which detector should i take

Same-same. They are both going to be great in the dry sand, and they will both lose sensitivity and depth in the wet sand. I would take the 705 just for giggles.

A single frequency "VLF" or very low frequency type metal detector (like yours),works extremely well on dry sand and in fresh water. It does not work as well in saltwater, or on wet ocean-water beaches. In highly conductive saltwater conditions pulse induction or "PI," and multi-frequency VLF "BBS" metal detectors excel.

Radio or "sine" waves bounce off everything that is conductive in the sand or water. This is why single-frequency VLF detectors must be "ground balanced" to work effectively in highly mineralized soil, or on highly conductive saltwater beaches. You must tune or adjust the machine to see through the "fog," or white-noise created by the salt and iron in the sand or water you are detecting. Unfortunately, this usually leads to a loss of depth and stability with most single-freq VLF detectors.

The magnetic iron sands (“Black Sands”), salt, and high concentrations of other minerals in the water and sand conspire to bounce the sine (radio) waves away from the target. Conductivity and mineralization act like a shield around the target and create white noise that must be filtered electronically. Think of it as turning on your bright headlights in a heavy fog at night. All that powerful light is diffused and causes a complete white out – you can’t see anything three-feet past the hood of your car! Your machine may "see" the wet sand as one target. So basically, your machine -may- false and chatter in the wet sand and salt water.
 

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