Download Frequently Asked Questions about magnetic shielding

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Magnetic Shield
- Frequently Asked Questions about magnetic shielding
What is Mumetal®? (or mu-metal or mewmetal, or moometal or newmetal?)
Mumetal® is one registered trade name for a high-permeability, magnetically "soft"
alloy used for magnetic shielding. Hipernom®, HyMu-80®. Permalloy®, and other
names refer to one of the two typical formulas for this alloy. Both include about 80%
nickel and 15% iron, with the balance being copper, molybdenum or chromium,
depending on the recipe being used. The term mumetal is often used as a generic
term, as Kleenex® is used for any facial tissue. Magnetic Shield Corporation's
registered trademark for our shielding alloy is CO-NETIC AA® (Other trademarks
shown are property of their respective owners).
How does magnetic shielding work?
All shielding materials work by diverting the magnetic flux to themselves, so although the
field from a magnet will be greatly reduced by a shield plate, the shield plate will itself be
attracted to the magnet. Closed shapes are the most efficient for magnetic shielding cylinders with caps, boxes with covers, and similar are the most effective. Magnetic
shielding materials offer a very high permeability path for magnetic field lines, directing
the magnetic field lines through the thickness of the alloy, and keeping them from going
where they are not wanted. It is important that the shield offers a complete path for the
field lines, so that they do not exit the material in a place where they will cause
unintended interference.
How can I stop the jitter on my CRT display?
The slow movement of the image on the CRT monitor screen, sometimes called jitter,
swimming, hula, or shimmy, is due to the interaction of an external magnetic field with
the sensitive components of the CRT. This can be stopped by enclosing the monitor in
an enclosure made from magnetic shielding alloy. Standard and custom-made (for
strong field and DC field applications) enclosures are available from our company.
Please see information on the ImageGuard™ CRT monitor enclosures elsewhere on our
website.
What is a magnetic field?
A magnetic field cannot be seen, heard, or felt. The magnetic field results from a source
of magnetic flux, which might be the Earth, a motor, transformer or electric power line, or
even a bar magnet. We can sense magnetic fields with measuring instruments, called
Gaussmeters, or something simpler, like a compass (for DC fields) or a pickup coil (for
AC fields). Magnetic fields make transformers and motors function, and have many other
uses.
What is the difference between DC and AC fields?
DC fields are non-varying, or perhaps slowly changing. A DC field might be from the
Earth, a permanent magnet, or a coil carrying direct current. AC magnetic fields oscillate
in direction at a frequency. The most common AC magnetic fields are 60 Hertz fields
emitted by electric power equipment.
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What is ELF?
ELF stands for Extremely Low Frequency and usually refers to magnetic fields at 0.5 to
100 Hertz. This range includes the 60 Hertz power line frequency commonly used in the
United States. In other countries, the power line frequency might be 50 Hertz.
What is the difference between RF and Magnetic shielding?
Radio frequency (or RF) shielding is required when it is necessary to block high
frequency - 100 kilohertz and above - interference fields. These shields typically use
copper, aluminum, galvanized steel, or conductive rubber, plastic or paints. These
materials work at high frequencies by means of their high conductivity, and little or no
magnetic permeability. Magnetic shields use their high permeability to attract magnetic
fields and divert them through themselves. Magnetic shielding alloys have the ability to
become broadband shields, performing shielding of both frequency ranges, when
properly constructed.
How are magnetic fields measured?
The units for measuring magnetic fields are Gauss and Oersted. Magnetic flux density is
measured in Gauss, while magnetic field intensity is measured in Oersted. The ratio of
B, magnetic flux, in Gauss, to H, magnetic field, in Oersted, is defined as permeability,
"µ" (pronounced "mew"). The B/H ratio, or "µ", is a measure of the material’s properties.
It is high for ferromagnetic materials. In air "µ" is equal to one, making Gauss and
Oersted identical numerically, adding to the confusion. The S/I or Metric system adds
the Tesla and Ampere-turns/meter units.
What blocks magnetic fields?
There is no known material that blocks magnetic fields without itself being attracted to
the magnet. Magnetic fields can only be redirected by using high permeability shielding
alloys. The magnetic field lines are strongly attracted into the shielding material.
Why are both NETIC S3-6 and CO-NETIC AA alloys offered?
The two primary magnetic shield alloys offered by Magnetic Shield Corporation have
differing characteristics that sometimes suggest that one or the other (or sometimes
both) be used. CO-NETIC AA alloy has the highest magnetic permeability and provides
the highest attenuation in a magnetic shield. Its only limit is the relatively low saturation
induction rating. This means that in very strong magnetic fields NETIC S3-6 alloy, with
its higher saturation induction must be used. NETIC S3-6 alloy cannot achieve the high
magnetic permeability of CO-NETIC AA, and so provides more modest attenuation
factors. If very high attenuation ratios must be achieved in a very strong field, sometimes
both alloys must be used. The NETIC S3-6 alloy is used closest to the source of the
field, to protect the CO-NETIC AA alloy from saturation.
Why is CO-NETIC AA alloy offered in both Perfection Annealed and Stress Annealed
types?
CO-NETIC AA alloy always requires a special annealing process to develop its full
magnetic properties. If fabrication, forming and welding operations are to be done, the
properties of the alloy will be affected, and annealing is required after the manufacturing
operations are complete. For this application CO-NETIC AA alloy Stress Annealed has
better formability characteristics and costs less. Final annealing after fabrication is done
according to the annealing specification. If the application does not require severe
forming or welding, then CO-NETIC AA alloy Perfection Annealed may be used, and the
final annealing step may be avoided. This is because CO-NETIC AA alloy Perfection
Annealed has been fully annealed before shipment and is ready to perform as a
magnetic shield. CO-NETIC AA alloy Perfection Annealed has a large, open grain
structure and must not be used when the shield fabrication involves severe drawing or
forming. CO-NETIC AA alloy Perfection Annealed can be an economical choice for
some flat or formed shields. All CO-NETIC AA alloy foil is Perfection Annealed.
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Should the source of interference or the sensitive device be shielded?
The answer to this question depends on several factors. Shielding the source may
involve stronger fields, and therefore thicker materials. One must be sure that all
interference sources are shielded, or the sensitive device will still be affected. The usual
approach is to shield the sensitive device. This prevents interference from both present
and future sources. Many magnetic shielding solutions begin with the magnetic shielding
Lab Kit. This kit enables a "hands-on" approach to solving a shielding problem.
How do I solve a magnetic shielding problem?
Shield calculation formulas do exist, but are usually valid only for theoretical conditions
of closed shield shapes and well-described interference fields. Remember, one test is
worth a thousand expert opinions. Many magnetic shielding problems are solved by
prototyping a magnetic shield from the materials in our Magnetic Shielding Lab Kit. The
hands-on approach offers the advantages of being able to immediately see the results of
the shielding in the item under test, and the opportunity to optimize the shield's thickness
and shape.
Does cutting the alloy destroy its properties?
This concern about the magnetic shielding alloys arises because they do have a
sensitivity to mechanical shock - bending, forming, even severe flexing may give some
reduction in the alloy permeability. Modern, vacuum-refined alloys have a lower
sensitivity to shock, and normally withstand regular handling without significant loss of
properties. Cutting by shearing, EDM, waterjet, photo-chemical etching, or blanking dies
typically only affect the alloy immediately adjacent to the edge, and the shield will exhibit
normal shielding performance. If the alloy had already received its final magnetic anneal,
reannealing should not be necessary.
How is attenuation defined?
The attenuation of a magnetic shield is the ratio of the measured magnetic field before
shielding to the field measured after shielding is applied. For example, if the field before
shielding is 450 milligauss, and the field measured inside the shield is 10 milligauss, the
attenuation ratio is 450/10 or 45 times. Attenuation ratios are sometimes expressed in
dB (decibels). The ratio in dB is 20 times the log (base 10) of the shielding ratio. In the
above example, the ratio in dB is 20*log(base 10) of 45, or 33 dB.
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