In September 2006, VitaTech Engineering was contracted by a univerisity to
perform an EMI Site Assessment & Mitigation Study of an existing classroom
to be rennovated for a placement of SEM / TEM Electron Microscopes. The before
EMI data was recorded on 22 September
2006, in
the classroom, adjacent hallways and
outside building areas. The
peak AC ELF emission recorded before installation was 2.6-mG (Brms). VitaTech
normally recommends less than 0.1 mG Brms (equivalent to 0.3 mG Bp-p) in
the imaging rooms and in clean room laboratory areas as the maximum acceptable
EMI threshold
for existing research tools.
The affected 800 sq.ft. classroom area was shielded over 13 days in February
2007. The shielding performance had a 0.1-mG requirement over 95% of the shielded
area.
The shielded
area was the future site of a SEM and TEM Electron Microscope in the ENGINEERING TECHNOLOGY & APPLIED SCIENCES (ETAS) building at the University of Arkansas at Little Rock, Arkansas.and required
a 0.1mG environment for
optimal performance. Below
are the before shield and after shield 3-D plots. After
the shield was installed the room had an average performance of 0.00-mG and
a peak of 0.4-mG by the unshielded doorway.
BEFORE SHIELD 3-D PLOT
AFTER SHIELD 3-D PLOT
FINAL AC ELF EMF SURVEY CONCLUSIONS
A final Shielding Factor (SF) of -48.3 dB was obtained and is very impressive considering the technical challenges to shield ground/net currents sources such as the underground medium voltage 13.8 kV distribution line beneath ETAS Room 152 with at least 1 to 2 amps of ground/net currents under average loads and twice the magnitude during peak summer loads. At 1-meter above the floor and walls in the shielded SEM/TEM Electron Microscope section of ETAS Room, the AC ELF magnetic shielded area fully complies with the 0.1 mG and less Brms (Bx, By and Bz axes) performance guarantee over 95% of the shielded area. |
