Prototype Of Kilogram Research Articles

The watt balance: determination of the Planck constant and redefinition of the kilogram

Since 1889, the international prototype of the kilogram has served as the definition of the unit of mass in the International System of Units (SI). It is the last material artefact to define a base unit of the SI, and it influences several other base units. This situation is no longer acceptable in a time of ever-increasing measurement precision. It is therefore planned to redefine the unit of mass by fixing the numerical value of the Planck constant. At the same time three other base units, the ampere, the kelvin and the mole, will be redefined. As a first step, the kilogram redefinition requires a highly accurate determination of the Planck constant in the present SI system, with a relative uncertainty of the order of 1 part in 10(8). The most promising experiment for this purpose, and for the future realization of the kilogram, is the watt balance. It compares mechanical and electrical power and makes use of two macroscopic quantum effects, thus creating a relationship between a macroscopic mass and the Planck constant. In this paper, the operating principle of watt balance experiments is explained and the existing experiments are reviewed. An overview is given of all available experimental determinations of the Planck constant, and it is shown that further investigation is needed before the redefinition of the kilogram can take place. Independent of this requirement, a consensus has been reached on the form that future definitions of the SI base units will take.

Adapting the International System of Units to the twenty-first century

We review the proposal of the International Committee for Weights and Measures (Comité International des Poids et Mesures, CIPM), currently being considered by the General Conference on Weights and Measures (Conférences Générales des Poids et Mesures, CGPM), to revise the International System of Units (Le Système International d'Unitès, SI). The proposal includes new definitions for four of the seven base units of the SI, and a new form of words to present the definitions of all the units. The objective of the proposed changes is to adopt definitions referenced to constants of nature, taken in the widest sense, so that the definitions may be based on what are believed to be true invariants. In particular, whereas in the current SI the kilogram, ampere, kelvin and mole are linked to exact numerical values of the mass of the international prototype of the kilogram, the magnetic constant (permeability of vacuum), the triple-point temperature of water and the molar mass of carbon-12, respectively, in the new SI these units are linked to exact numerical values of the Planck constant, the elementary charge, the Boltzmann constant and the Avogadro constant, respectively. The new wording used expresses the definitions in a simple and unambiguous manner without the need for the distinction between base and derived units. The importance of relations among the fundamental constants to the definitions, and the importance of establishing a mise en pratique for the realization of each definition, are also discussed.

State-of-the-art mass determination of 28Si spheres for the Avogadro project

For a new determination of the Avogadro constant to play a role in the future redefinition and realization of the mass unit, the mass measurements involved need to be carried out at a very high level of accuracy. From a mass comparison among two 1 kg 28Si spheres and the 1 kg platinum–iridium (Pt/Ir) mass standards, the mass of the spheres must be determined with a combined standard uncertainty of less than 5 µg. The BIPM, the PTB and the NMIJ have carried out such a state-of-the-art mass comparison in air and under vacuum in order to reach this target set by the International Avogadro Coordination (IAC). The results obtained for the spheres AVO28-S5 and AVO28-S8 involved in the comparison have demonstrated that by using air buoyancy artefacts and sorption artefacts it is possible to achieve a relative uncertainty of 4.1 × 10−9. The reference value for each sphere has been determined, taking into account the traceability of the masses to the International Prototype of the kilogram, , in due consideration of the correlations among 17 standards used directly or indirectly in this comparison.

Metallurgical characterization of materials for the reference mass standard of the French watt balance experiment

Watt balance experiments are intended to link the mass unit to the Planck constant. Therefore, in these experiments, a reference mass standard is necessary. However, in the French apparatus, the platinum–iridium alloy that forms the international prototype of the kilogram cannot be considered because of its magnetic susceptibility. Consequently, new materials were investigated, according to stringent specifications. This led to the selection of alloys based on precious metals. The stability of the materials used in mass metrology is strongly dependent on bulk homogeneity and cleanliness: after the investigation of different production modes and heating treatments, microstructural and analytical studies were performed. The critical properties of the different materials were examined and discussed in comparison with the platinum–iridium alloy.

Traceability of mass measurements in Estonia

Realization of the national mass scale for the optimal coverage of the industrial need for the mass calibrations at the Estonian national metrology institute, AS Metrosert, is described. The cost effective technical solutions to meet the calibration requirements of the advised OIML class E1 weights have been established. The traceability to the International Prototype Kilogram, the measurement capability achieved, and the results of participation in international comparisons are described in detail. The actual need for the mass calibrations in Estonia with respect to the measurement capability of the laboratory is estimated.

The determination of the atomic mass constant with ion accumulation: status and perspectives

An experiment is described for determining the atomic mass constant by accumulating ions from an ion beam up to a weighable mass. It establishes a link between the international prototype of the kilogram, the realization of the associated SI unit and an atomic mass. The items necessary for such an experiment are a high-current ion source, an ion optical system with high transmission, a suitable ion collector and a vacuum balance. With the most recent measurement, a mass of more than 320 mg of bismuth was accumulated and its atomic mass was determined with a relative standard uncertainty of 9.4 × 10−5. Special emphasis is placed on determining the mass loss of the accumulated ions caused through sputter effects in the ion collector.Although work on this experiment at the PTB has now been stopped, we conclude with a number of suggestions that could lead to much smaller relative uncertainty in a future experiment.

SI traceability: Current status and future trends for forces below 10 microNewtons

Measurements related to nano- and micro-scale science, technology, and manufacturing are pushing the limits of detectable mechanical, electrical, and chemical quantities to ever smaller values, raising questions about traceability on such scales. The case in small force measurement is illustrative. At present, the mechanical unit of force is linked to the International Prototype Kilogram, or a deadweight force of nearly 10 N. Although known with exquisite accuracy on this scale, such a mass-based force standard is of little use to investigators and manufacturers using instruments that can determine quantities twelve orders of magnitude smaller. Recognizing this situation, the world congress of the International Measurement Confederation (IMEKO) convened a round table of researchers from National Metrology Institutes representing the US, Europe, and Asia to provide an overview of the emerging field of low-force metrology. This paper captures the information shared in that round table and amplifies on its content.

Toward the Redefinition of the Kilogram: Measurements of Planck's Constant Using Watt Balances

The watt balance relates electrical and mechanical power to determine the SI watt, which, in conjunction with an SI ohm, can determine the SI ampere or volt. It can also be used to measure Planck's constant h in SI units, and if the SI is altered to fix the value of Planck's constant rather than the value of the mass of the prototype kilogram, the last artifact standard will be eliminated from the SI. An ensemble of watt balances, which is run by laboratories around the world, could then provide a way of maintaining the SI unit of mass. At present, there are three operational watt balances, with more planned or under construction. Two laboratories, namely, the National Institute for Science and Technology (NIST) and the National Physical Laboratory (NPL), have published results from their recent watt balance measurements. These results disagree by more than their combined standard uncertainties, and further work is required to discover the source of the discrepancy.

Scope for replacing the prototype kilogram by an atomic mass unit

The main versions of the redefinition for the international prototype kilogram are examined for best accuracy and transmitting the mass unit. It is concluded that it is most promising to use Avogadro’s number and the atomic mass unit in order to transfer to a new standard for the kilogram, i.e., to introduce an atomic kilogram.

Redefinition of the kilogram: a decision whose time has come

The kilogram, the base unit of mass in the International System of Units (SI), is defined as the mass of the international prototype of the kilogram. Clearly, this definition has the effect of fixing the value of to be one kilogram exactly. In this paper, we review the benefits that would accrue if the kilogram were redefined so as to fix the value of either the Planck constant h or the Avogadro constant NA instead of , without waiting for the experiments to determine h or NA currently underway to reach their desired relative standard uncertainty of about 10−8. A significant reduction in the uncertainties of the SI values of many other fundamental constants would result from either of these new definitions, at the expense of making the mass of the international prototype a quantity whose value would have to be determined by experiment. However, by assigning a conventional value to , the present highly precise worldwide uniformity of mass standards could still be retained. The advantages of redefining the kilogram immediately outweigh any apparent disadvantages, and we review the alternative forms that a new definition might take.

Redefining the kilogram

IN A VAULT on the outskirts of Paris there is a small cylinder made of platinum and iridium that has the unique property that its mass can neither increase nor decrease. This property is not the result of new physics, however, because the object is over 100 years old. Rather, the unique properties of this cylinder arise because it is the international prototype of the kilogram. And the definition of the kilogram in the SI system of units states that “the kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram”.

The SI unit of mass

The present definition of the unit of mass in the International System (SI) is based on the international prototype of the kilogram, an artefact dating back to the 1880s. At present there is considerable effort worldwide aimed at replacing this artefact definition by one based on physical constants. This paper gives a brief history of the SI unit and how it is currently realized. The focus is on historical information, often forgotten, which has current relevance.

Development of a silicon density standard and precision densitymeasurements of solid materials by hydrostatic weighing

The method and devices for absolute density measurements of siliconcrystals and relative density measurements of solid materials by hydrostaticweighing are described. A solid density standard based on single-crystalsilicon spheres has been developed, where the mass of the 1 kg sphere iscompared with that of the prototype kilogram through secondary 1 kg standards,and the volume is determined directly by measuring its diameters by opticalinterferometry, so that the unit of density is traceable to the definitionof mass and length in SI units. The relative combined standarduncertainty of this absolute density measurement is 1.0×10-7. Usingthese silicon spheres as a reference standard, relative density measurementsof other solid materials were performed by hydrostatic weighing. Results haveshown that the densities of other silicon crystals and stainless steel sampleshave been determined with relative combined standard uncertainties of1.1×10-7 and 5.8×10-7, respectively. Details on theevaluation of uncertainty in the density measurement arepresented.

Standard weights and measures

We trace the evolution of standards for weights and measures from olden days to their modern scientific definitions. Even in the era of modern science, standards till recently used to be defined by artifacts. It is only in the last few decades that we have gone over to definitions based on fundamental constants of nature. This has made the standards universally reproducible. The present standards for time, length, and electrical units are all based on physical constants. The mass standard, represented by the prototype kilogram, is the only remaining artifact, but there are promising proposals to replace that in the near future.

Comparative SIMS investigations on surface contamination of Pt–Ir alloy treated by different cleaning procedures

A comparative study of the surface contamination of Pt–Ir alloy, treated by different cleaning procedures has been carried out using Secondary Ion Mass Spectrometry (SIMS) technique. The Pt–Ir (10%) alloy is basically the standard mass material of the international prototype of the Kilogram. The effect of the conventional cleaning method on the surface of one of the Pt–Ir sample is examined, not only on the top most surface but also a little deep inside the surface, to identify the causes of long term instability of primary standard mass material. In addition to the usual atmospheric contaminants (carbon and oxygen), the results obtained show the presence of contaminants such as sodium, calcium, potassium, magnesium, etc. on the surface of Pt–Ir alloy samples not hitherto observed using other surface analytical techniques like AES, and XPS. In another experiment, four samples, one uncleaned and three cleaned by solvent and/or steam washing, were studied to compare the effect of cleaning procedures on the Pt–Ir samples. It is concluded from the above results that steam cleaning may be advantageous for standard mass stability compared to other cleaning procedures where solvents are used.

Absolute measurement of the density of silicon crystals in vacuo for a determination of the Avogadro constant

An absolute measurement of the density of silicon crystals used for a determination of Avogadro constant is described. Two 1-kg spheres of nearly perfect geometries were fabricated from a single-crystal silicon ingot both for measuring their densities and for determining the density distribution in the ingot. A scanning type optical interferometer was used to measure their diameters in vacuo. Their volumes were obtained by fitting the diameters to a series of spherical harmonics. Thickness of oxide layers on their surfaces was measured by using an ellipsometer to evaluate its effect on their volumes. The masses of the spheres were measured by using a precision balance for the prototype kilogram. The relative total uncertainty of the densities, determined by direct measurements of their masses and volumes in vacuo, is estimated to 1.1/spl times/10/sup -7/. >

Stability of Reference Masses III: Mechanism and Long-term Effects of Mercury Contamination on Platinum-Iridium Mass Standards

In a previous study (Part I) we showed that the material used for national prototype and reference masses, Pt-10% Ir, is susceptible to mercury contamination even at the very low levels of mercury present in air in weighing laboratories. In this study we measure the mass uptake per unit area of Pt-10% Ir exposed to mercury vapour, as a function of time, using the mass response of a quartz crystal microbalance with electrodes of Pt-10% Ir. This system can detect with accuracy mass increases equivalent to less than 0,1 μg on a prototype kilogram. It is shown that there is a rapid adsorption of the first monolayer of mercury, but this mercury then diffuses into the solid allowing further mercury to adsorb. The quantity of mercury absorbed does not tend towards any immediate limit, but instead continues to grow in proportion to the square root of time. A consideration of the morphology of polished surfaces suggests this growth will continue for between 10 and 600 years. We conclude that our earlier recommendation to clean all environments where reference masses are housed, particularly with regard to mercury, is essential for maintaining the stability of Pt-10% Ir masses. Significant mass instability due to mercury sorption will occur for atmospheric mercury levels well below current health and safety limits.

Cryogenics in fundamental metrology

During the last two decades new cryogenic phenomena, such as the ac Josephson Effect and the Quantum Hall Effect, have been applied to Metrology allowing the electrical units to be referred to the values of fundamental physical constants. To be consistent with SI, these constants must be determined in terms of mechanical quantities. The accuracy of these determinations is improving to such an extent that the constancy of the prototype Kilogram could be tested using electrical quantum standards in the foreseeable future.

Surface analytical study of the cause of mass gain of the kilogram prototype

AbstractThe International Prototype of the Kilogram, which is the only artificial material defining one of the fundamental constants, has been reported to gain mass as time elapses after it is cleaned. The surface of a Pt‐Ir alloy that has been used for the newer Kilogram prototypes was analysed by XPS and thermal desorption spectroscopy (TDS) to investigate the cause of mass gain, its time dependence and the effect of the cleaning method. The main cause of the mass gain was found to be adsorption of carbon (in the form of hydrocarbons) and oxygen (as water) from the atmosphere. The amounts of the two contaminant elements were almost saturated 6 months after sample cleaning. Cleaning methods do not make a large difference to adsorption behaviour. Although surface iridium is slightly oxidized, this effect is negligibly small.

Accurate determination of the density of a crystal silicon sphere

For an independent determination of the Avogadro constant, the density of a 1-kg crystal silicon sphere was determined by direct measurements of its mass and volume. A scanning-type interferometer was used to measure the diameters, and the volume was calculated from the mean of uniformly distributed diameters. The sphere was weighted using a balance for the prototype kilogram at the National Research Laboratory of Metrology (NRLM), and the mass was determined accurately by direct measurements of the buoyancy force acting on the sphere. The total uncertainty of the density is estimated to be 0.34 ppm. Effects of a thin oxide layer and impurities on the density of a pure crystal are evaluated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>