This is an excellent set of links since together they discuss four of the most commonly used technologies.
Fisher says, "Detects ferrous only FX-12 will detect ferromagnetic objects (like steel
and iron). When passing though the walk-through you do not need to
take of your jewelry etc.."
They're using the very simple fact that ferrous objects increase the inductance of a coil. (Many of the "wand" type detectors contain two metal plates and measure the capacitance between those two plates. A third metal object near the plates increases the capacitance between them. This technique works only close up; it's not suitable for "walk-through" applications. This difference explains why people often fail the walk-through detector and pass a seemingly more intensive wand search.). As Fisher points out, these techniques do not emit a field. I like the scare tactics Fisher uses, "Pulse induction is
extremely dangerous for patients using pacemakers and other
electronics devices and causing serious problems. There are a lot to be
studied at this point, but at least it is proven to be dangerous at some
points. On the other hand you might be happy that your school using a
detector and your kid is safer that way, but you do not know ; to
expose kids to daily radiation might be much more hazardous."
First of all, they're not quite being honest. Their products do create fields. But, those fields are "static" (not changing, except when a metal object is introduced into them). Many people feel that if there is any health risk associated with exposure to electromagnetic fields, it is probably mostly when exposed to dynamic (changing) fields.
What's the big deal with pacemakers? Three problems: First, strong electromagnetic fields can disrupt or even destroy any electrical circuit. The EMP from an atomic bomb is an extreme example of this. Devices that create reasonably electromagnetic fields are all around us. Cellular phones are a common example. One of these articles points out that even a common hair dryer creates a reasonably strong electromagnetic field. Perhaps you've noticed that when you drive under large electrical transmission lines, the radio in your car may experience some interference? So, if sources of electromagnetic fields are everywhere, then how does any electrical device function? Simple: circuits are designed so that the valid signals are significantly stronger than any expected external interference noise. Making those signals strong enough takes power. Pacemakers are designed to run for years on small batteries. So, they have to conserve power. Modern pacemakers, though, use better batteries and better circuitry such that this really isn't a problem anymore. As you might imagine, the FDA has set strict standards for pacemakers.
The second problem with pacemakers is that virtually all modern pacemakers are remarkably sophisticated devices. They not only pace the patient's heart, but they constantly collect a wealth of data about the patient's health and their own operation. Those data are stored in the pacemaker and can be downloaded for analysis by the patient's doctor at routine exams. At those same exams, the doctor can adjust the pacemaker's operation to meet the changing needs of the patient (keep in mind that modern pacemakers operate for five to ten years. During that time, the patient's needs will change. The doctors do not want to operate on the patient to physically access the pacemaker each time a change is needed.) There are two ways that the doctors can access the implanted pacemaker. They can either implant a USB port in the patient's naval, or they can use an electromagnetic transducer that is simply held over the patient's skin and communicates with the pacemaker with no pain or even sensation to the patient. Most patients opt for the electromagnetic transducer method. But, it is vaugely conceivable that a pacemaker could misinterpret the magnetic pulsed from a metal detector as programming commands. Of course, modern pacemakers are specifically designed to avoid this. Again, the FDA has rigid standards on such things.
Finally, many pacemakers actually have a second generator independent generator circuit in them. This second generator serves two functions. First, it's the backup in case the main one malfuctions. And second, it's designed so that it's pulse rate depends on the battery level. As the battery gets low, the rate of the backup generator drops slightly. So, periodically, it's a good idea to switch over to that backup generator briefly to make sure that it's working fine and also to get a reading on the battery level. In many designs, that switch is initiated by holding a strong magnet over the pacemaker. It is, again, vaugely possible that the magnetic field from a metal detector could falsely trip that circuit. This would not be a problem. The backup pacer is designed to keep the patient perfectly fine. But, if the person was under stress, late for an important flight, lugging a heavy bag, nervious about the metal detector, and had just sprinted a mile and a half from the ticket counter to try and make their flight, a sudden change of heart rhythm could be uncomfortable. Again, this is very unlikely to happen. The FDA has standards about these sorts of things too.
So, Fisher's radical claims about health hazards of metal detectors is just a crock. Fisher's key selling point is doubtlessly cost. So, they're trying to make their inferior product look good by attacking their competition with vauge and unsubstantiated claims that play on people's fears of technology.
Several of these sites nicely discuss the impulse/response method of detection. These methods detect electrically-conductive materials. That would include all known metals. However, the one paper hints at some of the weakness of these detectors. They are sensitive to the physical orientation of the item. Also, if the item can be shielded from the magnetic pulse, then it won't react and be detected. This can be done.
Densitometers are another method, but they false a lot. I, myself, have a hard time getting through densitometer metal detectors. I have to take every piece of metal off of me. Since I wear a necklace with a cross on it that is to small to take off and has no clasp (it was assembled around my neck), that is impossible. The guards are always amazed that that little bit of metal will trip the detectors. But, I have very dense bones (I was raised on whole milk). Every time I have to be x-rayed for anything, they always comment on my bones. Often, the x-rays have to be done twice since they under-expose the first one. Anyway, my very bones seem to upset the densitometer detectors.
There was a website that was posted in one of the previous metal detector threads about a company that had adapted a technology developed by the US Navy. Metal objects create a disturbance in the Earth's gavitational field. A knife for a gun is a very small object in comparison to the Earth, but it still creates a disturbance in the Earth's gravity, a very small disturbance, but a disturbance. That disturbance can be detected. In fact, it's even possible to make a general-shape image of the object causing the disturbance. The Navy uses this technology to try and detect submarines. The Army is now looking at it for land mine detection. It's also been used for mineral mining and to try and locate burried objects. This company had adapted this technology to detect concealed weapons. It's very expensive and still needs some development work.
Magnetic resonance has not been mentioned in any of these sites. It's not in use to screen people, but it is used at seventy some-odd airports in the US (including Portland's PDX) to screen a fraction of baggage. It's the same techniques used for medical MRI imaging. The machines are physically quite large, very expensive, and slow for security checkpoints. But, that will all change in the coming years.
Ultrasonic imaging is another technique that is impractical today, but could conceivable be used.
So, there are many technologies. Most detectors in use in airports and other higher-security situations will detect Ti.
------------------
Chuck
Balisongs -- because it don't mean a thing if it ain't got that swing!
http://www.balisongcollector.com
