Metal detectors are electronic devices that detect metal objects buried underground or hidden within other things. They have become a popular tool for treasure hunters, archaeologists, and security personnel. I also once owned a metal detector from the Soviet era, which we used to search for remnants of war on the battlefields of the Second World War. But have you ever wondered how does a metal detector work? In this blog post, we will briefly examine how metal detectors work and the technology behind them.
A metal detector is essentially a specialized type of radio that is designed to detect metal objects. It generates an electromagnetic field that induces an electric current in nearby metal objects. This electric current produces a magnetic field, which the metal detector can detect and interpret.
Main parts of a metal detector
Several components make up a metal detector. The main parts of the metal detector are the control box, the search coil, and the shaft. The control box contains the electronic circuitry that generates the magnetic field and interprets the signals received from the search coil. The search coil is part of the metal detector that detects metal objects. It is typically a round, flat coil connected to the control box via a cable. The shaft is part of the metal detector that connects the control box and the search coil. It is typically adjustable in length to accommodate different user heights.
How does a metal detector work and detect the presence of a metallic object?
The search coil of a metal detector consists of two different coils - the transmitter coil, and the receiver coil. When the metal detector is turned on, the transmitter coil carries alternating current, generating a magnetic field around it. This magnetic field extends out from the transmitter coil and into the ground. When the search coil passes over a metal object, such as a coin or a piece of jewelry, eddy currents are induced in the metal. Eddy currents are circular electrical currents that are created within a conductor when it is placed in a changing magnetic field. Eddy currents induced in the metal produce a secondary magnetic field, which opposes the original magnetic field generated by the transmitter coil. This causes a decrease in the amplitude of the original magnetic field, which is detected by the receiver coil in the metal detector.
The receiver coil then sends a signal to the control box, which analyzes the signal and determines whether it is coming from a metal object or something else, such as mineralization in the soil. If the signal is determined to be coming from a metal object, the control box emits an audible signal to alert the user that a metal object has been detected.
How metal detector distinguishes between different metals
Different metals interact differently with the magnetic field produced by the transmitter coil due to differences in their electrical conductivity and magnetic permeability. This enables the detector to distinguish between various metal types based on their characteristics. For instance, metals with high electrical conductivity, such as silver and copper, are good conductors of the magnetic field and produce a strong signal that the metal detector can detect. In contrast, metals like iron and nickel have a larger magnetic permeability than they have electrical conductivity, making them more amenable to magnetization. Because of this, the signal is weaker but still distinguishable from the signal produced by more conductive metals.
The detector's electronics then analyze the characteristics of the signal to determine the type of metal detected and can provide an indication to the user based on pre-programmed target identification settings or audio/visual signals.
Metal detector operating frequency
Single-frequency metal detectors use one frequency to detect metal objects. These detectors are usually less expensive and simpler to use than multi-frequency detectors. They work well in certain soil conditions and for certain types of metal detecting, such as coin shooting, but they can struggle in highly mineralized soil or when trying to detect smaller or deeper targets.
Multi-frequency metal detectors, on the other hand, use multiple frequencies to detect metal objects. These detectors are more advanced and can operate at different frequencies simultaneously or switch between frequencies as needed. They are typically more expensive than single-frequency detectors, but they offer greater versatility in different soil conditions and can detect a wider range of metal targets, including smaller or deeper targets. Multi-frequency metal detectors are preferred by treasure hunters, archaeologists, and hobbyists who want more advanced features and capabilities in their metal detecting equipment.
While shorter electromagnetic waves produced by higher frequencies are less effective at penetrating the ground, they are better at picking up tiny metal objects at shallower depths. Lower frequencies can detect larger metal items at greater depths because their electromagnetic waves are longer and can travel further underground. They might not be as sensitive to smaller items, though.
Metal detector detection depth
The detection depth is affected by several variables. The most obvious ones are probably object size and composition and metal detector quality and technology.
The detecting depth of entry-level metal detectors, which are suitable for beginners, ranges usually from 60-90 cm (2-3 feet) for big objects and roughly 15-20 cm (6-8 inches) for smaller objects like coins and jewelry.
The detecting depth of mid-range metal detectors is greater than that of entry-level models, ranging from 25 to 30 cm (10 to 12 inches) for tiny objects to up to 1.5 meters (3-5 feet) for bigger ones.
Professional-grade metal detectors are the most advanced types of metal detectors and have a detection depth of up to 30 cm (12 inches) or more for small objects like coins and jewelry and several meters (10 feet or more) for larger objects.
The actual depth that a metal detector can detect will depend on a number of variables, including the kind of soil, the presence of minerals or other metals, and the size and makeup of the object being detected.
Summary
In conclusion, metal detectors operate by creating a magnetic field that interacts with metal objects, causing a change in the magnetic field that is detected by the device. The most important parameters to consider when selecting a metal detector include the frequency of the device, the size and shape of the search coil, the sensitivity to different metals, and the discrimination capabilities. Understanding how these parameters affect the performance of a metal detector can help you choose the right device for your needs and increase your chances of finding valuable metal objects.