Posted on 11/08/2010 11:57:39 AM PST by zeugma
Taking the first steps of what would be a major historical advance in the science of measurement, the National Institute of Standards and Technology (NIST) is participating in a worldwide effort to recommend major revisions to the International System of Units (SI), the modern metric system that is the basis of global measurements in commerce, science and other aspects of everyday life. The new SI, which would be based on seven constants of nature, would enable researchers around the world to express the results of measurements at new levels of consistency and accuracy.
 |
|
In support of a proposal to revise the International System of Units (SI) so that it would be based on constants of nature, NIST efforts such as the watt balance experiment (pictured) offer new ways of determining an accurate value of the Planck constant h, thereby contributing to a definition of the kilogram based on physical constants. Credit: Steiner/NIST |
The most significant change in the possible future revision of the SI would be in the kilogram, the only one of the SI’s seven base units* still defined in terms of a material “artifact”: a 130-year-old platinum-iridium cylinder maintained at the International Bureau of Weights and Measures in France. The kilogram artifact poses long-term problems because its mass changes slightly over time. The proposed revision “puts the SI on a firm foundation,” says Ambler Thompson, a NIST scientist involved in the international effort. “We get rid of the last artifact.”
In the current SI, it’s not just the unit of mass that depends on the kilogram. The definitions of the ampere (electric current), mole (amount of substance) and candela (luminous intensity) ultimately depend on the platinum-iridium artifact. For example, a mole is currently defined as the number of carbon-12 atoms whose total mass is 12 grams.
The new proposal defines the kilogram in terms of the Planck constant h**, an important constant in quantum physics, which is expressed in units containing the kilogram. Efforts at NIST such as the watt balance experiment and determinations of the mass of one mole of silicon atoms offer new ways of determining an accurate value of h, thereby contributing to a more reliable definition of the kilogram.
The new SI would specify agreed-upon values of the seven constants, according to the results of an analysis published by CODATA (Committee on Data for Science and Technology)of all of the relevant data. Fixed values of constants would then define all base units. For example, the ampere would be formally defined in terms of the electric charge of a proton, the kelvin (temperature) in terms of the Boltzmann constant***, and the mole in terms of the Avogadro constant. However, before the revised SI could be implemented, additional experiments are needed to obtain more accurate values for some of the constants, especially the Planck, Avogadro, and Boltzmann constants.
The Consultative Committee for Units, of which NIST is a member and which is one of 10 advisory committees of the International Committee for Weights and Measures (CIPM), submitted its proposal for a revised SI for consideration by the CIPM during its meeting in Paris, France, earlier this month. The CIPM, whose membership includes Willie May, director of NIST's Material Measurement Laboratory, modified and approved this proposal on Oct. 15, 2010. Based on this proposal, the CIPM will soon submit a resolution on a possible future revision of the SI for consideration at the next meeting of the General Conference on Weights and Measures (CGPM), the international diplomatic body that has the authority under the Meter Convention to adopt such a sweeping change. If the resolution passes and all the technical requirements it sets out are in place, a new SI could be adopted later in the decade.
* The seven SI base units from which all others are derived are the second (time), the meter (length), the kilogram (mass), the ampere (electric current), the kelvin (thermodynamic temperature), the mole (amount of substance) and the candela (luminous intensity).
** Current accepted value: 6.626 068 96(33) x 10-34 kg m2/ s
***The Boltzmann constant relates the change in the temperature of a system (such as a group of atoms and molecules) to the change in its thermal energy.
Edited on Nov. 2, 2010, for minor wording changes requested by BIPM.
Media Contact: Ben Stein, bstein@nist.gov, (301) 975-3097
I propose a new unit of weight. We shall call it the “Obama.”
The weight of One Obama is the equivalent of the total amount of bovine excrement that can fit in the Oval Office at one time.
What’s that in English?
Basically all the measurments in the metric system are now based on measurable physical properties. For example, the meter is based on the wavelength of light emitted from the atom Krypton. The sole exception is the kilogram which as the article mentions is based on an actual block of material stored in France. There used to be an actual “mother of all” meter sticks in France. The problem with the kg block is it changes over time so the kg is not a constant measure. The idea is to find some physical property that can be defined as a kg.
how about The Pelosi for the weight filling the volume of Congress?
The SI system is global hegemony on the part of France.
All of the lab work that I oversee has to use instrumentation that is traceable to NIST standards.
I wonder how a scale is supposed to be calibrated to Plank’s constant?
IOW - it is commendable that NIST is attempting to use standards that have greater stability, but how practical is the new standard?
The last one we lost was the meter, which had some relation to the distance from the equator to the North Pole, and made a metal bar that length. Then they found out that Earth measurement wasn’t right, and kept the bar as the standard. Then they defined it as some wavelength of an atom that matched the bar’s length. Now they define it by the speed of light, with something about relativity thrown in.
The all of these will continue to get redefined as science improves our understanding of the world. The funny part is that they redefine it at about what we currently think it is, giving some strange definitions.
One "pelosi" has already been defined as the amount of lateral torque that can be applied to one square inch of facial skin before it disintegrates.
not sure I like the idea of all that Carbon 12-Platinum-Iridium being shot around at my local deli counter...
Think how much faster the day would go by if it were only 10 hours long...
I'd assume that you'd have a known good device against you would calibrate your own equipment.
As time progresses and you're able to devise better ways to measure the properties of the universe, your known good device would have a higher and higher level of accuracy.
IOW - it is commendable that NIST is attempting to use standards that have greater stability, but how practical is the new standard?
For all practical purposes, they can now measure this stuff with enough significant digits as to make practical application of the standard moderately meaningless.
SI units are way easier to use in science but a hassle in engineering. When I have to do higher level calculations of most any sort, I resort to SI units in the equations. Then though as an engineer, I will need to convert the final results to English units so that the audience will relate to the numbers. All of my lower level calculations are directly in English units though.
There is a metric calendar out there. I remember seeing a webpage dedicated to it when I was doing some surfing about the subject of time.
Thats what I'm wondering. The article notes that the standard for mass will be changed from a gold plated standard to one that is based on Planck's constant. How are the secondary standards calibrated against a primary standard that is based on a constant that has units of J-s?
I thought of that too. I think Murphy's rules of such things is as follows:
Constants aren't.
Variables dont.
I think Monweds and Saturthurs are two of the days of the metric week.
And this helps me to remember how many teaspoons are in a tablespoon how?
And what exactly will be the new serving size for ice cream. Are they fluid ounces or weight?
And why do the french get to hold onto the artifact. Don’t we have “top people...really, top people...” to hold onto important artifacts?
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.