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

[dbanner]

Modern construction nails are primarily composed of steel, often coated or treated to enhance durability and prevent corrosion. Common nails are typically made of low-carbon steel, while those used in harsh or outdoor environments may be galvanized or made of stainless steel. Specialized nails for masonry or concrete may have a higher carbon content and be tempered for strength. Additionally, nails made of copper or brass are used in wet climates for their resistance to corrosion.
Elaboration:

• Steel:
  The majority of construction nails are made from steel, with carbon steel being the most common for general use.
• Low-carbon steel:
  Nails for wood are often made of low-carbon steel, also known as "mild" steel, to ensure flexibility and ease of driving.
• Coating and Treatment:
  To protect against rust and corrosion, steel nails may be galvanized (coated with zinc) or made of stainless steel, which is naturally resistant to rust.
• Specialized Nails:
  Nails for specific applications, like masonry or concrete, may have a higher carbon content and be tempered to increase their strength and hardness.
• Other Metals:
  Nails made of copper or brass are sometimes used in very wet climates due to their excellent resistance to corrosion from moisture or salt spray.

[dbanner]

A magnetized nail will produce an increased eddy current response? As opposed to an non-magnetized nail.

• Once the external magnetic field is removed, the nail retains some degree of magnetism (magnetic retentivity), as it is a hard magnetic material.

Collecting data we must always start with a non-magnetized nail ie, de-magnetized before each testing? Or maybe the results are not so significantly impacted as to make much of a difference.?

[Carl-NC]

Magnetizing the nail creates a DC magnetic offset in both the magnetic hysteresis response and the eddy magnetic response. It is removed by the same mechanisms that remove Earth field effect.