PADI's Advanced Scuba Course
should be seriously considered by every certified diver. The various
components are designed to extend the knowledge and experience that an Open-Water
Scuba diver has. At Deep-Six it is an inexpensive way to continue diving,
as well as a way for a diver that has not been diving for a while to get
back into it.
As explained elsewhere, there are just 5 dives that have
to be completed for a diver to become Advanced. Two of them are core dives
and are required. The other 3 are elected by the diver.
The following information is used for specific dives.
A diver working on Advanced Scuba may be referred to this chapter to enhance
their knowledge about a specific dive.
SEARCH AND RECOVERY DIVE:
METAL DETECTORS:
It is quite simple to understand how a metal locator
or detector works. Most use electromagnetic fields to locate metallic objects.
Electromagnetism comes in a variety of forms such as light, x-rays, radio
waves, etc. Some electromagnetic energy is at a very low frequency such as
radio waves, and some is at a very high frequency such as gamma radiation.
There is an even lower frequency than radio waves and that is electrical
radiation. It travels at the speed of light, as do all of the types of
electromagnetism, but it should not be confused with electricity. However,
when electricity travels through a wire it emits electrical radiation. That
is why your radio emits static when you drive under a high voltage wire.
The very low frequency metal detectors (VLF Detectors)
have an outer coil of wire that has alternating electrical current traveling
through it. Electrical radiation is emitted from that coil of wire at right
angles to it. If that radiation hits a piece of metal a pulse of electrical
radiation is emitted by the metal but has an opposite polarity. A second
smaller coil in the detector detects that pulse and registers it on a meter,
with sound, or by light. Different metals change the electrical field in
different ways so some metal detectors are manufactured to tell one metal,
such as iron, from another, such as gold. They are discriminators.
The Beat Frequency Oscillators (BFO Detectors) emit higher
frequency radio waves. They also have a receiver for radio echos. Metals
change the frequency of the radio waves and the metal locator detects those
changes.
Pulse Induction Detectors: These metal detectors have
a single coil of wire. Direct current is pulsed out at rapid intervals. So,
the very low frequency electrical radiation is emitted in pulses and is distorted
by metals. This distortion is picked up between pulses and that is why only
one coil is needed. The detection depth is greater but the ability to
discriminate is usually less.
Magnetometers: These have a great detection range sometimes
measured in thousands of feet. Also, they are quite expensive when compared
to the other types of detectors. They are able to detect only iron and steel
objects. Magnetometers are able to measure the magnetic field of the earth.
That field is distorted by iron or steel. Just as a compass detects
and lines up with the earth's magnetic field, if you bring a piece of iron
near that compass it will deflect because of the change in the magnetic field.
A steel shipwreck will deflect the magnetic field of the earth and that can
be measure quite effectively by a magnetometer from quite a distance.
