increase if interference is present, and decrease
as you move away. As with the radio, move the
meter around, trying different angles, to deter-
mine the peak of the interference, then move
it away from the device to find the minimum
separation zone.
I use a gauss meter to measure relative mag-
netic field strength at various points around a
conductor or piece of equipment, as well as to92 electrical systems troubleshooting
EMF-200A, Sperry Instruments, http://www.awsperry.com
Model 480823, Extech Instruments, http://www.extech.comGAUSS METERS OR ELECTROMAGNETIC FIELD TESTERSprobe near large DC conductors and equipment
that delivers high amperage to high-current-
draw motors, such as starters and bow thrusters.
It is a handy tool, costing about $100.Using a gauss meter to check the separation zone near a
high-current-draw cable from a battery switch. Notice the
reading of 2.5 mG. The circuit in question (a bilge blower
fan) was drawing 6 amps, which is not a particularly high
current draw, but I wouldn’t mount a digital fluxgate com-
pass sensor near this wire.Here I used a small handheld compass to see how much
compass deviation I could induce, and compared that to
the mG reading shown in the previous photo. A mere 2.5
mG produced a 12-degree error on the compass. It took
10 inches of separation before the compass returned to
the correct reading. By comparison, some battery charg-
ers and inverters I’ve checked emit as much as 170 mG!
That’s enough to send a compass into an extreme level of
deviation from the actual heading. At a 170 mG reading,
a typical separation zone would equate to 3 to 6.5 feet
(1 to 2 m).