Issue 
Int. J. Metrol. Qual. Eng.
Volume 8, 2017



Article Number  27  
Number of page(s)  12  
DOI  https://doi.org/10.1051/ijmqe/2017022  
Published online  17 November 2017 
Research Article
The ITS90 after definition of neon isotopic reference composition: extent of the isotopic effect tested on previous intercomparison results
^{1}
INTiBS,
Wroclaw, Poland
^{2}
INRIM,
Torino, Italy
^{*} frpavese@gmail.com
Received:
17
July
2017
Received in final form:
19
October
2017
Accepted:
20
October
2017
Starting from the end of the past century, the importance has been recognised of the effect of isotopic composition on some of the temperature fixed points for the most accurate realisations of the ITS90. In the original definition of the latter, dating back to 1990, only a generic reference was made to “natural” composition of the substances used for the realisation of the fixed points, except for helium. The definition of a reference isotopic composition for three fixed points, eH_{2}, Ne and H_{2}O, while eliminating the nonuniqueness of the Scale in this respect, induced detectable differences in the present and future realisations of the Scale, at the highest accuracy level, with respect to the previous realisations, when they affected the results of past MRA key comparisons, namely the CCT K1 (and K1.1) and CCT K2 (and K2.1–K2.5) and the related regional and supplementary ones. The paper provides evidence of the extent of this effect by using the results of the relevant key comparisons for neon archived in the BIPM KCDB, and of other comparisons existing in the literature (1979–1984, 2007–2012 and 2009–2010 sealed cell comparisons), and discusses the meaning and the outcomes of this evaluation.
Key words: isotopes / isotopic correction / chemical corrections / neon / triple point / intercomparisons / ITS90
© F. Pavese et al., published by EDP Sciences, 2017
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1 Introductory notes
When the current version of the International Temperature Scale, the ITS90 [1], was promulgated in 1990, the isotopic effect on the Scale was basically ignored, except originally for helium. The Scale definition only made generic reference to a “natural” composition of the substances used for the realisation of the fixed points.
Since the end of the past century, also thanks to the decreased uncertainty of the best realisations of the ITS90, the effect of the natural variability of the isotopic composition of some of these substances was recognised as an appreciable contribution to the total uncertainty budget of the realisations of those fixed points, in some cases being the largest single contribution [2,3]. In Table 1 this fact is clearly shown. The goal of a total uncertainty budget of 30 μK for a measurement on a single cell was reached around 2010 at INRIM [4], PTB and NMIJ.
1.1 State of the art
Isotopic studies were undertaken, initially on eH_{2} (HD in H_{2}) [5,6] and H_{2}O (D_{2} and ^{18}O_{2}) [7–10], and later also on Ne [4,11–15].
They are relevant to ITS90, since the triple points of eH_{2} and Ne are required in SPRT subrange 2 (25–273.16) K. The vapour pressure points at ≈17 and ≈20.3 K of eH_{2} are required in the range (13.8–273.16) K. The use of the triple point of water (TPW), which can presently be realised also in small metallic sealed cells [16,17], is prescribed for the whole part of the ITS90 that is based on resistancethermometer ratios, extending below 273.16 K down to 13.8 K and above 273.16 K up to the silver point, so affecting also all comparisons including these ranges, based on the resistance ratios W = R(T_{90})/R(TPW).
In addition, the triple points of eH_{2} and Ne also affect the range covered by the interpolating gas thermometer (ICVGT), being two of the three fixed points of the ICVGT defined by the ITS90 in the range 3–25 K − the third being the boiling point of ^{4}He (or ^{3}He).
At the time when the key comparisons (KC) CCTK1 “Realisations of the ITS90, 0.65 K to 24.5561 K, using rhodium–iron resistance thermometers” (1997–2001) [18], CCTK2 “Key “Comparison of capsuletype standard platinum resistance thermometers from 13.8 K to 273.16 K” (1997–1999) [19] and CCTK7 “Key comparison of water triple point cells” (2002–04) [20], were organised and completed, the above issue was not yet recognised as important, so not yet formally included in the protocols. Subsequent CCTK1.1 (2006–14, results available in the BIPM KCDB only recently) [21] and EUROMET.TK1 (2008–12, similarly) [22] did not take the isotopic effect into account, except for INRIM. CCTK2.1 (2003) [23] and CCTK2.4 (2006) [24] did not take the isotopic effect into account; CCTK2.3 (2006) [25] did take the isotopic effects into account (official correction for eH_{2} and H_{2}O; VSL unofficial evaluation for Ne, see the separate file of the Online Supplementary Information (OSI)); also CCTK2.5 (2015) [26] did take the isotopic effects into account; CCTK2.2 (2014) [27], not yet completed, will also take the isotopic effects into account. The EUROMET.TK7.1 (2008–2009) [28] and APMP.TK7 [29] included (optionally in the former) the isotopic issue in the comparisons for water.
For water, the issue also involves the present definition of the kelvin, modified in 2005 to include a reference isotopic composition [30]. In the ITS90, for eH_{2} and H_{2}O, corrections to a reference composition were made formally available since the first version of the Technical Annex to the Mise en pratique of the kelvin in 2006; for neon it was since its 2014 version [31].
At present, several cases are known of ITS90 national realisations having adopted, at least partially, isotopic reference compositions: for example, NIST for the all ranges between 4 and 273.16 K only for eH_{2} and of H_{2}O [32]; INRIM for both eH_{2} and Ne affecting the ICVGT [33].
1.2 Aim of this paper
The study in this paper intends to provide evidence of the consequences of taking the isotopic effect into account. This is best done by using the outcomes of intercomparisons, because one can also understand to which extent such a correction have affected, and will possibly affect, the differences between laboratories, when they were obtained in studies not having taken that effect into account. In particular, the scrutiny of key comparisons already available from the BIPM KCDB is important, because that MRA exercise provides to metrology the most valuable results, also in respect to the CMC declarations. However, this paper does not intend to tackle any formal consequence that may arise from, or be related to, the isotopic corrections.
In general, a study on the effect that the correction for the isotopic composition may have on the realisation of the ITS90 in each laboratory is worthwhile if three conditions are met:

the isotopic composition of the samples used in a comparison are known;

the equation to compute the temperature correction is included in the current Annex to the MeP of the kelvin;

the correction can be applied to the results of a substantial number of participants to the comparison.
In addition, no significant effect due to possible remaining chemical impurities should exists, or it has to be taken in account (see for neon the OSI).
Hydrogen.The present information concerning the isotopic correction for hydrogen in CCTK1 and K1.1 comparisons is quite limited, so the third condition is not met. In addition, the effect of the correction on the latter is almost irrelevant with respect to the comparison uncertainty. Similarly, for the CCTK2.x the third condition is not met.
Water. The effect of water isotopic composition will not be analysed in this paper, being minimal in the temperature range below 25 K.
Neon. For neon it is possible to assign the isotopic composition to the gas samples used in a few opencell realisations or contained, in most cases, in permanentlysealed cryogenic metal cells [13]. In these cases, it is possible to apply the equations in the ITS90 Technical Annex [31] and compute the results at the reference composition. For neon all the above conditions are met for the CCTK2, K2.1, K2.3–K2.5 (for the chemical impurity corrections see later Table 3 and the OSI).
In addition, some data are also traceable to the first International Intercomparison of sealed cells performed in 1978–84 [34] or also ensures traceability for several results of the 1997–2005 Star Intercomparison [12].
Therefore, in this paper the computation of the corrections and the discussion of some consequences is limited to neon, as an example of the complexity of the information needed to perform sound corrections, which may also affect the same type of corrections for other substances. See [35] for the way the information drawn from [31] should be used to take isotopic composition into account in the calibration of SPRTs on the ITS90, and [15] for details about the needed isotopiccomposition assays and their outcomes.
In the OSI, the effects of the chemical impurities in neon are briefly presented, presently not subject to correction according to [31] but only considered as an uncertainty component, to compare the importance of their effect with the isotopic effect.
Data for neon used in this study and isotopic correction for CCTK2.x.
2 Isotopic effects on ITS90 for the neon triple point temperature (24.5561 K)
During a worldwide study lasted about 10 years, 26 different bottles of highpurity neon of commercial origin obtained from distillation of air, plus three certified reference mixtures, were studied, including isotopic composition and chemical impurities assays; thermal studies were performed on 34 samples drawn from them [13]. These studies and the subsequent ones on pure ^{20}Ne and ^{22}Ne samples [13] led to the equation, now included in [31], relating T_{90,ref} (ITS90 defined value) to the value T_{90} for the isotopic composition of the sample used, and allow to compute, from the measured resistanceratio value, the corresponding value at T_{90,ref} [35].
In Table 2 the data are reported for the outcomes of several comparisons concerning neon, and in Table 3 the results for the CCTK2.x of having taken into account the isotopic effect, based on the assay values selected after the critical evaluation of the assays, and their associated uncertainties [13,15].^{1} In Table 5 the results of the isotopic corrections for the Star Intercomparison are reported [12]. For important specific conditions concerning the way the data of each laboratory were obtained, see the OSI associated with this paper.
Data used in this study from comparisons involving the Neon triple point, T_{90tp} = 25.5661 K. Basic data.
2.1 Taking the effect of the isotopic composition into account
We recall here that, according to the MRA, the key comparison reference value (KCRV) of the comparison CCTK2 is common to all the subsequent integrations of its results with the results of the subsequent supplementary comparisons. It is not affected by uncertainty, as indicated in [19].
In order to take the effect of the isotopic composition on T_{tp,Ne} into account, it is useful to summarise the exact meaning of the CCTK2 results (not including the CCTK2.x), and the procedure for applying the isotopic correction to them

(a)
Each participant used a sample of neon whose effect of the isotopic composition, at that time, was taken into account in the uncertainty budget only. This contribution to uncertainty is reported in Table 4, whose mean value amounts to 305 μK out of 361 μK of the total mean laboratory budget (85%) and out of 517 μK of the total comparison mean budget (59%) − so being the dominant contribution.

(b)
The results of the realisation of the triple point temperatures were compared through exchange of thermometers calibrated without taking into account the isotopic effect. However, being the triple point of neon a fixed point of the ITS90, each participant laboratory associated to the provided measured value of the resistance ratio R_{tp,Ne}/R_{TPW} the ITS90 temperature value, 24.5661 K, exact. When the thermometers were compared in a comparison block at NRC, the measured resistance ratios did not exactly reproduce the supplied values − being that evidence the very reason of the comparison.

(c)
According to the CCTK2 protocol, although one cell (NRC F15) was taken as the reference, the value 24.5561 K was not associated to it as the KCRV of the comparison. Instead, the resulting differences in the results, expressed as ΔT_{meas}, were computed in [19] with respect to a T_{KCRV} being the weighted mean of the resulting temperatures.^{2}

(d)
Normally those differences would directly express the difference in the realisations of the fixed point between the participant laboratories, T_{thermal}, due to thermal or technical effects. However, in this case, the measured temperatures were instead T_{meas} = T_{thermal} + DT, where:

•
a DT_{x} is the temperature difference due to the isotopic composition of a sample with respect to the reference composition defined after 2014, the ^{IUPAC}x(Ne) one. Thus the corrections DT_{x} = T_{meas,x} – 24.5661 K.

•
all the remaining items of the uncertainty budget that are usual in a comparison, are taken into account for T_{thermal}. Notice that the KCRV_{bc} used in [19] is affected by the DT_{x} − see item (f) below.^{3} Thus, T_{thermal} = T_{meas} – DT_{x} = T_{meas} – (T_{x} – 24.5661 K) = 24.5661 K + (T_{meas} – T_{x}). However, the final aim of this paper is instead to find δT_{thermal} = T_{thermal} – KCRV_{ac}.

•

(e)
Let us start from the fact that ΔT = T_{meas} – T(KCRV_{bc}) = T_{meas} – wmean(T_{meas}). This can be approximated by replacing the weighted mean with the simple mean: ΔT = T_{meas} – mean(T_{meas}) = T_{meas} – mean(T_{thermal}) – mean(DT) = T_{thermal} + DT – mean(T_{thermal}) – mean(DT).

(f)
Then, one can compute the net contribution for each sample:
Uncertainties for neon of the intercomparisons CCTK2 and CCTK2.x with and without the contribution of the isotopic factor.
2.1.1 Main comparison (CCTK2)
The comparison did not define a “reference cell” to which assign the ITS90 value, 24.5561 K but, as recalled above, the temperature value of the KCRV of CCTK2 was computed as the weighted mean of the temperature values measured in the comparison block by each calibrated thermometer participating in the comparison, leading to the ΔT_{original} values in Table 2: the value of T_{90,K2} assigned to the KCRV was not indicated in the Final Report.
When instead the isotopic composition is taken into account, an arbitrary choice for T_{KCRV} is not allowed anymore, since the ITS90 definition was later integrated by attributing the value 24.5561 K to, and only to, neon having the reference isotopic composition, the one recommended by IUPAC, ^{IUPAC}x(Ne): ^{22}x = 0.0925; ^{21}x = 0.0027; ^{20}x = the rest [36].
This means that, in principle, the CCTK2 KCRV after correction is unlikely to be equal to the CCTK2 KCRV before correction, i.e. to the one used to express the differences in Table 2.
The T_{90}(KCRV_{ac}) and difference (KCRV_{ac} – KCRV_{bc}) can now be evaluated with good approximation. Should the KCRV be the simple mean of the T_{meas}, it would be exact to say that KCRV_{ac} = KCRV_{bc} + mean(DT_{x}); in this case it is a good approximation because the corrections are small with respect to the temperature values. In addition, as illustrated in Section 2.1, one is not interested in the KCRV_{ac}(T_{meas}), as it would directly come from the elaboration of the Final Report of CCTK2, but in the KCRV_{ac}(T_{thermal}), i.e. based on the measured values cleaned from the isotopic effect, T_{thermal} = T_{meas} – DT_{x}.
Being not all corrections necessarily exactly consistent with each other, the resulting value of the KCRV_{ac} can vary somewhat depending on the correction chosen as the reference (exact) one.
In order to first obtain the value of the KCRV_{bc}(T_{meas}), the method used in this paper is the following (where #1 and #2 indicate the thermometer set)^{4}:

(i)
the value T_{90}(Ne) = 24.5561 K, exact, corresponds to ^{IUPAC}x(Ne);

(ii)
a reference sample is chosen. The choice of the NRC F15 sample seems the most obvious, since NRC was the pilot in all K2.x comparisons;

(iii)
for NRC's last reference cell, CuM1, the isotopiceffect difference to ^{IUPAC}x(Ne) is DT_{CuM1} = –6(94) μK;

(iv)
thus, the ITS90 value of the NRC CuM1 cell is T_{90}(CuM1)_{ac} = 24.566 09_{4} K;

(v)
the NRC difference measured through cell F17 [T(CuM1) – T(F15)]_{bc} = –165(200) μK, so one gets T_{90}(F15)_{ac} = 24.566 25_{9} K;

(vi)
the differences ΔT_{F15} indicated in [19, exCCTK2] are ΔT_{F15#1} = T(F15 – KCRV)_{#1} = –0.06(44) mK and ΔT_{F15#2} = T(F15 – KCRV)_{#2} = –0.12(44) mK;

(vii)
thus T_{90}(KCRV_{bc})_{#1} ≈ 24.566 32 K and T_{90}(KCRV_{bc})_{#2} ≈ 24.566 38 K;

(viii)
incidentally, the isotopiceffect from the assays is [T(CuM1) – T(F15)] = –342(95) μK: this is not a discrepancy since it is a different component of the cell differences.
Figure 1 depicts graphically the above procedure.
The temperatures actually measured during the CCTK2, T_{meas}, are obtained by adding to T_{KCRVbc} the ΔT_{meas} values recorded under “results” in [19] for each sample.
One could then compute the T_{meas,ac} by simply adding to ΔT_{meas} the DT_{x} obtained from the ITS90 Technical Annex of [31], and then compute the weighted mean from the latter set, for both sets #1 and #2: δT_{meas,ac} = T_{meas,corr} – T(KCRV_{ac}). The isotopic corrections are reported in Table 3 in the column “Isotopic DT”, For the isotopic composition of the samples, see [13–15]. The KCRV_{ac} are reported in Table 3: T_{KCRVac} = 24.566 47_{1} K for thermometers #1, and T_{KCRVac} = 24.566 55_{8} K for thermometers #2, different, as expected, from the KCRVs before correction: notice that these values correspond to the values in item (viii) above well within the uncertainties. That change alone entails changes of +0.15 mK and +0.18 mK, respectively, to all the ΔT_{meas} = T_{90bc} – T_{KCRVbc} in Table 3 – and in Sections 2.1.4–2.1.6 – but note that pair differences are unaffected.^{5}
However, the above computation is of limited interest, since the T_{meas} are those biased by the isotopic effect through ΔT. They should be transformed into the T_{thermal}, according to the procedure indicated in Section 2.1, approximated by using the simple mean of the T_{meas}.
Starting from equation (1) in Section 2.1(f), the values known in it are those for: all ΔT_{x} from [19] and all DT_{x} from [31]. Note that equation (1), does not contain any absolute value of T, but only mean or relative values: however, one obtains the temperature values in Table 3 as T_{thermal} = 24.566 100 K + δT_{90,thermal}. The δT_{90,thermal} after correction replace the ΔT_{meas} before correction.
The summary of the uncertainties is reported separately in Table 4 – and commented in Section 2.2.
It is interesting to compare the δT_{90,thermal} with the δT_{meas,ac} computed before. Both are approximated: the latter because, as said, they use T_{meas}; the former because the simple mean replaced the weighted mean and they still use the ΔT. However, the difference between the two is fixed and only +40 μK for #1 and +95 μK for #2. The reason is that δT_{90,thermal} – δT_{meas,ac} = KCRV_{bc} – 24.5661 K – mean(DT_{x}).
It is to be noticed that, after correction for the isotopic effect, the NRC experimental difference (CuM1 – F15)_{NRC} = –165(200) μK becomes (CuM1 – F15)_{thermal} + 147(220) μK. However, this change does not require a correction in the procedure Section 2.1.1(v) nor an iteration of the calculations, since in (v) one must use the KCRV based on which the values of the ΔT_{or} in Table 2 were computed, as taken from [19].
Fig. 1 Graphical representation of the procedure described in Section 2.1.1 for set #1.The procedure starts from cell NRC CuM1, step (i), where T_{ref} = T_{90,Ne} = 24.5561 K. For the KCRV_{ac} see Table 3 and Figure 2 The KCRV_{ac} is 24.56647 K. 
2.1.2 Comparison K2.1 (VNIIFTRI, NRC)
In this comparison, the NRC reference cell was still F15. The isotopic composition of the VNIIFTRI sample used in the CCTK2 is unknown, so no computation is possible to take it into account. Therefore, the measured differences +0.28 mK (#1) and +0.22 mK (#2) remain unchanged.
Should one assume that the sample in question is from the same bottle that was used for the cell participating to the 1978–84 Intercomparison [34] and the more recent Star Intercomparison [12], an isotopic correction of −0.29 mK would apply, leading to a difference of −0.01 mK (#1) and −0.07 mK (#2), respectively.
2.1.3 Comparison K2.3 (NMIVSL, NRC)
In this comparison, the NRC reference cell was changed to the newest CuM1, whose uncorrected difference from cell F15 has been measured at NRC (though cell F17) to be [T(CuM1) – T(F15)]_{bc} = –165(200) μK. See Table 3 for the values before and after correction of T_{90}(12Ne), T_{90}(F15) and T_{90}(CuM1)_{.}
NMIVSL used INRIM cell 12Ne (5N gas sample from Messer Griesheim, with assay #11, [15] assigned isotopic correction 123 μK). Thus, from Table 3 the values after correction are [T(12Ne) – T(CuM1)] = –0.00055 K, [T(12Ne) – T(F15)] = –0.00040 K and [T(12Ne) – T_{90}] = –0.00055 K.^{6}
2.1.4 Comparison K2.4 (INTiBS, LNEINM, NRC)
In this comparison, the NRC reference cell was also the newest CuM1–see comparison K2.3.
INTiBS used INRIM cell E3Ne (5N gas sample from Messer Griesheim, with assay #11, [15] assigned isotopic correction 123 μK). Thus, from Table 3 one gets the value of T_{90}(E3Ne) and the values after correction are [T(E3Ne) – T(CuM1)] = –0.00015 K, [T(E3Ne) – T(F15)] = 0 K and [T(E3Ne) – T_{KCRV}] = –0.00015 K.^{7}
LNEINM used cell Ne02/1 (5N gas sample from Air Liquide, with assay #14, [15] assigned isotopic correction −32 μK). Thus, from Table 3 one gets the value of T_{90}(Ne02/1) and the values after correction are [T(Ne02/1) – T(CuM1)] = –0.00042 K, [T(Ne02/1)−T(F15)] = −0.00027 K and [T(Ne02/1) – T_{KCRV}] = –0.00042 K.^{7}
2.1.5 Comparison K2.5 (NMIJAIST, INRIM, NRC)
This comparison is the only one supplied with the results corrected for the isotopic composition of the samples. This requires an inverse computation in order to get the values before correction. For this comparison, the NRC reference cell was also the newest CuM1–as with comparisons K2.3 and K2.4.
NMIJAIST used its cell Ne5 (5N gas sample from AirWater, with assay #7, [15] assigned isotopic correction 4 μK). Thus, from Table 3 one gets the value of T_{90}(Ne5) and the values after correction are [T(Ne5) – T(CuM1)] = –0.00032 K, [T(Ne5) – T(F15)] = –0.00018 K and [T(Ne5) – T_{KCRV}] = 0.00032 K.^{7}
INRIM used cell Ec2Ne (5N gas sample from Messer Griesheim, with assay #11, [15] assigned isotopic correction 123 μK). Thus, from Table 3 one gets the value of T_{90}(Ec2Ne) and the values after correction are [T(Ec2Ne) – T(CuM1)] = 0.00059 K, [T(Ec2Ne) – T(F15)] = 0.00044 K and [T(Ec2Ne) – T_{KCRV}] = 0.00059 K.^{7}
2.2 Uncertainty of the CCTK2 comparisons
The uncertainty issue has been treated separately in Table 4, since its complex analysis requires a full table.
Table 3 shows an important issue: every comparison exercise adds uncertainty to the previous results, in average a 30% more when comparing U_{KC} to U_{TOTlab}. In addition, as expected, the increase is larger for the late K2.1–K2.5 (≈30%) than for the original K2 (≈20%).
Another very important issue is that, by strongly decreasing the uncertainty on the isotopic composition, one strongly affects the overall laboratory uncertainty budget of the comparison of neon samples: in fact the average contribution of the isotopic effect is of 305(97) μK out of a total of 361(145) μK, so accounting for more than half.
Since the isotopic uncertainty drops in average from 305(97) to 37(33) μK, the laboratory differences decrease by about 30% in average after compensating for the isotopic effect, and the comparison uncertainty accordingly: the benefit of the corrections amounts in average to 60(15)%, i.e. it cuts the comparison uncertainty by more than half.
3 Discussion and final remarks
Figure 2 shows a graphical representation of data reported in Table 3: the mean value of the original deviations ΔT_{or} is −147(268) μK^{7} for set #1 and −166(309) μK for set #2, while those after correction, δT_{iso} = (T_{90ac} – KCRV_{ac}), are −175(306) μK for set #1 and −187(388) μK for set #2, thus basically the same: this only means that the (obviously unknown) thermal contributions to the deviations are dominant.
However, when subtracting from the original differences the contribution of the isotopic effect, in Figure 4 one gets for δT_{thermal,ac} −167(233) μK for set #1 and −147(240) μK for set #2, where the uncertainty is reduced by 60% in average, as already observed in Table 4. In addition, apart for two samples, the deviations after correction are within the interval (+0.3, −0.2) mK, while in Figure 2 they were in the wider interval (+0.4, −0.8) mK.
Therefore, by taking into account the isotopic effect, one can have a substantial improvement in the quality of the comparison results of the CCTK2.x, though the uncertainty will necessarily increase progressively by adding up new comparisons, as it happened for the supplementary comparisons on the same fixed point − see Table 4 and Section 2.2.
In some cases, it is also possible to compare cell differences of INRIM production or of cells of other NMIs directly measured also at INRIM [4] with the values obtained from the K2.x ones.
In Figure 3 the following cells are shown, all sealed with gas taken from the same bottle of gas (Messer Greisheim, analysis #11 [15]): from the left, 2000: INTiBS (INRIM) cell 15Ne; 2000: VSL (INRIM) cell 12Ne [25] that was made in the same batch (21 Oct 1999) of cell 15Ne; 2001: INRIM cell E3Ne [24], that was made in the same batch (24 Aug 2000) of cell E4Ne; 2001: PTB (INRIM) E1Ne (12 Dec 1999) [12] that was sealed two months before the E2Ne to E4Ne batch; 2002: INRIM Ec1Ne and Ec2Ne (reference cell) [4]; finally, 2015: INRIM Ec29Ne that was sealed from the same bottle of gas after its return back to INRIM after the assays at IRMM, and measured in 2015. All results are compatible with each other except the last one with respect to the 2002 ones. The +94 μK increase of T_{tp} in 2015 is attributed in [15] to a possible change for unknown reasons of gas isotopic composition within the bottle during the years.
The results of the CCTK2.x can also be compared in Figure 4 with the results of the largest direct comparison of samples in sealed cells made after the Int84 [34]: the Star Intercomparison [12], whose data are compared in Table 5 using the data of Table 2.
Figure 4 (right part) makes selfevident the improvement of the Star data (right part) with respect to the K2.x data (left part). Only two samples are outlying: INM Ne02/1 and NIST 201. The latter is greyed in Table 5 because traceability back to the right filling gas is unsure. With its exclusion, the mean of the corrected differences is 56(68) μK (compared with 74(87) μK before isotopic correction), thus basically not significant at the U level being the measurement uncertainty (k = 1) of ≈47 μK. Except one, all deviations are now within ±50 μK.
Fig. 2 Graphical representation of the corrected data ΔT = (T_{meas,ac} – T_{KCRVac}) from Table 3 and the uncorrected data ΔT = ΔT_{original} from Table 2, for cells #1 to #15 and for thermometer sets #1 and #2. Gray dots and lines: uncorrected differences. Black squares and lines: isotopiccorrected differences. 
Fig. 3 Differences between samples drawn at INRIM at different times (from left to right) from the same bottle; zero of ΔT arbitrary, hydrostatic head correction applied. Sealing dates of INRIM cells: from left, cell 12Ne–15Ne; cell E2–E4Ne; cell Ec1Ne; cell Ec2Ne; cell Ec29Ne sealed and measured in 2015. Uncertainty of each determination is ±≈50 μK. 
Fig. 4 Graphical representation of differences from the KCRV of K2xx and Star directcell Intercomparison, uncorrected (gray dots and lines) and corrected (black squares and lines): (T – KCRV_{K2ac}) = DT_{thermal,ac} (Tab. 2); (T – KCRV_{K2bc}) = ΔT_{original} (Fig. 1). On the left until #28: K2xx differences for cells #1 to #15 and thermometer sets #1 and #2. ΔT_{original,#1} = –147(268) µK, ΔT_{original,#2} = –166(309) µK; DT_{thermal,ac,#1} = –167(233) µK, DT_{thermal,ac,#2} = –147(240) µK. On the right from #30 to end: Star differences (56(68) µK; 74(87) µK before isotopic correction), u = 47 µK [12]⋅ The dotted lines indicate the range of the isotopic effect for the studied samples, as obtained from the MeP [31] in Table 3. 
4 Conclusions
The intercomparisons were taken as the main basis of the analysis in this paper because, in order to evaluate the importance of the effect of the isotopic correction, one needs to have a number of “comparable” data.
This does not necessarily mean that the authors suggest to always “correct” previous data, e.g. due to uncorrected significant chemicalimpurity effects. However, in this way, one can get the correct understanding of the oftencomplex procedure to be used for performing the isotopic correction. Originally, that study was undertaken mainly for the latter purpose, then we considered it worth also for comparing different types of intercomparisons.
The technical conclusions are as follows.

The exercise made on the KC2 has shown that the total uncertainty of the results for neon can be reduced to less than half by making the isotopic correction, because up to 80% of the original uncertainty budget was due to the contribution of the unknown isotopic compositions (Tab. 3).

Being the original KC2 uncertainty budget made of two main components − the isotopic one and the instrumental one − the results of the corrected data show instead that the resulting intercomparison dispersion of the degrees of equivalence (DoE) for the corrected data is basically the same, though the values are obviously different. This means that the instrumental and thermal uncertainty components (including the notsmall comparison contribution) is of the same order of magnitude of the isotopic corrections. So one gets basically the same size of the DoEs, but less uncertain.

The same exercise made on other types of intercomparisons have shown the superiority of the directcell comparison, especially clear for the Star Intercomparison (Tab. 4 and related figure) − see also the OSI.

Concerning the effect of the chemical impurities (see the OSI), there is not yet, at present, a formal obligation from CCT to make a correction, at least for the cryogenic range: nothing on this matter is included yet in the Technical Annexe to the ITS90 [31], though CCT documents of the Working Groups already exist. Therefore that Section is placed in the OSI, together with more information on directcell intercomparisons. However, the contribution to the final values and uncertainties of the chemical impurities, though variable depending on their level, in the worst cases can be substantial and even larger than the isotopic effect.
Supplementary Material
Supplementary Material supplied by the authors. Access here
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In this paper, the Greek Δ is used for differences before isotopic correction (e.g., ΔT_{meas} = Δ_{or} in Table 2), while capital Roman D is used for the isotopic effect − see text in (d). In this paper the differences due to a different amount of chemical impurities is not considered − see the OSI.
The new KCRVs were obtained by omitting the INM datum, probably already omitted from the KCRV computation by NRC in the Final Report, and by including KRISS, whose datum was not processed in the Final Report [19].
The above values derive from considering the NRC F15 as the reference cell for the original CCT K2 [19]. The KCRV_{K2.x} remains that of the CCTK2.
Cite this article as: Franco Pavese, Anna SzmyrkaGrzebyk, Peter P.M. Steur, The ITS90 after definition of neon isotopic reference composition: extent of the isotopic effect tested on previous intercomparison results, Int. J. Metrol. Qual. Eng. 8, 27 (2017)
All Tables
Data used in this study from comparisons involving the Neon triple point, T_{90tp} = 25.5661 K. Basic data.
Uncertainties for neon of the intercomparisons CCTK2 and CCTK2.x with and without the contribution of the isotopic factor.
All Figures
Fig. 1 Graphical representation of the procedure described in Section 2.1.1 for set #1.The procedure starts from cell NRC CuM1, step (i), where T_{ref} = T_{90,Ne} = 24.5561 K. For the KCRV_{ac} see Table 3 and Figure 2 The KCRV_{ac} is 24.56647 K. 

In the text 
Fig. 2 Graphical representation of the corrected data ΔT = (T_{meas,ac} – T_{KCRVac}) from Table 3 and the uncorrected data ΔT = ΔT_{original} from Table 2, for cells #1 to #15 and for thermometer sets #1 and #2. Gray dots and lines: uncorrected differences. Black squares and lines: isotopiccorrected differences. 

In the text 
Fig. 3 Differences between samples drawn at INRIM at different times (from left to right) from the same bottle; zero of ΔT arbitrary, hydrostatic head correction applied. Sealing dates of INRIM cells: from left, cell 12Ne–15Ne; cell E2–E4Ne; cell Ec1Ne; cell Ec2Ne; cell Ec29Ne sealed and measured in 2015. Uncertainty of each determination is ±≈50 μK. 

In the text 
Fig. 4 Graphical representation of differences from the KCRV of K2xx and Star directcell Intercomparison, uncorrected (gray dots and lines) and corrected (black squares and lines): (T – KCRV_{K2ac}) = DT_{thermal,ac} (Tab. 2); (T – KCRV_{K2bc}) = ΔT_{original} (Fig. 1). On the left until #28: K2xx differences for cells #1 to #15 and thermometer sets #1 and #2. ΔT_{original,#1} = –147(268) µK, ΔT_{original,#2} = –166(309) µK; DT_{thermal,ac,#1} = –167(233) µK, DT_{thermal,ac,#2} = –147(240) µK. On the right from #30 to end: Star differences (56(68) µK; 74(87) µK before isotopic correction), u = 47 µK [12]⋅ The dotted lines indicate the range of the isotopic effect for the studied samples, as obtained from the MeP [31] in Table 3. 

In the text 
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