Combined pool of artefacts maintained by the BIPM and NMIs with primary realizations of the kilogram: Advantages and Difficulties

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1 Bureau International des Poids et Mesures CCM Workshop on the ise en pratique of the new definition of the kilogra Noveber 2012 Cobined pool of artefacts aintained by the BIPM and NMIs with priary realizations of the kilogra: Advantages and Difficulties Estefanía de Mirandés Mass Departent 1

2 BEFORE REDEFINITION BIPM IPK IPK 1 kg Overview of the disseination of the kilogra Watt balance 2 AFTER REDEFINITION Priary realizations t Watt balance 1 t Enseble of ass standards N BIPM/NMIs Avogadro silicon spheres NP NP National prototypes NP1 NP2 NPn NP National prototypes NP1 NP2 NPn 2

3 Advantages of having available a stable enseble of ass standards Readily available (not easily true for priary realizations). Can be routinely calibrated by several (all) priary realizations. Therefore its weighted average can be very well known. Can help copare priary realizations which are available at different ties. Mass differences of the eleents of the enseble can be very accurately known (~µg) and frequently easured. Between two successive calibrations of the enseble by priary realizations the average ass of the enseble can be aintained at the level of few icrogras. To prepare the redefinition, each eleent can be copared directly to IPK at the ~µg level. If coposed of several standards (and eventually different aterials), its weighted ean should be quite stable in tie. If a particular standard shows a ass jup, the weighted average should be less sensitive to it. 3

4 Characteristics of the ideal enseble of ass standards A sufficiently large nuber of standards should be included. Mass differences aong all the standards should be easured often. The standards should be kept in clean (and if possible controlled) environents to ensure the best possible ass stability. The storage environent of the standards should not be changed significantly to liit the possibility of jups in ass. 4

5 Bureau International des Poids et Mesures The BIPM enseble of ass standards: present status 5

6 WHY an enseble of reference ass standards at the BIPM? In 2011, the General Conference on Weights and Measures (CGPM) encouraged the BIPM "to develop a pool of reference standards to facilitate the disseination of the unit of ass when redefined" (resolution 1). Since then, the BIPM Mass Departent has been assebling a new enseble of 12 reference ass standards. 6

7 The BIPM enseble of reference ass standards Main purpose The enseble should have the best possible ass stability in the short and iddle ter. For that, the standards are stored in an uncontainated and continuously analyzed environent. The standards 12 reference ass standards + 4 stacks: 4: 1kg PtIr cylinders. 4: 1kg stainless steel cylinders. 4: 1kg natural silicon spheres. 4: 1kg stack of cylinders (1 PtIr, 1 Si, 2 Stainless steel). The stack of cylinders are designed to have the sae ass, volue and surface polish as the single-piece standard of sae aterial but with a larger surface. Mass changes due to surface effects ay be studied. Copare the ass stability of standards of different aterials. The storage conditions 4 different storage environents. (In each environent we store 1PtIr cylinder, 1 stainless steel cylinder, 1 nat Si sphere and 1 stack of cylinders): a low-flow argon gas (0.3 l/in per standard). a low-flow nitrogen gas (0.3 l/in per standard). in vacuu (10-5 bar). in air (abient atospheric pressure, in the sae storage conditions that have been traditionally used at the BIPM). Copare the ass stability of standards under different storage conditions. PtIr stack nat Si sphere 7

8 Laboratory for the BIPM enseble of ass standards Nitrogen generator (not visible) 4 standards stored in a low-flow argon gas Argon bottles 4 standards stored in a low-flow nitrogen gas (behind, not visible) 4 standards stored in vacuu Nitrogen bottles In preparation: 4 standards in air, to be kept inside the cabin Oxygen analyzer Huidity analyzer Hydrocarbons analyzer (Gas chroatograph + Flae Ionization Detector) Teperature, huidity and pressure are continuously onitored within the cabin. Outside the cabin, teperature is actively controlled. 8

9 The storage containers ass holder Storage containers are electro-polished inside. Inner volue about 4 litres Containers are designed to contain a ass holder with a ass standard inside. Containers have been constructed by the BIPM echanical workshop. 9

10 Gas storage network (I) reotely controlled electric valves bypass anual isolation valves A continuous gas flow (argon (left) and nitrogen (right) is aintained through all the containers. The gas network is fully autoated: 28 reotely controlled electric valves and 2 gas flow-eters allow to choose to analyze the gas passing through one container at a tie while ensuring an adequate gas flow through all the others. Gas containers are equipped with anual isolation valves which allow each container to be reoved and put back into the network without altering the storage conditions of the standard inside. Gas storage containers have a bypass tube which allows the tubes of the network to be purged without flowing the purge gas through the container. Mass flow controller 10

11 Gas storage network (II): gas sources Nitrogen gas bottles Nitrogen gas generator (below the floor, not visible) Argon gas bottles The argon gas coes fro gas bottles with contents certified to have a axiu of 50 nol/ol of hydrocarbons, oxygen and water ipurities. For the nitrogen network we use at present two different gas sources for coparison: ultrapure nitrogen in bottles (sae ipurity specifications as the argon bottles) and nitrogen gas coing fro a nitrogen generator which has up to 2 parts per illion of oisture. Autoatic switch between bottles. Autoatic switch between bottles 11

12 Gas storage network (III): gas analyzers Oxygen analyzer Water analyzer Hydrogen gas generator Gas chroatograph + Flae ionization detector filtered air generator After passage through the containers, the output gas is continuously analyzed and copared to the analysis of a control line. The purpose is to detect changes in the concentration of hydrocarbons, oxygen and water which could be due to the ass standards stored inside the containers. Our analyzers work at the hundreds of parts per billion level. 12

13 Vacuu storage network (I) Vacuu pup Residual gas analyzer (RGA) Vacuu storage network au 10 ppb level A residual gas analyzer (RGA) analyzes continuously the cheical coposition of the background vacuu. Eight valves allow the selection of the container(s) to be analyzed by the RGA while continuing to pup all containers without interruption. Notice that each container can be puped by two channels, each controlled by one closing valve. For each container only one channel is analyzed by the RGA. The other goes directly to the pup. This design allows to choose thanks to the closing valves which container's vacuu is analyzed by the RGA while continuing to pup all the others. 13

14 40: Ar 41: C3 H 5 44: CO2 45: CH3 CH 2 O 46: CH3 CH 2 OH 32: O2 A typical scan of our RGA 1: H 2: H2 12: C 16: O 14: N 19: F 20: Ne 22: 22 Ne 26: C2 H 2 CN 18: H2 17: OH, O NH3 28: N2 29: CH3 CH 2 30: C2 H 6 14

15 Typical easureents of oisture concentration Argon gas coes fro argon bottles and nitrogen gas coes fro nitrogen bottles Moisture concentration -Six onths of continuous operation. 4 containers under a low-flow Ar gas 4 containers under a low-flow Nitrogen gas Ar_1 Ar_2 Ar_3 Ar_4 N_1 N_2 N_3 N_4 -We easure sequentially and uninterruptedly the oisture concentration of the background of each container. -Between each two containers we easure the oisture content of a control line. Ar_1:epty Ar_2: ass holder Ar_3: epty Ar_4: ass holder N_1: 1 kg stainless steel + ass holder N_2: 1 kg stainless steel + ass holder N_3: ass holder N_4: epty We don t notice significant differences in the oisture content of those containers which have a ass holder or a ass standard inside. 15

16 Typical easureents of oxygen concentration N 2 N_2 N_3 N_4º Ar Ar_3 Ar_4 Ar_2 Ar_1 N_1 Ar_1:epty Ar_2: ass holder Ar_3: epty Ar_4: ass holder N_1: 1 kg stainless steel + ass holder N_2: 1 kg stainless steel + ass holder N_3: ass holder N_4: epty We don t notice significant differences in the oxygen content of those containers which have a ass holder or a ass standard inside. Argon gas coes fro argon bottles and nitrogen gas coes fro nitrogen bottles 16

17 Typical hydrocarbons spectru Retention ties in the GC colun - We follow the evolution in tie of the the area of each peak and for each container. -The gas having flown through each container is accuulated during 10 inutes in a cold trap before being analyzed by the gas chroatograph. The analysis takes about 20 inutes. -3 ties per year a saple is analyzed by a specialized copany in order to identify the containant corresponding to each retention tie. 17

18 Present status of the BIPM enseble of ass standards All storage containers (Argon, Nitrogen, vacuu) have been constructed and assebled. The easureents of the gas containants (oisture, oxygen, hydrocarbons) of each container are fully autoated and coputer controlled. The syste has been operating for six onths now. Soe containers are epty, others contain ass holders and others have test stainless steel standards inside. ALL the final standards of our enseble have been anufactured. Mass, volue and surface easureents are being ade. We expect to place the final standards inside the storage containers in the following onths. 18

19 Bureau International des Poids et Mesures Advantages and difficulties of a cobined pool of artefacts aintained by BIPM and NMIs with priary realizations 19

20 Advantages and difficulties of a cobined pool of artefacts aintained by BIPM and NMIs with priary realizations Potential advantages Larger nuber of ass standards More robust Enseble of 12 ass standards N BIPM NMIs ass standards Potential difficulties Necessity of coparing all those ass standards frequently with each other Bigger probability of having ass jups due to environent changes in the coparison process 20

21 Thank you for your attention 21