Following are the critical design
factors for manufacturing and use of a MuMETAL® Zero Gauss Chamber.
EXTERNAL
FIELD STRENGTH (HO)
Most Zero Gauss Chambers are operated
in Earth’s field, which is usually about 0.5 Gauss. In most environments,
however, there are additional sources of magnetic fields, usually 60 Hertz and
its harmonics. Since all magnetic fields around the chamber are part of the
ambient magnetic field, their magnitude must be considered as part of the
shielding requirement. For strong external fields, the outer layer may be
manufactured from our high saturation NETIC® Alloy.
ATTENUATION(A)
& LAYERS
The number of concentric shields (layers) required is a
function of the degree of Attenuation (A) desired. Typically three layers are
required, however, additional layers may be added. The external field strength
(HO) and the maximum allowable magnetic field inside the chamber (Hi), or the
required attenuation, are determined by the customer. To achieve internal
magnetic fields of 10 milliGauss or less, additional layers and a Degaussing
Coil are usually required.
INTERIOR
SIZE
For best results, the inside chamber
diameter should be as small as possible because attenuation is inversely
proportional to diameter. The inside depth should be 2-3 times longer than the
inside diameter to provide a workable area.
OPERATING
ENVIRONMENT
Normal room conditions usually do not
present any problems. High or low operating temperatures may affect the
insulation of an optional Degaussing Coil. The operating range of MuMETAL®
alloys is –452°F (–269°C) to 850°F (454°C). Operation in a vacuum will preclude
use of plastics and many finishes because of outgassing.
WALL
THICKNESS
Typically .025" [0,64mm] is the
minimal wall thickness for Zero Gauss Chamber construction. Thicker materials
are used on large diameter chambers to provide more attenuation. After
fabrication and Perfection Anneal, MuMETAL® alloy is relatively soft.
Consequently, the chambers must be fabricated with sufficient wall thickness to
maintain the physical integrity of the assembly. The chamber must be able to
support itself and provide sufficient strength to support the item being
shielded.
ACCESS
HOLES & COVERS
Holes are specified by the customer,
for connecting power and signal cables to the interior. Access holes should be
as few and as small as possible to minimize interfering field fringe into the
chamber. Hole axes should be perpendicular (transverse) to the ambient field.
The minimum distance between any two holes in a shield should be equal to the
diameter of the larger hole. Interfering field fringe can be substantially reduced
with cylindrical extensions welded onto the outer surface of the chamber
assembly, and by using our SPIRA-SHIELD conduit or Co-NETIC® AA CABLE SHIELD to
shield cables.
Typically, the close-fitting removable covers have a lip of
1.0" [25,4mm] to avoid magnetic field leakage and still allow access to
the chamber interior. For easy cover removal, each cover is typically provided
with a .88" [22mm] hole in the center to allow removal with a finger.
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