ETS-Lindgren HI-3627 ELF Magnetic Field Meter - Manual (Page 22)

Page -- 14 HI-3627 Manual
32 feet apart. The actual profile shape is dependent on the
exact geometry of the power line, its height above ground and
the current flowing in the line. The flux density peaks near a
point beneath the conductors of the line.
Locating and Mapping Underground Cables
The HI-3627 with its omni-directional response is an effective
and easy to use device for locating buried current carrying
bodies. The procedure is quite simple. Hold the probe near the
ground at a constant height and walk toward the highest field
reading. A peak value usually means that you are standing
above the current carrying body.
A similar approach may be used for measuring the flux density
produced by buried lines. In this case, the area must be explored
by walking about with the meter until the region of maximum
flux density is found. Then, as in mapping power lines, you
would make straight line passes perpendicular to the buried
cable recording readings at fixed intervals.
VDTs
VDTs produce ELF magnetic fields which come about from the
circuitry within the VDT which deflects the electron beam
vertically in the cathode-ray-tube (CRT). ELF fields may also be
generated by the VDT power supply. The magnetic fields are
used to deflect the beam and have a saw-tooth waveform with
frequencies ranging between approximately 45 Hz and 75 Hz,
depending on the particular design of the VDT. The HI-3627 is
designed to properly detect these magnetic fields because of its
RMS detector and wide bandwidth.
Two different methods for VDT magnetic field measurements
have been commonly used. The first is that of characterizing the
field at a fixed distance from the screen of the VDT.
Conventionally, distances of 30 cm and 50 cm have been used
for such measurements. Normally, the measurements are taken
from the front surface of the screen. Additional measurements