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Hi ! 
,
Every now and then I learn something new to me about metal detectors and how they work.
I recently became curious about the depth of a metal detector's signal penetration into coins and other metal objects buried in the ground. From my research I came to the conclusion that induction balance (IB) VLF detectors generally produce a signal that penetrates all the way through a target. However, recalling a term I had read about years ago, called "Skin effect", I wondered what affect that might have on the depth of penetration. So having a copy of George and Carl's book titled inside the METAL DETECTOR , I checked the index and found skin effect listed on pages 17 and 34.
Well, I was enlightened by the second paragraph on page 17 of the above book !
Paraphrasing here, when the frequency employed for a metal detector approaches 100kHz, it begins to affect the current, causing it to have a tendency to move along the outside surface of a target. This affect becomes even more noticeable above 100kHz. Since low frequency signals are generally thought to go deeper and high frequency signals shallower, then from the magic frequency of 100kHz and up, "skin effect" escalates. When the resulting signal penetration is shallow into a target surface , the signal might be interpreted incorrectly by the control box as ferrous, rather than non-ferrous, or in reverse.
In conclusion, for the majority of detectorists using VLF detectors with frequencies well below 100kHz, we can forget "skin effect" and assume that our coil signals are penetrating all the way through targets and the control box is generally reporting the target correctly......that is, most of the time.
ToddB66
,Every now and then I learn something new to me about metal detectors and how they work.
I recently became curious about the depth of a metal detector's signal penetration into coins and other metal objects buried in the ground. From my research I came to the conclusion that induction balance (IB) VLF detectors generally produce a signal that penetrates all the way through a target. However, recalling a term I had read about years ago, called "Skin effect", I wondered what affect that might have on the depth of penetration. So having a copy of George and Carl's book titled inside the METAL DETECTOR , I checked the index and found skin effect listed on pages 17 and 34.
Well, I was enlightened by the second paragraph on page 17 of the above book !
Paraphrasing here, when the frequency employed for a metal detector approaches 100kHz, it begins to affect the current, causing it to have a tendency to move along the outside surface of a target. This affect becomes even more noticeable above 100kHz. Since low frequency signals are generally thought to go deeper and high frequency signals shallower, then from the magic frequency of 100kHz and up, "skin effect" escalates. When the resulting signal penetration is shallow into a target surface , the signal might be interpreted incorrectly by the control box as ferrous, rather than non-ferrous, or in reverse.
In conclusion, for the majority of detectorists using VLF detectors with frequencies well below 100kHz, we can forget "skin effect" and assume that our coil signals are penetrating all the way through targets and the control box is generally reporting the target correctly......that is, most of the time.
ToddB66
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