Cause of nail falsing?

[Diga]

Hello.

What characteristic(s) of a nail can cause it to false? The head, the elongated shape, the pointed tip? How much of a factor is the nail's orientation in producing a false or not?

[Carl-NC]

I assume you mean false positive, where the nail gives a non-ferrous response. All iron targets have a magnetic response and an eddy response. The magnetic response is on the ferrous side and the eddy response is on the non-ferrous side. Most nails are dominated by the magnetic response so they ID as ferrous. A short nail with a large head (roofing nail) might have a dominant eddy response and show as non-ferrous. A nail that is laying flat and pointed perpendicular to the coil sweep is more likely to false non-ferrous, but when you do a 90° sweep you should then hear the tell-tale ferrous double-beep. A 90° bent nail can false even from different sweep directions. Also, catching a nail on the edge or outside the edge of the coil can give a non-ferrous response, but if you figure out its true location (AM mode) and then do cross sweeps it should be easy to figure out. My strategy is to always use AM mode to get the exact target location and then sweep from different directions.

[Diga]

Thank you for the reply Carl.

Yes, I did mean a false positive and was also aware of the magnetic and eddy response. I wasn't aware of what exact physical aspect of the nail can often cause that false. For example, I've come across posts that claimed it was the head of the nail, while others claimed it was the point of the nail. Some others said the main factor in nail falsing is the orientation of the nail.

So, when the iron bias control is lowered, does it simply just "suppress" the magnetic response which allows the eddy response to become dominant? Also, why is it that an iron bias control is typically found on SMF detectors but not single frequency detectors?

[Carl-NC]

When the frequency changes, iron behaves differently than normal eddy targets. Here are a couple of coins:



and here is a nail:



You can see that when the nail is vertical the magnetic response is always dominant, but when horizontal the eddy response can take over. You also see a very predictive movement of eddy responses along a semicircle that you don't get with nails; MF attempts to recognize these differences, whereas SF doesn't even have this information.

[Diga]

Are those illustrations from your book? If so, I've only read selective sections of your book so far. I'll check out that section.

[Carl-NC]

Yes