On the long-term stability of the triple-point-of-water cells

Abstract

Slow dissolution of the borosilicate container of triple-point-of-water (TPW) cell is widely recognized as the main cause of long-term drift in observed triple point temperature. We add to the available experimental data a comparison of two large batches of TPW cells (67 cells in total) of various ages (from 1 year to 64 years), manufacturers (NRC, VSL, Fluke, Isotech, etc), and materials (borosilicate glass and fused-silica) which was undertaken in 2018. After measuring the TPW temperatures realized by all 67 cells, 12 borosilicate cells were opened and their water was analyzed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) in order to correlate their impurity content with their respective age and their realized TPW temperature. No direct correlation was observed between the TPW cells age/impurity content and their measured triple-point temperature for neither borosilicate cells nor fused silica cells (Pearson’s correlation coefficient rxy is within the range −0.60 ≤ rxy≤ +0.40 for all the pairs considered). For fused-silica cells, the results indicate that after the isotopic variation in the water source is taken into account, the long-term drift due to the dissolution of glass envelope, if any, is negligibly small: (+0.4(±0.6) µK·yr−1 reported herein). Given that all the fused-silica cells realize the TPW temperature within 100 µK of NRC and VSL national reference cells and since the analyzed time period of 15 years is equal to the average lifespan of a TPW cell, we conclude that fused-silica TPW cells are superior to those made from borosilicate glass.

NRC measurement set-up and preparatory work for CCT-K7.2021 key comparison of triple-point-of-water cells

“…We acknowledge that the historical measurements in figure 1 correspond to different resistance ratio bridges and different quartz TPW cells, however we estimate the variations due to the bridge performance and to different immersion depth in the cell to be of the order of 10 μK. The dissolution of quartz over this time frame is expected to be minimal [6]. In principle, the pilot's measurement scheme for the CCT-K7.2021 comparison is robust against slow long-term changes (on a week to a month timescale) in the SPRT resistance (e.g.…”

“…All participants were asked to select a transfer TPW cell of the highest quality using preferably a fused silica cell, as the traditional borosilicate TPW cells were shown to drift more with age than fused silica cells [5,6]. The transfer cells had to be measured against the corresponding TPW national standard before shipping to the pilot and after receiving from the pilot.…”

“…This SPRT has a long history of TPW measurements dating back to at least 1997, when it was used by Hill to intercompare NRC TPW cells [5]. It was subsequently used by NRC for CCT-K7 measurements in 2002 [4], in two internal comparisons of NRC TPW cells in 2007 and 2013 (not published), and as a check-thermometer in a TPW cells investigation in 2018 [6]. To our knowledge, this SPRT was never used for any other measurements, was never annealed and always remained at room temperature when not used for TPW measurements.…”

“…The ICP-MS analysis was focused on 8 elements, which are known to constitute the large majority (99% of the total impurity content in [6]) of impurities found in TPW cell water: B, Na, Mg, Al, Si, K, Ca and Fe. The ICP-MS results were provided in terms of μg g −1 and transformed into μmol mol −1 , using the molar mass of each element.…”

No country for old borosilicate triple-point-of-water cells

“…However, for any given borosilicate TPW cell, the onset and the rate of the degradation are unpredictable. Given this known limitation, all major TPW manufacturers now offer TPW cells made from vitreous silica (fused silica or fused quartz, see additional discussion in section 4) that are believed to be more stable than their borosilicate counterparts [16] and most NMIs include vitreous-silica TPW cells in their national standards, often together with old borosilicate TPW cells.…”