There is nothing more exciting than discovering a hidden treasure, which is why metal-detecting has emerged as a popular hobby all over the world. Enthusiasts go scavenging for jewelry, caches of old silver, gold coins, or any other valuables rumored to have been buried somewhere. Besides the recreational uses, metal detectors have vast utilitarian functions that make them a useful tool for a multitude of reasons.
- Building security – This is for screening people entering a secured building, such as an office, a school, or a prison.
- Airport security – This is for screening passengers before they are allowed access to the airport flight boarding area and the plane.
- Event security – For screening people attending a concert, sporting event, or other large public gatherings.
- Archaeological exploration – This helps professionals locate metallic antiques of historical significance.
- Geological research- This shows what comprises the metal makeup of rock and soil.
Regardless of the nature of the application, every metal detector is based on the basic science of electromagnetism. So how exactly do these metal detectors work? Let’s talk about that.
Core Components of a Metal Detector
A recreational metal detector is typically a lightweight and portable device that consists of several key parts.
1. A control box – This is an integral component that houses the circuitry, microprocessor, controls, speaker, and batteries. The main goal of the control box is to generate the transmit signal, then process the received signal, and convert the received signal into a target response.
2. Shaft – This connects the coil and the control box. It is often designed to be adjustable, so you can make it comfortable for your height.
3. Search coil – Also known as the loop, search head, or even antennae, the search coil is the part that transmits the electromagnetic field into the ground and picks up the return signal from the detected target. The most common search coil shapes you’ll see are elliptical and round coils and come in open-web and solid designs. Elliptical coils provide better ground coverage and maneuverability while round ones are more stable and efficient in detecting greater depths.
4. Stabilizer – This part comes fitted at the top of the detector, where you can rest your arm or firmly hold the detector to keep it steady while sweeping it back and forth. This is most often found on more advanced and sophisticated models.
Different metal detector models feature different designs, with some having the control box and a display unit slightly below the shaft while others include a jack for connecting headphones.
What Technology Do Metal Detectors Use?
While there are many metal detectors models out there, all of them employ one or two of the following three technologies to successfully detect buried metals and treasures. If you’re starting to get a little headache from this techy talk I recommend taking a break and learning what exactly detectors can detect.
Very low frequency (VLF) technology
The VLF (Very Low Frequency) is the most popular and widely used detecting technology in more recent models. This utilizes two different coils to generate and receive an electromagnetic field. An outer coil, known as a transmitter coil, houses wires that pass an electric current through them in an alternating manner up to a 1000/second. The frequency of a VLF detector is determined by the number of times the electric current alternates per second.
Then, there is the inner coil, known as a receiver coil. This also contains a wire coil, which serves as antennae to pick up the frequencies of metal targets buried deep and amplify their signals.
How Do VLF Metal Detectors Work?
The electric current running along the transmitter coil generates an electromagnetic field. When the generated magnetic field encounters any conductive metal object as it pulses into and out of the ground, it causes it to create a small magnetic field.
The magnetic field coming from the transmitter coil isn’t received by the receiver coil. It can pick up the fields coming from the objects in the ground itself rather than the machine. That way you will only pick up objects and won’t be caught up by machinery.
The coil then amplifies the received frequency and transmits it to the metal detector’s control box, where built-in sensors further analyze the signal.
Based on the magnetic field strength, the detector can accurately determine approximately how deep you need to dig. An object closer to the surface will emit a stronger magnetic field, generating a stronger electric current. The field becomes weaker as objects go farther below the surface. Beyond a certain depth threshold, the object’s field becomes so weak that it’s undetectable by a receiver coil at the surface.
How a VLF metal detector differentiates metals?
The delay in timing (phase shifting) between the received and transmitted signal is what causes a VLF detector to distinguish between the various metal types.
Phase shifting is brought about by several factors, like inductance, which is the rate at which a metallic object conducts electricity. Typically, the easier an object conducts electricity, the slower it reacts to changes in the current. The other factor is resistance, which is an object’s inability to conduct electricity easily.
The phase shift of a highly inductive object will be larger because altering its magnetic field takes longer. An object with higher resistance will exhibit a relatively smaller phase shift. Since metals vary greatly in terms of inductance and resistance, a VLF detector analyzes the extent of phase-shifting of the detected targets to discriminate the various metals.
The metal detector can, therefore, notify you with a beeping sound or a visual indicator on the display panel as to what types of metal the object is most likely to be.
Most detectors even allow you to discriminate metal targets beyond a threshold phase-shift level. This comes in handy, especially when searching in highly mineralized grounds.
Pulse induction (PI) technology
Pulse Induction differs from the other technologies as it uses pulses to pick up buried objects and uses only a single coil for transmitting and receiving signals.
How do Pulse Induction Metal Detectors work?
This technology emits powerful, short pulses of current through a wire coil – with each pulse generating a temporary magnetic field. This discharge lasts only a few microseconds and results in another current running through the coil. This electric current is referred to as the reflected pulse and is usually extremely short-lived, lasting a mere 30 microseconds. Another pulse is then emitted, and the process starts all over again.
A standard PI-based metal detector emits about 100 pulses per second, but this figure can vary greatly depending on the detector model and manufacturer, reaching over a thousand pulses.
If the detector is held over a metallic object, the pulse induces an opposite magnetic field in the object. Once the pulse’s magnetic field dies, resulting in the reflected pulse, the item’s magnetic field makes the reflected pulse take longer to vanish completely.
A detector’s sampling circuit is programmed to monitor the extent of the reflected pulse. By relating it to the anticipated length, the circuit can tell if another magnetic field made the reflected pulse take much longer to decay. If it takes too long there is a metal object catching it up.
The sampling circuit emits small, weak signals, which it monitors on an integrator, a device that decodes the circuit’s signals, amplifies, as well as changes them to a direct current. The current is then converted into a beeping sound through a connected audio circuit, which is used to indicate the presence of a metal object beneath the ground.
Metal detectors that use PI technology are, however, not the best for discriminating the various metals due to the difficulty in separating the reflected pulse lengths of different metals.
However, they are still handy in hard-to-detect environments such as fields with highly conductive materials in the soil. Moreover, PI-based detectors can efficiently pick up metal objects deeper in the ground than other metal detectors.
BFO Technology
The beat-frequency oscillator (BFO) system utilizes two wire coils with a large coil on the search head and the control box housing the second smaller coil.
How BFO Metal Detectors Work
Both coils are connected to an oscillator that emits thousands of pulses every second. As the pulses traverse through the coils, it emits radio waves, which are picked up by the control box’s tiny receiver, producing audible tones depending on the discrepancy of the frequencies.
Once the search head’s coil encounters a metal target, the magnetic field generated by the flowing current produces a magnetic field within the object. Then the target’s magnetic field disrupts the radio waves frequency from the search-head coil. As the frequency shifts farther from the control box coil frequency, the audible sounds change in tone and duration.
Due to the simplicity in the functionality of BFO-based detectors, they are easily manufactured and available at incredibly affordable prices. However, these detectors do not achieve the high accuracy and control levels exhibited by the VLF or PI-based metal detectors.
How to Use A Metal Detector
Searching for buried metals and treasures using a metal detector is quite easy and simple for anyone. Upon turning on the unit, swing slowly and gently over the search area. This involves sweeping the coil frontwards, back and forth over your hunting field.
When the search coil comes across a potential target object, it emits an audible beeping signal to notify you. Some of the latest metal detectors even let you know how deep the target is located, and the type of metal target detected. Now that you are starting your detecting journey, you should check out some of my book recommendations.
So this is pretty much the overview on how metal detectors work. These detectors aren’t difficult to figure out once you take the time to get to know them and adjust them to your needs.
