CCT-K2.5: NRC/NMIJ/INRIM comparison of capsuletype standard platinum resistance thermometers from 13.8 K to K

CCT-K2.5: NRC/NMIJ/INRIM comparison of capsuletype standard platinum resistance thermometers from 13.8 K to 273.16 K K. D. Hill 1, T. Nakano 2, P. Steur 3 1 Institute for National Measurement Standards, National Research Council, Ottawa, Canada 2 National Metrology Institute of Japan (NMIJ), AIST, Tsukuba, Japan 3 Istituto Nazionale di Ricerca Metrologica (INRIM), Torino, Italy Abstract. A trilateral comparison of capsule-type standard platinum resistance thermometers is reported that links the NMIJ and INRIM realisations of the International Temperature Scale of 1990 to the results of the Consultative Committee for Thermometry Key Comparison 2 in the temperature range 13.8033 K to 273.16 K. 1. Introduction The Consultative Committee for Thermometry Key Comparison 2 (CCT-K2) results were published in 2002 [1], with the participants being the Bureau National de Métrologie - Institut National de Métrologie (BNM-INM, France), the Consiglio Nazionale delle Ricerche - Istituto di Metrologia G. Colonnetti (IMGC, Italy), the Korea Research Institute of Standards and Science (KRISS, Republic of Korea), the National Institute of Standards and Technology (NIST, United States), the National Physical Laboratory (NPL, United Kingdom), the National Research Council of Canada (NRC, Canada), and the Physikalisch-Technische Bundesanstalt (PTB, Germany). The topology of CCT-K2 was a star topology, requiring two capsule-style platinum resistance thermometers (CSPRTs) belonging to, and calibrated by, the participating national metrology institutes (NMIs) to be sent to the pilot laboratory (NRC) where they were then compared to one another. NRC remains able to carry out comparisons linked to the original results, as demonstrated by the linkages provided to the Institute for Physical-Technical and Radiotechnical Measurements, Gosstandart (VNIIFTRI, Russian Federation) via CCT-K2.1 [2], the Dutch Metrology Institute (VSL, Netherlands) via CCT-K2.3 [3], and the Institute of Low Temperature and Structure Research (INTiBS, Poland) and the Laboratoire National de Métrologie et d'essais (LNE, France) via CCT-K2.4 [4]. The participants of CCT-K2.5 are the Istituto Nazionale di Ricerca Metrologica (INRIM - formerly IMGC, Italy) and the National Metrology Institute of Japan (NMIJ-AIST, Japan), with NRC acting as the pilot and linking lab. In the intervening years since the CCT-K2 measurements were carried out, INRIM has improved the Italian national standards in the temperature range of the comparison, and was interested in the opportunity to update that assessment. The topology of CCT-K2.5 does not (strictly) follow the star topology of CCT-K2. Instead, NMIJ selected two CSPRTs, Chino serial numbers RS954-7 and RS85A-6, as the proxies for their local realization 1

of the International Temperature Scale of 1990 (ITS-90) in the temperature range from 13.8033 K to 273.16 K. These CSPRTs were sent to NRC in order to link the ITS-90 realized at NMIJ to the temperature standards maintained at NRC, and hence to CCT-K2. In addition to calibrating the two Chino CSPRTs, NMIJ also calibrated a CSPRT belonging to INRIM (Leeds and Northrup serial number 1860951). By this mechanism, INRIM can be linked to NRC via NMIJ. Given that INRIM (as the former IMGC) participated directly in CCT-K2 with two L&N CSPRTS (including S/N 1860951) serving as their proxy, NMIJ can also be linked to CCT-K2 through INRIM. 2. Experimental details 2.1 Measurements performed at NMIJ NMIJ selected two capsule-type standard platinum resistance thermometers (Serial Nos. Chino RS954-7, RS85A-6) and measured their resistances at the triple point of water and at the boiling point of liquid helium prior to sending them to NRC. When the CSPRTs returned from NRC, the resistances of the CSPRTs were measured again at the TPW and the He boiling point. The TPW resistance of RS954-7 remained stable throughout the comparison, and agreement of the NMIJ and NRC values was remarkable. On the other hand, upon its return to NMIJ, the TPW resistance of RS85A-6 was higher by the equivalent of 0.67 mk relative to the resistance measured prior to its shipment to NRC. Also, the agreement with the values measured at NRC was not as close as with RS954-7. The values measured at NMIJ are provided in Appendix A. Following the return of the CSPRTs from NRC, the CSPRTs were calibrated directly using NMIJ fixed-point cells at the Ne, Hg and H 2 O triple points, and were calibrated by comparison to a reference CSPRT at the Ar and O 2 triple points using a comparison cryostat. The realization and comparison procedures have been reported in [5-7] and the details will not be duplicated here. The thermometers were also calibrated at the temperatures 17.035 K, 20.27 K and the e-h 2 triple point by comparison to a reference RhFe thermometer using another comparison cryostat. Although the comparison system employed for these lower temperatures was different from that used at the triple points of O 2 and Ar, the procedure was the same as reported in [7]. The calibration of the reference SPRT is traceable to the cryogenic fixed points at NMIJ [5, 6]. The reference RhFe thermometer was calibrated directly at the e-h 2 triple point [5] and calibrated at 17.035 K and 20.27 K by comparison to another reference RhFe thermometer calibrated using the NMIJ interpolating constant volume gas thermometer [8]. The measurements of the e-h 2 triple point were corrected to 13.8033 K based on the isotopic composition of the sample gas and the information provided in the Technical Annex for the ITS-90. Since the isotopic concentration of the NMIJ H 2 sample (cell H-5) is 27.2 (2) mmol D per mole H, the magnitude of the isotopic correction is such that the realized triple point temperature has been estimated as 0.335 mk below the ITS-90 value 13.8033 K with a standard uncertainty of 0.02 mk. 2

The measurements of the Ne triple point were also corrected to 24.5561 K, based on the isotopic composition of the sample gas and the information provided in the Technical Annex for the ITS- 90. The isotopic concentration of the NMIJ Ne-5 (x( 21 Ne) = 0.002659 and x( 22 Ne) =0.092519) [13] results in a triple point temperature 1.7 µk above the ITS-90 value 24.5561 K. In addition, NMIJ calibrated INRIM thermometer L&N 1860951 using the fixed points indicated in Table 2 (these are the same fixed-point cells as used for the NMIJ CSPRTs) with one exception. In practice, cell NMIJ Ne-2 was used to calibrate the INRIM thermometer, but Ne-2 is known to be 0.031 mk hotter than cell Ne-5 [5] and the resistance ratio was corrected accordingly to put all of the measurements carried out at NMIJ on the same basis, so cell Ne-5 is identified in the table to define the traceability. The NMIJ TPW cell (LT-M0102) used to calibrate the CSPRTs was compared to the national reference, cell TR0201, and the two cells were found to be equivalent within the uncertainty of the comparison. Tamba et al. (Int. J. Thermophys (2008) 29 1749) studied the influence of isotopic composition on the temperatures of a group of TPW cells, and reported that cell TR0201 is 42 µk lower than the mean of seven new cells with their temperatures corrected to the reference isotopic composition. Therefore, cell LT-M0102 is also 42 µk lower than the TPW reference composition. The calibration temperatures, resistance ratios, and combined standard uncertainties (u L, for k = 1) are listed in Table 1 for the CSPRTs measured at NMIJ. The detailed uncertainty budgets are presented in Appendix A. Table 1. Calibration data for the CSPRTs measured at NMIJ corrected for the isotopic concentrations of the e-h 2, Ne and H 2 O cells. The values in italics at 273.16 K are the resistances of the thermometers (in ohms) at that temperature. Fixed point T / K Cell identification Chino RS954-7 Chino RS85A-6 W u L / mk W u L / mk e-h 2 13.8033 NMIJ H-5 0.001 494 404 0.076 0.001 230 288 0.076 CVGT 17.035 0.002 619 112 0.494 0.002 349 788 0.494 CVGT 20.270 0.004 571 906 0.495 0.004 300 012 0.495 Ne 24.5561 NMIJ Ne-5 0.008 799 967 0.142 0.008 523 066 0.145 O 2 54.3584 NMIJ O-2 0.092 072 640 0.069 0.091 790 375 0.069 Ar 83.8058 NMIJ A-4 0.216 167 373 0.099 0.215 919 495 0.100 Hg 234.3156 No. 3 0.844 205 549 0.160 0.844 152 947 0.163 H 2 O 273.16 LT-M0102 25.703 729 0.165 25.173 874 0.168 3

Fixed point T / K Cell identification L&N 1860951 W u L / mk e-h 2 13.8033 NMIJ H-5 0.001 215 825 0.096 CVGT 17.035 0.002 329 254 0.494 CVGT 20.270 0.004 277 239 0.495 Ne 24.5561 NMIJ Ne-5 0.008 504 069 0.142 O 2 54.3584 NMIJ O-2 0.091 810 606 0.071 Ar 83.8058 NMIJ A-4 0.215 945 418 0.100 Hg 234.3156 No. 3 0.844 159 782 0.161 H 2 O 273.16 LT-M0102 25.536 570 0.165 2.2 Measurements performed at INRIM At INRIM, a Leeds and Northrup CSPRT (Serial No. 1860951) was measured directly in the various fixed-point cells during 2014, and Table 2 summarizes the data. Further details are provided in Appendix B, including the detailed uncertainty budgets associated with each fixed point. INRIM measurement capabilities at low temperatures have evolved considerably since the CCT- K2 measurements took place more than 10 years ago. For CCT-K2.5, the measurements at e-h 2, Ne, O 2 and Ar were performed with the calorimeter and modular sealed cells developed during the MULTICELLS project [9,10] and improved throughout the duration of the Neon Project. However, the measurements at Hg and H 2 O were performed in the Medium Temperatures lab of INRIM, just as for CCT-K2. A recent fused-silica TPW cell with known isotopic composition was used for CCT-K2.5 because of the known difficulties associated with borosilicate glass cells. During the original CCT-K2 comparison, it was not possible to correct results obtained at the e- H 2 and Ne fixed points for their isotopic composition, since the information needed to do so was published much later. For CCT-K2.5, cell Ec1H2 contains 91.6 (1.0) ppm D/H, where 1 ppm D/H corresponds to 5.42 (31) µk. Based on these values, cell Ec1H2 realizes a temperature 0.014 (6) mk higher than the 13.8033 K ITS-90 temperature value associated with the reference composition 89.02 ppm D/H. The composition of the neon gas used for cell Ec2Ne is now known ( 21 x = 0.002649 (5); 22 x = 0.09336 (5)), leading to a temperature 0.125 (8) mk higher than the ITS-90 temperature of 24.5561 K for the reference isotopic composition for natural neon, based on information from the Technical Annex for the ITS-90. The isotopic composition of the cell used for the triple point of water, HART1015-Q, is (δ 2 H = -0.004; δ 18 O = -0.0003), resulting in a temperature that is 2.7 (9) µk lower than the ITS-90 temperature of 273.16 K corresponding to the reference isotopic composition of water. 4

Table 2. Resistance ratios obtained at INRIM with isotopic corrections for e-h 2, Ne and H 2 O. The value in italics is the resistance of the thermometer (in ohms) at 273.16 K. The uncertainties are detailed in Appendix B. Fixed point Cell T / K W(1860951) u L / mk identification e-h 2 Ec1H2 13.8033 0.001 215 819 0.083 Ne Ec2Ne 24.5561 0.008 503 849 0.055 O 2 Ec1O2 54.3584 0.091 810 533 0.033 Ar Ec1Ar 83.8058 0.215 945 226 0.076 Hg IMGC-CRYO-7 234.3156 0.844 159 364 0.063 H 2 O Hart 1015-Q 273.16 25.536 582 0.030 2.3 Measurements performed at NRC 2.3.1 Comparison measurements In the original CCT-K2 measurements, CSPRTs calibrated by the participating NMIs were compared at NRC in a nearly isothermal copper block. When the protocol for CCT-K2 was first envisaged, it was assumed that the reporting of the comparison results would include temperatures between the calibration fixed points. However, although such data was collected during the course of the CCT-K2 measurements, the final report focuses nearly exclusively on the results obtained near the defining fixed points of the ITS-90. For CCT-K2.5, the NRC measurements employed the superior thermal environment of the multiwell fixed-point cells that were used very effectively to calibrate the CSPRTs for the determination of the temperature of the triple point of xenon [11]. The measurements at NRC were carried out from January to May 2006. The TPW resistances of the CSPRTs were measured in January and again prior to their return to NMIJ, but following the fixed-point measurements. Both thermometers remained stable throughout the 3-month period, with the two values for RS954-7 essentially identical and those for RS85A-6 different by only 30 µk. In the years since the measurements for CCT-K2 were carried out, reference isotopic compositions and correction equations have refined the ITS-90 definitions for the fixed points e-h 2, Ne and H 2 O. The hydrogen cell used at NRC was filled from the same container of gas that contributed to the determination of the sensitivity coefficient for deuterium contamination [12], with an isotopic concentration of 103.8 µmol D/mol H implying that the triple point temperature of cell SS-M-2 is 80 µk higher than the e-h 2 reference temperature 13.8033 K. For neon, the isotopic mole fractions of the gas used to fill cell Cu-M-1 are x( 21 Ne) = 0.002669 and x( 22 Ne) =0.092275, with the balance 20 Ne [13]. Based on the data from the Technical Annex 5

for the ITS-90, the triple point temperature of cell Cu-M-1 is 33 µk lower than the Ne reference temperature 24.5561 K. The triple point of water measurements derived from cell Cu-M-6 are traceable to the NRC reference cell (Jarrett/Isotech B-11-2063) that participated in the CCT-K7 [14] comparison and known to have an isotopic composition that leads to a triple point temperature 6.4 µk above the reference temperature 273.16 K. A CSPRT whose resistance was measured in Cu-M-6 was also measured in a large, glass TPW cell (Hart 5901C-9-1032) having a thermowell of sufficient internal diameter to accommodate the CSPRT. The temperature of this cell was in turn compared to B-11-2063 using a long-stem SPRT (L&N 1504288). Through these comparisons, it was determined that Cu-M-6 realizes a temperature 135 µk colder than the TPW reference temperature 273.16 K. Table 3. Resistance ratios obtained at NRC corrected for the isotopic concentrations of the e-h 2, Ne, and H 2 O cells as detailed in the text. The values in italics are the resistances of the thermometers (in ohms) at 273.16 K. Fixed point Cell T / K W(RS954-7) W(RS85A-6) u L / mk identification e-h 2 SS-M-2 13.8033 0.001 494 463 0.001 230 289 0.20 IGT n/a 17.0324 0.002 618 025 0.002 348 643 0.58 IGT n/a 20.2700 0.004 572 228 0.004 300 228 0.58 Ne Cu-M-1 24.5561 0.008 800 181 0.008 523 210 0.20 O 2 Cu-M-3 54.3584 0.092 071 860 0.091 789 499 0.20 Ar Cu-M-7 83.8058 0.216 166 646 0.215 918 929 0.20 Hg SS-M-Hg 234.3156 0.844 205 460 0.844 153 600 0.20 H 2 O Cu-M-6 273.16 25.703 740 25.173 794 0.15 2.3.2 Resistance measurements The resistance measurements were made with the same Automatic Systems Laboratories Model F18 resistance bridge used for CCT-K2. A 25 Tinsley Model 5685A reference resistor was used, thermostatted at 25 C ± 2 mk in a Guildline 9732VT oil bath. From 13.8033 K to 24.5561 K, currents of 5 ma and 5 2 ma were used. At 54.3584 K, currents of 2 ma and 2 2 ma were used. From 83.8058 K to 273.16 K, currents of 1 ma and 2 ma were used. 2.3.3 Traceability to CCT-K2 NRC contributed to CCT-K2 a single Leeds and Northrup CSPRT, S/N 1872174, that had been calibrated in specific sealed fixed-point cells produced at NRC (e-h 2 cell 22, Ne cell F15, O 2 cell F10, Ar cell F13), as well as in a hydrogen vapour-pressure cryostat at temperatures near 17.0018 K and 20.2676 K, in a large glass cell suitable for long-stem thermometers (cell Hg-2), and in a large glass triple point of water cell. 6

The NRC thermometer (L&N 1872174) used in the original CCT-K2 comparison [1] was broken several years ago. In addition, establishing traceability to CCT-K2 via resistance ratios obtained more than 10 years ago for a single CSPRT would offer potentially dubious results without additional checks. The values in Table 3 reflect fixed-point realizations using the multi-well cells currently favoured at NRC. Nonetheless, we have linked the contemporary cells to the single-well sealed cells that were used to calibrate the NRC CSPRT for the CCT-K2 exercise. The resistance ratios of Leeds and Northrup CSPRT S/N 1876687 were measured in the multi-well cells at the same time that the resistance ratios were determined during a comparison of thermometers carried out in 2006. Immediately following the comparison exercise, S/N 1876687 was measured in cell 22 (e-h 2 ), cell F17 (Ne), cell F10 (O 2 ), and cell F13 (Ar). For e-h 2, O 2, and Ar, the linkage to the cells used for CCT-K2 is direct, assuming that the cells realize the same temperature as they did 10 years earlier. For Ne, the link to the CCT-K2 cell, F15, was established via measurements carried out with CSPRT S/N 1872174. Cell F17 was measured in September 2002, and 3 months later we determined that cell F15 was 0.053 mk hotter than cell F17. Linkage to CCT-K2 at 17 K and 20.3 K was lost when CSPRT 1872174 was broken, and the e- H2 vapour pressure points have not been realized at NRC since. Instead, traceability is to the NRC IGT scale via RhFe resistance thermometer A140, so for these temperatures a link to CCT- K1 is possible through the NRC data but not to CCT-K2. By this mechanism, we determined that: 1) Hydrogen SS-M-2 is 0.527 mk hotter than cell 22 2) Neon Cu-M-1 is 0.112 mk colder than F17, and F17 is 0.053 mk colder than F15 3) Oxygen Cu-M-3 is 0.098 mk hotter than F10 4) Argon Cu-M-7 is 0.120 mk colder than F13 3. Results For the comparison measurements carried out at NRC, the experimental uncertainty budget remains 0.12 mk at 20.3 K and below and 0.09 mk above 20.3 K, the same as in the CCT-K2 report [1]. This uncertainty component, u Exp, is summed in quadrature with each of the laboratory uncertainties, u L, to form a combined uncertainty, u C. The pair uncertainty for the comparison, u P, is obtained by summing in quadrature the combined uncertainties for each laboratory. The justification for treating the experimental uncertainty components as completely uncorrelated when evaluating the pair uncertainty is explained in further detail in the CCT-K2 report. The relevant uncertainty equations are as follows: u C u u (1) 2 L 2 Exp u p u u (2) 2 NRC 2 Lab 2 2u Exp 7

The results of the comparison measurements carried out at NRC with the NMIJ thermometers are summarized in Table 4 and Figure 1 with respect to the current NRC realization of the ITS-90. Only the TPW value for CSPRT RS85A-6 exceeds the expanded pair uncertainty. Table 4. Comparison data for the NMIJ thermometers expressed with respect to the 2006 NRC realization of the ITS-90 based on the new generation of multi-well fixed points. The table includes the expanded pair uncertainties (k = 2) for these temperature differences. The values at 273.16 K show the change in CSPRT RS85A-6 as detected at NMIJ. T NRC / K T NMIJ T NRC / mk RS954-7 RS85A-6 2u p 13.8033-0.159-0.004 0.55 17.0324-0.251-0.120 1.56 20.2700-0.426-0.286 1.56 24.5561-0.174-0.117 0.56 54.3584 0.200 0.224 0.49 83.8058 0.167 0.131 0.51 234.3156 0.022-0.162 0.58 273.1600-0.108 0.799 0.52 8

Figure 1. The comparison data for the NMIJ CSPRTs plotted with respect to the NRC realization of the ITS-90 based on the new generation of multi-well fixed points. The error bars represent the expanded uncertainties of the temperature differences, 2u p. The NMIJ data may be related to the CCT-K2 results by combining the results from Table 4 with the differences reported for the NRC thermometer with respect to the KCRV as determined by the CCT-K2 exercise. Since the NRC thermometer was used in both Groups A and B of CCT-K2, the degrees of equivalence for NMIJ with respect to the CCT-K2 KCRV were calculated for both determinations of (T NRC-1996 T KCRV ) according to Equation 3. These values, together with their average, are compiled in Table 5. D NMIJ T NMIJ TNRC 2006 ( TNRC 2006 TNRC 1996) T NRC 1996 TKCRV CCT K2 (3) The expanded uncertainty, U, of the NMIJ degree of equivalence (also listed in Table 5), is obtained by appropriately combining the uncertainties of the bracketed terms in Equation 3. The uncertainty of (T NMIJ T NRC ) is readily identified as u p from Equation 2, and reported in Table 4. Since the CCT-K2 KCRV has zero uncertainty by definition, the uncertainty of the third term in Equation 3, (T NRC-1996 T KCRV ), is determined solely by the uncertainty in T NRC-1996. This is simply the combined standard uncertainty of the NRC calibration in 1996, u NRC-1996, and the experimental comparison uncertainty of CCT-K2, which is equal to u Exp, calculated according to Equation 1. The NRC laboratory uncertainties for CCT-K2 are identical to those in Table 3. In combining the bracketed terms of Equation 3, we must consider the extent to which the two uncertainties for T NRC-2006 and T NRC-1996 are correlated. The 1996 and 2006 calibrations utilized the same resistance bridge and experimental apparatus, and links are maintained to the same fixedpoint cells. Under the assumption that the temperatures realized by the NRC sealed cells are stable in time, the Type B components of the uncertainty budgets for CCT-K2 and this bilateral key comparison can be considered to be completely correlated, and so only the Type A repeatability component from the full laboratory uncertainty budget should be counted twice once as the component of u NRC for this measurement comparison, and once to accommodate the uncorrelated component of u NRC-1996 reported in CCT-K2. This component is u TypeA = 0.07 mk for all five triple point temperatures. Equation 4 summarizes the calculation of the expanded uncertainties of Tables 5 and 6 for the NMIJ degree of equivalence to the Key Comparison Reference Value of CCT-K2. U 2 2 2 2 2 u u 2u u u (4) 2 NRC NMIJ Exp TypeA Exp Table 5 and Figures 2 and 3 show that the NMIJ measurements agree with the CCT-K2 KCRV within the expanded combined uncertainties, U, at all temperatures. 9

Table 5. The degrees of equivalence, D, for NMIJ thermometer RS954-7 expressed with respect to the KCRV for Groups A and B from the CCT-K2 report and their average, mediated by the NRC realization of the ITS-90. The table includes the expanded uncertainties of the degrees of equivalence, U (k = 2). The italic font indicates that the values at 17 K and 20.3 K are not, strictly speaking, traceable to CCT-K2, as explained previously. T / K D A / mk D B / mk D Avg / mk U / mk 13.8033 0.03 0.02 0.02 0.61 17.0324-0.25-0.14-0.20 1.63 20.2700-0.40-0.33-0.36 1.63 24.5561-0.40-0.46-0.43 0.60 54.3584 0.48 0.54 0.51 0.54 83.8058 0.23 0.29 0.26 0.56 234.3156-0.12-0.12-0.12 0.62 Table 6. The degrees of equivalence, D, for NMIJ thermometer RS85A-6 expressed with respect to the KCRV for Groups A and B from the CCT-K2 report and their average, mediated by the NRC realization of the ITS-90. The table includes the expanded uncertainties of the degrees of equivalence, U (k = 2). The italic font indicates that the values at 17 K and 20.3 K are not, strictly speaking, traceable to CCT-K2, as explained previously. T / K D A / mk D B / mk D Avg / mk U / mk 13.8033 0.18 0.17 0.18 0.61 17.0324-0.12-0.01-0.07 1.63 20.2700-0.26-0.19-0.22 1.63 24.5561-0.34-0.40-0.37 0.61 54.3584 0.50 0.56 0.53 0.54 83.8058 0.19 0.25 0.22 0.56 234.3156-0.30-0.30-0.30 0.62 10

Figure 2. The comparison data for the NMIJ CSPRT RS954-7 plotted with respect to the KCRV determined from the two Groups of measurements from the CCT-K2 report, mediated by the NRC realization of the ITS-90. The error bars represent the expanded uncertainties, U. Figure 3. The comparison data for the NMIJ CSPRT RS85A-6 plotted with respect to the KCRV determined from the two Groups of measurements from the CCT-K2 report, mediated by the NRC realization of the ITS-90. The error bars represent the expanded uncertainties, U. 11

A similar analysis can be carried out for the INRIM measurements with CSPRT S/N 1860951. First, consider the comparison results between INRIM and NMIJ. Table 7 shows the differences and twice the pair-difference uncertainty, calculated on the basis of Eq. (1) and Eq. (2), but with u Exp = 0 lacking any information that additional uncertainty components beyond the laboratory budgets, u L, are needed to characterize the comparison. The temperature differences are well within the expanded uncertainties, demonstrating consistency between INRIM and NMIJ. Table 7. Comparison data for the INRIM thermometer expressed with respect to the NMIJ realization of the ITS-90 (averaged over the two thermometers). These are the direct, bilateral differences between the two laboratories, without reference to NRC. The table includes the expanded pair uncertainties (k = 2) for these temperature differences. The value at 273.16 K expresses the consistency in the triple point of water realizations at the participating laboratories as carried on the thermometer. T NMIJ / K T INRIM T NMIJ / mk 1860951 2u p 13.8033-0.024 0.25 24.5561-0.179 0.30 54.3584-0.019 0.16 83.8058-0.044 0.25 234.3156-0.103 0.35 273.1600 0.109 0.34 Figure 4. The comparison data for the INRIM CSPRT plotted with respect to the NMIJ realization of the ITS-90. The error bars represent the expanded uncertainties of the temperature differences, 2u p. 12

In order to link the INRIM results to the CCT-K2 KCRV, we simply extend Eq. (3) by adding a term for the difference between INRIM and NMIJ. D INRIM T T T T (5) INRIM NMIJ NMIJ KCRV The uncertainty of (T NMIJ T NRC ) is u p from Table 7 and the uncertainty of (T NMIJ T KCRV ) is from Table 5 (or 6). In order to make the linkage manageable, the differences from the KCRV for the two NMIJ thermometers have been averaged as a best estimate of (T NMIJ T KCRV ) from the differences enumerated in Tables 5 and 6. Equation 6 summarizes the calculation of the expanded uncertainties of Table 8 for the INRIM degree of equivalence to the Key Comparison Reference Value of CCT-K2. U 2 2 2 2 2 2 2 u u u u 2u u u (6) 2 INRIM NMIJ NRC NMIJ Exp TypeA Exp Table 8 and Figure 5 summarize the INRIM data with respect to the CCT-K2 KCRV. Table 8. The degrees of equivalence, D, for the measurements carried out at INRIM using thermometer 1860951 expressed with respect to the KCRV for Groups A and B from the CCT- K2 report and their average, mediated by the NRC realization of ITS-90. The table includes the expanded uncertainties of the degrees of equivalence, U (k = 2). T / K D A / mk D B / mk D Avg / mk U / mk 13.8033 0.04 0.03 0.03 0.66 24.5561-0.52-0.58-0.55 0.68 54.3584 0.46 0.52 0.49 0.57 83.8058 0.14 0.20 0.17 0.62 234.3156-0.43-0.43-0.43 0.71 13

Figure 5. The comparison data for the INRIM CSPRT plotted with respect to the KCRV determined from the two Groups of measurements from the CCT-K2 report, mediated by the NRC realization of the ITS-90. The error bars represent the expanded uncertainties, U. 4. Conclusion The NRC/NMIJ comparison of capsule-style platinum resistance thermometers over the range 13.8 K to 273.16 K has revealed calibrations at NMIJ to be in agreement with the KCRV of CCT- K2 within the expanded uncertainty for all temperatures of the comparison. The linkage to the CCT-K2 data supports the inclusion of the NMIJ CMCs in Appendix C of the KCDB. The INRIM data is also consistent with the KCRV of CCT-K2 within the expanded uncertainty of the comparison. References 1. Steele A G, Fellmuth B, Head D I, Hermier Y, Kang K H, Steur P P M and Tew W L 2002 Key Comparison: CCT-K2: Key comparison of capsule-type standard platinum resistance thermometers from 13.8 K to 273.16 K Metrologia 39 551-571 2. Hill K D, Steele A G, Dedikov Y A and Shkraba V T 2005 CCT-K2.1: NRC/VNIIFTRI bilateral comparison of capsule-type standard platinum resistance thermometers from 13.8 K to 273.16 K Metrologia 42 03001 doi:10.1088/0026-1394/42/1a/03001 3. Hill K D, Peruzzi A and Bosma R 2012 CCT-K2.3: NRC/NMi-VSL bilateral comparison of capsule-type standard platinum resistance thermometers from 13.8 K to 273.16 K Metrologia 49 03004 doi:10.1088/0026-1394/49/1a/03004 4. Hill K D, Szmyrka-Grzebyk A, Lipinski L, Hermier Y, Pitre L and Sparasci F 2012 CCT- K2.4: NRC/INTiBS/LNE-Cnam trilateral comparison of capsule-type standard platinum 14

resistance thermometers from 13.8 K to 273.16 K Metrologia 49 03005 doi:10.1088/0026-1394/49/1a/03005 5. Nakano T, Tamura O and Sakurai H 2007 Realization of Low-Temperature Fixed Points of the ITS-90 at NMIJ/AIST Int. J. Thermophys. 28 1893-1903 6. Sakurai H 2002 Calorimetric Realization of the triple Point of Mercury Using a Pulse-Tube Refrigerator in Temperature, Its Measurement and Control in Science and Industry, Vol. 7 part 1, ed. by D.C. Ripple (AIP, New York) 209-214 7. Nakano T, Tamura O and Sakurai H 2008 Comparison System for the Calibration of Capsule- Type Standard Platinum Resistance Thermometers at NMIJ/AIST Int. J. Thermophys. 29 881-889 8. Tamura O, Takasu S, Nakano T and Sakurai H 2008 NMIJ Constant-Volume Gas Thermometer for Realization of the ITS-90 and Thermodynamic Temperature Measurement Int. J. Thermophys. 29 31-41 9. Pavese F, Ferri D, Peroni I, Pugliese A, Steur P P M, Fellmuth B, Head D I, Lipinski L, Peruzzi A, Szmyrka-Grzebyk A and Wolber L 2003 Cryogenic temperature sealed fixed points: IMGC new-generation modular cells in Temperature, Its Measurement and Control in Science and Industry, Vol. 7 part 1, ed. by D.C. Ripple (AIP, New York) 173 178 10. Ferri D, Ichim D, Pavese F, Peroni I, Pugliese A, Sparasci F and Steur P P M 2003 A Closedcycle Refrigerator for Realizing Low-Temperature Fixed Points in 2nd Seminar on Low- Temperature Thermometry, Wroclaw (Poland), 102 107 11. Hill K D and Steele A G 2005 The triple point of xenon Metrologia 42 278-288 12. Fellmuth B, Wolber L, Hermier Y, Pavese F, Steur P P M, Peroni I, Szmyrka-Grzebyk A, Lipinski L, Tew W L, Nakano T, Sakurai H, Tamura O, Head D I, Hill K D and Steele A G 2005 Isotopic and other influences on the realization of the triple point of hydrogen Metrologia 42 171-193 13. Pavese F, Steur P P M, Hermier Y, Hill K D, Kim J S, Lipinski L, Nagao K, Nakano T, Peruzzi A, Sparasci F, Szmyrka-Grzebyk A, Tamura O, Tew W L, Valkiers S and van Geel J 2013 Dependence of the triple point temperature of neon on isotopic composition and its implications for the ITS-90 Temperature, Its Measurement and Control in Science and Industry, Vol. 8 part 1, ed. by C.W. Meyer AIP Conference Proceedings 1552 192 197 DOI: 10.1063/1.4821378 14. Stock M, Solve S, del Campo D, Chimenti V, Méndez-Lango E, Liedberg H, Steur P P M, Marcarino P, Dematteis R, Filipe E, Lobo I, Kang K H, Gam K S, Kim Y- G, Renaot E, Bonnier G, Valin M, White R, Dransfield T D, Duan Y, Xiaoke Y, Strouse G, Ballico M, Sukkar D, Arai M, Mans A, de Groot M, Kerkhof O, Rusby R, Gray J, Head D, Hill K, Tegeler E, Noatsch U, Ďuriš S, Kho H Y, Ugur S, Pokhodun A, Gerasimov S F 2006 Final Report on CCT-K7 Key comparison of water triple point cells http://www.bipm.org/utils/common/pdf/final_reports/t/k7/cct-k7.pdf Address of the Corresponding Author K. D. Hill, National Research Council of Canada (NRC), Measurement Science and Standards, Montreal Road, M-36, Ottawa, Ontario, Canada, K1A 0R6. Tel: (613) 998-6077; fax: (613) 952-1394 e-mail: ken.hill@nrc.ca; website: www.nrc.ca 15

Appendix A - Measurements at NMIJ NMIJ reported the calibration data and uncertainty budgets at e-h 2, Ne, O 2, Ar, Hg and H 2 O triple points and 17.035 K and 20.27 K. NMIJ also reported CSPRT resistances measured at the TPW before and after transportation and before and after cryogenic measurements at NMIJ. The resistances of the NMIJ thermometers measured at 4.2 K before and after transportation were also reported. To measure the resistances, a Measurements International Model 6010B dc bridge was used. The excitation currents for measurements at the TPW and in liquid He were 1 ma and 5 ma, respectively. Tinsley Model 5685A standard resistors of 10 ohm and 1 ohm were used for the measurements at the TPW and in liquid He, respectively. Before sending the thermometers to NRC, the following resistances were measured: Thermal environment R(RS85A-6) / ohms R(RS954-7) / ohms TPW before measurement in liquid He 25.173 798 25.703 719 Liquid He (4.2 K) 0.009 392 0.015 836 TPW after measurement in liquid He 25.173 806 25.703 724 After the thermometers returned to NMIJ, we obtained the following resistances at the TPW: Thermal environment R(RS85A-6) / ohms R(RS954-7) / ohms TPW before measurement in liquid He 25.173 878 25.703 725 Liquid He (4.2 K) 0.009 391 0.015 834 TPW after measurement in liquid He 25.173 873 25.703 726 NMIJ also reported the results for e-h 2 triple point with the isotopic correction (values at 13.8033 K) based on the Technical Annex for the ITS-90. Since the isotopic concentration of the NMIJ H 2 sample is 27.2 (2) mmol D per mole H, the magnitude of the isotopic correction for the realized triple point temperature of e-h 2 at NMIJ and its standard uncertainty have been estimated to be -0.335 mk and 0.02 mk, respectively. The measurements of the Ne triple point were also corrected to 24.5561 K, based on the isotopic composition of the sample gas and the information provided in the Technical Annex for the ITS- 90. The isotopic concentration of the NMIJ Ne-5 (x(21ne) = 0.002659 and x(22ne) =0.092519) results in a triple point temperature 1.7 µk above the ITS-90 value 24.5561 K. The NMIJ TPW cell (LT-M0102) used to calibrate the CSPRTs was compared to the national reference, cell TR0201, and the two cells were found to be equivalent within the uncertainty of the comparison. Tamba et al. (Int. J. Thermophys (2008) 29 1749) studied the influence of isotopic composition on the temperatures of a group of TPW cells, and reported that cell TR0201 is 42 µk lower than the mean of seven new cells with their temperatures corrected to the reference isotopic composition. Therefore, cell LT-M0102 is also 42 µk lower than the TPW reference composition. At NMIJ, we calibrated two capsule type standard platinum resistance thermometers SPRTs of NMIJ (Serial Nos. Chino RS954-7, RS85A-6), and one CSPRT (Serial No. LN 1860951) and one rhodium-iron resistance thermometer (Serial No. 229821) of INRiM. 16

CSPRTs RS954-7, RS85A-6 and LN1860951 were calibrated directly using the NMIJ fixed-point cells for the Ne, Hg and H 2 O triple points, and calibrated by comparison to the reference CSPRT using a comparison cryostat at Ar and O 2 triple points. The procedure of the realization and comparison was reported in refs. [1-3]. The thermometers were also calibrated by comparison to the reference RhFe thermometer using another comparison cryostat at 17.035 K, 20.27 K and the e-h 2 triple point. Although the comparison system is different from that used at the triple points of O 2 and Ar, the procedure of the comparison was same as that reported in [3]. The reference SPRT was calibrated to be traceable to the cryogenic fixed points at NMIJ [1, 2]. The reference RhFe thermometer was calibrated directly for the e-h 2 triple point [1] and calibrated by comparison to another reference RhFe thermometer calibrated directly using the NMIJ interpolation constant volume gas thermometer at 17.035 K and 20.27 K [4]. Although the data for LN1860951 was obtained by the different runs of the NMIJ CSPRTs, we used the same reference thermometer for the LN1860951 and NMIJ CSPRTs for the 17.035K and 20.27 K. Rhodium iron thermometer 229821 of INRIM was calibrated directly using the NMIJ fixed-point cell for the Ne triple point, and calibrated by comparison at the e-h 2 triple point in the same way as for the SPRTs RS954-7, RS85A-6 and LN1860951. NMIJ used an automatic direct-current-comparator bridge for the measurements of the SPRTs. The correction for self-heating effects was obtained from the observations at I and I 2. Results are given for zero thermometer current. NMIJ used a 10 ohm standard resistor for the calibration of the triple points of Hg and water. For the calibration of the other fixed points, a 1 ohm standard resistor was used. The standard resistors were calibrated by the Electricity and Magnetic Division of NMIJ to be traceable to the national resistance standards. As to the uncertainty component thermal gradient for the comparison K2.5, we estimated the value directly for each measurement of the K2.5 from the effect of the change of the heater power on the temperature shift of the SPRT in the comparison block (Fig. 3 of IJT 29 (2008) p. 886) and the heater powers induced during measurement of the K2.5. Since we tried to reduce the heater power for the K2.5, the uncertainty component thermal gradient for the comparison K2.5 is smaller than that of the IJT 29 (2008) article. References 1. T. Nakano, O. Tamura, H. Sakurai, Int. J. Thermophys. 28 (2007) 1893-1903. 2. H. Sakurai, Temperature Vol. 7 part 1 (2002) 209-214. 3. T. Nakano, O. Tamura, H. Sakurai, Int. J. Thermophys. 29 (2008) 881-889. 4. O. Tamura, S. Takasu, T. Nakano and H. Sakurai, Int. J. Thermophys. 29 (2008) 31-41. 17

Uncertainty for Chino RS954-7 H 2 13.8033 K (H 2 * ) 17 20.3 Ne O 2 Ar Hg H 2 O Realization reproducibility 0.01 0.01 0.01 0.01 0.011 0.012 0.012 Reference Scale(CVGT) 0.492 0.493 Chemical impurity and Isotopes 0.2 0.02 0.14 0.009 0.022 0.057 Hydrostatic 0.005 0.005 0.02 0.02 0.04 0.007 0.02 determination of FP value 0.008 0.008 0.01 0.011 0.007 0.03 0.16 # spin equilibrium and catalysis 0.041 0.041 thermal equilibrium 0.012 0.012 0.006 0.008 0.017 0.04 measurement bridge accuracy 0.04 0.04 0.021 0.013 0.002 0.001 0.002 0.011 0.013 Standard resistor 0.002 0.002 0.002 0.002 0.003 0.009 0.019 0.015 0.03 propagation from TPW 0.001 0.001 0.001 0.001 0.001 0.015 0.036 0.139 self-heating 0.003 0.003 0.005 0.008 0.004 0.003 0.006 0.005 0.008 Comparison Temperature stability 0.023 0.023 0.023 0.023 0.023 0.023 thermal gradient 0.001 0.001 0.004 0.004 0.01 0.034 ref. thermometer measurements 0.028 0.028 0.024 0.027 0.054 0.062 temperature corrections 0.019 0.019 0.01 0.008 0.009 0.008 Combined (k=1) 0.213 0.076 0.494 0.495 0.142 0.069 0.099 0.160 0.165 * Isotopic correction based on the technical annex for the ITS-90 # Uncertainty of calibration of TPW cell by national standard at NMIJ 18

Uncertainty for Chino RS85A-6 H 2 13.8033 K (H 2 * ) 17 20.3 Ne O 2 Ar Hg H 2 O Realization reproducibility 0.01 0.01 0.01 0.01 0.011 0.012 0.023 Reference Scale(CVGT) 0.492 0.493 Chemical impurity 0.2 0.02 0.14 0.009 0.022 0.057 and Isotopes Hydrostatic 0.005 0.005 0.02 0.02 0.04 0.007 0.02 determination of FP 0.008 0.008 0.01 0.011 0.007 0.03 0.16 # value spin equilibrium and catalysis 0.041 0.041 thermal equilibrium 0.012 0.012 0.006 0.008 0.017 0.04 measurement bridge accuracy 0.041 0.041 0.022 0.013 0.002 0.001 0.002 0.011 0.013 Standard resistor 0.002 0.002 0.002 0.002 0.003 0.009 0.019 0.015 0.03 propagation from TPW 0.001 0.001 0.001 0.001 0.001 0.015 0.036 0.142 self-heating 0.002 0.002 0.004 0.006 0.003 0.006 0.014 0.005 0.025 Comparison Temperature 0.023 0.023 0.023 0.023 0.023 0.023 stability thermal gradient 0.001 0.001 0.004 0.004 0.01 0.034 ref. thermometer 0.028 0.028 0.024 0.027 0.054 0.062 measurements temperature corrections 0.017 0.017 0.011 0.006 0.012 0.01 Combined (k=1) 0.213 0.076 0.494 0.495 0.145 0.069 0.100 0.163 0.168 * Isotopic correction based on the technical annex for the ITS-90 # Uncertainty of calibration of TPW cell by national standard at NMIJ 19

Uncertainty for Leeds and Northrup 1860951 H 2 13.8033 K (H 2 * ) 17 20.3 Ne O 2 Ar Hg H 2 O Realization reproducibility 0.01 0.01 0.01 0.01 0.011 0.012 0.01 Reference Scale(CVGT) 0.492 0.493 Chemical impurity 0.2 0.02 0.14 0.009 0.022 0.057 and Isotopes Hydrostatic 0.005 0.005 0.02 0.02 0.04 0.007 0.02 determination of FP 0.008 0.008 0.01 0.011 0.007 0.03 0.16 # value spin equilibrium and catalysis 0.041 0.041 thermal equilibrium 0.012 0.012 0.006 0.008 0.017 0.04 measurement bridge accuracy 0.041 0.041 0.02 0.013 0.002 0.001 0.003 0.011 0.013 Standard resistor 0.002 0.002 0.002 0.002 0.003 0.009 0.019 0.015 0.03 propagation from TPW 0.001 0.001 0.001 0.001 0.001 0.015 0.036 0.14 self-heating 0.005 0.005 0.003 0.005 0.003 0.003 0.006 0.005 0.014 Comparison Temperature 0.023 0.023 0.023 0.023 0.023 0.023 stability thermal gradient 0.001 0.001 0.004 0.004 0.01 0.034 ref. thermometer 0.028 0.028 0.024 0.027 0.054 0.062 measurements temperature corrections 0.062 0.062 0.016 0.012 0.022 0.016 Combined (k=1) 0.221 0.096 0.494 0.495 0.142 0.071 0.1 0.161 0.165 * Isotopic correction based on the technical annex for the ITS-90 # Uncertainty of calibration of TPW cell by national standard at 20

Appendix B Final INRIM Results for Comparison CCT-K2.5 Key Comparison of Capsule Standard Resistance Thermometers FINAL calibration results for INRIM thermometers LN1857277 and LN1860951 F = 100% (uncertainties at 95% confidence level) [these thermometers participated in the original CCT-K2 comparison] Premise During the original CCT-K2 comparison, it was not possible to correct results obtained at the e- H 2 and Ne fixed points for their isotopic composition, since the information needed to do so was published much later. For full compatibility with CCT-K2, the results are presented here at first (Table 1) without the isotopic composition corrections (also in the uncertainty budget), except for neon, u = ± 0.050 mk), and in Table 2 the results are presented with the information at the present state-of-the-art. The associated uncertainty budgets are given after the actual resistance and W values. Equipment Unlike the original CCT-K2 comparison [1], in which IMGC (now INRIM) participated, the measurements at e-h 2, Ne, O 2 and Ar for CCT-K2.5 were performed with the use of the same calorimeter and modular sealed cells designed during the MULTICELLS project [2,3], and significantly improved during the Neon Project. Details on the equipment and the dataacquisition software can be found in various publications [e.g. 4,5]. Exactly the same two standard Platinum Resistance Thermometers were used during CCT-K2 and CCT-K2.5. They have been heavily used during the Neon Project (Euromet 770), see [4-9], while also for Euromet.T-K1.1 (to be published). The measurements at Hg and H 2 O were performed, as during CCT-K2, in the Medium Temperatures lab of INRIM. Because of the known difficulties associated with borosilicate cells for the Triple Point of Water (TPW), for CCT-K2.5 a recent fused-silica cell was used, with known isotopic composition. 21

Data Treatment The data obtained for e-h 2, Ne, O 2 and Ar were treated with the LSFME method, described in [10-12]. For e-h 2, overall fitting yielded standard errors of 20.8 µk and 21.1 µk for a linear and a quadratic fit in 1/F, respectively, where F stands for the melted fraction. The linear fit was chosen. For Ne, standard fitting errors of 25.4 µk and 25.8 µk were found for a linear and a quadratic fit in 1/F, respectively. The linear fit was chosen. For O 2, the standard fitting errors were 21.8 µk and 10.5 µk for a linear and a quadratic fit in 1/F, respectively. The quadratic fit was chosen. For Ar, the standard fitting errors were 39.6 µk and 16.0 µk for a linear and a quadratic fit in 1/F, respectively. The quadratic fit was chosen. These uncertainties are considered to be included in the overall plateau repeatability. Traceability to CCT-K2 Hydrogen For CCT-K2, a sealed cell of type S, 2H2, was used, filled with hydrogen of 6N nominal purity. If the whole nominal impurity of 1 ppm is attributed to He (unlikely), this would cause a depression of 0.011 mk only. No correction for impurities was applied. For CCT-K2.5, cell Ec1H2 was used. This cell was sealed during the MULTICELLS project [2] with hydrogen of certified isotopic composition (Messer ISOTOP -41.26%) and 5N nominal purity, and known (from a batch analysis to contain 7.4 ppm 1 N 2 and 0.2 ppm CO 2, while Ne was below threshold (5 ppm). The former two do not affect the triple point temperature, while taking the threshold value for Ne for a maximum estimate one would obtain a depression of 0.010 mk. No correction for impurities is applied, but impurity is taken into account in the uncertainty budget with a (one-sided) contribution of 0.003 mk, based on the nominal purity and evaluated with the one-sided OME effect [16]. Any temperature difference between the two cells due to chemical impurities is estimated to be within 0.01 mk. Isotopic composition Since 2005, a method is available for taking the presence of HD in hydrogen into account see the Technical Annex for the Mise en pratique of the kelvin. 1 1 ppm = 10-6 22

Cell 2H2 is known to contain 101.2 (1.0) 2 ppm D/H and cell Ec1H2 91.6 (1.0) ppm D/H, where 1 ppm D/H corresponds to 5.4 2 (3 1 ) µk. Based on these values, cell 2H2 realizes a temperature 0.066 (7) mk higher than the reference, while the value for cell Ec1H2 is 0.014 (6) mk. Therefore, based on the known isotopic compositions, the realized temperature for cell Ec1H2 is 0.052 (9) mk lower than for cell 2H2. Neon For CCT-K2, a sealed cell of type S, 3Ne, was used but the National Standard was declared to be cell 1Ne. The difference between the two cells was stated to be 0.0 (2) mk. Cell 1Ne was filled with gas of 4N5 nominal purity, and a batch analysis yielded 11.3 ppm N 2 and 2.7 ppm O 2, while He and H 2 were below threshold (50 and 25 ppm, respectively). Due to the filling method (cryogenic pumping), He impurities are unlikely. The sensitivity coefficients for H 2 and N 2 are -7 and -8 µk/ppm [13], respectively, while for O 2 there is no effect. The only certain effect would derive from N 2 : a depression of 0.090 mk. Assuming an uncertainty of 25 % for the batch analysis, the latter figure will have an uncertainty of 0.006 mk, evaluated with the one-sided OME effect [16]. No correction for impurities was applied. Cell Ec2Ne, sealed during the MULTICELLS project [2] and used here for CCT-K2.5, was filled with gas of 5N nominal purity. Chemical analysis of this gas yielded He, H 2, N 2 and O 2 all below threshold (2 ppm for N 2, 1 ppm for the other gases). Using the threshold values for a maximum estimate one arrives at a depression of 0.025 mk. No correction for chemical impurities is applied. A contribution from this is added to the uncertainty budget, with a value of 0.002 mk, evaluated with the one-sided OME effect [16]. Any temperature difference between the two cells, due to chemical impurities, is estimated to be within 0.03 mk. Isotopic composition Until 2012, it was not possible to correct for the isotopic composition of neon. However, since the composition of the gas used for cells 3Ne and Ec2Ne is known, the difference in realized temperature between the two cells, arising from this effect, is determined based on the equations published in the Technical Annex to the Mise en pratique for the kelvin. The isotopic composition of the gas in Cell 3Ne is ( 21 x = 0.002686; 22 x = 0.094602) [8], yielding a temperature 0.310 (8) mk higher than the reference, while for cell Ec2Ne the composition of the gas is ( 21 x = 0.002649 (5); 22 x = 0.09336 (5)) [8], leading to a temperature 0.125 (8) mk higher than the reference. Thus, based on the known isotopic composition, cell Ec2Ne realizes a temperature 0.186 (11) mk lower than cell 3Ne. For assay uncertainty, see [14]. 2 Uncertainties in parentheses are given for k = 1. 23

Oxygen For CCT-K2, a sealed cell of type S, 8O2, was used, while for CCT-K2.5 a miniature sealed cell, Ec1O2, was used. The latter type was developed and sealed during the MULTICELLS project [2]. Since the then National Standard, 1O2, could not house more than one thermometer, cell 8O2 was used which could house up to three thermometers. The difference from 1O2, due to impurities in 8O2, was measured to be +0.48 (15) mk, assumed to be mainly due to the effect of argon. The W-value reported for CCT-K2 applies to cell 1O2. No correction for impurities was applied. A contribution ± 0.15 mk due to the measured difference was added to the uncertainty budget. On the contrary, cell Ec1O2, sealed during the MULTICELLS project and used here for CCT- K2.5, is filled with 5N5 nominal purity oxygen, and a chemical analysis yielded traces of Ne, N 2, Ar and CH 4 all below the measurement threshold (5, 5, 3 and 0.1 ppm, respectively). Using these thresholds for a maximum estimate, a depression of at most 0.079 (20) mk can be expected. No correction has been applied. A contribution from this has been added to the uncertainty budget, with a value of 0.013 mk, evaluated with the one-sided OME effect [16]. Cell Ec1O2 is therefore expected to realize the same temperature value as cell 1O2, with ± 0.15 mk uncertainty arising totally from cell 1O2. Isotopic effects are < 10 µk [17]. Argon During CCT-K2, a sealed cell of type S, 4Ar, was used, while for CCT-K2.5 a miniature sealed cell, Ec1Ar, was used. This latter type was developed and sealed during the MULTICELLS project [2]. Cell 4Ar contains gas of 5N7 nominal purity. At the time, it was assumed that 3 ppm are N 2 +O 2 causing a depression of 0.066 mk. No correction for impurities was applied, implying an uncertainty contribution of 0.011 mk, evaluated with the one-sided OME effect [16]. Cell Ec1Ar of 6N nominal purity, used for CCT-K2.5, was found to contain 0.75 ppm N 2 and 0.92 ppm O 2. Using sensitivities of 0.022 mk/ppm for both gases [13], one obtains a total depression of 0.037 mk. No correction for impurities was applied, implying an uncertainty contribution of 0.011 mk, evaluated with the one-sided OME effect [16]. Cell Ec1Ar is therefore expected to realize a temperature 0.029 mk higher than cell 4Ar. Isotopic effects are < 10 µk [17]. Mercury The same cell, CRYO-7, was used for both CCT-K2 and CCT-K2.5. The filling of this cell is described in [15]. 24

Water During CCT-K2, (borosilicate) cell CRYO-8 was used. In 2004, during the measurements for CCT-K7, where borosilicate cell HART-1322 was used, CRYO-8 was compared with HART- 1322 and a difference between the latter and the former was found to be 0.096 (38) mk. Later on, fused-silica cell HART1015-Q was compared with HART-1322, which resulted in a difference of 0.145 (8) mk. Thus, the difference between HART1015-Q and CRYO-8 amounts to 0.241 (39) mk. The uncertainties indicated here are for k = 1. Isotopic composition Cell HART1015-Q has a certified isotopic composition of (δ 2 H=-0.004; δ 18 O=-0.0003), resulting in a temperature change of 2.7 (9) µk. Bibliography 1. A.G.Steele, B.Fellmuth, D.I.Head, Y.Hermier, K.H.Kang, P.P.M.Steur, W.L.Tew, 2002: Key Comparison CCT-K2: Key Comparison of Capsule-type Standard Platinum Resistance Thermometers from 13.8 K to 273.16 K, Metrologia 39 (6), 551 571. 2. F.Pavese, D.Ferri, I.Peroni, A.Pugliese, P.P.M.Steur, B.Fellmuth, D.I.Head, L.Lipinski, A.Peruzzi, A.Szmyrka-Grzebyk, L.Wolber, 2003: Cryogenic temperature sealed fixed points: IMGC new-generation modular cells, Proc. Temperature, its Measurement and Control in Science and Industry, Vol.7 (Am.Inst.of Phys., New York), 173 178. 3. D. Ferri, D. Ichim, F. Pavese, I. Peroni, A. Pugliese, F. Sparasci, P.P.M. Steur, 2003: A Closed-cycle Refrigerator for Realizing Low-Temperature Fixed Points, 2 nd Seminar on Low-Temperature Thermometry, Wroclaw (Poland), 102 107. 4. F. Pavese, P.P.M. Steur, N. Bancone, D. Ferri, D. Giraudi, 2010, Comparison with U 50 µk of neon samples of different isotopic composition, Metrologia 47 (5), 499 517. http://dx.doi.org/10.1088/0026-1394/47/001. 5. F. Pavese, S. Valkiers, P.P.M. Steur, D. Giraudi, D. Ferri, 2010 An accurate determination of the triple point temperature of pure 20 Ne and 22 Ne, J. Chem. Thermodynamics 42, 1222 1229. http://dx.doi.org/10.1016/j.jct2010.04.024. 6. F. Pavese, P.P.M. Steur, Jin Seog Kim, D. Giraudi, 2011, Further results on the triple point temperature of pure 20 Ne and 22 Ne, J. Chem. Thermodynamics 43 (12), 1977 1983, http://dx.doi.org/10.1016/j.jct.2011.07.011. 25