While reviewing metal detectors you will likely find yourself coming across mentions of various technologies like VLF, Pulse Induction, and BFO (Beat Frequency Oscillation). These various labels describe the type of technology leveraged by the detector to send, receive, and interpret electromagnetic signals.
To better learn how to properly select a metal detector, I thought it would be best to have a deeper look at the most common technologies seen in the market. Below you will find a breakdown of each technology, along with a final comparison between the three.
Very Low Frequency (VLF) Metal Detectors
How does a VLF Metal Detector Operate?
Transmitting and receiving coils
Very Low Frequency (VLF) metal detectors operate using two distinct coils: a transmitter coil and a receiver coil.
The transmitter coil generates a continuous, low-frequency magnetic field that penetrates the ground. When this magnetic field interacts with a metallic object, it induces eddy currents in the object, which in turn generates a secondary magnetic field.
The receiver coil then detects this new magnetic field and produces an electrical signal proportional to the strength of the field. This signal is then sent to the control box for interpretation and display.
Phase shift detection
VLF metal detectors rely on the concept of phase shift detection to discriminate between different types of metals. Phase shift refers to the difference between the transmitted and received signals.
This phase shift is influenced by the conductivity and magnetic permeability of the target metal, allowing the VLF detector to distinguish between ferrous and non-ferrous metals. By analyzing the phase shift, VLF detectors can provide information about the probable composition and size of the object.
Advantages of VLF metal detectors
One of the most notable advantages of VLF metal detectors is their improved ability to discriminate between various types of metals. This discrimination capability allows users to ignore unwanted targets, such as iron or steel, and focus on more valuable items like gold, silver, or copper.
VLF metal detectors typically come equipped with advanced electronics that can provide additional information about the detected target, such as its depth and size.
Some VLF detectors even have built-in visual or audio indicators that help users identify the type of metal detected, further enhancing the user experience and increasing the chances of successful finds.
Disadvantages of VLF metal detectors
Sensitivity to ground mineralization
VLF metal detectors are more sensitive to ground mineralization than PI Detectors, which can lead to false signals and decreased performance in highly mineralized areas. Ground mineralization can cause the detector to produce signals for non-metallic objects, like minerals or wet saltwater sand, making it challenging to identify actual metal targets.
To offset this disadvantage, users may need to opt for a metal detector with ground balance capabilities. This allows for more optimal performance in various ground conditions.
Limited depth detection
While VLF detectors are effective at finding metal objects near the surface, they may struggle to locate deeper targets. The strength of the magnetic field generated by the transmitter coil decreases with depth, limiting the detection range of VLF metal detectors.
Pulse Induction (PI) Metal Detectors
How does a Pulse Induction (PI) Metal Detector Operate?
Pulse generation and decay
Pulse Induction (PI) metal detectors operate by generating a series of short, powerful electromagnetic pulses instead of a continuous magnetic field. These pulses are transmitted into the ground using a single coil, which also serves as the receiver.
When the pulse encounters a metallic object, it induces eddy currents in the target, creating a secondary magnetic field that decays over time. The decay rate is dependent on the target’s conductivity and magnetic permeability.
Time delay measurement
The PI metal detector measures the time delay between the transmitted pulse and the received decay signal. This time delay information is used to determine the presence of a metallic object and estimate its depth.
Since the decay rate of the secondary magnetic field is primarily affected by the target’s properties, PI detectors can detect various types of metals, including those with low conductivity.
Advantages of PI metal detectors
One of the primary advantages of PI metal detectors is their superior depth penetration compared to VLF detectors. The powerful electromagnetic pulses allow PI detectors to locate targets at greater depths.
This depth advantage can be particularly useful in areas where valuable objects have been covered by layers of soil, rock, or sediment over time.
Insensitivity to ground mineralization
PI metal detectors are generally less sensitive to ground mineralization than VLF detectors. This insensitivity allows them to maintain their performance in highly mineralized areas or environments with high salt content, such as beaches or salt flats. As a result, PI detectors can be used in a broader range of conditions and locations without requiring frequent adjustments to ground balance settings.
Disadvantages of PI metal detectors
Lack of discrimination capabilities
A significant disadvantage of PI metal detectors is their limited ability to discriminate between different types of metals. Due to the nature of the pulse-based detection system, PI detectors struggle to differentiate between ferrous and non-ferrous metals, making it challenging to avoid unwanted targets. This limitation can be particularly frustrating when you are in a high trash area wanting to avoid iron targets.
Higher power consumption
PI metal detectors generally consume more power than VLF detectors due to the high-energy pulses they generate. This increased power consumption can lead to shorter battery life and increased operating costs. Users of PI detectors may need to carry additional batteries or invest in high-capacity rechargeable battery systems to ensure sufficient power during extended detecting sessions.
Beat Frequency Oscillation (BFO) Metal Detectors
How does a Beat Frequency Oscillation (BFO) Metal Detector Operate?
Radio frequency oscillator circuits
Beat Frequency Oscillation (BFO) metal detectors use two separate radio frequency oscillator circuits to detect the presence of metallic objects. One oscillator, called the reference oscillator, maintains a constant frequency, while the other, known as the search oscillator, is connected to the search coil.
When the search coil encounters a metallic object, the object’s presence affects the magnetic field and causes a change in the search oscillator’s frequency.
Beat frequency detection
The frequencies produced by the reference and search oscillators are combined in a mixer circuit, generating an audible beat frequency that corresponds to the difference between the two oscillator frequencies.
When a metallic object is detected, the change in the search oscillator’s frequency causes the beat frequency to change, alerting the user of the target.
Advantages of BFO metal detectors
Simplicity and Cost
One of the main advantages of BFO metal detectors is their simplicity and affordability. Due to their straightforward design and fewer components, BFO detectors are generally less expensive than VLF or PI detectors.
It is important to note that BFO metal detectors are no longer produced by most brands. They are relatively easy to find on online marketplaces such as ebay or local thrift stores and peddlers’ malls.
Ease of use
BFO metal detectors are relatively easy to use, with minimal adjustments and settings required. Their simple design allows users to start detecting with little to no experience or knowledge of metal detecting techniques.
Disadvantages of BFO metal detectors
Limited depth and sensitivity
BFO metal detectors are generally less sensitive and have a more limited detection depth compared to VLF or PI detectors. Their reduced sensitivity can make it challenging to locate smaller or deeply buried targets, which may be frustrating for users searching for specific objects or valuable items.
Poor discrimination capabilities
Another disadvantage of BFO metal detectors is their limited ability to discriminate between different types of metals. Unlike VLF detectors, BFO detectors cannot accurately distinguish between ferrous and non-ferrous metals, making it difficult for users to ignore unwanted targets such as iron or steel.
Comparing the Different Types of Metal Detectors
When comparing the detection depth of the three types of metal detectors, Pulse Induction (PI) detectors generally offer the greatest depth penetration, making them ideal for locating deeply buried targets.
Very Low Frequency (VLF) detectors provide moderate depth capabilities and are suitable for detecting objects near the surface.
Beat Frequency Oscillation (BFO) detectors, on the other hand, have the most limited depth range, making them less effective for finding deep or small targets.
In terms of discrimination capabilities, VLF detectors stand out as the most effective at differentiating between various types of metals, allowing users to focus on specific targets and ignore unwanted objects.
PI detectors have limited discrimination abilities, which can make it challenging to avoid undesirable targets during detection.
BFO detectors have the poorest discrimination capabilities, making it difficult for users to distinguish between valuable and non-valuable items.
For hobbyists and casual users, VLF detectors are often the preferred choice due to their discrimination capabilities, target identification features, and ease of use.
BFO detectors can be suitable for beginners or those on a tight budget, but their limited depth and discrimination may hinder the user’s overall experience.
PI detectors are typically more popular among experienced treasure hunters with a specific focus or those searching for deeply buried items or natural gold deposits.
Cost and ease of use
VLF detectors are moderately priced and offer a good balance between performance and cost.
PI detectors are typically the most expensive, but their superior depth capabilities and insensitivity to ground mineralization can make them worth the investment for specific applications or experienced users.
While BFO detectors are generally the most affordable option, making them attractive for beginners or those on a budget, modern options are rare and they need to be purchased through second-hand marketplaces.
Maintenance and operation
In terms of ease of use, BFO detectors are the simplest to operate, with minimal adjustments and settings required.
VLF detectors may require a little more setup and calibration but are still user-friendly, with advanced quality-of-life features that can significantly enhance your experience.
PI detectors can be more complex to use and require a better understanding of metal detecting techniques, but their performance benefits may outweigh the learning curve for some users. Additionally, PI detectors may have higher power consumption, which can result in increased operating costs or the need for additional batteries during extended detecting sessions.
Final Thoughts on the various types of metal detectors
Each type of detector operates on a unique system and offers distinct advantages and disadvantages. VLF detectors excel in discrimination capabilities and target identification but can be sensitive to ground mineralization and have limited depth detection. PI detectors provide superior depth penetration and insensitivity to ground mineralization but lack discrimination capabilities and consume more power. BFO detectors are simple, affordable, and easy to use but offer limited depth and sensitivity, along with poor discrimination capabilities.
New metal detectorists should consider their specific needs, budget, and intended applications when choosing a metal detector type. For those seeking a well-rounded, user-friendly detector with good discrimination capabilities, a VLF detector is the best choice. For experienced users or those with specialized requirements, such as deep target detection or operation in highly mineralized environments, PI detectors may be worth the investment.