From: John De Armond
Subject: Re: Demagnatizing to defeat detectors
Organization: Dixie Communications Public Access.  The Mouth of the South.

gary@ke4zv.atl.ga.us (Gary Coffman) writes:

#In article <16F81A8B3.JMCMAHON@NUACVM.ACNS.NWU.EDU>
#JMCMAHON@NUACVM.ACNS.NWU.EDU writes:
#
##metal detectors.  My question for you is this: if a stainless steel rod the
##size of 46mm long by 1 and 1/2 to 2 cm in diameter can be demagnatized, why
##can't the steel for a handgun be demagnatized?
#
#The reason the metal pin doesn't set off the metal detector is that
#it's shielded by body tissue, mostly water, which attenuates the search
#fields, and doesn't detune the coil enough to be detected. If you carried
#the pin in your pocket, it would set off the detector.

Water does not affect the operation of a metal detector.  While I don't
claim to be a metal detector expert, I have been responsible for
writing calibration and surveillance procedures for nuclear power
plant portal monitors (exactly the same detectors as used in airports)
and training the public safety officers (guards) in the setup and operational
testing.  I also use a metal detector for beachcombing and
treasure hunting and I know that when I go into the water on a beach,
the detector's response is not affected and I do not have to re-balance it.

According to the Metal-tek manual, the portal monitor's sensitivity is
roughly proportional to the mass of the metal.  This isn't strictly
true but it is good enough for this discussion.  The portal is zoned,
normally into 3 zones, so that the sensitivity can be adjusted for
different parts of the body.  For high traffic areas such as airports
where the detection is mostly for show and where it is undesirable
to have lots of false alarms, it is usual to slightly detune the
lower zone so that steel-toed shoes, metal arch supports and the
like won't set it off.

The functional test at the nuclear plant, performed twice a shift,
is for a guard to pass through the portal with a small gun strapped
to his ankle and again with the same gun strapped up high in his
crotch (apparently a lot of people think the body will shield a gun.)
The sensitivity is adjusted until the detector just alarms.

The reason the 1.8" long by .7" diameter pin in that fellow's leg
didn't set the detector off is simply that it is not large enough,
particularly if it happens to lay such that it splits the lower
and middle zones.  A friend who has an approx 18" long pin in one leg,
also compliments of a motorcycle accident DOES set the detector off every
time.

John

From: John De Armond
Subject: Re: Demagnatizing to defeat detectors
Organization: Dixie Communications Public Access.  The Mouth of the South.

gary@ke4zv.atl.ga.us (Gary Coffman) writes:

#A NMR machine works by putting the subject in a big very strong magnet.
#If you embedded magnetic material in the subject, it would try very hard
#to reach the magnet. That would *hurt* a lot.

Yet another false urban myth.  Having spent hours with my legs sticking
in an NMR machine compliments of old basketball injuries, I have a
pretty good idea about this.  My first trip in, I wore my Rockport
street shoes in order to keep my feet warm.  The look completely
non-metallic but in reality contain steel staples, as I verified with
an X-ray later.  As I entered the machine, I felt only the most
gentle tug at my foot, so light as to almost be confused with
vibration.  According to the NMR tech I asked, the only metal of
concern is metal located in extremely soft tissue, things like
metal that may have been left in the eye by a previous injury and the like.
They do NOT take any special precautions when they NMR my father who
still carries 12 pieces of steel shrapnel in his butt compliments
of a German artillery shell, the largest piece being about the size
of a thumb.

(unrelated to guns and I promise I'll sit on my fingers now but I just
can't sit by and watch these myths float by.)

John

From: John De Armond
Subject: Re: Demagnatizing to defeat detectors
Organization: Dixie Communications Public Access.  The Mouth of the South.

gary@ke4zv.atl.ga.us (Gary Coffman) writes:

#No, you don't (usually) have to rebalance it, but its sensitivity is
#affected. In water it won't detect the same sized coin at the same
#distance as it can in air. If you think about this for a bit, you'll
#also realize that your HT (assuming it was waterproof) doesn't transmit
#or receive very far under water either. That's why subs have to come
#to periscope depth and stick up an antenna to carry on radio communications.
#The water attenuates the RF fields.

But since there is NO RF involved with metal detectors and they work
strictly based on magnetic disturbance, RF propagation in water is
irrelevant.  And regardless of your imagined experience with metal
detectors, water does NOT attenuate the sensitivity.  Instead of making
this stuff up, you really ought to TRY it sometime.  Or at
least go to the library and read up on it. (For those wondering how
magnetic disturbance can work on non-ferrous metal, it relies on
eddy currents being induced in the metal.  The phase (and therefore the
POLARITY of the output of the phase detector in my treasure detector)
is the opposite of ferrous, a fact used to discriminate between ferrous
and non-ferrous metal.  "All metal" detectors, such as those in
airports don't discriminate and so don't care whether the metal is ferrous
or not.)

##The functional test at the nuclear plant, performed twice a shift,
##is for a guard to pass through the portal with a small gun strapped
##to his ankle and again with the same gun strapped up high in his
##crotch (apparently a lot of people think the body will shield a gun.)
##The sensitivity is adjusted until the detector just alarms.

#Holding the gun in his crotch doesn't help. It's not *completely*
#surrounded by tissue. The RF fields follow the contours of the
#dielectric (the body) and still reach the gun. It has to be inside
#the body cavity to be effectively shielded. Again you can think of
#this as a Faraday cage with the door open. It doesn't work very well.

Again, since the metal detection process does not involve RF (Airport
metal detectors typically run at 15 khz, my treasure detector has a
selectable 15 or 40 khz carrier.  15 khz works better in soil with a
high iron content.) your comments are irrelevant and wrong.  You are
also wrong because the airport metal detectors generate the sensitive
magnetic field from the sides and the mass of the thighs ARE between the
gun and the detector coil.  Does not matter, in any event, since the
body mass does NOT shield a pistol concealed in the crotch.

#Now this has merit. It *is* likely that the sensitivity of the detectors
#is set too low to pick up such a small piece of metal. I'd note that my
#keys set off the detectors though, and they aren't any bigger, but they
#are outside my body. They aren't magnetic either, magnetism has nothing
#to do with the way metal detectors work.

Nope.  Your keys set off the detector because you carry them in the
part of your body where the midzone detector is and that one is
almost always set more sensitive since there is no legitimate
metal in that area of the body other than belt buckles which can easily
be removed.  Hang those same keys down around your ankle and try it
again.

If you want to see a metal detector operating at its limits, come up
to the Cobb County court house.  In their fits of paranoia, they
have that detector set to catch just about anything.  It triggered on
a single quarter in my pocket AND on my HP95, a device that does NOT
trip the detectors at the airport.

This whole airport metal detector game is a public relations and propaganda
sham.  The public would NOT stand for the delays involved with detectors
set sensitive enough to catch small weapons.  They are therefore set to
catch the most common stuff and rely on the fact that almost no one hijaacks
planes anymore to cover for the rest.  I *KNOW* the detectors at the
Atlanta airport will not detect a small gun because I accidentally
carried my NAA .22 mag pistol onboard a flight a few years ago.  I didn't
realize it until I crossed my legs and felt my ankle holster.  The feeling
of utter panic was indescribable.  Longest flight I *EVER* had.

John