How Do Metal Detectors Work

by Chad Eicher

How metal detectors work

As you know, a metal detector can detect the presence of metal objects without contacting them at all.

The operator is informed about the presence of metal with the help of special signals: sound, movement of the arrow, changes in the indicator, etc.

Below we will consider the principles of how metal detectors work.

How the metal detector works?

Regardless of how the device detects metal in the ground, all metal detectors can be divided into a processor and analog ones.

It is necessary to understand the difference between these concepts because, in the literature, there is confusion and the replacement of some words with others.

Pulse metal detectors are sometimes called analog detectors. This is true, but only partially.

What’s the difference?

If the metal detector has a processor that processes the signal, then such a metal detector is called a processor.

If there is no processor, and the signal is not processed in any way, that is, it goes directly to the operator (to the speaker or headphones), then such a metal detector is called analog.

Analog metal detectors have no delays and inform the operator at the moment when the target is under the coil. And processor ones have a delay. The coil is already away from the target, and the signal has just arrived.

From this point of view, analog metal detectors are preferable. Still, the processor gives more search capabilities: additional search programs, graphical representation, special signal processing to filter out unwanted interference, both from the ground and from targets that are discriminated against. In addition, selective discrimination (on analog discrimination is sequential).

Next, we will describe the principles of operation of metal detectors, so they can be both processor and analog—the issue of technology implementation.

General operating principle of a metal detector

Metal detector in operation

All metal detector operating technologies are based on the following principle:

  1. the metal detector coil generates electromagnetic waves;
  2. these waves produce their own eddy currents in a metal object;
  3. these eddy currents generate their own electromagnetic waves;
  4. these waves from the object are registered by the metal detector.

The next question is how the metal detector will register and process them. By phase difference, by frequency difference, etc.

PI-metal detector (pulse)

The PI metal detector does not send the signal from the coil to the ground all the time. It uses pulses. First, it sends a signal, then it is silent and receives a signal from the target on the same coil.

It is clear that the reflected signal also comes from the ground. But it fades away from it faster than from the target.

The usual operating frequency of such metal detectors is from 50 to 400 Hz.

TR-metal detectors

TR-metal detectors during operation use 2 balanced coils located in the same plane: one transmits, the second receives. The signal from the first coil enters the ground, and the second registers the return signal. Based on the signal’s phase difference, a conclusion is made about the presence (or absence) of the target under the coil—operating frequency about 20 kHz.

VLF/TR – metal detectors

VLF (Very Low Frequency) metal detector working principle is the most modern today. This is a type of TR-metal detector.

There are also two coils (but more stringent requirements are imposed on them, according to consistency). They are also located in the same plane, one transmits, the other receives. The phase shift is used to conclude that there is a target—operating frequency from 1 kHz to 10 kHz.

RF-metal detectors

RF (Radio Frequency) are metal detectors that work on the same principle as TR. Only their operating frequency is higher: from 50 to 500 kHz. And the coils are not located in the same plane, as was the case in VLF and TR, but are perpendicular and spaced at a certain distance.

BFO-metal detectors

Such metal detectors work on the beat’s principle—an old technology used in the 60s and 70s.

There is a frequency generator, there is an incoming frequency from the target. A comparison of 2 frequencies is made. Based on this, it is concluded that there is a target.

The frequency of these devices is from 40 to 500 kHz.

Advantages and disadvantages of various operating principles of metal detectors

  • BFO-metal detectors – low sensitivity, low stability, problematic operation on mineralized and wet soils.
  • TR-metal detectors – high sensitivity, good metal discrimination, good ground balancing. Disadvantage – with increasing depth, sensitivity to small targets is lost.
  • RF-metal detectors – extremely low sensitivity to small targets. They are used in deep metal detectors.
  • PI-metal detectors – insensitive to the ground, poor target recognition, high energy consumption.

Thus, of all the listed methods, VLF is the most progressive and modern.

Accordingly, VLF metal detectors can be either processor-based or analog.

Features of technical specifications

When choosing a metal detector, you must clearly understand the environment in which it will work. It is also important to consider the size of the object and at what depth it will search. Metal detectors whose technical characteristics make it easy to find a coin under a 10ft layer of soil may not see it on the surface, and vice versa.

Let’s look at the main technical characteristics that you should pay attention to when buying:

  • operating principle
  • operating frequency (kHz)
  • sensitivity
  • discriminator
  • ground balancing
  • target designation
  • weight
  • additional functions

The operating principle and operating frequency give the main characteristic of the capabilities of the device. They show to what type the device can be attributed: from the simple ground detector to professional. Without special configuration skills, a professional metal detector is not much different from simpler models, so beginners are better off starting with budget ground metal detectors. The effectiveness of their work will be sufficient for a successful search for treasures.

Sensitivity – indicates at what depth the device can find an object the size of a coin. Looking at the technical datasheet, you can see two numbers – the minimum and maximum detection depth. Often, this indicator varies from 4-20 inches to 25-60 inches. However, there are deep models created to detect objects under a 15ft layer of the earth.

Discriminator – allows the metal detector to only respond to a specific type of metal. Do you know how much iron debris can be found underground? Foil from cigarette packs, aluminum cans, bottle caps – a detector that reacts to all this adds significantly to the operator’s work. Having set up the discriminator, you can skip all this junk and focus on finding only gold, or only copper.

Thanks to the discriminator, the operator’s work can be significantly simplified, so you should pay special attention to this indicator. The more programs are stored in the discriminator’s memory, the easier it is to work with the metal detector.

Ground balance – allows you to tune the device to the type of soil the target is in. Differences in ground mineralization can give false signals, which complicates the operation of the device. Most detectors adjust this value automatically.

Targeting can be attributed to additional functions. The essence of this program boils down to adjusting the search for specific target size.

All of the above technical specifications help to understand the capabilities of the metal detector. However, the success of its work depends on the settings that the user makes.

Photo of author

Article by

Chad Eicher

Detectorist Chad is passionate about metal detecting since he got his first metal detector as a gift when he was 12 years old. He created Metal Pursuits to share his knowledge and create the ultimate metal detecting resource on the web.

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