IV the estimates of Table I1 have been extended to 2800" C, the highest calibratioil temperature for most commercial optical pyrometers, and to 4000" C, which would cover almost all temperature measurements of practical interest.…”

A pan-European investigation of the Pt-40%Rh/Pt-20%Rh (Land–Jewell) thermocouple reference function

“…To convert IPTS-68 temperatures to ITS-90 values at about 1770 °C, 0.58 °C should be subtracted from the IPTS-68 value. Early determinations of the melting point of platinum in the 1930s by Schofield at NPL [23], Roeser et al at NBS (now NIST) [24], and Hoffmann and Tingwaldt at PTR (now PTB) [25] have been shown to be consistently higher by about 3 °C than more recent determinations, if all temperatures are expressed in terms of ITS-90. This was brought to light by the measurements of Quinn and Chandler at NPL [45] which were later retracted and replaced with [46].…”

“…Note that in his determination of the reference function of Land-Jewell thermocouples in 1965 [15], Bedford took the value of the melting temperature of Pt to be 1769 °C (ITS-48), based on the best available information at the time [23][24][25]. To convert from ITS-48 to ITS-90 at this temperature, 2.16 °C should be added to the ITS-48 temperature, which gives 1771.2 °C.…”

“…The second reason is that, to the authors' knowledge, the origin of the coefficients in ASTM standard E1751-09 [16], while thought to originate from the measurements of Bedford [15], is not documented, either in the standard itself, nor, to the authors' knowledge, in the publicly available literature. The third reason is that in his determination of the reference function [15], Bedford took the value of the melting temperature of Pt to be 1769 °C (ITS-48), based on the best available information at the time [23][24][25]. To convert from ITS-48 to ITS-90 [26], 2.16 °C should be added to the ITS-48 temperature, which gives 1771.2 °C.…”