biological effects of AC fields are detectable at much
lower levels than for DC fields. It is therefore inappropriate
to use the AC field Guidelines for DC fields.
The International Commission on Non-Ionizing Radiation
Protection (ICNIRP) Guidelines for Occupational Static
Magnetic Fields are 200mT for continuous exposure, 2000mT
for short-term whole-body and 5000mT for exposure to
arms & legs. These field levels are high and indicate
the lack of evidence for biological effects from DC
fields. The ICNIRP level for persons with pacemakers
and other implanted devices is set at the much lower
level 0.5mT (5gauss). This level is apparently not related
to biological effects but rather to possible effects
on electrical or electronic devices (particularly reed-relays)
or metal prosthesis.
Adoption of the 0.5mT (5gauss) level and installing
shielding or access restrictions avoids potential problems
with operators or visitors having implanted devices
and also avoids malfunction of most electronic control
& test equipment used in the vicinity of magnets. Setting
DC Field exposure levels at less than 0.5mT seems unnecessary
except for special, magnetically sensitive processes.
(Some electron beam equipment such as display devices
or electron lithography systems require lower fields.)
The US Food and Drug Administration (USFDA) in "Guidance
for the Submission of Premarket Notifications for Magnetic
Resonance Devices" requires iso-field contours
at 0.5mT (5gauss), 1mT, 10mT, 20mT, 40mT, and 200mT
(see Section 4, Site Planning) in planes parallel and
perpendicular to the magnetic field. Entry into regions
where the magnetic field is in excess of 0.5mT is only
allowed for authorized personnel.
The earth's geomagnetic field is about 0.05mT (50µT
or 0.5gauss). Inside buildings there can be variations
due to steel in the building construction. GMW has measured
DC fields to about 0.1mT in the open space of buildings
and to 0.5mT at the corners of large steel components
such as machine tools.
In measuring the actual fringing magnetic field it is
necessary to remember that the magnetic field (magnetic
flux density, B) is a vector quantity with three components.
The typical Hall effect Gaussmeter or Teslameter only
measures one component of the field. To measure the
total field it is necessary to measure the three components
Bx, By & Bz and then calculate the vector total (square
root of the sum of the squares:
The three component measurement can be difficult and
time consuming with a standard Hall gaussmeter or teslameter.
The Metrolab THM 7025 Three Axis Hall Effect Teslameter measures
the three components simultaneously & provides the vector
total with a resolution down to 0.01mT (0.1gauss). Senis Three Axis Magnetic Field Transducers provide three analog components with a frequency response from dc to approximately 1kHz and can be used with a data logger to map or monitor magnetic fields in the range 0.01mT to 2T. For high resolution measurement or mapping of magnetic fields of less than 1mT, the Bartington Mag-03 range provides very high resolution with frequency response from dc to 3kHz. Acquisition and spectral analysis in a portable instrument is provided by the Bartington Spectramag-6.
For more details, please refer to:
Ian J. Walker, August 2000, revised October 2015.