WJI November 2019 Feature Sample

WIRE JOURNAL NOVEMBER 2019

®

INTERNATIONAL www.wirenet.org

APPLICATIONS WJI’S MOST ACCURATE STORY EVER

Wrapups: IWCS wire Southeast Asia wire South America Energized about WAI membership? Page 29.

O F F I C I A L P U B L I C AT I O N O F T H E W I R E A S S O C I AT I O N I N T E R N AT I O N A L

FEATURE

APPLICATIONS The cover blurb was no exaggeration: this feature begins with the most accurate story (by weight) that has ever appeared in WJI, followed by other entries, including a little-known wire use whose future is threatened. Of note, a new version of this feature could be done every month, as the world is entwined by wire and cable, in ways both expected and unexpected.

WJI’S MOST ACCURATE STORY EVER

The Kibble Balance: the ultimate kg At its essence, the Kibble Balance, named after its inventor, Bryan Kibble of the U.K.’s National Physical Laboratory (NPL), is the world’s most accurate weighing device. Conceptualized in 1975, it has enabled the figurative mothballing of the “official” 1 kg (see next page). Below is a group staff response supplied by Dr. Jon Pratt, an engineer at the National Institute of Standards and Technology (NIST), to questions WJI posed about the role of wire in the device.

Pratt: The process was creative, so there was a fair WJI: What kind of rope is used for the Kibble Balance, amount of trial and error, but cumulatively lots of thought and how was it chosen? has gone into the “rope” over the years. The first piece Pratt: The “rope” that spools from our “pulley” is a of our rope is actually a 50 mm wide ribbon of Grade highly engineered, hybrid mechanical device designed to 4 titanium foil, 75 μm thick (0.003 in.), etched through support the weight of our mass pan, spider, and moving to produce a multifilament band of 60 equally spaced coil (15 kg) while being as flexible as possible, so that the “wires,” each having a rectangular cross section of 125 μm entire assembly will hang like a pendulum, and self-align by 75 μm. This multifilament to gravity. Our “pulley” is a band “spools” off the ultranominally 50-mm wide (2 flat rim of the wheel and is in.), 610 mm diameter alumilong enough to provide 80 num wheel (2 ft), the rim of mm (about 3 in.) of travel which was machined on an between full extension and ultra-precision lathe using a retraction. We originally single-point diamond stylus assembled the multifilament tool that produces an optical band from individual wires, quality surface, flat to much but found that working with less than one wavelength of an etched foil provides better visible light. We joke that geometric consistency, and is it is one of the most precise far less hassle to make since wheels on the planet, since its we can order it from a vendor diameter is consistent everywho will make the part to where around its circumour specifications. The next ference to within less than From l-r, Kibble masters display their tattoos: Stephan 700 nanometers, or about 30 Schlamminger, Frank Seifert, Darine Haddad, Leon Chao, segment of the “rope” is still handmade. It is a precision millionths of an inch. The Jon Prat and, David Newell. bundle of 40 125 μm diamepoint of the precision (and ter (0.005 in.) platinum tungsten alloy wires that we assemyes we need it!) is to allow us to translate the coil along ble into a part we call the rotational de-coupler. This is a a straight line defined by gravity with deviations that are wire rope, but it is not braided. Instead, all of the filaments less than one or two μm as the wheel rotates. Interestingly, are parallel, packed alongside each other and silver solder the bearing on which the wheel turns is a precision knife brazed together at each end. edge, which is exactly like the balance pivots originally used in ancient Egyptian instruments to weigh grain, or, as WJI: What characteristics must your cables have? depicted on the walls of pyramids, to weigh souls.

Pratt: We need the wires to be nonmagnetic, stress relieved, vacuum compatible and to hang absolutely WJI: The Kibble Balance is amazing in scope and techstraight, even after brazing. The goal is to achieve a nology…but what thought went into the wire? coupler that is very rigid axially, but that is otherwise very 46 | WIRE JOURNAL INTERNATIONAL

WJI: Is there a predicted “life” for the cable used for the Kibble Balance?

Pratt: We gave some thought to the life of the cable, but we do not have a predicted “life” model. The loads are static in nature, and we designed them with a factor of safety of three in mind. Through various load tests, we have witnessed catastrophic failures, but only when the load exceeded the factor of safety or for large, sharp impulses. Occasionally, one or two filaments will fail during use, at which point we replace the part.

WJI: Your Kibble Balance offers absolutely incredible accuracy ... but again, is that really, really needed?

Pratt: The ultimate value of being able to weigh a kg to better than 50 μg with the NIST 4 Kibble Balance is that it allows the instrument to function just as our previous national standard kg did, preserving the precision of the existing mass dissemination infrastructure. Although it seems excessive, the balance must be able to support a legal metrology infrastructure accustomed to certified mass standards that claim uncertainties of less than 83 μg. The certified mass standards serve as the basis of a vast, mass measurement infrastructure across a range of academic, industrial, and government primary standards laboratories.

Such primary standards labs use their certified standards to calibrate other less-precise mass standards to carry out 

The history of the “official kg” ... and why it had to go The kilogram (kg), which equals 2.205 lb, has been the base unit of mass in the metric system. Since 1889, a kg of platinum-iridium alloy at the International Bureau of Weights and Measures in Sèvres, France, has served as the “official weight.” That is, until the definition of a kg officially changed on May 20, 2019. Below is a Wikipedia summation of how that came to be.

The kg, a widely used measure in science, engineering, and commerce worldwide, is almost exactly the mass of one liter of water. In fact, it was originally defined in 1795 as the mass of a liter of water, a simple definition but one that was hard to precisely replicate. In 1799, the “Kilogramme des Archives,” a platinum artifact, replaced it as the standard of mass.

In 1879, a cylinder of platinum-iridium, the International Prototype of the Kilogram (IPK) became the standard.

Why the change? Despite best efforts to maintain it, evidence accumulated that the IPK mass had been changing: the IPK diverged from its replicas by approximately 50 micrograms since their manufacture late in the 19th century. Eventually, a more precise measurement was needed, which led to the redefining of the kg not as a physical weight but with a definition based directly on physical fundamental constants.

How could the weight of the official kg change? Dozens of official IPKs exists, including seven at the Gaithersburg, Maryland, location of the National Institute of Standards and Technology (NIST). One of the IPKs is located in an underground laboratory that requires three separate keys to enter. The kg can be seen but not

touched as the touch of a human hand could transfer oil from the skin and add to the weight. Of note, the IPK was believed to have lost weight, some 50 micrograms, yet paradoxically, because it is the definition of a kg it has to be correct.

Discussion eventually led to an agreement that the kg should not be a physical weight, which led to the adoption of the Planck constant, which was approved by the General Conference on Weights and Measures (CGPM) on Nov. 16, 2018.

The Planck’s constant, symbolized by h, relates the energy in one quantum (photon) of electromagnetic radiation to the frequency of that radiation. In the International System of units (SI), the constant is equal to approximately 6.626176 x 10-34 joule-seconds. Basically, the Planck constant describes the kg by quantum mechanics that can never vary.

The Kibble Balance, named after Bryan Kibble of the U.K.’s National Physical Laboratory (NPL), embodies the actual application of the Planck constant.

Much has been written in science journals, but this feature may be the first one to focus solely on the role of wire.

NOVEMBER 2019 | 47

FEATURE

compliant. While someone could obviously make this for us, we need so few specimens that the cost would likely be prohibitive. However, if your readers have suggestions, we are always open to a new idea.