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► Target ID read-outs, such as the VDI numeric display, is based on a combination of many things, such as the metal object's:
• Size
• Shape
• Orientation to the search coil.
• Depth or distance from the coil and the stronger or more intense portion of the EMF.
• Conductivity.
► And we can add to that the following Detector and Coil considerations:
• Detector Used
• Search Coil Used
• Settings Used, to include Sensitivity, Discrimination, Ground Balance, and functions related to Recovery Speed, Iron handling, and so forth.
• Sweep Speed or Search Coil Presentation.
• Search Coil Type, such as Concentric or Double-D internal winding.
• Operating Frequency
► And we shouldn't ignore any outside effects we have no control over, to include:
• Mineralization Level
• Ground or Soil make-up, such as consistently uniform to a good depth or layered, and then the differences between layers.
• Undisturbed Ground for a good period, or Disturbed Ground, such as very loose material or plowed / tilled ground, etc.
• Moisture Content: Dry, Damp, Wet, and to what depth, or even Saturated ground down several inches.
Factor all these things together and I can guarantee you that there is NO detector, regardless of what a price tag it has, that will ever be 100% accurate. In reality, you will find that for 'common' or 'average' day-to-day Coin Hunting in typical parks, schools or yards, and when going after coins that are located in the upper surface to 4" depth range (where probably 90% of them are) most detectors can run from being about 15% accurate to perhaps 85% accurate on most US coins. And I emphasize USA coins because too many foreign countries use nickel, steel or some other magnetic-based ferrous material to make their coins so they won't be as nice-and-proper consistent like an air-tested US coin.
Also, I am referring to any detector's potential 'correctness' to identify a US Coin. The problem, as related in the first set of cautions, is 'Conductivity' and there are so many pieces of objects or whole objects that are lost or discarded that share a similar 'Conductivity' to a US Coin that it becomes almost impossible for the operator and detector to agree on what is likely a good or bad target, A VDI read-out suggests an objects Visual Discrimination Indication of what it might potentially be. Quite often, in less favorable environments, the 'VD' portion might just stand for Very Discouraging.
Best advice from me, and I've only been at it for over half-a-century, is BE PATIENT! Learn the detector, listen to the audio response, hunt as often as you can and recover as many targets as you can. When you do, relate the audio and visual experience to what was actually recovered. Results will be site-dependent, but one thing I'd discourage is trying to put full faith in any blasted Target ID display read-out. It's only a tool, not the final decision maker.
Monte