Studies have been carried out of the effects of using
three different cleaning methods on 1 kg stainless steel mass
standards, all three methods being suitable for use in a mass
calibration laboratory. The methods investigated were manual
and ultrasonic cleaning using solvents, and boiling in pure
water. Boiling in pure water proved the most effective and
additionally produced the most stable weights with stabilities
of better than 1 part in 108 over a period of months.
Ultrasonic cleaning was almost as effective as boiling but the
weights were less stable after the cleaning process. Manual
cleaning proved the least effective and the mass change
produced was very dependent on the specific cleaning technique.
Finally, a summary of the key features of each cleaning
technique is given.
Highly polished spheres, manufactured from silicon single-crystal material, are used in the X-ray crystal density method (XCDM) to determine the Avogadro constant. If the measurement uncertainty associated with this method can be reduced to 0.01 p.p.m., it would be possible to redefine the SI unit of mass, the kilogram, in terms of a fixed number of atoms of a definite species. The spheres are manufactured with a nominal mass of 1 kg and nominal diameter of 90 mm and a surface roughness of 0.5 nm (peak to valley). A goniometer has been constructed to enable the crystallographic orientation of these spheres to be determined using the back-reflection Laue technique. Two spheres have been successfully orientated in this manner by identifying two orthogonal 〈100〉 directions.
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