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DC (GEO-MAGNETIC, QUASI-DC) INTERFERENCE
Although the term “DC” refers to a static, non-varying field (0 Hz), the term is also applied, in common use to a range of exceedingly low frequency fields, typically between 0 Hz and 10 Hz. Inasmuch as these fields are time-varying, they are technically AC fields. In common use, these exceedingly low frequency fields are also – and more correctly – referred to as “Quasi-DC” or “Near-DC” fields.
The most common natural source of DC fields is the Earth which is essentially a large DC magnet, with magnetic lines of flux emanating from the South to the North Pole. Sometimes referred to as the geomagnetic field, this field provides a compass with the ability to indicate the direction of magnetic North.
The “field strength” of the Earth’s magnetic field varies from place to place on the globe, but is generally between 350 – 500 milliGauss (mG). Through the characteristic of permeability, all magnetic fields, including the Earth’s prefer to accumulate in ferromagnetic materials (iron, nickel or steel) rather than air. Air has a permeability of 1, while steel has a permeability of approximately 300. High-permeable metals, like mu-metal, can have permeability in the hundreds of thousands. Much as a conductive rod will attract an electric field, (e.g., “lightning”), DC magnetic lines of flux (from the Earth) will accumulate inside and near ferromagnetic materials (the steel structure of a building) and field levels in the vicinity will be elevated.
The phenomenon of elevated DC fields in structural steel can be man-made: DC welding cables can magnetize structural steel during construction, and MRIs (a source of powerful DC fields, can permanently magnetize steel in the immediate vicinity and, if physically connected, at considerable distances from the magnet. Areas of a building with magnetized steel can have DC magnetic field levels in the range of 2,000 mG or more. As a measurement instrument is moved away from the concentration of fields in the steel structure, the field levels will decline.
Direct Current (DC) from a battery (or the traction power of a subway or light rail) will also create a DC magnetic field. However, to the extent that the demand (load) on a DC electrical circuit will vary with time, so too will the frequency of the “Quasi-DC” field which it produces.
Generally, DC magnetic fields do not present an EMI threat to most electronic equipment such as office and household electronics, although in certain circumstances the ambient DC field can exceed the tolerances of sensitive instruments whose accuracy is in part based on the assumption of a stable, uniform DC field environment. And, although the earth’s magnetic field is relatively stable, there are inherent variations in both the field direction and strength that occur over time. These temporal instabilities in the earth’s magnetic field can be a source of DC interference for long-term operations, like E-Beam lithographic systems and certain electron microscope operations.
More commonly problematic are the Quasi-DC fields, which are produced by either a change in DC current (above) or by the relative movement of a ferromagnetic mass through the Earth’s DC field (below).
Quasi-DC Field Interference can disrupt the proper functioning of sensitive laboratory equipment and is a growing problem for two reasons. First, research, medical and laboratory instruments are not only proliferating, they are becoming more and more sensitive and therefore more vulnerable to EMI. Many instruments must be shielded from changes in the Earth’s magnetic field.
Second, two sources of Quasi-DC fields are growing. The first is medical and research instruments themselves; MRI’s and NMR’s emit extremely high, and occasionally ramping, DC fields, creating Quasi-DC fields.
In addition, research campuses are being built in urban areas, near public transportation. Subways, light rail, buses and other large ferromagnetic bodies create quasi-DC fields which can disrupt sensitive instruments at surprisingly long distances.
This urban/internal source of DC field interference is of growing importance: the movement of large ferromagnetic bodies through the Earth’s magnetic field produces a momentary shift or perturbation in the geomagnetic field with a consequent “Quasi” or “slowly-varying” DC field. Trucks and trains outside the building add to changes caused by the movement of elevators. When the vehicles are powered by DC current (busses, trollies, rail), the changes in the current load add to the perturbation AND substantially increase the affected area. This is particularly troublesome in urban areas with subways and light rail systems.
FMS has had to mitigate fields from subways which were over 1,000 feet from a sensitive instrument.
As an offset to these risks, a careful analysis of present and future DC field conditions, during the early design stage should be performed so that (where necessary) mitigation/shielding strategies can be designed and implemented during construction to ensure each instrument’s performance at the lowest possible cost.
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