There are two major types of metal detector sensing technologies. VLF and PI. Here are the differences, along with their pros and cons.
VLF stands for very low frequency and PI stands for pulse induction. VLF technology is found in most metal detectors. The VLF transmitter and receiver coils measure the phase shift between transmitted and returned frequencies. PI detectors send pulses of current through their coil(s) and measure the time length difference between the expected reflected pulse and the actual reflected pulse.
More detail on that in a second, plus the truly practical point of it all. First I wanted to mention that there’s a third technology called BFO, which stands for beat frequency oscillator. BFO technology is found in the very basic, low cost metal detectors that generally don’t have any discrimination circuitry. This means they’ll alert on any/all metal in the ground. I just wanted you to know that, since this article will only cover the two most prevalent types of detectors: VLF and PI.
How Do PI Metal Detectors Work?
Metal targets are repeatedly hit by magnetic pulses from the detector. PI metal detectors put out about 100 pulses per second, but there can be less or more depending on the model. This short duration pulsating transmission causes relatively longer response signals to comeback from a metal target than if there were no target. This is seen as a form of echo to the detector, which results in the audio alert you hear.
Buckle up – this is a longer version of that:
PI metal detectors sometimes use one coil to transmit and receive, sometimes have two or three coils that work together. Nevertheless, the detector delivers a short pulse of current into the coil(s) which causes a magnetic field to emanate from the detector coil(s) into the ground. When this short pulse shuts off, the transmitted magnetic field quickly collapses. This collapse creates a return current (back-emf) in the coil as a “reflected” pulse.” This whole process repeats itself hundreds of time a second.
Assuming there is no target in the transmitted magnetic field, a sort of steady-state relationship exists between the duration of sent and received pulses.
However, if there’s a target in the ground, the transmitted pulse creates an opposing magnetic field (eddy current) in the object. When the transmitted pulse’s magnetic field collapses (resulting in the reflected pulse), the opposing magnetic field of the target object makes the reflected pulse take longer to completely disappear. So the detector sees it as a longer-duration reflected pulse than usual.
During each repeated pulse cycle, this time difference is measured by the metal detector’s circuitry, compared to the steady state “expected length”, and a target alert occurs when the difference meets specified thresholds.
Interesting fact: a German engineer named Claus Colani invented the modern pulse induction principle and even had a “pulse patent” for it in 1961!
How Do VLF Metal Detectors Work?
As the name indicates, these detectors use very low frequency coils (generally below 30 kHz.) The outer coil serves as the transmitter and the inner coil serves as a receiver. The VLF detector produces alternating current to induce a magnetic field in the transmitter coil.
This field, when striking a metal target, sets up small “eddy currents” in the target. The detector’s receiver coil then reads the eddy current’s magnetic field and the detector’s control box electronics sense the phase shift in the reflected field. It interprets that difference as a target object and produces an alert audio tone. The size of the phase shift helps in the classification of target type.
GEEK OUT: Eddy currents are cool, check here for some deep science. It’s amazing the way a relatively simple thing like the eddy current can be adopted into practical use like our metal detectors.
Difference Between PI and VLF Detectors
|Best for the average user||Not effected by mineralization, so best choice for mineralized soil and salt water|
|Well designed discrimination features||Detects targets at greater depth|
|Wide price range depending on features||Best technology for gold detection|
|Typically used for coins, jewelry, relics, and other metals||Very expensive, tend to be heavier than VLF models|
|Lightweight||No discrimination mode, will alert on every metal target; not great in urbanized areas|
|Most versatile metal detector type||Can be impacted by Earth’s magnetic field, may alert at the end of each swing|
Electricity and Magnetism Working Together – “It’s Science!”
You read here about electric frequencies and pulses and eddy currents and collapsing fields.
Here’s why that relates to metal detecting:
Electricity and magnetism go hand in hand. Magnetic fields can generate electric fields and vice-versa.
- That is a simple summary of Ampere-Maxwell’s law which states that an electric field that is changing with time produces a magnetic field.
When a magnetic field moves across a metal target, it induces an opposing electric current in the target. These are referred to as “eddy currents” and, since they are currents of electrons moving in metal, they produce a magnetic field.
The detector’s receiving coil can then sense this magnetic field as it moves through it. This in turn causes an electric current to flow in the coil. This current travels to the control box and through the detector’s circuitry, setting off the target alert tone.