1. New TPW cells with isotopic composition versus old cells: quantifying the difference M. Dobre SMD Belgium S ervice Etalonnage M ETROLOGIE D ienst Kalibratie

2. http://economie.fgov.be Motivation  Toward more accurate fixed points: a step froward has already been done for TPW with the CIPM decision to take into account the isotopic composition of water.  How does that affect the main metrology instituts mission: maintain and transfer national definitions of units? S ervice Etalonnage M ETROLOGIE D ienst Kalibratie

3. http://economie.fgov.be Background S ervice Etalonnage M ETROLOGIE D ienst Kalibratie « Old » cells: 1 NPL & 1 Hart Scientific cell without specifications about isotopic composition. First attempt to take into account the isotopic composition: a Nmi cell with isotopic composition is added to the batch. « New » cells: two more cells with isotopic composition (one from Hart Scientific and one from Isotech) are added to the batch.

4. http://economie.fgov.be Experiments  First experimental investigation: compare NMi cell with old batch of cells in order to determine a correction value: –Measurements were taken daily in the 3 cells with 3 SPRT’s and on two different mantles.  Second set of measurements (6 months later) involved 3 cells with isotopic composition. S ervice Etalonnage M ETROLOGIE D ienst Kalibratie

5. http://economie.fgov.be Old cells are lower by as much as 140 µK than reference cell S ervice Etalonnage M ETROLOGIE D ienst Kalibratie Mean of differences between cells: Hart1 - NMi: -84 µK NPL – NMi : -102 µK When isotopic correction is applied to Nmi cell: Hart1 - NMi : -121 µK NPL – NMi : -139 µK

6. http://economie.fgov.be S ervice Etalonnage M ETROLOGIE D ienst Kalibratie Mean of differences between cells: Hart2 - NMi: +57 µK Isotech - NMi : +98 µK Mean of differences between cells when isotopic correction is applied: Hart2 - NMi : +22 µK Isotech - NMi : +39 µK Isotopic corrections: Nmi : +37 µK Isotech : -22 µK Hart2 : +2,2 µK New cells are higher by as much as 40 µK than reference cell

7. http://economie.fgov.be Data exploitation  One way to analyse the measurements: compare the old standard based on three cells with the new three cells with isotopic composition standard;  Another way: treat the Nmi cell, present in both groups, as a « client cell » to be calibrated versus a national standard composed of two cells in order to quantify the influence of the new standard on calibrations. S ervice Etalonnage M ETROLOGIE D ienst Kalibratie

8. http://economie.fgov.be National standard based on the mean of three cells: new versus old cells S ervice Etalonnage M ETROLOGIE D ienst Kalibratie Old standard for TPW: Mean of Hart1, NPL & NMi cells New standard for TPW: Mean of Hart2, Isotech & NMi cells The Nmi cell is the reference cell and is compared with the old and new standard. The temperature difference of the reference cell from the mean is expressed as:

9. http://economie.fgov.be A cell from the batch compared with the new national definition will show a temperature difference 74µK lower S ervice Etalonnage M ETROLOGIE D ienst Kalibratie

10. http://economie.fgov.be TPW cell calibration: estimate the influence of changing standard S ervice Etalonnage M ETROLOGIE D ienst Kalibratie Old standard for TPW: Mean of Hart1 & NPL cells New standard for TPW: Mean of Hart2 & Isotech cell The Nmi cell is the tested cell and is compared with the old and new standard. The temperature difference of the tested cell from the mean is expressed as:

11. http://economie.fgov.be A test cell compared with the new national standard based on two cells mean will show a temperature difference 132µK lower S ervice Etalonnage M ETROLOGIE D ienst Kalibratie

12. http://economie.fgov.be Conclusions  The decision of changing the TPW cells in order to take into account the isotopic composition has consequences on calibrations: it should be documented on the calibration certificats.  For our batch, the same cell shows a difference from the mean 74 µK lower when compared to new cells with isotopic composition.  If only two cells compose the batch the difference is even greater (132 µK). S ervice Etalonnage M ETROLOGIE D ienst Kalibratie … and questions  How to communicate with clients on changes made on national standards?  Could one mix cells with and without isotopic composition to form a larger group.  What about the uncertainity of isotopic analysis?