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93-522: The scientific community examined several approaches to redefining the kilogram before deciding on a revision of the SI in November 2018. Each approach had advantages and disadvantages. Prior to the redefinition, the kilogram and several other SI units based on the kilogram were defined by an artificial metal object called the international prototype of the kilogram (IPK). There was broad agreement that

186-409: A Michelson interferometer . By counting and timing the interference fringes, the acceleration of the mass can be measured. A more recent development is a "rise and fall" version that tosses the mass upward and measures both upward and downward motion. This allows cancellation of some measurement errors ; however, "rise and fall" gravimeters are not yet in common use. Absolute gravimeters are used in

279-512: A quantum Hall effect resistance standard . The Kibble balance requires extremely precise measurement of the local gravitational acceleration g in the laboratory, using a gravimeter . For instance when the elevation of the centre of the gravimeter differs from that of the nearby test mass in the Kibble balance, the NIST compensates for Earth's gravity gradient of 309  μGal /m , which affects

372-579: A C-based definition proposes to define the Avogadro constant as being precisely 84 446 889 (≈   6.022 141 62 × 10 ) atoms. An imaginary realisation of a 12-gram mass prototype would be a cube of C atoms measuring precisely 84 446 889 atoms across on a side. With this proposal, the kilogram would be defined as "the mass equal to 84 446 889   × ⁠83 + 1 / 3 ⁠ atoms of C." Another Avogadro-based approach, ion accumulation, since abandoned, would have defined and delineated

465-407: A current of only 10 μA demonstrated a relative uncertainty of 1.5%. Follow-on experiments using bismuth ions and a current of 30   mA were expected to accumulate a mass of 30   g in six days and to have a relative uncertainty of better than 1 ppm. Ultimately, ion‑accumulation approaches proved to be unsuitable. Measurements required months and the data proved too erratic for

558-415: A given point in an electric circuit. The virtue of a practical realisation based upon this definition is that unlike the Kibble balance and other scale-based methods, all of which require the careful characterisation of gravity in the laboratory, this method delineates the magnitude of the kilogram directly in the very terms that define the nature of mass: acceleration due to an applied force. Unfortunately, it

651-603: A gold-based definition of the kilogram for instance, the relative atomic mass of gold could have been fixed as precisely 196.966 5687 , from the current value of 196.966 5687 (6) . As with a definition based upon carbon‑12, the Avogadro constant would also have been fixed. The kilogram would then have been defined as "the mass equal to that of precisely ⁠ 1000 / 196.966 5687 ⁠ × 6.022 141 79 × 10 atoms of gold" (precisely 3 057 443 620 887 933 963 384 315 atoms of gold or about 5.077 003 71 fixed moles). In 2003, German experiments with gold at

744-560: A much lower drift due to elongation over time. The test mass is sealed in an air-tight container so that tiny changes of barometric pressure from blowing wind and other weather do not change the buoyancy of the test mass in air. Spring gravimeters are, in practice, relative instruments that measure the difference in gravity between different locations. A relative instrument also requires calibration by comparing instrument readings taken at locations with known absolute values of gravity. Absolute gravimeters provide such measurements by determining

837-477: A number of standing committees and strategic executive led initiatives, and through its task groups and working groups. CODATA also works closely with member unions and associations of ISC to promote the efforts on open data and open science. CODATA supports the Data Science Journal and collaborates on major data conferences like SciDataCon and International Data Week. In October 2020 CODATA

930-402: A practical realisation, this definition would precisely define the magnitude of the kilogram in terms of a certain number of carbon‑12 atoms. Carbon‑12 (C) is an isotope of carbon. The mole is currently defined as "the quantity of entities (elementary particles like atoms or molecules) equal to the number of atoms in 12 grams of carbon‑12". Thus, the current definition of

1023-516: A practical realisation. The apparatus necessarily required that the deposition chamber have an integral balance system to enable the convenient calibration of a reasonable quantity of transfer standards relative to any single internal ion-deposited prototype. Furthermore, the mass prototypes produced by ion deposition techniques would have been nothing like the freestanding platinum-iridium prototypes currently in use; they would have been deposited onto—and become part of—an electrode imbedded into one pan of

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1116-573: A silicon sphere-based kilogram proposes to use isotopic separation techniques to enrich the silicon until it is nearly pure Si, which has a relative atomic mass of 27.976 926 5325 (19) . With this approach, the Avogadro constant would not only be fixed, but so too would the atomic mass of Si. As such, the definition of the kilogram would be decoupled from C and the kilogram would instead be defined as ⁠ 1000 / 27.976 926 5325 ⁠ ⋅ 6.022 141 79 × 10 atoms of Si (≈ 35.743 740 43 fixed moles of Si atoms). Physicists could elect to define

1209-428: A special balance integrated into the device. Moreover, the ion-deposited mass wouldn't have had a hard, highly polished surface that can be vigorously cleaned like those of current prototypes. Gold, while dense and a noble metal (resistant to oxidation and the formation of other compounds), is extremely soft so an internal gold prototype would have to be kept well isolated and scrupulously clean to avoid contamination and

1302-548: A target mass of one kilogram. Experiments are being performed on the Avogadro Project's silicon spheres to determine whether their masses are most stable when stored in a vacuum, a partial vacuum, or ambient pressure. However, no technical means currently exist to prove a long-term stability any better than that of the IPK's, because the most sensitive and accurate measurements of mass are made with dual-pan balances like

1395-624: A vacuum), vibration, and magnetic forces. Such instruments are capable of an accuracy of about 2 ppb or 0.002 mGal and reference their measurement to atomic standards of length and time. Their primary use is for calibrating relative instruments, monitoring crustal deformation , and in geophysical studies requiring high accuracy and stability. However, absolute instruments are somewhat larger and significantly more expensive than relative spring gravimeters and are thus relatively rare. Relative gravimeter usually refer to comparisons of gravity from one place to another. They are designed to subtract

1488-423: A weight measurement to a mass, and therefore require the precise measurement of the strength of gravity in laboratories. All approaches would have precisely fixed one or more constants of nature at a defined value. The Kibble balance (known as a "watt balance" before 2016) is essentially a single-pan weighing scale that measures the electric power necessary to oppose the weight of a kilogram test mass as it

1581-453: A weight on a spring, and by measuring the amount by which the weight stretches the spring, local gravity can be measured. However, the strength of the spring must be calibrated by placing the instrument in a location with a known gravitational acceleration. The current standard for sensitive gravimeters are the superconducting gravimeters , which operate by suspending a superconducting niobium sphere in an extremely stable magnetic field ;

1674-405: Is a force. A gravimeter measures this gravitational force. For a small body, general relativity predicts gravitational effects indistinguishable from the effects of acceleration by the equivalence principle . Thus, gravimeters can be regarded as special-purpose accelerometers . Many weighing scales may be regarded as simple gravimeters. In one common form, a spring is used to counteract

1767-468: Is an important branch developed on the foundation of classical gravimetry. Microgravity investigations are carried out in order to solve various problems of engineering geology, mainly location of voids and their monitoring. Very detailed measurements of high accuracy can indicate voids of any origin, provided the size and depth are large enough to produce gravity effect stronger than is the level of confidence of relevant gravity signal. The modern gravimeter

1860-580: Is co-organising an International FAIR Symposium together with the GO FAIR initiative to provide a forum for advancing international and cross-domain convergence around FAIR . The event will bring together a global data community with an interest in combining data across domains for a host of research issues – including major global challenges, such as those relating to the Sustainable Development Goals . Outcomes will directly link to

1953-468: Is extremely difficult to develop a practical realisation based upon accelerating masses. Experiments over a period of years in Japan with a superconducting , 30   g mass supported by diamagnetic levitation never achieved an uncertainty better than ten parts per million. Magnetic hysteresis was one of the limiting issues. Other groups performed similar research that used different techniques to levitate

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2046-459: Is not only in reducing their uncertainty, but also in making them truly practical realisations of the kilogram. Nearly every aspect of Kibble balances and their support equipment requires such extraordinarily precise and accurate, state-of-the-art technology that—unlike a device like an atomic clock—few countries would currently choose to fund their operation. For instance, the NIST's Kibble balance used four resistance standards in 2007, each of which

2139-668: Is often hundreds or thousands of times stronger than the changes in gravity being measured. The Lunar Surface Gravimeter was deployed on the surface of the Moon during the 1972 Apollo 17 mission but did not work due to a design error. A second device carried on the same mission, the Lunar Traverse Gravimeter , functioned as anticipated. Gravimeters are used for petroleum and mineral prospecting , seismology , geodesy , geophysical surveys and other geophysical research, and for metrology . Their fundamental purpose

2232-507: Is possible to solve for their position and orientation, by either tracking the arrival time and pattern of seismic waves from earthquakes, or by referencing them to the Sun and Moon tidal gravity. Recently, the SGs, and broadband three-axis seismometers operated in gravimeter mode, have begun to detect and characterize the small gravity signals from earthquakes. These signals arrive at the gravimeter at

2325-399: Is pulled by Earth's gravity. It is a variation of an ampere balance , with an extra calibration step that eliminates the effect of geometry. The electric potential in the Kibble balance is delineated by a Josephson voltage standard , which allows voltage to be linked to an invariant constant of nature with extremely high precision and stability. Its circuit resistance is calibrated against

2418-581: Is somewhat arbitrary. The subsystems of the gravitational radiation experiments are very sensitive to changes in the gradient of the potential. The local gravity signals on Earth that interfere with gravitational wave experiments are disparagingly referred to as "Newtonian noise", since Newtonian gravity calculations are sufficient to characterize many of the local (earth-based) signals. There are many methods for displaying acceleration fields, also called gravity fields . This includes traditional 2D maps, but increasingly 3D video. Since gravity and acceleration are

2511-562: Is the acceleration. Precise GPS stations can be operated as gravimeters since they are increasingly measuring three-axis positions over time, which, when differentiated twice, give an acceleration signal. The satellite borne gravimeters GOCE , GRACE , mostly operated in gravity gradiometer mode. They yielded detailed information about the Earth's time-varying gravity field. The spherical harmonic gravitational potential models are slowly improving in both spatial and temporal resolution. Taking

2604-492: Is the measurement of the strength of a gravitational field . Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest. The study of gravity changes belongs to geodynamics . Gravity is usually measured in units of acceleration . In the SI system of units, the standard unit of acceleration is metres per second squared (m/s ). Other units include

2697-420: Is to map the gravity field in space and time. Most current work is Earth-based, with a few satellites around Earth, but gravimeters are also applicable to the Moon, Sun, planets, asteroids, stars, galaxies and other bodies. Gravitational wave experiments monitor the changes with time in the gravitational potential itself, rather than the gradient of the potential that the gravimeter is tracking. This distinction

2790-549: The Czochralski process , to service the semiconductor industry. To make a practical realisation of the kilogram, a silicon boule (a rod-like, single-crystal ingot) would be produced. Its isotopic composition would be measured with a mass spectrometer to determine its average relative atomic mass. The boule would be cut, ground, and polished into spheres. The size of a select sphere would be measured using optical interferometry to an uncertainty of about 0.3   nm on

2883-654: The National Research Council ), where research and development with the device could continue. The virtue of electronic realisations like the Kibble balance is that the definition and dissemination of the kilogram no longer depends upon the stability of kilogram prototypes, which must be very carefully handled and stored. It frees physicists from the need to rely on assumptions about the stability of those prototypes. Instead, hand-tuned, close-approximation mass standards can simply be weighed and documented as being equal to one kilogram plus an offset value. With

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2976-532: The cgs gal (sometimes known as a galileo , in either case with symbol Gal), which equals 1 centimetre per second squared, and the g ( g n ), equal to 9.80665 m/s . The value of the g n is defined as approximately equal to the acceleration due to gravity at the Earth's surface , although the actual acceleration varies slightly by location. A gravimeter is an instrument used to measure gravitational acceleration . Every mass has an associated gravitational potential. The gradient of this potential

3069-420: The definitions of the second and the metre. The definition of the second depends on a single defined physical constant: the ground state hyperfine splitting frequency of the caesium-133 atom Δ ν (Cs) hfs . The metre depends on the second and on an additional defined physical constant: the speed of light c . With the kilogram redefined in this manner, physical objects such as the IPK are no longer part of

3162-442: The fundamental physical constants and closely related conversion factors for use worldwide. The first such CODATA set was published in 1973. Later versions are named based on the year of the data incorporated; the 1986 CODATA (published April 1987) used data up to 1 January 1986. All subsequent releases use data up to the end of the stated year, and are necessarily published a year or two later, with an additional gap between

3255-400: The speed of light , so have the potential to improve earthquake early warning methods. There is some activity to design purpose-built gravimeters of sufficient sensitivity and bandwidth to detect these prompt gravity signals from earthquakes. Not just the magnitude 7+ events, but also the smaller, much more frequent, events. Newer MEMS gravimeters , atom gravimeters – MEMS gravimeters offer

3348-732: The 94th Meeting of the International Committee for Weights and Measures (CIPM) in 2005, it was recommended that the same be done with the kilogram. In October 2010, the CIPM voted to submit a resolution for consideration at the General Conference on Weights and Measures (CGPM), to "take note of an intention" that the kilogram be defined in terms of the Planck constant , h (which has dimensions of energy times time) together with other physical constants. This resolution

3441-431: The Avogadro constant at precisely 6.022 14 X × 10 mol with the kilogram being defined as "the mass equal to that of ⁠ 1000 / 12 ⁠ × 6.022 14 X × 10 atoms of C". The accuracy of the measured value of the Avogadro constant is currently limited by the uncertainty in the value of the Planck constant . That relative standard uncertainty has been 50   parts per billion (ppb) since 2006. By fixing

3534-482: The Avogadro constant, the practical effect of this proposal would be that the uncertainty in the mass of a C atom—and the magnitude of the kilogram—could be no better than the current 50   ppb uncertainty in the Planck constant. Under this proposal, the magnitude of the kilogram would be subject to future refinement as improved measurements of the value of the Planck constant become available; electronic realisations of

3627-490: The BIPM's FB‑2 flexure-strip balance (see § External links , below). Balances can only compare the mass of a silicon sphere to that of a reference mass. Given the latest understanding of the lack of long-term mass stability with the IPK and its replicas, there is no known, perfectly stable mass artefact to compare against. Single-pan scales , which measure weight relative to an invariant of nature, are not precise to

3720-721: The CODATA Decadal Programme Data for the Planet: making data work for cross-domain grand challenges and to the developments of GO FAIR community towards the Internet of FAIR data and services. One of the CODATA strategic Initiatives and Task Groups concentrates on Fundamental Physical Constants. Established in 1969, its purpose is to periodically provide the international scientific and technological communities with an internationally accepted set of values of

3813-653: The Earth's oblateness and geocenter motion are best determined from satellite laser ranging . Large-scale gravity anomalies can be detected from space, as a by-product of satellite gravity missions, e.g., GOCE . These satellite missions aim at the recovery of a detailed gravity field model of the Earth, typically presented in the form of a spherical-harmonic expansion of the Earth's gravitational potential, but alternative presentations, such as maps of geoid undulations or gravity anomalies, are also produced. The Gravity Recovery and Climate Experiment (GRACE) consisted of two satellites that detected gravitational changes across

Alternative approaches to redefining the kilogram - Misplaced Pages Continue

3906-411: The Earth. The term "absolute" does not convey the instrument's stability, sensitivity, accuracy, ease of use, and bandwidth. The words "Absolute" and "relative" should not be used when more specific characteristics can be given. The most common gravimeters are spring -based. They are used in gravity surveys over large areas for establishing the figure of the geoid over those areas. They are basically

3999-499: The Earth. Though similar in design to other accelerometers, gravimeters are typically designed to be much more sensitive. Their first uses were to measure the changes in gravity from the varying densities and distribution of masses inside the Earth, from temporal tidal variations in the shape and distribution of mass in the oceans, atmosphere and earth. The resolution of gravimeters can be increased by averaging samples over longer periods. Fundamental characteristics of gravimeters are

4092-407: The Kibble balance, while the kilogram is delineated in electrical and gravity terms, all of which are traceable to invariants of nature; it is defined in a manner that is directly traceable to three fundamental constants of nature. The Planck constant defines the kilogram in terms of the second and the metre. By fixing the Planck constant, the definition of the kilogram depends in addition only on

4185-586: The Planck constant that was consistent with and independent of the Kibble balance method. The alternative approaches included: One Avogadro constant-based approach, known as the International Avogadro Coordination's Avogadro project , would define and delineate the kilogram as a 93.6   mm diameter sphere of silicon atoms. Silicon was chosen because a commercial infrastructure with mature technology for creating defect-free, ultra-pure monocrystalline silicon already exists,

4278-467: The accuracy of a single measurement (a single sample ) and the sampling rate . for example: Besides precision , stability is also an important property for a gravimeter as it allows the monitoring of gravity changes . These changes can be the result of mass displacements inside the Earth, or of vertical movements of the Earth's crust on which measurements are being made. The first gravimeters were vertical accelerometers , specialized for measuring

4371-409: The ampere as a derivative of the kilogram. This redefinition of the kilogram would specify elementary charge ( e ) as precisely 1.602 17 × 10 ^ coulomb rather than the current recommended value of 1.602 176 634 × 10 C . It would necessarily follow that the ampere (one coulomb per second) would also become an electric current of this precise quantity of elementary charges per second passing

4464-491: The atomic mass of gold or bismuth have to have been precisely fixed, but also the value of the elementary charge ( e ), likely to 1.602 17 X × 10  C (from the currently recommended value of 1.602 176 634 × 10 C ‍ ). Doing so would have effectively defined the ampere as a flow of ⁠ 1 / 1.602 17 X × 10 ⁠ electrons per second past a fixed point in an electric circuit. The SI unit of mass would have been fully defined by having precisely fixed

4557-406: The average vertical gravity automatically. They can be calibrated at a location where the gravity is known accurately and then transported to where gravity is to be measured. Or they can be calibrated in absolute units at their operating location. Researchers use more sophisticated gravimeters when precise measurements are needed. When measuring Earth's gravitational field , measurements are made to

4650-442: The calibration of relative gravimeters, surveying for gravity anomalies (voids), and for establishing the vertical control network . Atom interferometric and atomic fountain methods are used for precise measurement of the Earth's gravity, and atomic clocks and purpose-built instruments can use time dilation (also called general relativistic) measurements to track changes in the gravitational potential and gravitational acceleration on

4743-442: The central frequency of the instrument is often given by The term for the "force constant" changes if the restoring force is electrostatic, magnetostatic, electromagnetic, optical, microwave, acoustic, or any of dozens of different ways to keep the mass stationary. The "force constant" is just the coefficient of the displacement term in the equation of motion: F is the force being measured, and ⁠ F / m ⁠

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4836-451: The constant downward acceleration of gravity on the Earth's surface. The Earth's vertical gravity varies from place to place over its surface by about ±0.5%. It varies by about ±1000 ⁠ nm / s ⁠ (nanometers per second squared) at any location because of the changing positions of the Sun and Moon relative to the Earth. The majority of modern gravimeters use specially designed metal or quartz zero-length springs to support

4929-463: The current required to generate the magnetic field that suspends the niobium sphere is proportional to the strength of the Earth's gravitational acceleration. The superconducting gravimeter achieves sensitivities of 10 m·s (one nanogal ), approximately one trillionth (10 ) of the Earth surface gravity. In a demonstration of the sensitivity of the superconducting gravimeter, Virtanen (2006), describes how an instrument at Metsähovi, Finland, detected

5022-421: The definition, but instead become transfer standards . Scales like the Kibble balance also permit more flexibility in choosing materials with especially desirable properties for mass standards. For instance, Pt‑10Ir could continue to be used so that the specific gravity of newly produced mass standards would be the same as existing national primary and check standards (≈21.55   g/ml). This would reduce

5115-477: The density of the atmosphere, the effect of changes in the shape of the surface of the ocean, the effect of the atmosphere's pressure on the Earth, changes in the rate of rotation of the Earth, oscillations of the Earth's core, distant and nearby seismic events, and more. Many broadband three-axis seismometers in common use are sensitive enough to track the Sun and Moon. When operated to report acceleration, they are useful gravimeters. Because they have three axes, it

5208-500: The final numerical values of h , e , k , and N A that are used for the new SI definitions. The regular version with a closing date of 31 December 2018 was used to produce the new 2018 CODATA values that were made available by the time the revised SI came into force on 20 May 2019. This was necessary because the redefinitions have a significant (mostly beneficial) effect on the uncertainties and correlation coefficients reported by CODATA. Gravimeter Gravimetry

5301-426: The force of gravity pulling on an object. The change in length of the spring may be calibrated to the force required to balance the gravitational pull. The resulting measurement may be made in units of force (such as the newton ), however, gravimeters display their measurements in units of gals (cm/s ), and parts per million, parts per billion, or parts per trillion of the average vertical acceleration with respect to

5394-432: The gradient of the potentials gives estimate of local acceleration which are what is measured by the gravimeter arrays. The superconducting gravimeter network has been used to ground truth the satellite potentials. This should eventually improve both the satellite and Earth-based methods and intercomparisons. Transportable relative gravimeters also exist; they employ an extremely stable inertial platform to compensate for

5487-419: The gradual increase in surface gravity as workmen cleared snow from its laboratory roof. The largest component of the signal recorded by a superconducting gravimeter is the tidal gravity of the Sun and Moon acting at the station. This is roughly ±1000 ⁠ nm / s ⁠ (nanometers per second squared) at most locations. "SGs", as they are called, can detect and characterize Earth tides , changes in

5580-410: The gravitational acceleration of a test mass in a vacuum. A test mass is allowed to fall freely inside a vacuum chamber and its position is measured with a laser interferometer and timed with an atomic clock . The laser wavelength is known to ±0.025  ppb and the clock is stable to ±0.03 ppb. Care must be taken to minimize the effects of perturbing forces such as residual air resistance (even in

5673-403: The interests of the researcher or operator, this might be counteracted by integral vibration isolation and signal processing . Gravimeters have been designed to mount in vehicles, including aircraft (note the field of aerogravity ), ships and submarines. These special gravimeters isolate acceleration from the vehicle's movement and subtract it from measurements. The acceleration of the vehicles

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5766-488: The interoperability and the usability of such data; research data should be FAIR (findable, accessible, interoperable and reusable). By promoting the policy, technological, and cultural changes that are essential to promote open science, CODATA helps advance ISC's vision and mission of advancing science as a global public good. The CODATA Strategic Plan 2015 and Prospectus of Strategy and Achievement 2016 identify three priority areas: CODATA achieves these objectives through

5859-489: The kilogram by precisely creating new metal prototypes on demand. It would have done so by accumulating gold or bismuth ions (atoms stripped of an electron) and counting them by measuring the electric current required to neutralise the ions. Gold (Au) and bismuth (Bi) were chosen because they can be safely handled and have the two highest atomic masses among the mononuclidic elements that are stable (gold) or effectively so (bismuth). See also Table of nuclides . With

5952-435: The kilogram in terms of Si even when kilogram prototypes are made of natural silicon (all three isotopes present). Even with a kilogram definition based on theoretically pure Si, a silicon-sphere prototype made of only nearly pure Si would necessarily deviate slightly from the defined number of moles of silicon to compensate for various chemical and isotopic impurities as well as the effect of surface oxides. Though not offering

6045-440: The kilogram would be recalibrated as required. Conversely, an electronic definition of the kilogram (see § Electronic approaches , below), which would precisely fix the Planck constant, would continue to allow ⁠83 + 1 / 3 ⁠ moles of C to have a mass of precisely one kilogram but the number of atoms comprising a mole (the Avogadro constant) would continue to be subject to future refinement. A variation on

6138-471: The laboratory. The presence of the thin oxide layer on a silicon-sphere mass prototype places additional restrictions on the procedures that might be suitable to clean it to avoid changing the layer's thickness or oxide stoichiometry . All silicon-based approaches would fix the Avogadro constant but vary in the details of the definition of the kilogram. One approach would use silicon with all three of its natural isotopes present. About 7.78% of silicon comprises

6231-446: The masking effects of motion and vibration, a difficult engineering feat. The first transportable relative gravimeters were, reportedly, a secret military technology developed in the 1950–1960s as a navigational aid for nuclear submarines . Subsequently in the 1980s, transportable relative gravimeters were reverse engineered by the civilian sector for use on ship, then in air and finally satellite-borne gravity surveys. Microgravimetry

6324-414: The mass. Committee on Data for Science and Technology CODATA exists to promote global collaboration to advance open science and to improve the availability and usability of data for all areas of research . CODATA supports the principle that data produced by research and susceptible to being used for research should be as open as possible and as closed as necessary. CODATA works also to advance

6417-406: The metre, previously similarly having been defined with reference to a single platinum-iridium bar with two marks on it, was redefined in terms of an invariant physical constant (the wavelength of a particular emission of light emitted by krypton , and later the speed of light ) so that the standard can be independently reproduced in different laboratories by following a written specification. At

6510-454: The mole requires that ⁠ 1000 / 12 ⁠ moles ( ⁠83 + 1 / 3 ⁠  mol) of C has a mass of precisely one kilogram. The number of atoms in a mole, a quantity known as the Avogadro constant , is experimentally determined, and the current best estimate of its value is 6.022 140 76 × 10  entities per mole. This new definition of the kilogram proposed to fix

6603-559: The necessary long-term uncertainty of 10–20 parts per billion. Another issue to be overcome is that silicon oxidises and forms a thin layer (equivalent to 5–20 silicon atoms deep) of silicon dioxide ( quartz ) and silicon monoxide . This layer slightly increases the mass of the sphere, an effect that must be accounted for when polishing the sphere to its finished size. Oxidation is not an issue with platinum and iridium, both of which are noble metals that are roughly as cathodic as oxygen and therefore don't oxidise unless coaxed to do so in

6696-434: The new definition takes effect, it is likely there will only be a few—at most—Kibble balances initially operating in the world. Several alternative approaches to redefining the kilogram that were fundamentally different from the Kibble balance were explored to varying degrees, with some abandoned. The Avogadro project, in particular, was important for the 2018 redefinition decision because it provided an accurate measurement of

6789-419: The older definition of the kilogram should be replaced. The International Committee for Weights and Measures (CIPM) approved a redefinition of the SI base units in November 2018 that defines the kilogram by defining the Planck constant to be exactly 6.626 070 15 × 10 kg⋅m⋅s . This approach effectively defines the kilogram in terms of the second and the metre , and took effect on 20 May 2019. In 1960,

6882-401: The per-metre force between two straight parallel conductors of infinite length, of negligible circular cross section, placed one metre apart in vacuum, through which flow a constant current of ⁠ 1 / 1.602 17 × 10 ^ ⁠ elementary charges per second". Effectively, this would define the kilogram as a derivative of the ampere rather than the present relationship, which defines

6975-405: The potential for low-cost arrays of sensors. MEMS gravimeters are currently variations on spring type accelerometers where the motions of a tiny cantilever or mass are tracked to report acceleration. Much of the research is focused on different methods of detecting the position and movements of these small masses. In Atom gravimeters, the mass is a collection of atoms. For a given restoring force,

7068-478: The potential of wear from having to remove the contamination. Bismuth, which is an inexpensive metal used in low-temperature solders, slowly oxidises when exposed to room-temperature air and forms other chemical compounds and so would not have produced stable reference masses unless it was continually maintained in a vacuum or inert atmosphere. This approach would define the kilogram as "the mass which would be accelerated at precisely 2 × 10 m/s when subjected to

7161-399: The precision of microgals to find density variations in the rocks making up the Earth. Several types of gravimeters exist for making these measurements, including some that are essentially refined versions of the spring scale described above. These measurements are used to quantify gravity anomalies . Gravimeters can detect vibrations and gravity changes from human activities. Depending on

7254-513: The radius—roughly a single atomic layer. The precise lattice spacing between the atoms in its crystal structure (≈   192   pm) would be measured using a scanning X-ray interferometer . This permits its atomic spacing to be determined with an uncertainty of only three parts per billion. With the size of the sphere, its average atomic mass, and its atomic spacing known, the required sphere diameter can be calculated with sufficient precision and low uncertainty to enable it to be finish-polished to

7347-639: The relative uncertainty when making mass comparisons in air . Alternatively, entirely different materials and constructions could be explored with the objective of producing mass standards with greater stability. For instance, osmium -iridium alloys could be investigated if platinum's propensity to absorb hydrogen (due to catalysis of VOCs and hydrocarbon-based cleaning solvents) and atmospheric mercury proved to be sources of instability. Also, vapor-deposited, protective ceramic coatings like nitrides could be investigated for their suitability for chemically isolating these new alloys. The challenge with Kibble balances

7440-467: The reproducible production of new, kilogram-mass prototypes on demand using measurement techniques and material properties that are ultimately based on, or traceable to, physical constants. Others were based on devices that measured either the acceleration or weight of hand-tuned kilogram test masses and which expressed their magnitudes in electrical terms via special components that permit traceability to physical constants. Such approaches depend on converting

7533-409: The same, "acceleration field" might be preferable, since "gravity" is an oft-misused prefix. Gravimeters for measuring the Earth's gravity as precisely as possible are getting smaller and more portable. A common type measures the acceleration of small masses free falling in a vacuum , when the accelerometer is firmly attached to the ground. The mass includes a retroreflector and terminates one arm of

7626-418: The target electrode or even dislodged atoms that had already been deposited. The deposited mass fraction in the 2003 German experiments only approached very close to 100% at ion energies of less than around 1  eV (<   1   km/s for gold). If the kilogram had been defined as a precise quantity of gold or bismuth atoms deposited with an electric current, not only would the Avogadro constant and

7719-455: The task group has produced a new version every four years, incorporating results published up to the end of the specified year. In order to support the 2019 revision of the SI , adopted at the 26th General Conference on Weights and Measures on 16 November 2018, CODATA made a special release that was published in October 2017. It incorporates all data up to 1 July 2017, and determines

7812-509: The technique to be considered a viable future replacement to the IPK. Among the many technical challenges of the ion-deposition apparatus was obtaining a sufficiently high ion current (mass deposition rate) while simultaneously decelerating the ions so they could all deposit onto a target electrode embedded in a balance pan. Experiments with gold showed the ions had to be decelerated to very low energies to avoid sputtering effects—a phenomenon whereby ions that had already been counted ricochet off

7905-486: The test mass. The special property of these springs is that the natural resonant period of oscillation of the spring–mass system can be made very long – approaching a thousand seconds. This detunes the test mass from most local vibration and mechanical noise, increasing the sensitivity and utility of the gravimeter. Quartz and metal springs are chosen for different reasons; quartz springs are less affected by magnetic and electric fields while metal springs have

7998-523: The two heavier isotopes: Si and Si. As described in § Carbon-12 below, this method would define the magnitude of the kilogram in terms of a certain number of C atoms by fixing the Avogadro constant; the silicon sphere would be the practical realisation . This approach could accurately delineate the magnitude of the kilogram because the masses of the three silicon nuclides relative to C are known with great precision (relative uncertainties of 1   ppb or better). An alternative method for creating

8091-427: The values of the Avogadro constant and elementary charge, and by exploiting the fact that the atomic masses of bismuth and gold atoms are invariant, universal constants of nature. Beyond the slowness of making a new mass standard and the poor reproducibility, there were other intrinsic shortcomings to the ion‑accumulation approach that proved to be formidable obstacles to ion-accumulation-based techniques becoming

8184-491: The values themselves and the paper explaining how they were arrived at: 1998 (April 2000), 2002 (January 2005), 2006 (June 2008), 2010 (November 2012), 2014 (June 2015), 2018 (May 2019), and 2022 (May/August 2024). The CODATA recommended values of fundamental physical constants are published at the National Institute of Standards and Technology Reference on Constants, Units, and Uncertainty. Since 1998,

8277-486: The weight of a one-kilogram test mass by about 316 μg/m . In April 2007, the NIST's implementation of the Kibble balance demonstrated a combined relative standard uncertainty (CRSU) of 36   μg. The UK's National Physical Laboratory's Kibble balance demonstrated a CRSU of 70.3   μg in 2007. That Kibble balance was disassembled and shipped in 2009 to Canada's Institute for National Measurement Standards (part of

8370-559: Was accepted by the 24th conference of the CGPM in October 2011 and further discussed at the 25th conference in 2014. Although the Committee recognised that significant progress had been made, they concluded that the data did not yet appear sufficiently robust to adopt the revised definition, and that work should continue to enable the adoption at the 26th meeting, scheduled for 2018. Such a definition would theoretically permit any apparatus that

8463-410: Was capable of delineating the kilogram in terms of the Planck constant to be used as long as it possessed sufficient precision, accuracy and stability. The Kibble balance is one way do this. As part of this project, a variety of very different technologies and approaches were considered and explored over many years. Some of these approaches were based on equipment and procedures that would have enabled

8556-572: Was developed by Lucien LaCoste and Arnold Romberg in 1936. They also invented most subsequent refinements, including the ship-mounted gravimeter, in 1965, temperature-resistant instruments for deep boreholes, and lightweight hand-carried instruments. Most of their designs remain in use with refinements in data collection and data processing. Currently, the static and time-variable Earth's gravity field parameters are determined using modern satellite missions, such as GOCE , CHAMP , Swarm , GRACE and GRACE-FO . The lowest-degree parameters, including

8649-537: Was rotated through the Kibble balance every two to six weeks after being calibrated in a different part of NIST headquarters facility in Gaithersburg, Maryland . It was found that simply moving the resistance standards down the hall to the Kibble balance after calibration altered their values 10   ppb (equivalent to 10   μg) or more. Present-day technology is insufficient to permit stable operation of Kibble balances between even biannual calibrations. When

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