About Pulse Induction type metal detectors (draft)

I am an avid water, sand and dirt treasure hunter and I have 4 metal detectors with various coils for each one.

• Garrett At Pro VLF 15khz
• Garrett GTA350 VLF 7.5khz
• Fisher 1280-X VLF 2.5 khz
• Garrett Infinium Pulse Induction

I have found in my travels, different environments can render 3 of my detectors useless if not at least totally aggravating to try to use while looking for certain types of objects. So with these 4 units, I always have one that is best for the ground conditions and what I am looking for. Before I had them all, I was consistently frustrated when I did not have the right unit for the area.

Now when they start making a VLF where you can program your own frequency and it is waterproof, maybe I will sell all my VLFs and buy it. Then I want them to combine VLF with PI, I will then only need that 1 magic metal detector!.

As for programmable frequency VLFs, we really should have had one by now that costs under $1000, the technology has been here long enough. By programmable I mean letting me select almost any frequency I want or multiple frequencies at the same time and be able to save and tweak them.

Anyway getting back to Pulse Induction units. Every metal detector enthusiast should have an API Pulse Induction unit if they will be hunting near or in salt water or old homesteads. Since the APIs (Advanced Pulse Induction) will give a unique target signal for Silver and copper (a low-to-high pitch beep) and if you dig only those signals, it can be used to find deeply buried Silver, Copper and clad coins anywhere and it will go deeper than most all VLFs finding these items. Since you will not be digging high-low signals, you will not be digging much garbage.

There are other brands of PIs that have this feature and I think we are calling them all APIs at this point. While this feature is not particularly useful in the water at the beach, it is a game changer for PIs in the sand or dirt. You will just have to deal with lots of garbage when looking for gold. (sound familiar?) may as well get out your VLF for that!

From what I understand, basically the fundamental difference between VLF and Pulse Induction (PI) is:

• VLF units transmit a frequency energy pulse, then gives target information to the user based on the attributes of the received signal that bounces off the metal object.
• Pulse Induction units transmit a frequency energy pulse, then gives target information to the user based on the attributes of the received signal that the metal object itself transmits after receiving the energy pulse.

These 2 types of metal detector circuits each can do things the other cannot.

Pulse Induction Metal Detectors transmit a series of quick electronic pulses in to the ground. These electronic currents are not affected by wet salt sand and ground minerals (like VLFs that give false target signals, little relative depth and require constant ground balancing). A Pulse detector is best used for salt water beaches (near the water) and diving. The Pulse detector is very deep seeking, and is great for tough ground mineral conditions but has limited discrimination capabilities, so you will need to dig more trash items if you want to locate the maximum number of precious objects.

Here is a small list of some Pulse Induction underwater metal detectors:

• Garrett Infinium LS
• Garrett Sea Hunter Mark II
• Fisher Impulse
• DetectorPro Headhunter Pulse
• JW Fisher Pulse 8X
• Tesoro Sand Shark

About Pulse Induction Theory (cached from somewhere on the net)

All types of metal locators are "electromagnetic" in nature, and share a certain amount in common: the search head contains one or more coils carrying a time-varying electric current, and this generates a time-varying magnetic field which propagates towards the metal target (and in other directions as well of course). This primary field reacts with the electrical and/or magnetic properties of the target which responds to it by either modifying the primary field or, as a more general and more accurate description, generating a secondary magnetic field; one way or another, the effect links back into the coils in the search head (sometimes the same coil as the transmitting one, sometimes a different one), and induces an electrical voltage in the receiver coil(s). Beyond this basic similarity, there are a wide range of different variations used: in the number of coils (one, two or three); the "shape" (spatial extent) of the primary magnetic field; the frequency of the transmitter; the waveform transmitted (sinusoidal or pulsed); the dominant target property responded to (magnetic permeability or electrical conductivity); whether the head coils(s) have a magnetic core or are air-cored; and how the electronics separate the (very weak) received voltage out from the (potentially much larger) voltages present in the search coils even in the absence of any metal target. Although all these factors can affect the sensitivity to any one particular target, the last factor is probably the most important, as it determines the stability or "zero-drift" of the instrument:-- if the zero-point is unstable, high sensitivity will never be achieved, however much the other factors are optimized.

Pulse Induction.




If your browser supports animated GIFs, you should see below a "movie" of the Pulse Induction process locating a steel bar (This is in slow-motion: things actually happen 5000 times faster!)

A pulse of current is sent (repeatedly) through a coil in the search head. This current tends to start up fairly gently (and is allowed to do so); see figure 1a. However, at the end of the pulse, it is arranged that the current turns off very rapidly (within a few microseconds); this (briefly) induces a very large "voltage spike" or "back-e.m.f" across the coil (rather like the induction coil used to generate the spark for a car engine ignition, though in this case the voltage is only(!) about 100 volts); see figure 1b. After the mayhem of this transient is over, there is no current flow through the coil and no voltage across it. After about a millisecond (or less or more, depending on the particular model) the whole cycle is repeated. The primary (or transmitted) magnetic field will vary with time exactly in step with the figure 1a current waveform, and propagates (rapidly -- at the speed of light) down to and through the target. When the pulse is switched off, and if the target is a conductor, eddy currents are induced to flow in the target. These eddy currents always flow in such a direction as to try to re-create the magnetic field that has just disappeared, and, initially at least, they actually succeed in this; but once the primary field has all gone, there is no source of energy to maintain these currents, so they decay gently away -- nevertheless persisting for about a hundred microseconds; see figure 1c.



The eddy currents generate a secondary magnetic field which propagates in all directions, including back towards the search head, where it induces a (small) voltage in the coil; this voltage also decays away at the same rate (see figure 1d), and has the same sign (polarity) as the back-emf spike. The received voltage from a target at the limit of the detection range may only be a few micro volts: one ten-millionth of the back-emf spike! It would be quite out of the question for the electronics to notice such a tiny change actually during the back-emf spike, and that is not the way it's done. The signal is "sampled" by an electronic switch which ignores the signal during the transmit pulse and immediately after (during the back-emf), and only "looks at" the signal after a short delay which ensures that the switch-off transient is over (see figure 1e). In this way, the transmitted and received signals are separated from each other. If the target had been purely magnetic, but non-conductive, it would have become magnetized by the transmit pulse, and then de-magnetize just as promptly at switch-off; by the time of the delayed sample pulse, nothing would be happening down at the target, and therefore nothing would be happening up at the search coil. If the target is both conductive and magnetic (eg a ferrous metal), the eddy currents would be produced exactly as in the purely conductive case; the effect of the target's magnetic permeability is to enhance the magnitude of the effect (and also to modify the "time-constant" of the decay of the eddy currents). If there is no target at all . . . . . . . . nothing happens! Actually, there will always be a certain inescapable amount of electrical "noise" in the receiver coil and circuitry, and three techniques are used to filter this out to produce a final signal (in the absence of a target) which is very close to zero and absolutely rock-steady. The decay time-constant (persistence) of the eddy-currents, and hence received signal, depends (predominantly) on the target's electrical conductivity and size. Targets such as low-conductivity alloys or thin foils have a very short decay time; and the choice of a short or long delay between switch-off and sample can be arranged to either detect or ignore such targets. The ionic conductivity of sea- or brackish water is so low, and its decay time so short, that such signals have always decayed away before the sample is taken; so the P.I. technique is not affected by moisture.