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�                 � METAL DETECTORS - HOW DO THEY WORK? (4).                                   Heavy Black Sand                                   Heavy black sand is an exception. It will cause a VLF coil to overload,                                   making metal detector penetration poor at best. A PI detector will work in                                  black sand however, some false signals may result if the coil is held very                                  close to the ground. Ground responses can be minimised by using a longer                                  time delay between the shut-off and sample point (pulse delay). Advancing                                  the time delay slightly will help to smooth out the noises caused by most                                  mineralization.                                 �Automatic vs. Manual Tuning                                  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                                  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 mineralised                                  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 responding.                                 �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 specialised 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 in rural areas where iron trash is not present in                                  large quantities, or is desired. They are intended for maximum depth under                                  extreme search conditions such as salt water beaches and highly mineralised                                  ground. In such conditions PI type metal detectors produce superior results                                  when compared to VLF models, particularly in the ability to ignore such                                  extreme ground and penetrate it for maximum depth.

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