Download Metal Detector PDF

TitleMetal Detector
TagsElectric Current Inductor Detector (Radio) Metal Detector Ferromagnetism
File Size188.7 KB
Total Pages13
Document Text Contents
Page 2


(By Mark Rowan & William Lahr)

Metal detectors are fascination machines. Many of the people who use them are

just as enthusiastic about extolling the virtues of their favorite metal detector as

they are about setting off in search of buried treasure. Those of us who design and

build these instruments for a living listen carefully when one of our customers talks

about his or her experience in the field, because this is the primary means by which

we determine how well we are doing our jobs, and what sort of things we need to

do better. Sometimes though, communication is difficult. Almost as though we and

our customers speak different languages. Which in a sense, we do. The purpose of

this page is to try to narrow that communication gap a little. And, to resolve some

of that "typical curiosity" metal detector operators have regarding what is going on

inside their instruments.

Is it necessary to know how a metal detector works in order to use it effectively?

Absolutely not. Will knowing how it works help someone to use it more

effectively in the future? Quite possibly yes, but only with persistence and practice.

The best metal detector available is still only as good as the person using it.

VLF (Very Low Frequency) Transmitter & Receiver


Inside the metal detector's loop (sometimes called a search head, coil, antenna,

etc.) is a coil of wire called the transmit coil. Electronic current is driven through

the coil to create an electromagnetic field. The direction of the current flow is

reversed several thousand times every second; the transmit frequency "operating

frequency" refers to the number of times per second that the current flow goes

from clockwise to counterclockwise and back to clockwise again.

When the current flows in a given direction, a magnetic field is produced whose

polarity (like the north and south poles of a magnet) points into the ground; when

the current flow is reversed, the field's polarity points out of the ground. Any

metallic (or other electrically conductive) object which happens to be nearby will

have a flow of current induced inside of it by the influence of the changing

magnetic field, in much the same way that an electric generator produces

electricity by moving a coil of wire inside a fixed magnetic field. This current flow

inside a metal object in turn produces its own magnetic field, with a polarity that

tends to be pointed opposite to the transmit field.

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Most PI detectors are manually tuned. This means the operator has to adjust a

control until a clicking or buzzing sound is heard in the headphones. If the search

conditions change, such as when moving from black sand to neutral sand or from

dry land to salt water, the tuning must be re-adjusted. Failure to do so can result in

reduced detection depth and missed targets. Manual tuning is very difficult with

short integration time constants, so most manually tuned models use long time

constants and the search coil must be swept slowly.

This is not a problem when a PI is used for scuba diving because the coil cannot be

swept quickly underwater. When used at the surf line, where the coil will be in and

out of salt water, a manually tuned metal detector can be very frustrating to use.

The tuner must be adjusted continually to maintain a threshold. Some operators

elect to set it slightly below the threshold however, that can result in a reduction in

depth as the ground conditions change.

Automatic tuning, or S.A.T. (Self Adjusting Threshold) offers a significant

advantage when searching in and out of salt water or over mineralized ground.

S.A.T. helps keep the metal detector operating at maximum sensitivity without

requiring constant adjustments by the operator. It improves the stability, reduces

noise, and allows higher gain settings to be used. PI metal detectors do not emit

strong, negative signals like a VLF. As such they do not "overshoot" on pockets of

mineralization. With S.A.T. the coil must be kept in motion while detecting a

target. Stopping over a target will cause the S.A.T. to tune it out or cease


Audio Circuits

PI audio circuits generally fall into two categories: variable pitch and variable

volume. Variable pitch or V.C.O. (Voltage Controlled Oscillator) audio has the

advantage for faint targets because the change in pitch is easier to hear than a

change in volume at lower aud io levels. This is primarily true for manually tuned

models. The "fire siren" sounds can become annoying and many have trouble

hearing the higher tones. A variant of this is the mechanical vibrator device

primarily used for deep water. It emits a slow clicking sound and vibration that

increases to a buzz to signal a find. The mechanical device is easier to hear and feel

over the sound of an underwater air supply.

Many people prefer a more conventional audio tone that increases in volume rather

than pitch to signal a find. This audio system works best with a PI metal detector

that has a fast target response and automatic tuning (S.A.T.). Automatic tuning

makes the PI sound and respond similar to a typical VLF metal detector.

PI Summary Pulse Induction metal detectors are specialized instruments. They are

generally not suitable for coin hunting urban areas because they do not have the

ability to identify or reject ferrous (iron) trash. They can be used for relic hunting

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