REPORT ON VMI-NMISA BILATERAL COMPARISON: … · comparison, CCT-K3, was carried out from 1997 to 2001 by representative laboratories in North America, Europe and Asia. Following

REPORT ON VMI-NMISA BILATERAL COMPARISON: REALISATIONS OF THE ITS-90 OVER THE RANGE

-38.8344 °C TO 419.527 °C APMP.T-K3.1

Final report (7 October 2010) Prepared by Hans Liedberg

National Metrology Institute of South Africa Private Bag X34, 0040 Lynnwood Ridge, South Africa

VMI-NMISA bilateral comparison (-38.8344 °C to 419.527 °C)

APMP.T-K3.1 Final report (7 October 2010)

Page 2 of 22

CONTENTS LIST

1. INTRODUCTION ............................................................ 3 2. PARTICIPANTS ............................................................ 3 3. ARTEFACT ................................................................ 3 4. MEASUREMENT PROCEDURE ................................................... 3 5. MEASUREMENT DATA ........................................................ 4 6. UNCERTAINTY OF MEASUREMENTS ............................................. 7 7. LINK TO APMP.T-K3 (STABILITY OF NMISA’S FIXED POINT CELLS) ............. 10 8. DIFFERENCE FROM PILOT LABORATORY AND CCT-K3 ARV ........................ 11 9. DIFFERENCES FROM APMP.T-K3 AND CCT-K3 PARTICIPANTS ..................... 12 10. CONCLUSIONS ............................................................ 14 11. REFERENCES ............................................................. 14 APPENDIX 1: PROTOCOL FOR VMI-NMISA BILATERAL COMPARISON ...................... 15 APPENDIX 2: INSTRUMENTATION .................................................. 21



Page 3 of 22

1. INTRODUCTION The Comité Consulatif de Thermometrie (CCT) has organised several key comparisons to compare realisations of the ITS-90 in different national metrology institutes. To keep the organisation, time scale and data processing of such a comparison manageable, the number of participants in a CCT Key Comparison (CCT KC) is limited to a few laboratories in each major economic region. Subsequent regional key comparisons are linked to the applicable CCT KC by two or more link laboratories. For the temperature range from 83.8058 K (triple point of Ar) to 933.473 K (freezing point of Al), a key comparison, CCT-K3, was carried out from 1997 to 2001 by representative laboratories in North America, Europe and Asia. Following CCT-K3, the Asia-Pacific Metrology Program Key Comparison 3 (APMP.T-K3) was organised for national metrology institutes in the Asia/Pacific region. NMIA (Australia) and KRISS (South Korea) provided the link between CCT-K3 and APMP.T-K3. APMP.T-K3, which took place from February 2000 to June 2003, covered the temperature range from –38.8344 °C (triple point of Hg) to 419.527 °C (freezing point of Zn), using a standard platinum resistance thermometer (SPRT) as the artefact [1]. In June 2007 the Vietnam Metrology Institute (VMI) requested a bilateral comparison to link their SPRT calibration capabilities to APMP.T-K3, and in October 2007 the National Metrology Institute of South Africa (NMISA) agreed to provide the link to APMP.T-K3. Like APMP.T-K3, the comparison was restricted to the Hg to Zn temperature range to reduce the chance of drift in the SPRT artefact. The comparison was carried out in a participant-pilot-participant topology (with NMISA as the pilot and VMI as the participant). VMI’s results in the comparison were linked to the Average Reference Values (ARV) of CCT-K3 via NMISA’s results in APMP.T-K3. (The APMP.T-K3 results were linked to the CCT-K3 ARV via the results of NMIA and KRISS in the two comparisons [1].) 2. PARTICIPANTS Laboratory Contact person Measurement period Vietnam Metrology Institute (VMI) Vu Quang Cuong 1 – 14 May 2008 National Metrology Institute of South Africa (NMISA) Hans Liedberg 24 June – 17 July 2008 Vietnam Metrology Institute (VMI) Vu Quang Cuong 18 August – 5 September 2008 3. ARTEFACT The travelling SPRT details were as follows:

Serial number: 1357 Model: 5681 Manufacturer: Hart Scientific, USA. Sensor position: 10 mm to 38 mm from tip of sheath.

Before measurements for the comparison started, VMI annealed the SPRT for a total of 24 hours at 450 °C, performing regular measurements of the resistance at the water triple point (Rtp) during the annealing process. The changes in Rtp, expressed in °C, are shown in Figure 1 below. 4. MEASUREMENT PROCEDURE Following the annealing process to stabilise the artefact, the SPRT was calibrated at Zn, Sn, Ga and Hg fixed points by VMI, then it was sent to NMISA. At NMISA, it was annealed for a total of 4.5 hours at 440 °C, then calibrated at Zn, Sn, Ga and Hg fixed points, twice at each fixed point. (Only the results obtained using the same fixed point cells as were used in APMP.T-K3 are used in the analysis of results below.) The SPRT was returned to VMI, where it was recalibrated at Zn, Sn, Ga and Hg fixed points.



Page 4 of 22

5. MEASUREMENT DATA The variation in the measured resistance at the water triple point (corrected for self-heating and hydrostatic head and expressed in temperature units) is shown in Figure 1.

VMI-NMISA comparison: stability of Rtp

-0.000 6

0.000 4

0.001 4

23-Feb-08 26-Mar-08 27-Apr-08 29-May-08 30-Jun-08 1-Aug-08 2-Sep-08Date

Rtp

- R

tp(i

nit

ial)

(°C

)

VMI (anneal) VMI (initial) NMISA (anneal) NMISA (meas) VMI (final)

Figure 1. Variation of water triple point values during the comparison. The measurement data at the fixed points are presented in Table 1, and the differences between measured resistance ratios at the fixed points (expressed in temperature units) are shown in Figure 2.

Table 1. Measurement data received from the participants.

Lab Fixed point

R (1 mA) (Ω)

Self-heating correction

(Ω)

Hydrostatic head

correction (Ω)

Gas pressure correction

(Ω) R (0 mA) W (0 mA)

VMI Zn 65.468 822 -0.000 235 -0.000 046 0.000 000 65.468 541 2.568 755 08 (initial) WTP 25.486 638 -0.000 169 0.000 017 0.000 000 25.486 486

Sn 48.238 878 -0.000 223 -0.000 037 0.000 000 48.238 617 1.892 713 43 WTP 25.486 639 -0.000 169 0.000 017 0.000 000 25.486 488 Ga 28.497 479 -0.000 231 0.000 017 0.000 000 28.497 265 1.118 130 85 WTP 25.486 672 -0.000 169 0.000 017 0.000 000 25.486 520 Hg 21.514 766 -0.000 161 -0.000 133 0.000 000 21.514 472 0.844 151 08 WTP 25.486 672 -0.000 170 0.000 017 0.000 000 25.486 519

NMISA Zn 65.469 083 -0.000 230 -0.000 038 0.000 000 65.468 815 2.568 755 36 WTP 25.486 739 -0.000 168 0.000 020 0.000 000 25.486 590 Sn 48.239 106 -0.000 243 -0.000 032 0.000 000 48.238 831 1.892 712 81 WTP 25.486 757 -0.000 169 0.000 020 0.000 000 25.486 609 Ga 28.497 507 -0.000 176 0.000 012 0.000 000 28.497 342 1.118 128 99 WTP 25.486 779 -0.000 167 0.000 020 0.000 000 25.486 632 Hg 21.514 844 -0.000 158 -0.000 114 0.000 000 21.514 572 0.844 150 97 WTP 25.486 787 -0.000 167 0.000 020 0.000 000 25.486 640

VMI Zn 65.469 044 -0.000 229 -0.000 046 0.000 000 65.468 769 2.568 753 29 (final) WTP 25.486 745 -0.000 170 0.000 017 0.000 000 25.486 593

Sn 48.239 042 -0.000 230 -0.000 037 0.000 000 48.238 775 1.892 712 28 WTP 25.486 739 -0.000 170 0.000 017 0.000 000 25.486 586 Ga 28.497 589 -0.000 233 0.000 017 0.000 000 28.497 373 1.118 130 84 WTP 25.486 769 -0.000 169 0.000 017 0.000 000 25.486 617 Hg 21.514 867 -0.000 174 -0.000 133 0.000 000 21.514 559 0.844 151 17 WTP 25.486 770 -0.000 168 0.000 017 0.000 000 25.486 619



Page 5 of 22

VMI-NMISA comparison: differences between measured W(fixed point) values

0

0.0005

0.001

0.0015

VMI (initial) NMISA VMI (final)

Laboratory

Dif

fere

nce

of

W(F

P)

fro

m

arb

itra

ry b

asel

ine

(°C

)

Zn Sn Ga Hg

Figure 2. Differences between measured resistance ratios at the fixed points. The immersion profiles measured by the participants in their fixed point cells are shown in Table 2 and in Figures 3 through 7.

Table 2. Vertical temperature gradients, T – T(bottom), in mK. Distance from VMI NMISA bottom (cm) Zn Sn Ga WTP Hg Zn Sn Ga WTP Hg

0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2 -0.06 -0.02 -0.01 0.03 -0.09 -0.02 -0.03 0.01 0.00 -0.10 4 -0.02 -0.04 0.00 0.04 -0.21 -0.12 -0.13 0.30 0.04 -0.18 6 -0.31 -0.23 -0.03 0.05 -0.31 -0.38 -0.48 2.24 0.01 -0.03 8 -0.47 -0.31 -0.13 0.06 -0.42 -2.26 -2.97 20.73 0.03 -0.30

Immersion profiles: Zn

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0 2 4 6 8 10

Distance from bottom (cm)

T -

T(b

ott

om

) (m

K)

ITS-90 VMI NMISA

Immersion profiles: Sn

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0 2 4 6 8 10


T -

T(b

ott

om

) (m

K)

ITS-90 VMI NMISA

Figure 3. Immersion profiles in Zn cells. Figure 4. Immersion profiles in Sn cells.



Page 6 of 22

Immersion profiles: Ga

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0 2 4 6 8 10


T -

T(b

ott

om

) (m

K)

ITS-90 VMI NMISA

Immersion profiles: WTP

0

0.01

0.02

0.03

0.04

0.05

0.06

0 2 4 6 8 10


T -

T(b

ott

om

) (m

K)

ITS-90 VMI NMISA

Figure 5. Immersion profiles in Ga cells. Figure 6. Immersion profiles in WTP cells.

Immersion profiles: Hg

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0 2 4 6 8 10


T -

T(b

ott

om

) (m

K)

ITS-90 VMI NMISA

Figure 7. Immersion profiles in Hg cells.



Page 7 of 22

6. UNCERTAINTY OF MEASUREMENTS The uncertainty budgets submitted by the participants are presented in Tables 3 through 5. Dominant components of uncertainty are shown in red. Where participants have assigned infinite degrees of freedom to a component, a value of 500 has been used for calculation purposes.

Table 3. Uncertainty analysis: VMI. (All values are in mK.) VMI Zn Sn Ga WTP Hg Component Type ui νi ui νi ui νi ui νi ui νi Measurement scatter A 0.005 49 0.004 49 0.002 49 0.001 49 0.004 49 Chemical impurity B 0.098 500 0.018 500 0.004 500 0.039 500FP realisation B 0.200 50 0.150 50 0.100 50 0.080 50 Isotope (WTP only) B 0.040 50 Reference resistor B 0.175 50 0.175 50 0.175 50 0.175 50 0.175 50 Self-heating B 0.059 17430 0.041 1814 0.009 5737 0.013 41665 0.015 514Quadrature effect B 0.003 500 0.003 500 0.003 500 0.003 500 0.009 500Hydrostatic head B 0.039 500 0.032 500 0.017 500 0.011 500 0.103 500Conduction B 0.000 1 0.000 1 0.000 1 0.000 1 0.000 1 Insulation degradation B 0.000 1 0.000 1 0.000 1 0.000 1 0.000 1 Gas pressure B 0.203 500 0.150 500 0.087 500 0.013 500 0.016 500PRT instability at TPW B 0.090 500 Propagation of PRT instability B 0.206 500 0.151 500 0.089 500 0.068 500WTP realisation B 0.080 50 Propagation of WTP realisation B 0.054 500 0.019 500 0.088 500 0.003 500Bridge linearity & resolution B 0.058 500 0.040 500 0.003 500 0.006 500 0.012 500uc(k=1) (mK): 0.418 0.322 0.254 0.217 0.233 Eff. deg. of freedom 527 346 196 113 149 Coverage factor for 95.45 % C.L. 2.00 2.00 2.00 2.00 2.00 Expanded uncertainty (mK): 0.84 0.64 0.51 0.43 0.47

During the technical peer review of VMI by NMIJ in November 2008, the components for chemical impurity were revised, as shown in Table 4. The values for “propagation of PRT instability at the water triple point” and “propagation of water triple point realisation” were reported in incorrect positions in the initial submission, and were subsequently exchanged.

Table 4. Amended uncertainty analysis: VMI. (All values are in mK.) VMI Zn Sn Ga WTP Hg Component Type ui νi ui νi ui νi ui νi ui νi Measurement scatter A 0.005 49 0.004 49 0.002 49 0.001 49 0.004 49 Chemical impurity B 0.310 500 0.173 500 0.079 500 0.081 500 FP realisation B 0.200 50 0.150 50 0.100 50 0.080 50 Isotope (WTP only) B 0.040 50 Reference resistor B 0.175 50 0.175 50 0.175 50 0.175 50 0.175 50 Self-heating B 0.059 17430 0.041 1814 0.009 5737 0.013 41665 0.015 514 Quadrature effect B 0.003 500 0.003 500 0.003 500 0.003 500 0.009 500 Hydrostatic head B 0.039 500 0.032 500 0.017 500 0.011 500 0.103 500 Conduction B 0.000 1 0.000 1 0.000 1 0.000 1 0.000 1 Insulation degradation B 0.000 1 0.000 1 0.000 1 0.000 1 0.000 1 Gas pressure B 0.203 500 0.150 500 0.087 500 0.013 500 0.016 500 PRT instability at TPW B 0.090 500 Propagation of PRT instability B 0.054 500 0.019 500 0.088 500 0.003 500 WTP realisation B 0.080 50 Propagation of WTP realisation B 0.206 500 0.151 500 0.089 500 0.068 500 Bridge linearity & resolution B 0.058 500 0.040 500 0.003 500 0.006 500 0.012 500 uc(k=1) (mK): 0.511 0.365 0.266 0.217 0.244 Eff. deg. of freedom 897 541 235 113 177 Coverage factor for 95.45 % C.L. 2.00 2.00 2.00 2.00 2.00 Expanded uncertainty (mK): 1.02 0.73 0.53 0.43 0.49



Page 8 of 22

Table 5. Uncertainty analysis: NMISA (2008). (All values are in mK.)

NMISA Zn Sn Ga WTP Hg Component Type ui νi ui νi ui νi ui νi ui νi Measurement scatter A 0.016 25 0.004 7 0.002 6 Chemical impurity B 0.266 500 0.023 500 0.087 500 0.044 500FP realisation B 0.179 500 0.133 500 0.040 500 0.032 114 0.058 500Isotope (WTP only) B 0.038 500 Reference resistor B 0.065 500 0.061 500 0.057 500 0.057 500 0.056 500Self-heating B 0.039 500 0.037 500 0.026 500 0.024 500 0.022 500Quadrature effect B Hydrostatic head B 0.031 500 0.025 500 0.007 500 0.004 500 0.041 500Conduction B 0.029 500 0.023 500 0.150 500 0.021 500 0.052 500Insulation degradation B Gas pressure B 0.025 500 0.019 500 0.081 500 0.000 500 0.031 500PRT instability at TPW B Propagation of PRT instability B 0.233 500 0.110 500 0.038 500 0.009 500WTP realisation B Propagation of WTP realisation B 0.211 500 0.155 500 0.092 500 0.069 500Bridge linearity & resolution B 0.112 41 0.106 41 0.099 41 0.097 500Reproducibility B 0.248 500 0.144 500 0.081 500 0.095 500uc(k=1) (mK): 0.533 0.305 0.262 0.082 0.194 Eff. deg. of freedom 2414 1428 1237 1287 4782 Coverage factor for 95.45 % C.L. 2.00 2.00 2.00 2.00 2.00 Expanded uncertainty (mK): 1.07 0.61 0.52 0.16 0.39

The methods used to estimate certain components of uncertainty were as follows: Chemical impurity: VMI: Raoult’s law of dilute solutions was used to calculate the freezing-point depression, using

impurity components in the certificates of the cell manufacturer, Hart Scientific. As mentioned above, these components were revised during the technical peer review of November 2008.

NMISA: Differences between the temperatures of duplicate cells were measured using one or more SPRTs. In the case of zinc, the difference measured by the travelling SPRT was included in this uncertainty component, although its Rtp values after Zn cell 1 and after Zn cell 2 differed by the equivalent of 0.4 mK. (Rtp values of the two local SPRTs used were stable to within 0.1 mK.) In the analysis of cell stability in Section 7 below, only the two local SPRTs were considered.

Fixed point realisation: VMI: This component was calculated from the reproducibility of the times of realisation of fixed points

at VMI. NMISA: The temperature difference between the liquidus point (start of freeze or end of melt) and

the middle of the plateau, T(F=1) – T(F=0.5), was used. Reference resistor: VMI: The calibration uncertainty of the 100 Ω resistor (35 µΩ at a coverage factor of k = 2) and the

uncertainty due to the resistor bath were used. NMISA: The uncertainty in resistor temperature was estimated to be 0.2 K (k = √3). The resistor was

maintained in a thermally insulated box, with a small PRT inserted in the central well of the resistor to measure its temperature. Its measured temperature coefficient (~0.5 µΩ/Ω/K) was used to correct its resistance for measured variations in its temperature.

Reproducibility: NMISA: The difference between melting and freezing temperatures, or, in the case of gallium, the

difference between two melts in different cells, was used. The method of linking VMI’s measurements to the ARV of CCT-K3 will be described in the following section. For this calculation, correlated uncertainty components in NMISA’s 2008 budget (above) and 2000 budget (APMP.T-K3) need to be removed. (NMISA used the same fixed point cells for the 2000 and 2008 measurements.) Table 6 presents the 2008 budget with correlated components (in bold) removed, the 2000 budget is reproduced in Table 7 and correlated components in the 2000 budget are removed in Table 8.



Page 9 of 22

Table 6. Uncertainty analysis: NMISA (2008), with correlated components removed. (All values are in mK.)

NMISA (correlated removed) Zn Sn Ga WTP Hg Component Type ui νi ui νi ui νi ui νi ui νi Measurement scatter A 0.016 25 0.004 7 0.002 6 Chemical impurity B FP realisation B 0.032 114 Isotope (WTP only) B 0.038 500 Reference resistor B 0.065 500 0.061 500 0.057 500 0.057 500 0.056 500 Self-heating B 0.039 500 0.037 500 0.026 500 0.024 500 0.022 500 Quadrature effect B Hydrostatic head B 0.031 500 0.025 500 0.007 500 0.004 500 0.041 500 Conduction B 0.029 500 0.023 500 0.150 500 0.021 500 0.052 500 Insulation degradation B Gas pressure B 0.025 500 0.019 500 0.081 500 0.000 500 0.031 500 PRT instability at TPW B Propagation of PRT instability B 0.233 500 0.110 500 0.038 500 0.009 500 WTP realisation B Propagation of WTP realisation B Bridge linearity & resolution B Reproducibility B 0.248 500 0.144 500 0.081 500 0.095 500 uc(k=1) (mK): 0.352 0.199 0.203 0.082 0.134 Eff. deg. of freedom 1138 1312 1405 1287 7729 Coverage factor for 95.45 % C.L. 2.00 2.00 2.00 2.00 2.00 Expanded uncertainty (mK): 0.70 0.40 0.41 0.16 0.27

Table 7. Uncertainty analysis: NMISA (2000). (All values are in mK.)

NMISA: APMP.T-K3 (2000) Zn Sn Ga WTP Hg Component Type ui νi ui νi ui νi ui νi ui νi Measurement scatter A 0.008 4 0.008 10 0.023 7 0.018 6 0.014 6 Chemical impurity B 0.243 738.7 0.154 51.8 0.020 9.9 0.085 1 FP realisation B 0.223 215 0.187 106 0.073 236 0.077 154 Isotope (WTP only) B 0.066 5.6 Reference resistor B 0.074 8 0.051 8 0.028 8 0.021 8 Self-heating B 0.064 16.6 0.089 16.8 0.051 11 0.058 19 0.052 13 Quadrature effect B Hydrostatic head B 0.027 8 0.022 8 0.012 8 0.007 8 0.071 8 Conduction B 0.188 3 0.274 3 0.101 3 0.010 2 0.029 3 Insulation degradation B Gas pressure B 0.043 2 0.033 2 0.020 2 0.038 2 0.054 2 PRT instability at TPW B Propagation of PRT instability B 0.102 2 0.071 2 0.110 2 0.046 2 WTP realisation B 0.054 4 Propagation of WTP realisation B 0.299 25.3 0.220 25.3 0.130 25.3 0.098 25.3Bridge linearity & resolution B 0.089 8.3 0.062 8.5 0.035 9.4 0.032 9.8 0.027 10.6Reproducibility B uc(k=1) (mK): 0.513 0.451 0.225 0.116 0.194 Eff. deg. of freedom 85 20 21 25 22 Coverage factor for 95.45 % C.L. 2.03 2.13 2.13 2.11 2.13 Expanded uncertainty (mK): 1.04 0.96 0.48 0.25 0.41



Page 10 of 22

Table 8. Uncertainty analysis: NMISA (2000), with correlated components removed. (All values are in mK.)

NMISA (2000) (correl. removed) Zn Sn Ga WTP Hg Component Type ui νi ui νi ui νi ui νi ui νi Measurement scatter A 0.008 4 0.008 10 0.023 7 0.018 6 0.014 6 Chemical impurity B 738.7 51.8 9.9 1 FP realisation B 215 106 236 154 Isotope (WTP only) B 0.066 5.6 Reference resistor B 0.074 8 0.051 8 0.028 8 0.021 8 Self-heating B 0.064 16.6 0.089 16.8 0.051 11 0.058 19 0.052 13 Quadrature effect B Hydrostatic head B 0.027 8 0.022 8 0.012 8 0.007 8 0.071 8 Conduction B 0.188 3 0.274 3 0.101 3 0.010 2 0.029 3 Insulation degradation B Gas pressure B 0.043 2 0.033 2 0.020 2 0.038 2 0.054 2 PRT instability at TPW B Propagation of PRT instability B 0.102 2 0.071 2 0.110 2 0.046 2 WTP realisation B 0.054 4 Propagation of WTP realisation B Bridge linearity & resolution B 0.032 9.8 Reproducibility B uc(k=1) (mK): 0.240 0.304 0.163 0.116 0.119 Eff. deg. of freedom 7 4 7 25 19 Coverage factor for 95.45 % C.L. 2.43 2.87 2.52 2.11 2.14 Expanded uncertainty (mK): 0.58 0.87 0.41 0.25 0.26

7. LINK TO APMP.T-K3 (STABILITY OF NMISA’S FIXED POINT CELLS) NMISA used the same fixed point cells for the 2000 and 2008 measurements, namely Zn118 (zinc, sealed), Sn77 (tin, sealed), Ga224 (gallium, sealed) and M013 (mercury, sealed), all manufactured by Isotech. Evidence of their stability in the interim period is presented below. All data were corrected for gas pressure (where appropriate) and hydrostatic head. No step change to the NMISA WTP definition occurred after the 2005 CIPM resolution to clarify the SI text relating to the isotopic composition of the water defining the WTP. A home-made, open zinc cell, TS-005, has been in use at NMISA over the entire period 2000 to 2008. The temperature difference T(Zn118) – T(TS-005) was measured to be -0.1 mK in July 2000 and 0.0 mK in July 2008. Apart from Sn77, no other tin cells have been in use at NMISA over the whole period 2000 to 2008. Measurements of W(Sn) of a Tinsley model 5187SA SPRT, serial number 269581, in Sn77 are presented in Table 9 below.

Table 9. Measurements of W(Sn) of SPRT 269581 in cell Sn77. To facilitate comparison of the data, the resistance ratios are also expressed in temperature units.

Date W(Sn77) Equivalent temperature (°C) December 2000 1.892 658 4 231.928 0

May 2003 1.892 659 8 231.928 4 April 2005 1.892 659 0 231.928 2 July 2008 1.892 659 3 231.928 2

Much of the variation in W(Sn) may be due to instability of the SPRT (rather than the cell temperature), as this PRT is used regularly up to 550 °C. In addition to the above data, the stability of Sn77 in the latter half of the period 2000 to 2008 is demonstrated by comparisons with an open cell, Sn134. T(Sn77) – T(Sn134) was measured to be 0.0 mK in April 2005 and 0.0 mK in July 2008. The gallium cell Ga224 was compared to a home-made cell, TS-010, in April 1998 and July 2008. T(Ga224) – T(TS-010) was measured to be 0.1 mK in April 1998 and -0.2 mK in July 2008. Apart from M013, no other mercury cells have been in use at NMISA over the whole period 2000 to 2008. Measurements of W(Hg) of a Rosemount model 162CE SPRT, serial number 4203, in M013 are presented



Page 11 of 22

in Table 10 below.

Table 10. Measurements of W(Hg) of SPRT 4203 in cell M013. To facilitate comparison of the data, the resistance ratios are also expressed in temperature units.

Date W(M013) Equivalent temperature (°C) December 2000 0.844 175 8 -38.834 4

June 2003 0.844 175 4 -38.834 5 May 2005 0.844 175 8 -38.834 4

In addition to the above data, the stability of M013 in the latter half of the period 2000 to 2008 is demonstrated by comparisons with another sealed cell, Hg156. T(M013) – T(Hg156) was measured to be 0.0 mK in May 2005 and -0.1 mK in July 2008. In all cases except gallium, the estimated instability of NMISA’s fixed point cells used for the 2000 and 2008 comparisons is negligible, compared to the expanded uncertainties of VMI and NMISA. In the case of

gallium, an additional uncertainty linkU = 0.3 mK is included in equation (5) below.

8. DIFFERENCE FROM PILOT LABORATORY AND CCT-K3 ARV In Table 11, the means of VMI’s initial and final resistance ratios at the fixed points are compared to those measured by NMISA. The expanded uncertainties of the participants are combined as follows:

22NMISAVMINMISAVMI UUU (1)

NMISA’s differences from the ARV of CCT-K3, as well as their expanded uncertainties, are reproduced from [1]. The correlated components are removed from these uncertainties as follows:

22000,

22000,)____( NMISAARVNMISA UUARVtoNMISAoflinkU (2)

22000,

22000,

22000,2000, )()( eduncorrelatUUUeduncorrelatU NMISANMISAARVNMISAARVNMISA (3)

VMI’s differences from the ARV of CCT-K3 are calculated as follows:

)()( 2000,2008, ARVNMISANMISAVMIARVVMI WWWWWW (4)

The uncertainties of these differences are calculated as follows: 22

2008,2

2000,2 )()( linkNMISAARVNMISAVMIARVVMI UeduncorrelatUeduncorrelatUUU (5)

Table 11. Differences of VMI results from those of NMISA and from ARV of CCT-K3. (All values are in mK,

and uncertainties are reported for a coverage factor of k = 2.) Zn Sn Ga Hg

W(VMI) - W(NMISA, 2008) -0.34 0.01 0.47 0.04 U(VMI) 1.02 0.73 0.53 0.49

U(NMISA,2008) 1.07 0.61 0.52 0.39 U(VMI - NMISA,2008) 1.48 0.95 0.75 0.62

W(NMISA,2000) - W(ARV) -0.21 -0.53 0.31 0.29 U(NMISA,2000 - ARV) 1.19 1.15 0.50 0.45

U(NMISA,2000) 1.04 0.96 0.48 0.41 U(link of NMISA to ARV) 0.57 0.63 0.14 0.18

U(NMISA,2000) uncorrelated 0.58 0.87 0.41 0.26 U(NMISA,2000 - ARV) uncorrelated 0.82 1.08 0.44 0.31

W(VMI) - W(ARV) -0.55 -0.52 0.78 0.33 U(NMISA,2008) uncorrelated 0.70 0.40 0.41 0.27

U(link) 0.00 0.00 0.30 0.00 U(VMI-ARV) 1.49 1.36 0.85 0.64

dT/U -0.37 -0.38 0.91 0.51 VMI’s result at Ga agrees with that of NMISA within the uncertainty of the difference, but the difference is large enough to warrant further investigation. If NMISA’s Ga cell (a sealed Isotech mini-cell) had developed a leak between 2000 and 2008, the gas pressure in the cell would have dropped (atmospheric pressure at NMISA is 87 kPa) and the melting temperature would have risen (dT/dP = -2 x 10-8 K.Pa-1 for Ga), so such a leak would not explain the observed difference. Measurements in NMISA’s sealed Isotech mini-cell and their



Page 12 of 22

home-made sealed large cell (performed using the transfer artefact SPRT and three others, at the same time as this comparison) yielded a mean temperature difference of Thome-made – TIsotech = 0.44 mK. (The transfer SPRT was measured at the beginning and end of each melt: its measurements at the end of the cells’ melts yielded Thome-made – TIsotech = 0.25 mK.) Since the seal on the home-made cell is poor, its gas pressure may now be 87 kPa: applying a +14.3 kPa (equivalent to -0.29 mK) correction to Thome-made would reduce the mean difference between the cells to 0.15 mK. The large difference from VMI’s result remains unexplained, and it is recommended that a Ga cell (preferably a full-size one, for ease of realisation) be circulated to VMI, NMISA and one other institute (preferably NMIA or KRISS, since they are the institutes linking APMP.T-K3 (2000) to CCT-K3) for comparison to the local Ga cells. 9. DIFFERENCES FROM APMP.T-K3 AND CCT-K3 PARTICIPANTS

In Table 12, the differences ilabNMISA WW _ , as well as their expanded uncertainties, are reproduced from

[1]. Table 12. Bilateral differences of NMISA results in APMP.T-K3 from those of other APMP.T-K3 participants,

as well as participants in CCT-K3 [1]. (All values are in mK, and uncertainties are reported for a coverage factor of k = 2.)

Zn Sn Ga Hg PSB 1.83 0.91 -0.55 0.04 dT

3.30 2.85 1.43 2.52 U NIMT 0.95 0.75 0.59 -0.49 dT

2.72 2.83 0.73 0.63 U NPL -3.72 -2.43 -0.25 -1.96 dT

1.41 1.16 0.66 0.63 U MUSSD 1.23 -0.30 0.06 -0.52 dT

2.55 1.91 1.05 1.91 U SIRIM 2.37 -1.18 1.08 -0.28 dT

1.81 1.64 0.88 0.81 U KRISS -0.12 -0.41 0.39 0.04 dT

1.50 1.14 0.50 0.45 U NMIA 0.08 0.10 0.32 0.18 dT

1.33 1.34 0.55 0.51 U KIMLIPI -3.13 -3.18 #VALUE! #VALUE! dT

2.18 1.49 #VALUE! #VALUE! U SCL -0.16 -0.30 0.60 0.15 dT

1.76 1.54 0.72 0.76 U CMS 1.51 -0.82 0.38 0.00 dT

2.04 1.52 0.67 0.68 U ITDI #VALUE! #VALUE! #VALUE! #VALUE! dT

#VALUE! #VALUE! #VALUE! #VALUE! U

BIPM #VALUE! #VALUE! 0.36 #VALUE! #VALUE! #VALUE! 0.56 #VALUE!

BNM -0.82 -0.49 0.29 0.71 dT 1.53 1.32 0.59 0.64 U

IMGC 0.47 -0.86 0.18 0.43 dT 1.50 1.19 0.56 0.53 U

MSL -0.04 -0.03 0.17 0.20 dT 4.07 1.41 0.61 0.57 U

NIM -1.21 0.06 0.80 0.01 dT 1.59 4.15 0.68 0.61 U

NIST -0.84 -0.99 0.27 0.34 dT 1.37 1.14 0.55 0.50 U

NPL -0.70 -0.72 0.43 0.24 dT 1.50 1.25 0.62 0.56 U

NRC 0.32 0.36 0.41 0.12 dT 1.43 1.26 0.58 0.51 U

NRLM 1.64 0.34 0.71 0.57 dT 1.99 1.29 0.57 0.64 U

PTB 0.35 -1.19 0.06 0.40 dT 1.64 1.29 0.58 0.54 U

SMU -0.36 -0.86 0.23 #VALUE! dT



Page 13 of 22

1.48 1.29 0.57 #VALUE! U VNIIM -0.86 -1.19 0.26 #VALUE! dT

1.70 1.25 0.57 #VALUE! U VSL -0.53 -0.56 0.49 0.30 dT

1.53 1.26 0.62 0.58 U The correlated components are removed from these uncertainties as follows:

22000,

22000,

2_2000,_2000, )()( eduncorrelatUUUeduncorrelatU NMISANMISAilabNMISAilabNMISA (6)

VMI’s differences from each laboratory are calculated as follows:

)()( _2000,2008,_ ilabNMISANMISAVMIilabVMI WWWWWW (7)

The uncertainties of these differences are calculated as follows: 22

2008,2

_2000,2

_ )()( linkNMISAilabNMISAVMIilabVMI UeduncorrelatUeduncorrelatUUU (8)

Table 13. Differences of VMI results from those of APMP.T-K3 and CCT-K3 participants. (All values are in

mK, and uncertainties are reported for a coverage factor of k = 2.) Zn Sn Ga Hg

PSB 1.50 0.93 -0.08 0.08 dT 3.41 2.94 1.59 2.56 U

NIMT 0.61 0.76 1.06 -0.45 dT 2.86 2.92 1.00 0.77 U

NPL -4.05 -2.41 0.22 -1.92 dT 1.67 1.37 0.96 0.78 U

MUSSD 0.90 -0.29 0.53 -0.48 dT 2.70 2.04 1.26 1.96 U

SIRIM 2.03 -1.17 1.55 -0.25 dT 2.02 1.79 1.12 0.92 U

KRISS -0.45 -0.40 0.86 0.08 dT 1.75 1.35 0.85 0.64 U

NMIA -0.26 0.11 0.79 0.22 dT 1.60 1.52 0.88 0.68 U

KIMLIPI -3.47 -3.17 #VALUE! #VALUE! dT 2.35 1.66 #VALUE! #VALUE! U

SCL -0.49 -0.29 1.07 0.18 dT 1.97 1.70 1.00 0.88 U

CMS 1.17 -0.81 0.85 0.04 dT 2.23 1.68 0.96 0.81 U

ITDI #VALUE! #VALUE! #VALUE! #VALUE! dT #VALUE! #VALUE! #VALUE! #VALUE! U

BIPM #VALUE! #VALUE! 0.83 #VALUE! #VALUE! #VALUE! 0.89 #VALUE!

BNM -1.15 -0.48 0.76 0.74 dT 1.77 1.51 0.91 0.78 U

IMGC 0.13 -0.85 0.65 0.46 dT 1.75 1.39 0.89 0.70 U

MSL -0.38 -0.02 0.64 0.23 dT 4.17 1.59 0.92 0.73 U

NIM -1.54 0.07 1.27 0.04 dT 1.82 4.22 0.97 0.76 U

NIST -1.17 -0.98 0.74 0.37 dT 1.64 1.36 0.88 0.67 U

NPL -1.03 -0.71 0.90 0.27 dT 1.75 1.45 0.93 0.72 U

NRC -0.01 0.37 0.88 0.16 dT 1.68 1.46 0.90 0.68 U

NRLM 1.31 0.35 1.18 0.60 dT 2.18 1.48 0.90 0.79 U

PTB 0.02 -1.18 0.53 0.43 dT 1.87 1.48 0.90 0.70 U

SMU -0.69 -0.85 0.70 #VALUE! dT



Page 14 of 22

1.73 1.48 0.89 #VALUE! U VNIIM -1.19 -1.18 0.73 #VALUE! dT

1.92 1.44 0.89 #VALUE! U VSL -0.86 -0.55 0.96 0.33 dT

1.77 1.45 0.93 0.73 U 10. CONCLUSIONS The resistance ratios measured by VMI and NMISA at Zn, Sn, Ga and Hg agree within their combined uncertainties, and VMI’s results also agree with the CCT-K3 ARVs at these fixed points. 11. REFERENCES [1] APMP-KC3: Key comparison of realization of the ITS-90 over the range -38.8344 °C to 419.527 °C

(final report, prepared by M. K. Nguyen and M. J. Ballico, NMIA, Lindfield, Australia. 7 Oct. 2005).



Page 15 of 22

APPENDIX 1: PROTOCOL FOR VMI-NMISA BILATERAL COMPARISON Protocol for bilateral comparison between VMI (Vietnam) and NMISA (South Africa) of realisations of the ITS-90 over the range -38.8344 °C to 419.527 °C (version 3) Vu Quang Cuong (VMI) and Hans Liedberg (NMISA) 22 December 2007 Contents 1. Introduction 2. Objectives of the comparison 3. Detailed description 4. Reporting of data to pilot laboratory 5. Uncertainty analysis 6. Organization of the comparison 7. Timeline for the comparison 8. References Appendix Appendix 1: Measurement Data Appendix 2: Uncertainty Analysis Appendix 3: Immersion curves Appendix 4: Measurement equipment and standards Appendix 5: Customs declaration Appendix 6: Circulation schedule Appendix 7: Artefact received Appendix 8: Artefact shipped 1. Introduction The Comité Consulatif de Thermometrie (CCT) has organised several key comparisons to compare realisations of the ITS-90 in different national metrology institutes. To keep the organisation, time scale and data processing of such a comparison manageable, the number of participants in a CCT Key Comparison (CCT KC) is limited to a few laboratories in each major economic region. Subsequent regional key comparisons are linked to the applicable CCT KC by two or more link laboratories. For the temperature range from 83.8058 K (triple point of Ar) to 933.473 K (freezing point of Al), a key comparison, CCT-K3, was carried out from 1997 to 2001 by representative laboratories in North America, Europe and Asia. Following CCT-KC3, the Asia-Pacific Metrology Program Key Comparison 3 (APMP-KC3) was organised for national metrology institutes in the Asia/Pacific region. NMIA (Australia) and KRISS (South Korea) provided the link between CCT-KC3 and APMP-KC3. APMP-KC3, which took place from February 2000 to June 2003, covered the temperature range from –38.8344 °C (triple point of Hg) to 419.527 °C (freezing point of Zn), using a standard platinum resistance thermometer (SPRT) as the artefact [1]. In June 2007 the Vietnam Metrology Institute (VMI) requested a bilateral comparison to link their SPRT calibration capabilities to APMP-KC3, and in October 2007 the National Metrology Institute of South Africa (NMISA) agreed to provide the link to APMP-KC3. The comparison will be carried out in a participant-pilot-participant topology (with NMISA as the pilot and VMI as the participant) to reduce the workload on the link laboratory. This comparison is intended to be a bilateral key comparison, equivalent to CCT-K3. Like APMP-KC3, the comparison has been restricted to the Hg to Zn temperature range to reduce the chance of drift in the SPRT artefact. The results of the link laboratory in the APMP-KC3 RMO key comparison will be used to link VMI’s results to those of CCT-KC3. As a preliminary step, VMI prepared an initial draft of the comparison protocol, which was modified by NMISA.



Page 16 of 22

This protocol describes the objectives of the comparison, its organization and the procedures to be followed by the participants. It follows the 'Guidelines for key comparisons' established by the BIPM [2], and is based on current best practice. 2. Objectives of the comparison The objective of this comparison is to assess the procedures and standards used in calibrating an unknown SPRT. The travelling SPRT is to pass through the following sequence:

1) a measurement at the triple point of water (TPW) 2) a stabilisation procedure 3) a second measurement at the triple point of water 4) measurements at metal fixed points in order of decreasing temperatures alternating with a measurement at the triple point of water. This includes:

Zn point, TPW, Sn point, TPW, Ga point, TPW, Hg point and TPW. The travelling SPRT details are as follows (travelling SPRT artefact from VMI):

Serial number: 1357 Model: 5681 Manufacturer: Hart Scientific, USA. Description: Nominal resistance 25.5 Ω at 0 °C, 4 wires with spade terminal, quartz glass sheath, 520 mm long and 7 mm OD.

The SPRT artefact is very fragile so it must be handled with extreme care. When not in use, it should be stored horizontally in the groove of the protecting foam inside its hard travelling case. The case is 94 cm long × 16 cm wide × 7 cm high and weighs about 2 kg. For transport, the SPRT must be shipped via the international express service DHL in original wooden box to the next destination by the manner door-to-door, in accordance with the procedures required by the Department of Customs of the two countries. The box is 115 cm long × 30 cm wide × 30 cm high and weighs about 10 kg. The total weight of SPRT, case and wooden box is about 12-14 kg. 3. Detailed instructions 1. Before the bilateral comparison begins, VMI will assess the stability of the artefact under thermal cycling by determining the change in its W(Zn) value caused by 2 hour anneals at 440 °C. Annealing will be repeated until the difference between W(Zn) values measured before and after an anneal is smaller than 1 mK. The W(Zn) values will be corrected to zero current. 2. Following the annealing process to stabilise the artefact, VMI will perform an initial calibration of the artefact. VMI will first measure the resistance of the travelling SPRT in a triple point of water (TPW) cell at two measuring currents (to determine the zero-current value), before and after annealing at 440 °C for 2 hours. These values must satisfy the following condition:

∆RTPW = (RTPW)before – (RTPW)after ≤ 0,5 mK (1)

If the SPRT resistance values do not satisfy the inequality (1), the travelling SPRT must be annealed again until

(RTPW)i-1 – (RTPW)i ≤ 0,2 mK 3. VMI will then calibrate the SPRT over the entire temperature range. Existing techniques as practised by VMI must be used. For each metal fixed point, W = Rt/RTPW will be determined, where Rt is the resistance at the fixed point and RTPW is the resistance at the TPW obtained immediately after the measurement of Rt. Both Rt and RTPW should have been corrected for self-heating, the hydrostatic head and, if any, the pressure effect. 4. After completing the calibration, the VMI temperature laboratory must transmit the data and computations



Page 17 of 22

to NMISA (the pilot laboratory). 5. Upon receipt of the SPRT (travelling artefact from VMI), NMISA (pilot laboratory) must inspect the artefact for damage. Then NMISA must complete and forward (by e-mail or fax) the attached Artefact Received Form to VMI to report the condition of the artefact. If the travelling SPRT is damaged, VMI and NMISA will discuss how to proceed. 6. If no damage has been sustained, NMISA will measure the resistance of the travelling SPRT in a TPW cell at two measuring currents (to determine the zero-current value). The ice mantle of the TPW cell must have been prepared and aged according to the host’s existing practice. The resistance at zero current will be corrected for hydrostatic head to obtain RTPW1, expressed in terms of the NMISA’s national ohm. 7. NMISA will anneal the SPRT for two hours at 440 °C, then carefully remove the SPRT from the annealing furnace directly to the room environment. After the SPRT has cooled to room temperature, its TPW resistance will be re-determined as per instruction 6 to give RTPW2. 8. If the decrease in the calculated TPW resistance of the SPRT after annealing, (RTPW2 - RTPW1), is equivalent to 0.5 mK or greater, proceed to instruction 10. If the decrease is less than 0.5 mK, proceed to instruction 12. 9. If the resistance at TPW increases after annealing, i.e. if RTPW2 is larger than RTPW1, contact VMI for further instructions. 10. Repeat step 7 to give RTPW3. If the additional decrease in the resistance at the TPW, (RTPW3 - RTPW2), is equal to or less than the equivalent of 0.2 mK, continue with instruction 12. If not, communicate with VMI for further instructions. 11. If the resistance at TPW increases after the second annealing, i.e. if RTPW3 is larger than RTPW2, contact VMI for further instructions. 12. NMISA calibrates the SPRT over the entire temperature range. Existing techniques as practised by NMISA must be used. For each metal fixed point, determine W = Rt/RTPW, where Rt is the resistance at the fixed point and RTPW is the resistance at the TPW obtained immediately after the measurement of Rt. Both Rt and RTPW should have been corrected for self-heating, the hydrostatic head and, if any, the pressure effect. 13. After completing the calibration, NMISA must pack the transfer SPRT in its original case and wooden box and call express services DHL’s local branch for transport of the SPRT to VMI. After that, NMISA must inform VMI in advance of the shipment of the SPRT. 14. The transfer SPRT will sent through the international express Services DHL in manner door – to – door, in accordance with the procedures required by the Department of Customs of two countries. 15. Upon receipt of the artefact, VMI will perform a final calibration (as in instructions 2 to 3), to allow assessment of the stability of the artefact during the comparison. 4. Reporting of data to pilot laboratory (PL) VMI must send to NMISA the following information. 1. For each fixed point cell that was used in the comparison, determine (using the host’s own SPRT) and plot the change of phase transition temperature, dT, versus immersion, dh. On the same graph, plot the theoretical dT/dh curve using the hydrostatic pressure coefficients (mK/m of liquid) given in the ITS-90 text. 2. Freezing curves of the travelling SPRT in Sn and Zn cells, freezing/melting curve in Hg cell and melting curve in Ga cell. 3. Using the attached spreadsheet named ‘Calibrationdata.xls’ to report the resistance ratio W = Rt / RTPW where Rt is the resistance of the SPRT at each of the fixed points, and RTPW is the resistance in the TPW cell obtained after the measurement of Rt. The values of Rt and RTPW must be corrected for self heating, the hydrostatic head and, if applicable, the pressure effect.



Page 18 of 22

4. Uncertainty analysis using the attached spreadsheet named ‘Uncertainty.xls.’ 5. Details of instrumentation, fixed point cells and techniques used in the realisation of the fixed points for this comparison should be given in the attached sheet ‘Instrument.xls.’ The immersion curves, the freezing/melting curves and the completed forms ‘Calibrationdata.xls’, ‘Uncertainty.xls’ and ‘Instrument.xls’ should be e-mailed to NMISA. 5. Uncertainty analysis Participants are requested to use the attached spreadsheet ‘Uncertainty.xls’ to calculate and report their estimated uncertainties for the determination of resistance ratios obtained from the SPRT artefact at the fixed points that were used in this comparison. Calculations of uncertainties should follow the guidelines set out in the ISO Guide to the Expression of Uncertainty in Measurement. For each uncertainty component, a standard uncertainty ui and its associated degrees of freedom must be provided. The value of ui should be given in terms of temperature, µK. For type A evaluation, the number of degrees of freedom, i(A) is n-1 where n is the number of measurements. For type B evaluation, the number of degrees of freedom, i(B), is based on the relative uncertainty of the component and defined by the ISO Guide as i(B) 1/2 [∆ui/ui]

-2. The combined uncertainty Uc, the effective degrees of freedom eff and subsequently, the expanded uncertainty Uexp at 95% level of confidence are calculated as set out in the Guide. To assist with the determination of measurement uncertainties, the following section explains the meanings of the uncertainty components given in ‘Uncertainty.xls.’ Participants should note that not all components are applicable to every fixed point. 1. Measurement scatter This type A evaluation is the random variation in n readings of resistances, or resistance ratios, with a standard deviation of the mean of s. 2. Chemical impurity Considered a type B evaluation, the uncertainty due to the impurity in the metal sample used in the fixed point is estimated from the manufacturer’s essay, the melting range of the fixed point, the flatness of the freezing curve and the agreement between melting/freezing temperatures. 3. FP realisation This type B uncertainty is associated with the realisation of the fixed point. It may be taken as the resistance values believed to be of the flattest region of the melting/freezing curve. 4. Isotope variations This, regarded as type B, refers to the variation in the concentration of deuterium in water hence is applicable only to the triple point of water. 5. Reference resistor Another type B component, this arises from the instability (such as temporal drift) and the fluctuation in temperature of the reference resistor used with the resistance measuring devices. 6. Self-heating Resistance ratio W is determined from zero-current values that are extrapolated from measurements at two values of the sensing current, I and √2 of I. The self-heating correction obtained from a SPRT in a fixed point tends to vary slightly. This type B uncertainty may be due to the variation in the sensing current (covered in item 1) and the √2 multiplier. 7. Quadrature effect This effect, a type B consideration, is applicable only to AC bridges, due to imperfect quadrature balance in such bridges.



Page 19 of 22

8. Hydrostatic head The effect of the hydrostatic head pressure of a column of metallic liquid on the SPRT sensor is corrected for, according to Clausius-Clapeyron. However, this type B factor arises from the uncertainty in determining the hydrostatic head of liquid above the mid point of the SPRT sensor in the fixed point cell. 9. Conduction error If the change in the temperature, dT, to the change in the immersion, dh, of the SPRT follows the hydrostatic head pressure effect then it can be assumed that thermal conduction error is negligible. Otherwise, it can be determined as a type B deviation from the dT/dh line. 10. Insulation degradation This, type B evaluation, is mainly due to the presence of moisture in the insulating material, causing shunting of the platinum sensor. 11. Gas pressure The temperature of the freezing/melting point is defined at a pressure of 1 atm. A flow of inert gas at 1 atm is usually used to regulate the pressure of open cells. This item relates to the uncertainty in maintaining a constant pressure at 1 atm in the cell at the freezing/melting temperature. It is more difficult to determine the pressure inside a closed cell. If such a cell is used in this comparison, then the measured resistance should be corrected for the effect of deviation from 1 atm to the fixed point temperature and a type B uncertainty associated with such determination is included. 12. SPRT instability at TPW Prior to calibration, a SPRT is subject to a stabilisation process by annealing to remove mechanical shock and defects in the Pt wire through use. Even so, there remains some detectable instability in the SPRT indicated by variations in values of RTPW after repeated exposures to a given fixed point. This is considered a type B uncertainty. 13. Propagation of SPRT instability into the fixed point ratios Resistance ratio W = Rt/RTPW requires the RTPW value obtained immediately after each fixed point determination. The drift in RTPW after each fixed point (item 12) results in a type B uncertainty component to the resistance ratios. 14. TPW realisation Similarly to item 3, this is a type B uncertainty in the realisation of the triple point of water. 15. Propagation of TPW uncertainty into the fixed point ratios Uncertainty incurred at the triple point of water causes an additional type B uncertainty component to the resistance ratios since W= Rt/RTPW. 16. Other factors that are applicable to a particular laboratory should be added with a brief explanation for their inclusion. 6. Organization of the comparison The participants in this comparison are 1. NMISA (South Africa) (pilot laboratory):

Hans Liedberg National Metrology Institute of South Africa (NMISA) Building 6, CSIR campus Meiring Naudé Road 0184 Brummeria Pretoria SOUTH AFRICA Tel: +27-12-841 2753 Fax: +27-12-841 2131 / 4458 Email: [email protected]



Page 20 of 22

2. VMI (Vietnam) Vu Quang Cuong Vietnam Metrology Institute (VMI) No 8 Hoang Quoc Viet Road Cau Giay District Hanoi VIETNAM Tel: +844-836 1872 / 3242 Fax: +844-756 4260 Email: [email protected] / [email protected]

The temperature laboratory of VMI will select an SPRT, assess its stability and suitability as a

transfer standard and compare this transfer SPRT to its national measurement standards. VMI will transmit its data showing the artefact’s stability with thermal cycling and initial calibration data to NMISA.

The transfer SPRT will be sent to NMISA (Pilot Lab), who will compare this transfer SPRT against its own national measurement standards.

The transfer SPRT will be returned to the temperature laboratory of VMI, who will again compare the transfer SPRT to its national measurement standards.

The VMI will report both its initial and final calibration results on the artefact to the pilot laboratory (NMISA), and NMISA will prepare the draft A report. NMISA will not release its calibration data to VMI prior to the release of the first version of the draft A report.

7. Timeline for the comparison 1. January 2008: VMI initial measurements to stabilise the artefact SPRT 2. February 2008: VMI initial calibration 3. March 2008: NMISA calibration 4. April 2008: VMI final calibration 5. May 2008: NMISA draft A preparation 6. NMISA sends the draft A report to TCT chair and VMI 8. References [1] APMP-KC3: Key comparison of realization of the ITS-90 over the range -38.8344 °C to 419.527 °C (final report, prepared by M. K. Nguyen and M. J. Ballico, NMIA, Lindfield, Australia. 7 Oct. 2005). [2] T. J. Quinn, "Guidelines for key comparisons carried out by Consultative Committees", 1 March 1999,

BIPM, Paris.



Page 21 of 22

APPENDIX 2: INSTRUMENTATION Resistance measuring device

Laboratory Name VMI NMISA (Pilot) Bridge Manufacturer ASL ASL AC/DC F900 AC (F18) If AC, give: Frequency 25 Hz 25 Hz Bandwidth 0.1 0.1 Gain 105 105 Quad gain 10 10 output IEEE 488 IEEE 488 Normal measuring current 1 mA 1 mA Self-heating current 1.414 mA 1.414 mA Unity reading 1.000000004 Zero reading 0.000000000 Complement check error If DC, give: Gain Period of reversal output Reference Resistor Type AC/DC 5430/100 5685A (Wilkins) Manufacturer Tinsley & Co Tinsley Temperature? 25 °C oil bath ~ 21 °C (measured by IPRT and compensated for) Temperature coefficient? (2 µΩ/Ω)/°C (-0.5 µΩ/Ω)/°C Linearity of bridge

Water triple point cell

Water triple point cell VMI NMISA (Pilot) Cell Manufacturer Hart Scientific Jarrett (type A: NBS design) Water source and purity Well Diameter 12.6 mm 11 mm Immersion Depth 260 mm ~ 240 mm Heat transfer liquid: water? Distilled water Distilled water Cell maintained in: Ice Bath Ice Bath Water Bath Isotech 18233 WTP maintenance bath Ice Mantle: Method of preparation Cool alcohol Methanol heat pipe cooled with solid CO2 Annealing time before use 1 to 2 days 7 days

Other fixed points Laboratory Name: VMI

Cell Zn Sn Ga Hg Cell manufacturer Hart Scientific Hart Scientific Hart Scientific Hart Scientific

Open/closed closed closed closed closed Pressure in cell 85.00 kPa 85.6 kPa 86.2 kPa

Crucible Crucible Material Graphite Graphite Stainless Steel Stainless Steel

Crucible Manufacturer Crucible length

Metal Sample Sample source Sample purity 6N 6N 7N 7N sample weight 960 g 960 g 2000 g

Thermometer well Well material Quartz Quartz Stainless Steel Stainless steel



Page 22 of 22

Well ID (mm) 8 8 8.2 8 Immersion depth of SPRT 195 mm 195 mm 168 mm 213 mm Furnace/Bath

Manufacturer Hart Scientific Hart Scientific Hart Scientific Hart Scientific Control Type PID PID Programmed PID

How many zones? 3-zones 3-zones Deep-well Bath Furnace heater

AC/DC AC AC External heater AC Heat pipe liner No No No No

ITS-90 Realization Freeze/Melt Freeze Freeze Melt Freeze/Melt Technique Cold alumina Rods Cold alumina Rods Programmed Quartz rod

Heat transfer fluid Air Air Air Air Duration of freeze/melt 8 hours 8 hours 8 hours 8 hours

Cell used as FP/MP/TP? FP FP MP TP Laboratory Name: NMISA

Cell Zn Sn Ga Hg Cell manufacturer Isotech Isotech Isotech Isotech

Open/closed sealed sealed sealed sealed Pressure in cell 101 kPa 101 kPa 101 kPa

Crucible Crucible Material Graphite Graphite PTFE Stainless Steel

Crucible Manufacturer Isotech Isotech Isotech Isotech Crucible length

Metal Sample Sample source Sample purity sample weight

Thermometer well Well material Quartz Quartz PTFE Stainless steel Well ID (mm) 8 8 8 8

Immersion depth of SPRT 180 mm 180 mm 120 mm 180 mm Furnace/Bath

Manufacturer Isotech Isotech Heto Heto Control Type PID PID Analogue PID Analogue PID

How many zones? 1 1 1 Furnace heater

AC/DC AC AC AC AC Heat pipe liner No No No No

ITS-90 Realization Freeze/Melt Freeze Freeze Melt Freeze Technique Cold alumina rod Cold alumina rod 40 °C water Room temperature steel rod

Heat transfer fluid Air Air Water Ethanol Duration of freeze/melt 6 hours 2.5 hours 20 hours 1 hour

Cell used as FP/MP/TP? FP FP MP TP

Documents

REPORT ON VMI-NMISA BILATERAL COMPARISON: … · comparison, CCT-K3, was carried out from 1997 to 2001 by representative laboratories in North America, Europe and Asia. Following