# Disseminating the kilogram, no strings attached

##### August 28, 2012, National Institute of Standards and Technology

(Phys.org)—The impending redefinition of the kilogram presents a weighty dilemma. Methods to be used to realize the redefined kilogram are based on the Planck constant and the Avogadro constant respectively and realize the kilogram in vacuum. But secondary standards, as well as transfer standards for laboratory and industrial use, must be deployed in air.

Devising a practical method of resolving this difference is extremely complicated. Two masses can easily be compared when both are in vacuum. But an 's measured in air is affected by air as well as the amount of water and other adsorbed onto its surface. Correcting for adsorbed water requires precise determination of the volume of the mass and the pressure, temperature, and humidity of the air. Those values are then entered into a model that takes account of the surface properties of the metal or alloy from which the mass is made.

To date, there are no firm consensus data on which to base those models. Experimental determinations of the adsorption of water onto stainless steel, for example, have recorded mass changes that differ by as much as a factor of 15.

So PML's Division has devised an alternative system for direct comparison of a reference mass in vacuum to an unknown mass artifact in air, using magnetic levitation. Patrick Abbott and colleagues in the Mass and Force Group have demonstrated proof of principle using a small tabletop model, and are now at work on a full-scale apparatus about three meters tall.

"No one is really considering us right now," Abbott says, "and most of the effort around the world is going into adsorption models. But once it gets out that we've got this design and it works really well, I suspect that others would want to adopt it because it's a direct comparison that doesn't depend on any particular type of material or assumptions of adsorption models."

The full-scale device, constructed entirely of aluminum, will house two adjacent enclosures. At the top is a vacuum chamber containing the reference mass and a sophisticated mass comparator capable of resolving a difference of 10 micrograms out of a , or 10 parts in 109. The comparator has been specially modified at NIST to be able to couple its weighing pan directly to a reference mass or to a combination solenoid and permanent magnet that hangs just above the floor of the vacuum chamber.

Directly below the vacuum enclosure, separated by an airtight barrier, is an air chamber containing the mass artifact to be calibrated. The mass is mounted on a suspension carriage at the top of which is a permanent magnet with one exposed pole. That pole is attracted to the opposite polarity of the magnet/solenoid combination in the vacuum chamber above, and the attraction force levitates the mass artifact and carriage when the poles are the right distance apart (typically in the range of 15 mm to 17 mm).

The system is brought into equilibrium by using feedback sensors to carefully adjust the current in the solenoid. The magnets in each chamber are encased in shielding so no stray flux disturbs other parts of the system.

Magnetic levitation, of course, is not a new technology. "Objects spanning the gamut from passenger trains to live frogs have been magnetically suspended or levitated," Abbott says, and the technique has previously been used to weigh objects of up to 200 grams in small commercial systems. "But our device is new in the sense that we're attempting to weigh much more mass with higher precision than has ever been done with this technique—up to several kg—and also to go between vacuum and air in order to disseminate mass values. has never before been used for dissemination purposes."

Levitating the carriage and mass artifact is a very delicate process. If the magnetic force between the two poles is greater than the weight of the artifact, the poles will be pulled together. If the magnetic force is weaker than the downward force of gravity on the mass artifact and its carriage, then they will descend in the air chamber. For each load and distance between poles, there is a specific field strength that will produce near-motionless levitation.

To maintain equilibrium in the balance system, the device uses a computer-implemented feedback controller to adjust the strength of the solenoid's magnetic field. In the initial design, feedback was provided by a laser-and-receptor combination that monitored the position of the mass in air. Later the PML researchers switched to a Hall-effect sensor on a cantilever placed near the exposed pole of the permanent magnet in the air chamber. That device tracks the magnetic field between the poles, which is a function of the distance between the poles.

The Hall sensor output is sent to the controller, which uses that signal to vary the current passing through some 2500 windings in the solenoid. This fine-tuning goes on 20,000 times per second, maintaining the levitated artifact at equilibrium.

A 50 cm tall working model which can measure loads from about 100g to a little over a kg was used for proof of principle. The stability of measurements with that device, constrained by the resolution of the balance, is about 1 mg in 1 kg, or about one part in 106. The project's target uncertainty is no more than a few parts in 108. That is only slightly more than the BIPM's Consultative Committee for Mass and Related Quantities has recommended for realization of the kilogram for primary standards.

The full-scale apparatus currently under construction "will really tell us where we are right now," Abbott says. "My suspicion is that our stability is better than a milligram at present, but not much better. The new balance, once working, will tell us. Then it becomes a matter of chasing down systematic uncertainties." The researchers hope to have initial results around December 2012.

Although the PML device avoids the need for modeling, it is expected to play an important role in that effort as well. The Mass and Force Group, headed by Zeina Kubarych, is in the process of procuring equipment necessary to duplicate the measurements that are being used to create absorption models for different materials.

"So in an interesting way," Abbott says, "our endgame is going to be coming at the modeling problem from a different point of view. When we have the system working, we are going to make direct measurements on this apparatus and then compare the results with what people using absorption models are getting. That'll be verification for them and verification for us."

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not rated yet Aug 28, 2012
Um, what? The advantage with a new artifact free standard is that it is universal and ... artifact free.

Maybe a solution to (initial) cost concerns.
##### ant_oacute_nio354
1.8 / 5 (5) Aug 28, 2012
The mass is the electric dipole moment.

kilogram = Coulomb . meter

Antobio Saraiva
##### cantdrive85
1 / 5 (5) Aug 28, 2012
"What we call mass would seem to be nothing but an appearance, and all inertia to be of electromagnetic origin." Henri Poincaré, Science and Method
##### El_Nose
5 / 5 (3) Aug 28, 2012
@cantdrive

yeah he said that before Higg's made his conjecture about a mechanism that interacts with bosons.

But given the understanding of physics at that point -- he was remarkably close to being correct. He figured it was a force and that mass was just a consequence of events.
##### ValeriaT
3 / 5 (2) Aug 28, 2012
all inertia to be of electromagnetic origin." Henri Poincaré
It's not true: we know about many particles, which are massive, but they have no electric charge.
##### Infinion
1 / 5 (6) Aug 28, 2012
all inertia to be of electromagnetic origin." Henri Poincaré
It's not true: we know about many particles, which are massive, but they have no electric charge.

A man on the fringes of physics, name of Ilya Prigogine, questioned in modern times long accepted principles of Thermodynamics – a very old and a very closed subject. He faced much persecution. Today it is known that he is essentially correct. A great many workers today are coming to question Electromagnetic Theory – another very old and a very closed subject. Do you think it is wise to assume that all the physics that has been established to date is without flaw and beyond question?

The point is this. A certain idea may serve well for a long time, until some difficulty – even if subjective difficulty – surfaces. Then it is ready for a new assessment.
##### Eikka
3.5 / 5 (2) Aug 28, 2012
This gives me an idea of precisely calibrating a scale with a mass prototype.

Suppose you have the exactly vacuum-calibrated metal weight. Simply transport it into a glass cylinder under vacuum and seal it in.

Now, when you want to calibrate a scale with it, place the cylinder on the scale, levitate the metal artifact inside with an external electromagnet to prevent it from touching the glass, and zero the scale to the weight of the glass cylinder only. When you release the weight, a known amount is added on the scale without interference from adsorbed materials or buyoancy, because the artifact is never removed from the vacuum while being lifted off the scale.
##### PPihkala
not rated yet Aug 28, 2012
Now, when you want to calibrate a scale with it, place the cylinder on the scale, levitate the metal artifact inside with an external electromagnet to prevent it from touching the glass, and zero the scale to the weight of the glass cylinder only. When you release the weight, a known amount is added on the scale without interference from adsorbed materials or buyoancy, because the artifact is never removed from the vacuum while being lifted off the scale.

This might work if the scale to be calibrated is non-magnetic. If it is magnetic, then your lifting magnetic field will have effect that might give errors.
##### Parsec
5 / 5 (1) Aug 28, 2012
This gives me an idea of precisely calibrating a scale with a mass prototype.

Suppose you have the exactly vacuum-calibrated metal weight. Simply transport it into a glass cylinder under vacuum and seal it in.

Now, when you want to calibrate a scale with it, place the cylinder on the scale, levitate the metal artifact inside with an external electromagnet to prevent it from touching the glass, and zero the scale to the weight of the glass cylinder only. When you release the weight, a known amount is added on the scale without interference from adsorbed materials or buyoancy, because the artifact is never removed from the vacuum while being lifted off the scale.

This would probably work well. I suspect this is what the article has in mind however.
##### Eikka
not rated yet Aug 29, 2012
This might work if the scale to be calibrated is non-magnetic. If it is magnetic, then your lifting magnetic field will have effect that might give errors.

Depends on the shape of the lifting field. Besides, the force of the magnetic field diminishes in the fourth power of distance, so separating the scale and the prototype mechanically will quickly reduce the error.
##### Eikka
not rated yet Aug 29, 2012
This would probably work well. I suspect this is what the article has in mind however.

The device in the article has a separate scale in the vacuum with the mass prototype, and it compares the pull of the solenoid on the levitated object below to the weight of the mass prototype.

My version merely calibrates the scale which is then used to measure the artifact.
##### Eikka
not rated yet Aug 29, 2012
How I think it works: Suppose you have two solenoids, one hollow solenoid that levitates the mass prototype in vacuum, and another that levitates the artifact in air.

When the two solenoids are wired in series, the mutual lifting current in both solenoids lifts the prototype up in proportion to the weight of the artifact being lifted. No matter how strong the current, the hollow solenoid can never lift the mass prototype all the way up to the middle - it will always sag a little, but you know how much it should sag given a certain current and a certain mass.

Then you measure how much it sags, and you know how much weight is being levitated by the second solenoid.
##### axemaster
not rated yet Sep 02, 2012
I'm a bit confused here... wouldn't an oscillating (differential) balance give much more accurate mass readings? Seems like you should be able to do a lot better than 10 ppm.