How Metal Detectors Work: A Step-by-Step Guide

Published: 15 Sep 2024

Metal detectors have become an integral part of modern-day security and are used in multiple fields, such as security, treasure hunting, archeology, and construction. These miraculous devices can identify the presence of metals that can not be seen with the naked eye. These devices are quite famous, yet people rarely wonder how metal detectors work. In this article, we will provide step-by-step guidance about the working of detectors and their effective usage.

how metal detectors work

Introduction to Metal Detectors

Metal detectors are effective electronic devices used to detect metal objects. Although Alexander Graham Bell is generally believed to Have invented them in 1881, they were first produced for industrial use in the 1960s. The importance of these devices increased with the rise of terrorism worldwide. Moreover, metal detectors are also used today in multiple other fields, like hunting for treasure, construction, and landmine cleaning.

The key components of a typical metal detector are a search coil, control box, audio output, and discrimination control.

How Does a Metal Detector Work?

Metal detectors are based on the principles of electromagnetism and electromagnetic Induction, which are universal laws that explain how currents and magnetic fields influence each other. For this reason, it is possible to demystify the procedures into basic principles of science that occur sequentially to enable the functionality of the metal detectors.

how metal detectors work

Step 1: Creation of Electromagnetic Fields

  1. Most detectors use very low frequency (VLF), typically between 3 kHz and 30 kHz, for better metal detection.
  2. Metal detectors create an electromagnetic field using a coil of wire.
  3. When electricity flows through the coil, it generates a magnetic field around it.
  4. The coil at the detector’s end called the search coil, is powered by alternating current (AC).
  5. The AC causes the magnetic field to oscillate, moving back and forth.

Step 2: Interaction with Metallic Objects

  1. Because of their free electrons, metals like coins, jewelry, and other objects are good electrical conductors.
  2. When a metal object enters the detector’s oscillating magnetic field, electromagnetic induction occurs.
  3. The magnetic field induces small electric currents, known as eddy currents, within the metal object.
  4. These eddy currents are created as the field forces the free electrons in the metal to move.
  5. The eddy currents produce their own secondary magnetic field, which is different from the original field of the detector’s search coil.

Step 3: Detecting the Eddy Currents

1. Eddy Currents and Magnetic Field Interference:

  • Metal objects create eddy currents when exposed to the electromagnetic field from a metal detector.
  • These eddy currents generate a secondary magnetic field that interferes with the detector’s primary field.

2. Role of the Receiver Coil:

  • The metal detector’s receiver coil detects changes caused by the interference of the secondary magnetic field.
  • This coil may be integrated with the transmitter coil or operate as a separate component.

3. Signal Detection:

  • The receiver coil senses the changes and sends a signal to the detector’s control box.
  • The signal’s intensity and characteristics depend on various factors.

4. Factors Affecting Signal Strength:

  • Object Size: Larger objects produce stronger signals due to larger induced currents.
  • Metal Conductivity: Metals with higher conductivity generate stronger eddy currents, leading to distinct signals.
  • Object Depth: Signals weaken as the object’s distance from the coil increases because the magnetic field intensity diminishes with depth.

Step 4: Signal Processing

  1. When the receiver coil picks up a changed signal, the detector’s control box processes it to determine the appropriate response.
  2. The control box uses microprocessors and circuits to filter raw signals and make them useful for the user.
  3. The processed signal is displayed or transmitted in two ways:
  • Audio Feedback: A unique sound with specific pitch and loudness indicates the type, size, and depth of the metal.
  • Visual Display: Some detectors show basic information, like the type of metal and its approximate depth, on a screen.

4. Modern metal detectors can distinguish between different metals using a feature called discrimination:

  • The phase shift reveals how well the metal conducts electricity, helping to differentiate between ferrous (iron-based) and nonferrous metals like gold, silver, and copper.
  • This works by analyzing the phase shift, which measures the time difference between the transmitted signal and the received signal.

Step 5: Differentiating Metal Types

One of the essential characteristics of many metal detectors is the choice of Discrimination. The discrimination function works based on how various metals react to the induced electromagnetic field:

  • Ferrous metals: These are metals that contain iron, like steel and iron. They are also likely to generate more intense and rather sharp message types. The ferrous group of metals is paramagnetic, which implies that the metals have high magnetic permeability and hence, easily get affected by magnetic fields.
  • Non-ferrous metals: These elements and metals include gold, silver, aluminum, copper, and so on. Non-ferrous metals conduct electricity well but are not influenced by the magnetic field in the same way and are not attracted to magnets, so their interference is mild compared to that of ferrous metals.

Step 6: Depth and Sensitivity

Several factors affect the performance of the metal detector, more especially the depth of detection and sensitivity. These factors are primarily dictated by the following:

  • Coil size: A large coil produces a larger field and, therefore, can pick metal objects deeper into the ground. However, it may not be as accurate when finding small items. It can pick up small objects because the coils used are relatively small, but the depth range is suboptimal.
  • Frequency: One type of detector, higher-frequency detectors, tends to target small objects and lack penetration capabilities. The lower-frequency detectors, however, can go deeper while missing thin and small objects of interest.
  • Ground conditions: As for ground conditions, the existence of minerals, such as iron-bearing soils or salt influence, can negatively affect the operation of the device. This is why pulse induction (PI) detectors, which have a short pulse of current rather than a continuing wave, are preferable under such circumstances.

Step 7: EMI and Grounding

  1. A disadvantage of metal detectors is interference from electrical or magnetic fields, like power lines or highly mineralized ground.
  2. To address this, many detectors have a feature called ground balance:
  • Ground Balance Function: Reduces sensitivity to ground minerals, minimizing interference.
  • In areas with high mineral content, lack of ground balance can cause poor depth detection and false signals.
  • Ground balance helps the detector recognize and ignore irrelevant signals from the ground, focusing only on actual metal targets.

Best Tips for Getting the Maximum out of Metal Detectors

  • Begin at a location with the least disturbance, such as a field or beach.
  • Move the detector smoothly and slowly over the ground without jerky movements. The detector should always be at the same height as the ground.
  • They should employ headsets as they can give better results in sensing the signals, especially in noisy zones.
  • Learn how discrimination is done and how to set it up so as not to detect unnecessary metals.
  • Get acquainted with other conditions on and in the soil that may jeopardize performance, such as highly mineralized soils.
  • Remove the dirt gently in order not to destroy other artifacts that might be helpful.

FAQs

How Does a Walk-Through Metal Detector Work?

Walk-through metal detectors are very commonly used nowadays. They use pulse induction (PI) technology. When a person passes through the detector, it sends out pulses of magnetic fields. If there’s any metal on the individual, these pulses are disturbed, and the detector senses the change.

Although installed in several places, they dominate sensitive places, such as airports, courthouses, and concert halls. The sensitivity of the detector is changeable depending on the kind of area and security level required.

How Does a Hand-Held Metal Detector Work?

 A hand-held metal detector is usually used during a security check. It works in the same way as the larger detectors, but it is more accurate and compact. The security officer waves the device over the body, touching or hovering near the person. When in contact with metal, the detector gives out a signal. 

It is widely employed in security facilities to search for weapons or other metallic items prohibited in secured zones. It is effective in the presence of metals at close range, 5-6 inches deep. 

How Does a Metal Detector Work on Gold?

Metals used in gold detecting are different from those used in other detecting devices. Gold-detecting devices work at higher frequencies to detect small gold nuggets. Gold is mostly located at high concentrations of minerals in the soil, so the detector must distinguish between the mineral concentration and gold. 

Gold detectors operate based on a principle that involves the usage of an electromagnetic pulse that is sent to the ground. When these pulses come across a gold object, they form an opposing field that the detector comes across. It is also important to note that many gold detectors in the market also contain ground balancing options, making their use possible even in conditions where the mineralization is high. 

Final Thoughts

Metal detectors produce electromagnetic fields, read the eddy currents generated by metal in the ground or other structures, and pass this signal through a processor to locate the metal. While today’s models can discern between positive and negative signals and different types of metals, hobbyists, security workers, and archaeologists use metal detectors.

Knowing the interrelation of these scientific principles puts one in a better position and increases one’s chances of success.

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