In this issue we will be going over the basics of metal detector operation. The information in this article is for those who are contemplating purchasing a metal detector for the first time and for those who are still learning how to operate their metal detector. Since this article is only on the basics and the VLF type of metal detector is the most popular among enthusiasts that is the type we will be discussing here.
A metal detector creates an electromagnetic field surrounding the search coil. The search coil consists of two coils, the transmitter coil and the receiver coil. The transmitter coil is the outer loop and it is the frequency of this coil, which determines the frequency of the unit. The receiver coil acts like an antenna picking up and amplifying the frequencies coming from the targets in the ground. The current moving through the transmitter coil creates an electromagnetic field surrounding the search coil. This field moves with the motion of the search coil and interacts with any conductive objects it encounters, not only metal objects, but also naturally occurring iron called ferrous oxides and natural salts in the soil. When the electromagnetic field encounters these objects it causes them to generate a weak magnetic field of their own. The receiver coil is shielded from the electromagnetic field generated by the transmitter coil but it isnít shielded from the magnetic fields coming from the objects in the ground. When the receiver coil passes over an object in the ground emitting a magnetic field, a small electrical current goes through the coil. The coil then amplifies the signal and sends it to the control box to be analyzed. The strength of the objects magnetic field is then used to approximate the depth of the object in the ground. The stronger the objects magnetic field is, the closer it is to the surface. The deeper the object, the weaker the magnetic field is.
A VLF metal detector can also distinguish between different types of metals due to phase shifting. Phase shifting is the difference in timing between the frequency in the transmitter coil and the frequency of the target object. Phase shifting is the result of inductance and resistance. Inductance is how easily an object conducts electricity. Most metals are highly inductive. Resistance is how much an object resists the flow of electricity through it. Most metals have a low resistance. As a result of this, objects that are made of metal will have a low resistance and a high inductance, which results in a large phase shift. The size of the phase shift is how a VLF metal detector can determine what type of metal the object in the ground is made of.
The discrimination and ground balance circuits use these signals by comparing them against the control set points and tells the how to respond. If a metal object is detected in the ground your detector will respond favorably with an either an audio or a visual signal, or with both. When you receive a favorable signal, use it as a guideline only. This is where experience with your detector comes into play. The more you use your detector, the better the feel you will have for it. The reason for using the signals as a guideline are that smaller objects also have weaker magnetic fields and it is possible that what your detector says is 6 inches deep could only be 2 or 3 inches deep.
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