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How Metal Detectors work
While there are three distinct types of metal detector circuitry, they all detect the presence of an object by looking at how the object reacts when it is subjected to an electromagnetic field. In other words, the metal detector sends out an electromagnetic field that strikes the object and then looks to see what happens.
The three types of medal detectors are VLF (Very Low Frequency), PI (Pulse Induction) and
BFO (Beat- Frequency Oscillator). The majority of metal detectors made today are VLF units.
VLF metal detectors (Very Low Frequency) use two different coils; one for transmitting the detectors electromagnetic signal into the ground, and one to detect the signal produced by the object. VLF detectors can not only tell how deep an object is buried in the ground and tell you what type of metal they found, but thru a process called “notching,” you can tell the metal detector just what type of metal you are searching for. In fact, you can program several “notches” at a time. This allows you to tell the metal detector that you are not looking for beer cans (one notch setting) and nails (a second notch setting), but for to alert you if it finds other metals.
PI (Pulse Induction) metal detectors generally use a single coil for both sending the signal into the ground and detecting the response from the object it detects. Different metals send back slightly different pulse lengths that can be hard to distinguish. PI detectors are better to use in areas that are high in conductive material in the land or the general environment, such as salt water exploration, and PI detectors can detect items deeper in the ground, but they do not have the sophisticated detection and identification capabilities that VLF detectors have. They are less common.
BFO (Beat- Frequency Oscillator) metal detectorrs are very simplistic, inexpensive to manufacture(you can even build one yourself) and can not offer the level of accruacy or control that VLF or PI ditectors offer.
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