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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Tests for the correctness of the search and the quality of the metal detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors First observation. In tests in the air, a coin is found, say, at 30 cm, and in the ground deeper than 18-20 cm it cannot be found. Everything is natural - the probing signal is greatly weakened in the ground. Second observation - a significant deterioration in the quality of object discrimination in the ground. The device reacts to the ground as well as to the coin lying in it. That is, the metal detector needs to distinguish signals from two objects at the same time. Therefore, the signal reflected from the ground begins to "clog" the weak signal of our coin. In this case, the quality of discrimination deteriorates sharply compared to air tests. Is it worth trusting metal detector tests carried out in the air at all? And what is the best way to test in real conditions? Test 1. Measuring the depth of detection of objects. Metal detector manufacturer Fisher uses a plastic tube buried in the ground at an angle of 45 degrees to measure the depth of detection of objects. Inside the tube, special "sleds" move, on which a target is placed parallel to the surface. Using such a simple device, you can quickly assess the sensitivity of the device to various targets at different depths. Test 2. A simple test for determining the depth of detection of a patch. In many cases, you can do it easier. We take a test target, for example, a Soviet penny (it most often appears in such rapid depth tests). We put it in a small plastic bag with a clip. With a sharp sapper spatula, carefully remove the layer of earth, lower our bag with a coin to the bottom of the pit and lay it parallel to the surface of the earth. We measure the depth with a ruler and carefully return the removed lump to its place. The soil does not need to be heavily trampled. What do we get as a result? The coin lies in practically undisturbed and homogeneous ground; if we dug a hole and then filled it with loose earth, then the soil conductivity parameters would change, which would affect the detection depth of the object. Lightly trampling on the ground makes it easier to retrieve the coin back into the light of day and ensures that it does not go to great depths. After all the experiments, the dirty bag is thrown away, the coin remains pristine and untouched. Test 3. Coin detection depth by sound. Now you can arm yourself with several devices of various brands and conduct experiments. We run tests in the following order. We turn on the device. We are waiting for 5 minutes to establish its temperature regime. We carefully balance the device at maximum sensitivity. If balancing is not possible, then reduce its sensitivity until an acceptable ground compensation is achieved. For devices with built-in auto-tracking (i.e., the device automatically monitors the ground balance during operation and adjusts it itself), this option is disabled. For what? Autotracking is not very stable at maximum sensitivity and slightly reduces the search depth. Turn off the discriminator and work in the "All metals" mode. Consistently, by changing the depth of the object, we find one where it is still possible to detect it by sound (but not identify it by the display!). At this depth, the discriminator can no longer correctly determine the type of metal. It is advisable to carry out tests with different speeds of the coil, along different trajectories, to simulate the search process, i.e. start the test about a meter before the target. Test 4. Checking the temperature stability of ground balancing. You need to put the device for half an hour in direct sunlight, so that it warms up. The purpose is to test the temperature stability of the ground balance. If the "ground is gone", then this is almost a XNUMX% guarantee that you will not find anything at the previously measured depth, since the ground signals will clog the weak target signal. You will simply miss a weak signal from a deep coin against the background of constant triggering of an unbalanced device. There are two ways out of this situation:
Here we come to the most important conclusion: it is not the supersensitivity of the metal detector that is very important, but the stability of its operation! You can make a device that will "smell" the same penny half a meter through the air, but this is of little use. It will hardly be possible to balance this metal detector with such sensitivity. And if it also doesn’t care about temperature stability, then in the process of searching you will often have to adjust the ground balance, and this will be very distracting and tiring. Test 5. Determining the maximum depth of object discrimination. It is performed similarly to the previous one. But it will be necessary to include the discriminator. In this case, it will be necessary to look at the display (or navigate by sound) and determine the depth of the object, at which it begins to be correctly identified. Depending on the device, the object discrimination depth is reduced by 20%-50% of the maximum (measured in the previous test). Test 6. How to distinguish a signal from a coin and a beer cork lying next to it. Bury a coin, and nearby at a distance equal to the diameter of the coil - a beer cork. So you can imitate the most common metal waste today. The cork does not need to be buried deep, because in reality they lie almost on the surface. Make such movements with the coil in order to scan both the cork and the coin in one stroke. Memorize the signal and the picture on the display. In the case when the coil passes first over the coin and then over the cork, the identification quality will be higher. Test 7. Determination of detection depth in static mode. Switch the device to a static mode of operation (if its design allows it) and carry out a second test. In static mode, the detection depth of most devices will be greater. Test 8. Evaluation of search technique and scanning frequency. And the latest experiment. For example, you found that with your device you can find a Soviet nickel at a depth of 25 cm. Select a piece of land. Ask a friend to bury a coin at this depth in an unknown place for you. Then you can try to find it. In this test, you can already see for yourself how important the search technique and scanning frequency are. Training and learning function of tests These tests can be repeated on different types of soil, such as clay soil, loose black soil, sand. If this is your first time holding a metal detector in your hands, then such preliminary tests are very important. You will be able to evaluate the real, and not the declared characteristics of the device on real ground and in real working conditions. When conducting tests, try to notice the slightest features of the work:
Evaluate the influence of the nature of the movements of the coil, the influence of the ground level difference and metal debris on the quality of object identification. To begin with, the following may be noted:
The dependence of the correctness of identification on the speed of the coil and its trajectory becomes stronger. Discrimination worsens at very fast, very slow or uneven speeds. Try to move the coil not parallel to the ground, but along a gentle path, because when in extreme positions the coil does not remain strictly parallel to the ground and rises slightly. This is how inexperienced search engines usually work. The quality of discrimination will deteriorate sharply. By moving the coil with a small amplitude exactly over the center of the target, you will observe the best quality of identification. Use this technique to refine the identification of an object. There is such a situation when there is a small hollow above the coin or there is a difference in ground level on either side. In this case, object discrimination also worsens. You can reduce the amplitude of the oscillation of the coil so that in extreme positions you do not go into the bumps. You can try scanning from a different angle. It is very important to press the coil as close to the ground as possible, as if to "stroke" it. Do not sacrifice search depth at the expense of speed. It often happens that when moving in one direction, the device shows that there is an object made of non-ferrous metal in the ground, while moving in the opposite direction, it is silent. In this case, determine the exact location of the object and change the trajectory of the coil so that it moves exactly over the center of the object. You can remove the top layer of soil, the signal level will increase, and the identification will become more accurate. Or move the coil perpendicular to the original direction. In any case, such signals should not be ignored. Author: Dubrovsky S.L.
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