How are Fe Co Nubmers determined?

[Sir Gala Clad]

If I understand it correctly the FE (Ferrous - Iron) Numbers measure how difficult it is for the induced current to flow in an item under the coil (it's resistance) while the Co (Conductive number measure the ireactance to the induced current flowing in at item under a coil.

What I am trying to come to terms with is why one usually gets an FE number of 12 when air testing a target with the search coil several feet above the ground. I know there is pollution (smog) in the air, but not aware of iron in the air or iron in copper/ clad/silver coins.

Stated differently why is an FE number of 12 common for most beaches?
Is there a common CO number, if so what is it?

I assume that the FE number of 12 has to do with the factory calibration by Minelab?

Further, I assume that the discrimation circuits are alway in (never bypassed) , even with a wide open screen as you can here the different tones depenent upon what tone ID profile is selected.

 

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[WaterWalker]

If I remember it all form by training some 50 years ago:

It is easier to INDUCE a current into IRON than SILVER. To reduce circuit losses; transformers, motors, and generators have IRON cores. You can make a magnet from an iron nail but not copper nail. Copper is used in house wiring to CONDUCT electricity as was aluminum (back in the 1970s) and gold is used on contacts in electronics to reduce losses due to their low resistance to conducting electrons (current flow). Gold is also used for its non-oxidizing characteristic. If a metal has a low resistance to current flow it has a high resistance to INDUCING a current flow. With detectors we are trying to INDUCE a current flow or distort a field.

VLF detectors
Think this way the Radio Frequency Field is similar to a magnetic field. When passed over a piece of iron, the iron offers very high resistance to the the Field which will be greatly distorted. This distortion is "seen" and analyzed by the detectors circuits and offers you a LOW number.
If the Field passes over a piece of silver, very little field distortion is "seen" by the detector circuits and offers you a HIGH number because of the low field distortion.

Pulse detectors: (are a bit different)
When a pulse is sent out its energy is readily absorbed by the iron, the induced energy is then tries to collapse and create its own field. This returned energy is "seen" by the PI detector with a time delay and a reduction in amplitude. These changes yield a target signal. The opposite is true when a pulse "hits" a piece of silver. Very little energy is absorbed and a very small return bit of energy with a longer delay is "seen" by the PI detector which yields a target signal. The time it takes a piece of IRON to return its "signal" is different from that of SILVER. The difference in the delay can be used by some detector circuits to give a HI-LO or Lo-HI tone. Other detectors use a "delay" sensing circuit to "discriminate" against IRON.

RESISTANCE is the reciprocal of CONDUCTIVITY. One needs to know if a statement is referring to the INDUCTION of a current or the PASSAGE of a current.
These two measures, resistivity and conductivity, are directly related to each other mathematically, where one is the reciprocal of the other.

The number 12 at the beach is most likely due to the beache's mineralization - salt and iron content in the sand. Drag a magnet through the sand and look at all the black iron bits that adhered to the magnet. ATPro owners typically set the Ground Balance to a number around 15 due to the salt water beach mineralization.

I don't know of any "common" numbers for FE or CO numbers.

Hope this is not too confusing, I tried to make it easy to understand.
These two measures, resistivity and conductivity, are directly related to each other mathematically, where one is the reciprocal of the other.

 

[Randyg12]

Here is an article I copied that may help.
By Steve Herschbach
Which metal detectors have the most reliable target ID numbers?

Target ID is a function of depth - the deeper the target, the more difficult it is to get a clean target ID as the ground signal interferes. Other items directly adjacent to the desired target can also cause inaccurate numbers. The more conductive the item, the higher the resulting ID number, but also the larger the item the higher the number. Silver is more conductive than gold, so a gold item will give a lower number than the same size silver item. But a very large gold item can give a higher number than a small silver item, so numbers do not identify types of metal. Gold and aluminum read the same and vary in size so to dig one you dig the other. Only mass produced items like coins produce numbers that are more or less the same over the years but a zinc penny will read lower than a copper penny due to the change in composition.

In general iron or ferrous targets produce negative numbers or low numbers. Aluminum, gold, and US nickels produce mid-range numbers. And most other US coins produce high numbers. Other countries coins, like Canadian coins with ferrous content, can read all over the place.

The scale applied varies according to manufacturer so the number produced by each detector will vary according to the scale used. The 0-100 range for non-ferrous targets is most common but there are others. Minelab employs a dual number system on a 2D scale with thousands of possible numbers, but they are now normalizing the results produced to conform more closely to the linear scale used by other manufacturers.

Increasing ground mineralization has a huge effect on the ability to get a good target ID. Ground mineralization is nearly always from iron mineralization, and this tends to make weak targets, whether very small targets or very deep targets, misidentify. The target numbers get dragged lower, and many non-ferrous targets will eventually be identified as iron if buried deep enough. Small non-ferrous readings and iron readings actually overlap. That is why any discrimination at all is particularly risky for gold nugget hunters.

If you want target ID numbers to settle down, lower sensitivity and practice consistent coil control. The target number will often vary depending on how well the target is centered and how fast the coil moves.

Higher sensitivity settings lead to jumpier numbers as the detectors become less stable at higher levels. The interference from the ground signal increases and interference from outside electrical sources also increases, leading to less stable numbers.

Higher frequency detectors are inherently more sensitive and are jumpier. So lean lower frequency for more solid results. Multi frequency detectors act like low frequency detectors and tend to have more solid target numbers due to the ability to analyze a target with different frequencies.

Another issue is the number of target categories, or ID segments, or VDIs, or notches, or bins (all names for the same thing) that a detector offers.

For instance here are the number of possible target id categories or segments each detector below offers:

Fisher CZ-3D = 7

Garrett Ace 250 = 12

Minelab X-Terra 305 = 12

Minelab X-Terra 505 = 19

Minelab X-Terra 705 = 28

Fisher Gold Bug Pro = 99

White's MXT and many other models = 190

Minelab CTX 3030 = 1750

Fewer target categories means more possible items get lumped together under a single reading, but that the reading is more stable. Many detectors will tell you the difference between a dime and a quarter. The Fisher CZ assumes you want to dig both so puts them under one segment along with most other coins.

People who use detectors with many target numbers usually just watch the numbers jump around and mentally average the results. Some high end detectors can actually do this averaging for you! But I think there is something to be said for owning a detector that simplifies things and offers less possible numbers to start with. The old Fisher CZ method still appeals to me, especially for coin detecting. So do detectors like the Garrett Ace 250 or Minelab X-Terra 505 for the same reason.

The problem is that as people strive to dig deeper targets or smaller targets the numbers will always get less reliable. But if you want to have a quiet performing metal detecting with solid, reliable target numbers look more for coin type detectors running at lower frequencies under 10 kHz or at multiple frequencies and possibly consider getting a detector with fewer possible target segments. And with any detector no matter what just back that sensitivity setting off and you will get more reliable target numbers.

Detectors often use tones to identify targets and often use far fewer tones than indicated by the possible visual target id numbers. The X-Terra 705 for instance can use 28 tones, one for each segment. However, most people find this too busy, and so simple tone schemes of two, three, or four tones may be selected. I think it is instructive that many people often end up ignoring screen readings and hunting by ear, using just a few tones. This ends up just being an ultra simple target id system much like the simpler units offer. Reality is that most people do not need or care about huge numbers of target numbers. For many just three ranges suffice, low tone for iron, mid tone for most gold items, and high tone for most US coins. The meter could do the same thing, but for marketing purposes more is better and so we get sold on detectors with hundreds of possible target ID numbers. Perhaps that represents a digital representation of an old analog meter with its nearly infinite range of response but the reality is we do not need that level of differentiation to make a simple dig or no dig decision.

Finally, a picture often says it all. Below we have a shot of the White's M6 meter. I like it because the decal below illustrates a lot. You see the possible numerical range of -95 to 95 laid out in the middle. Over it is the simplified iron/gold/silver range. Note the slants where they overlap to indicate the readings really do overlap. Then you get the probable target icons. -95 is noted as "hot rock" because many do read there.
The M6 can generate 7 tones depending on the target category. I have added red lines to the image to show where these tones sit in relation to the scale. It breaks down as follows: (FYI: I didn't post the picture of the M6 meter)

-95 = 57 Hz (Very Low) Hot Rock
-94 to -6 = 128 Hz (Low) Iron Junk
-5 to 7 = 145 Hz (Med Low) Gold Earrings, Chains - Foil
8 to 26 = 182 Hz (Medium) Women's Gold Rings/Nickel - Small Pull Tabs
27 to 49 = 259 Hz (Med Hi) Men's Gold Rings - Large Pull Tabs
50 to 70 = 411 Hz (High) Zinc Penny/Indian Head Penny - Screw Caps
71 to 95 = 900 Hz (Very High) Copper Penny/Dime/Quarter/Dollar

Note that the screen reading of +14 is noted as being a nickel or ring but it can also be the "beaver tail" part of an aluminum pull tab or the aluminum ring that holds an eraser on a pencil, among other things.

The best book ever written on the subject of discrimination is "Taking A Closer Look At Metal Detector Discrimination" by Robert C. Brockett. It is out of print but if you find a copy grab it, assuming the topic interests you.

Always remember - when in doubt, dig it out! Your eyes are the best target ID method available

 

[cudamark]

I'm not sure how this fits into the discussion, but, I always found it curious that a tiny piece of iron, like a staple or brad, will sing out loud, whereas a small platinum ring (bigger than it's iron counterpart) will only give out a weak broken signal. They're both close to being in the same range on the detector but have different aural characteristics. If it were truly dependent on conductivity or ferrous content, then size wouldn't matter. Clearly there is something else in the equation such as the amount of distortion present affecting the readings and tones.