Depth of detection
One of the main parameters of the detector - depth of detection of objects. It the most obscure and the most burning question for beginners. And this is not surprising - not in the same directory, advertising metal detectors, you will find instructions on what distance the device detects coin, metal jug, etc. Indicates, at best, what is the weight of the device and what time he can to work without battery changes. Sometimes it is the best theoretically possible depth of detection, which is several times greater than the one values, which a person thinks, holding a metal detector.
In order to clarify this issue, the information provided below is broken for the convenience of the size scale of various items search. Set forth below is not relates to magnetometers!
Coin
Let's start with the fact that under the coin in the descriptions of many foreign devices usually means a coin with a diameter of 25 mm. For those who remember, for example, coin USSR are in denominations of 5 cents, or the current "iron" 5 rubles. Depth the discovery of this coin is from 10 cm to frivolous devices, up to 50 cm - for very serious. Depth of detection is highly dependent on the size of the sensor. Roughly, for such coin detection depth approximately equal to the diameter of the sensor (if it is in the form of a disk). The smaller coins will be be detected at smaller distances. For example, the depth of detection of the coin worth 1 penny will be about two times smaller than indicated above for Nickel.
Bronze statuette, gun
Naturally, the larger the metal detector detects objects at a greater distance. To estimate this increase, usually cite the example of the bronze statue or the gun (as relics of Russian history we can mention the Cannonball). The detection of such objects is about 1 m Depth of detection is only slightly dependent on the size and type of the sensor (it may be coplanar coils, and orthogonal to the rod). For sensor with larger diameter coils and sensor system with orthogonal coils on the rod, the depth of detection will be slightly higher (by 20% at the transition from the disk sensor with a diameter of 20 cm to the sensor with a diameter of 30 cm).
Metal shield, bells
Everything said in the previous paragraph is true for these objects, for with the exception of the depth, which is in the range of 1.5-2.5 m...
Tank, engine, aircraft
In serious literature, the possible detection of such objects is called: "the theoretical maximum depth of detection". In other words, there is a theoretical limit beyond which e the metal detector is not able to register objects in principle, despite arbitrarily large sizes. The existence of this limit on the depth of detection is easily explained if we consider, that the amplitude of the reflected signal is inversely proportional to the 6-7-th degree distance, and the size of the object depends only 3-5 degrees.
The value of the theoretical maximum depth of detection is 1 m for the simplest of devices with disk sensor, up to 4 m for complex devices with the large sensor of orthogonal coils on the rod. Unfortunately, it is this the value commonly adopted for advertising purposes, but at this depth you will not find nothing!
Remember! The most modern electronic metal detector can detect a medium coin at the depth of maximum 50 cm, very large massive all-metal nonferrous subject to a maximum of 2.5 m.
Selectivity for metals
Unlike the depth, on a very modest values which ignorant people usually do not realize that the selectivity for metals seems to be more clear question - so many people think. And they are wrong.
The source of confusion is, as usual, is not quite accurate advertising. In many catalogues and prospectus of firms, given closeup pictures of scales arrow indicators metal detectors. Immediately apparent that the scale the instrument is graduated by sector indicating the type of the metal: "metal", "not iron", "gold", "silver" etc., But! Nowhere explicitly stated that this calibration refers mainly to small items and unsuitable for long-range large items.
The inconsistency of the statements about the selectivity of the detectors can be installed even purely speculative. In fact, what happens if for experiment to try to determine the detector with the selector on metals a target, consisting of two small items - one of iron, the other of silver? It is clear that any indicator is false. Moreover, in depending on the ratio of masses, shape and size of these two objects to get any result. For example, the meter pointer may stop at the sector of "gold"! With the same ease the detectors selective metals "confused" iron nonferrous items, depending on their shape and mass. Imagine the feelings of the person, instead of the promised Golden mountains detects after hours of excavation a piece of train rail!!!
A special attention deserves outright misinformation that appeared in recently and has spread thanks to the Internet. We are talking about "molecular" and other detectors, capable of 100% to determine the gold. In some cases when describing the principle of operation featured some mysterious bundles particles, in other - some electrostatic (?!) field and sometimes pathetic attempts to describe something like a mass spectrometer or device on the effect of nuclear magnetic resonance. Indicative of country of origin data "masterpieces" - Brazil, Turkey, Greece...
In summary, we can say the following. Advertised e metal detectors exaggerated. However, they still remain the only available class of devices, permitting to see the metal the objects through the earth."
Author: A. I. Shchedrin