
A precise instrument to determine the Planck constant, and the future kilogram - igravious
http://scitation.aip.org/content/aip/journal/rsi/87/6/10.1063/1.4953825
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dekhn
That's quite the environmental setup:

The fourth generation watt balance apparatus, abbreviated NIST-4, shown in
Figs. 2 and 3, is located inside a radio-frequency shielded laboratory, 12 m
underground. For later reference, observe that the balance is oriented along a
diagonal with respect to the compass direction of North as shown in the
figure, with the main pivot axis falling along a line oriented from the north-
west to south-east corners of the room. The room itself is temperature
stabilized using an air jacket between the inner and outer exterior walls.
Inside the room, the apparatus sits on the center of a concrete block that is
4 m long and 4 m wide and has a total mass of 67 metric tons. This block is
isolated from the building’s foundation and, for additional vibration
isolation, can be floated off the ground using eight air springs. The
apparatus is housed inside a 1.6 m diameter and 2 m tall stainless steel
vacuum chamber which is pumped to a vacuum pressure on the order of 0.1 mPa.
The watt balance apparatus is supported at three points in a pseudo-kinematic
fashion on top of two stainless steel, sand-filled, parallel tubes that run
from south-west to north-east through the chamber. The tubes are structurally
decoupled from the vacuum chamber via four flexible bellows. This minimizes
the transmission of chamber vibrations to the apparatus and maintains the
apparatus alignments from air to vacuum. We have verified the alignment of the
top of the magnet in four different configurations: (1) with the vacuum
chamber top lid open, (2) closed, (3) during the evacuation of the chamber and
(4) in vacuum using an autocollimator with the beam reflecting from a mirror
on the tear drop plate. Between these four cases, the angle between the top of
the magnet and a horizontal plane did not change by more 30 μrad.

