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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING The real possibilities of metal detectors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors Depth of detection One of the main parameters of a metal detector is the depth of detection of objects. This is the most obscure and most burning question for beginners. And this is not surprising - in any catalog advertising metal detectors, you will not find an indication at what distance the device detects a coin, a metal jug, etc. It is indicated, at best, how much the device weighs and how long it can work without changing the battery. Sometimes the parameter "the maximum theoretically possible detection depth" is given, which is several times greater than the value that a person holding a metal detector thinks about. In order to clarify this issue, the information below is broken down for convenience into a scale of sizes for various search items. The following does not apply to magnetometers! Coin Let's start with the fact that a coin in the descriptions of many foreign devices usually means a coin with a diameter of 25 mm. For those who still remember - this is, for example, a coin of the USSR in denominations of 5 kopecks, or the current "iron" 5 rubles. The detection depth of such a coin ranges from 10 cm for frivolous devices, up to 50 cm for very serious ones. The depth of detection is highly dependent on the size of the sensor. Approximately, for such a coin, the detection depth is approximately equal to the diameter of the sensor (if it is in the form of a disk). Smaller coins will be detected at shorter distances. For example, the detection depth of a coin with a denomination of 1 kopeck will be approximately two times less than indicated above for a nickel. Bronze figurine, pistol Naturally, the metal detector detects larger objects at a greater distance. In order to appreciate this increase, a bronze figurine or a pistol is usually cited as an example (a cannonball can be mentioned as a relic from Russian history). The detection depth of such objects is about 1 m. The detection depth depends little on the size and type of the sensor (it can be either with coplanar coils or with orthogonal coils on the rod). For a sensor with larger diameter coils and for a sensor with an orthogonal coil system on the shaft, the detection depth will be somewhat greater (by 20% when moving from a 20 cm diameter disk sensor to a 30 cm diameter sensor). Metal shield, bells Everything said in the previous paragraph is also true for these objects, with the exception of the detection depth, which is in the range of 1,5 ... 2,5 m. Tank, locomotive, aircraft In serious literature, the depth of detection of such objects is called as follows: "the maximum theoretically possible depth of detection." In other words, there is a theoretical limit beyond which an electronic metal detector is not able to register objects in principle, despite their arbitrarily large sizes. The existence of this limit on the depth of detection is easily explained, given that the amplitude of the reflected signal is inversely proportional to the 6th-7th power of the distance, and depends on the size of the object only to the 3rd-5th power. The value of the maximum theoretically possible detection depth ranges from 1 m for the simplest devices with a disk sensor, up to 4 m - for complex devices with a large sensor of orthogonal coils on a rod. Unfortunately, it is this value that usually appears in advertising purposes, but at this depth you will not find anything! Remember! The most modern electronic metal detector can detect an average coin at a maximum depth of 50 cm, a very large massive all-metal non-iron object - at a maximum of 2,5 m. Selectivity for metals In contrast to the depth of detection, the very modest values of which the ignorant person usually does not guess, metal selectivity seems to be a clearer issue - this is how many people think. And they are wrong. The source of misconceptions is, as usual, not quite reliable advertising. In many catalogs and prospectuses of firms, there are close-up photographs of the dial indicators of metal detectors. It immediately catches the eye that the scale of the device is graduated by sectors indicating the type of metal: "iron", "not iron", "gold", "silver", etc. However! Nowhere is it explicitly stated that this graduation applies mainly to small objects and is unsuitable for long-range searches for large objects. The inconsistency of statements about the selectivity of metal detectors can be established even purely speculatively. In fact, what will happen if, for an experiment, we try to determine with a metal detector with a metal selector a target consisting of two small objects - one of iron, the other of silver? It is clear that any indication of the indicator will be false. Moreover, depending on the ratio of masses, shapes and sizes of these two objects, any result can be obtained. For example, the arrow of the device may stop at the "gold" sector! With the same ease, metal detectors with metal selection "confuse" iron objects with non-iron ones, depending on their shape and weight. Imagine the feelings of a person who, instead of the promised golden mountains, discovers, after many hours of excavation, a piece of a railway rail!!! A separate discussion deserves frank disinformation that has appeared recently and has spread thanks to the Internet. We are talking about "molecular" and other metal detectors capable of 100% detecting gold. In some cases, when describing the principle of operation, some mysterious particle beams appear, in others - some electrostatic (?!) fields, and sometimes - pathetic attempts to describe something like a mass spectrometer or a device based on the effect of nuclear magnetic resonance. The countries of origin of these "masterpieces" are indicative - Brazil, Türkiye, Greece ... Summing up, we can say the following. The advertised capabilities of electronic metal detectors are exaggerated. However, they are still the only available class of devices that allow "seeing metal objects through the ground." Author: Shchedrin A.I.
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