 |
 |
 |
 |
 |
 |
 |
 |
 |
Increasingly, computers have become an essential tool in the workplace. They are used in virtually every industry and have changed the way we do business. However, this "computer revolution" has brought with it the growing awareness that computers and their peripheral equipment can be extremely sensitive to very low level magnetic fields.
The most sensitive piece of computer equipment is the monitor or Video Display Terminal (VDT). AC magnetic fields as low as 5 milligauss or DC fields greater than 500 milligauss are often strong enough to create image distortion on a terminal. While monitors are sensitive to both AC and DC magnetic fields, the symptoms of the interference are very different.
AC Interference
The following are common symptoms of AC interference:
Jitter
"Jitter" can be characterized as a vertical oscillation
of text or images with an amplitude from 1/16" up to
1/2"-60 times per second.
Waviness
This effect describes a horizontal oscillation or swimming
of text and images and is symptomatic of magnetic fields "hitting"
the side of a monitor.
Scrolling Bars
These bars are the result of "crosstalk" between
monitors which are located too close to one another. The bars,
which can scroll top to bottom or bottom to top are a result
of the difference in power frequency and the refresh or scan
rate of the individual units.
Monitors in AC Magnetic Fields
Most AC image distortion on monitors is the result of elevated
magnetic fields from 60Hz power distribution equipment. This
equipment includes overhead power lines, transformers, switchgears,
busways and cabling. Smaller, more isolated sources such as
motors, office equipment and adjacent monitors can also cause
distortion when in close proximity to the monitor in question.
DC Interference
Most monitors are designed and manufactured to operate in
DC magnetic fields that approximate the earth's own magnetic
field. However, in the presence of strong DC magnetic fields
from sources such as MRI's, shipboard magnetization coils,
DC arc furnaces, building elevators, and subway trains, monitors
will exhibit the following types of interference:
Color Blooming
Color blooming describes the most common symptom of DC magnetic
interference. In these cases, the color integrity of the monitor's
image is lost. The colors "wash out" and change
over time, usually beginning in the corners and then moving
inward.
Image Rotation
Image rotation or shift is a phenomenon that occurs if the
incoming magnetic field is entering the monitor from either
the front or the rear.
Squeezed Image
A Squeezed image is very similar to image rotation except
the magnetic field is typically more intense and localized.
MSE Designs and Solutions
Regardless of interference type, the problem can be solved
using an Amuneal Magnetic Shield Enclosure (MSE). The shields,
as shown on the left, have been designed for cost-effective
shielding while maintaining an aesthetic and ergonomic value.
Each shield is custom manufactured according to the monitor's
physical size and the strength of the interference field.
In many cases, tooling already exists for industry standard
monitors. Each unit is designed to taper and slope to the
dimensions of your monitor, is painted in the color of your
choice and comes complete with air ventilation patterns to
insure cooling, five non-slip rubber feet and rear cut-outs
as required for power cords and data leads.
Every Amuneal MSE is guaranteed to eliminate all visual interference caused by low frequency magnetic fields.
For more information on Amuneal's MSE's please request our Information Bulletin #241.
MSEs for Harsh/Industrial Applications (AC & DC)
There are times when a single-layer or standard MSE is not
sufficient to attenuate or provide saturation protection from
very large magnetic fields. For units to be installed in harsh
industrial environments such as those encountered in metal-processing
plants, multi-layer enclosures are required.
The design of such shields is based on a combination of materials,
each chosen for their respective saturation and permeability
characteristics. Utilizing these materials with proper spacing
achieves very high levels of protection. The four-layer shield,
pictured right, was designed to operate in a DC magnetic field
in excess of 40 Gauss.