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Any Familiar With Induction Balance?

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How does IB detectors compare to VLF and PI's? Worth trying to build one or just a waste of time?

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VLF's are induction balance detectors.  IB is general; VLF originally meant specifically 3kHz to 30 kHz IB detector, but it's gotten sloppy since then.  As a result you can think of IB and VLF as synonyms in today's metal detecting world.

If you really like building things, then you can have fun building a detector.  If you really like detecting then you're probably better off buying one (used or new).



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Here is a summary I posted back in 2002 that must have been ok because it got quoted in at least one patent I have seen.

Hi Guys,

Here is something I wrote in response to a question on the Alaska Gold Forum. It is of course just my take of what I've read in lots of places, including this forum. I'd like to use it on my website, but I'm asking for your expert advice on anything I may have oversimplified or am simply wrong on. The main goal was to try and keep it simple, however.

Here it is:

What follows is a layman's description of the technologies involved. I'm trying to do this in as few words as possible while hopefully imparting some of the general concepts accurately. 

There are two types of detector technology currently available for hobby use. Today's metal detectors are basically radio transmitter and receiver devices. 

The first, Induction Balance (IB) technology, relies on a transmitter and receiver coil operating simultaneously in a state of electrical balance. Any conductive or magnetic object entering the field disturbs the balance and generates a signal. Plus, eddy currents are induced into conductive targets creating a detectable signal. 

Most people refer to these as VLF detectors, but all VLF means is Very Low Frequency and refers to a particular subset of IB detectors. Some detectors referred to as VLF today are actually operating in the LF (Low Frequency) range. Induction Balance is more accurate as it refers to the technology, not just a frequency range. Older style T/R (transmitter/receiver) detectors are also IB units. 

The second, Pulse Induction technology, has been around a long time and every one of the major manufacturers makes a PI machine. There is the Fisher Impulse, Garrett Sea Hunter XL500 Pulse, Tesoro Sand Shark, and White's Surf PI. These are all units designed for use in saltwater/black sand beach environments. 

Simple Pulse Induction units use a single coil to alternately transmit and receive. The transmitter segment creates an electromagnetic field that induces eddy currents in conductive targets. These eddy currents actually take some time to ''decay'' and so when the coil switches to the receive mode it picks up the weak eddy currents just induced in the transmit mode. 

Induction balance units constantly bathe the search area in an electromagnetic field. Smaller items can be detected because of this, but the IB units also detect more ground mineral. If there is too much ground mineral it makes it hard for the machine to detect the target. IB detectors have superior target id capabilities as they use a combination of the eddy currents and the electrical imbalance in the search field as information to identify targets. 

Pulse Induction units use eddy currents only. When the transmitter shuts off the electromagnetic field collapses and so there is only the weak eddy currents to work with. The good news is that eddy currents flow much longer in metals and for a much shorter period of time in ground minerals and salt water. A delay between the transmit and receive modes allows the eddy currents in low conductive targets to decay enough they will not be detected. The stronger eddy currents in more conductive items is detected. 

This makes most saltwater and certain mineral environments almost invisible to PI detectors. It also means very small targets are harder to detect as they cannot hold an eddy current for long. Since PI units have less information to work with than IB devices they have little or no discrimination capability by comparison. 

Although common PI units do better with certain mineral environments that IB units they still can be affected by mineralization. By using multiple pulse periods and using the information gained from some of the pulses to modify the others, Minelab developed ground balancing pulse induction technology. The called this MPS for ''multi-period sensing''. This allows the Minelab units to work in mineralized soils other PI units would have problems with. This in turn also allows for higher powered units with better sensitivity to small gold. And so the Minelab SD/GP series was born. 

The Explorer, Fisher CZ series, and White's DFX are all Induction Balance units but are working with information received from multiple frequencies. A very rough generality is that low frequencies detect larger objects better at deeper depths. Higher frequencies tend to hit smaller items better at shallower depths. Multi-frequency units attempt to use information received from multiple frequencies to achieve superior detection characteristics including better target id. Most of what multi-frequency is about is how many frequencies are received and compared, not how many are transmitted. 

But the key thing is that there is always a signal being transmitted and received simultaneously. The DFX is my favorite example. It can be run in 3kHz only or it can be run in 15kHz only. Or it can be run at both 3kHz and 15kHz simultaneously. Multi-frequency technology is just another type of IB technology, and they are not PI units. 

Steve Herschbach

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The website Reference Library has many free books that go into detail about how and why metal detectors work. A good place to begin is How Metal Detectors Work by Mark Rowan & William Lahr - Originally published by White's Electronics as a booklet P/N 621-0395. Basic but rather technical information on how induction balance and pulse induction metal detectors work.

The key for people who want to make a metal detector is the Geotech website and forum. This page has do it yourself projects.

You can easily make a simple metal detector. Making anything I would want to use would take a skilled person. I am not but I think I could tackle it so I am sure you could, but I am not into that. I like to use what people lots smarter than I can make! :smile:

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That's what I thought but was confused when I was looking at different circuit types and some had frequencies 150 and way higher.

I always tinker around here, have all sorts of things I have made, some need patents when I get the extra $$$. I wanted to make a nice waterproof analog machine with switchable frequency ie 2.4 to 16 or so, what ever divides out and can use the same coil with minimal fuss. I thought there might be a market though small, for a simple manually operated machine that can be brought to the park, corn fields or the beach. I see all the digital machines out there with all sorts of beeps, whistles, backlight displays because you can't run one well without looking and the bigger price tags they carry but they tend to lose the smoothness in tones that the analog have. The new machines kind of take the skill factor out of detecting and to me takes away from the hunt. I like a little bit of mystery when I swing my detector and come across a nice clean signal. Having just a hunch that it might be a nickel or maybe, just maybe a gold ring seems to be more rewarding than reading an lcd panel with big words it's a nickel.

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16 hours ago, kac said:

I always tinker around here....

You need to get the following book (by the Geotech founders):


It includes DiY detectors, but it's much more than that.

As far as analog detectors, they already exist since that was the technology in use in the early years (prior to the 90's, anyway).  You can find all of them on Ebay to your heart's content.  Although not all purely analog, the Tesoro detectors are the 21st Century version (throwback) of the early analog detectors.

This isn't meant to discourage you from building your own analog metal detector.  Just giving you options & research material.


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I have a Tesoro, love the machine but it isn't waterproof. I will look at the costs to build one and the time, might not be worth the aggravation.

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