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 |