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Fisher F75 Ferrous Tone Quirk

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Once upon a time all metal detectors went beep, and you dug up a metal object. Then a simple form of discrimination was developed based roughly on the conductivity scale. The main feature of this scale is that ferrous (iron or steel) items read lower on the scale than non-ferrous items. In a perfect world all ferrous readings could be set as a negative number, and all non-ferrous items set as a positive number.

The reality is not so perfect however. Some steel items, especially items with a hole like a steel washer, will read up in the middle or high end of the scale, and show up right where only non-ferrous readings should appear. Thin sheet steel (bottle caps, flat section of rusted cans or roofing material) can show up in the mid range, and hardened steel items like bolts or ax heads can read way up in the silver range. These types of targets can trouble coin hunters in particular.

Gold reads much lower on the discrimination scale normally due to a fairly low conductivity for gold. The gold range overlaps entirely with the lead and aluminum ranges, and these items are arranged on the scale based more on size than anything. Small gold, aluminum, and lead reads very low, and larger gold, aluminum, and lead tends to read in the low to middle portion of the scale.

Those who chase these low end targets run into another problem with ferrous. Very small gold, lead, aluminum, and other small non-ferrous low conductors actually overlap with small ferrous items and so the clean ferrous to non-ferrous "breakpoint" does not actually exist,. The breakpoint is more of a "breakzone" i.e. a fuzzy zone where items overlap. The ground itself contains ferrous materials in the form of iron minerals. Ground minerals can act to confuse the detector further, enhancing the chance that a small non-ferrous reading will be interpreted as ferrous. Another way to say that is that in highly mineralized ground the overlap between ferrous and non-ferrous targets gets larger. The ground mineralization is critical to how this all works and so air testing is not recommended for testing the ferrous/non-ferrous overlap region on any particular detector.

Note that this does not apply just to very small items. The deeper an item is, the smaller it appears to a detector. In other words a deep large item can sound just like a shallow small item. When you bury items of any size in highly iron mineralized ground, the deeper they are, the more chance the ground mineral signal will overlap and cause the item to read as ferrous right at the edge of detection range.

Fisher F75 metal detector

The early model discrimination detectors usually had a knob that adjusted all the discrimination. Everything below the knob setting was ignored, and everything above the setting accepted. The discrimination pioneers rapidly discovered that the dividing line between ferrous and non-ferrous is "fuzzy". The knobs could be set to reject nearly all ferrous readings, but then some good non-ferrous targets would get missed. The solution was to use a little bit lower discrimination setting, which meant more ferrous trash was dug, but more non-ferrous items were revealed.

Managing the ferrous to non-ferrous breakpoint is critical. There is no setting that rejects all ferrous while detecting all non-ferrous, and the more mineralized the ground is, the less reliable the settings become. As a rule of thumb, the more aggressive the iron rejection, the more chance of non-ferrous items being misidentified.

Detector technology advanced, and tone schemes were developed that divide the discrimination scale up into segments or "bins" where all numbers within a specific range make a specific tone. These tone schemes are often preset at the factory. The ranges can be arbitrary and arranged in many ways, but all share one common factor. Where is the setting that divides low ferrous tones from the higher non-ferrous tones? This is the "ferrous breakpoint". Everything below this point will give a "ferrous tone" and everything higher a "non-ferrous tone".

The detector engineers are well aware of the overlap between ferrous and non-ferrous items. In choosing one setting to define what is in reality a zone the engineers have to make a hard choice. If the setting is too low, the operator will get many non-ferrous readings that turn out to be ferrous. That really irritates people. Or they can set the breakpoint higher. That way less ferrous gets dug. Some good non-ferrous items will also be missed, but only in the rarest cases does anyone ever know what they are missing. The odds are there will be more complaints if the ferrous breakpoint is too low than too high. The goal is not to find every non-ferrous item, but to keep from digging too much trash identified as good.

This diagram is shows the common discrimination range employed in nearly all metal detectors. This particular model (Garrett) sets 40 as the point where ferrous items separate from non-ferrous items. Yet the chart reveals the overlap zone runs from about 35 to 45, a solid ten point spread. Small gold can identify as ferrous, especially in iron mineralized ground. Many detectors identify this zone on the meter via overlapping diagonal lines.

The ferrous/non-ferrous overlap region

What this means is that any detector that employs a preset tone scheme with no ability to adjust the "ferrous tone breakpoint" is assured to be missing at least some items due to an overly aggressive setting dialed in at the factory. This was eventually recognized, and now quite a few detectors allow the point where ferrous tones flip to non-ferrous tones to be adjusted. Some models are now even allowing for multiple volume controls for each separate tone, are at least the ferrous tone. This is most often called a "ferrous volume" setting.

The Fisher F75 is an earlier tone based model and as such the tone schemes are preset at the factory. You can choose between the schemes, but the tone settings of where the tones occur cannot be adjusted. The F75 employs a target id scale that ranges from 1 to 99 with the 0 - 15 range defined as ferrous. From the F75 Users Manual page 20:

1. 1-7 iron
2. 8-15 iron

3. 16-20 foil
4. 21-25 foil
5. 26-30 nickel
6. 31-35 nickel
7. 36-45 tab
8. 46-55 tab
9. 56-60 zinc
10. 61-65 zinc

and from page 25:

Most iron objects 4-12
foil from gum wrapper 16-25

U.S. nickel (5¢ coin) typically 30
aluminum pull-tab 33-55
aluminum screw cap 60 - 70
zinc penny (dated after 1982) typically 60
aluminum soda pop can most often 63-69,
but can vary widely
copper penny, clad dime typically 70
U.S. quarter (25¢ coin), clad typically 80
50¢ coin, modern clad typically 86
old silver dollar coin typically 90
US silver Eagle $1 coin typically 91

The implication is that non-ferrous items will only read 16 and above. Any readings of 15 and lower are deemed ferrous.

The F75 has several preset tone schemes, the basics being monotone, two tone, three tone, four tone, and Delta Pitch (separate tone for each target id number i.e. multitone).

The quirk is simple. The two, three, and four tone schemes all have a non-adjustable factory preset low tone for ferrous at 15 and below. The tone schemes override any other discriminations settings. In other words, if you have manually set the discrimination for ferrous to be a lower setting, switching to any two, three, or four tone scheme will automatically change the low tone setting to be at 15 and lower.

The problem is that with time it was revealed that the F75 will detect some non-ferrous items at much lower settings than 16. Tom Dankowski finally put it all together and determined that a reading of 7 or higher would reveal additional non-ferrous items that are rejected when the setting is at 15. Tom's recommendation for the F75 while hunting ferrous is therefore to not use the tones, but to use the monotone setting and adjust the discrimination manually to 6. That way items 7 and higher signal as a non-ferrous target instead of delivering a low ferrous tone via the tone schemes. Again, going to a tone setting will automatically override a manual discrimination setting if one has been set.

Tom wrote this all up as a great article in the 2009 Fisher Labs World Treasure News on page 11.

I actually had the chance to see this in person in my own use of the F75. Early on I trusted the tone settings and two tone is quite handy for those simply wanting to dig all non-ferrous. Yet on my trip to England with the F75 I encountered a mystery. A gold coin was found and another F75 newbie was telling me about how he tested it with his F75 and it gave a nice ferrous tone. He was quite upset and worried his detector was defective. He did not have the coin however and so I could not see what he was describing and at the time I have to admit I was clueless. I know now that he was using tones, and that the gold coin was reading lower than 16 and so being identified as ferrous!

My early use of the F75 was more for gold nuggets, and I usually used all metal mode. Yet my favorite feature on the F75 was full time target id while in all metal mode. My method was to acquire all targets, then dig any that flickered even once above my mental ferrous breakpoint. Unfortunately I leaned too much on the user manual initially and tended to pass on targets reading under 16. My early writing on the subject reflected that. After I discovered on my own that gold was reading lower I started adjusting my mental settings lower. Then I bumped into Tom's writing on the subject and it all came together.

The bottom line in that non-ferrous items can read as low as 7 on the F75 yet the ferrous tone break is set at 15. This is just fine for most Park coin detecting, but problematic for those hunting low conductors of any sort or coins in dense ferrous. Either use monotone and decide where you want the setting to be (6 as Tom recommends or maybe somewhere in between 6 and 15 if 6 has you digging too much trash) or hunt in all metal and use the target id numbers to decide when to dig keeping in mind non-ferrous can read lower than 16, especially in high mineral ground.

This is not a flaw in the F75 but just a function of any detector using a preset tone scheme. There are many detectors like this on the market. They tend to be less expensive models, or older models, as most new detectors now feature an adjustable tone break for the ferrous/non-ferrous overlap zone.

Another take on the subject.

And down the rabbit hole - Tune Out Nails - You Will Miss Gold!

Fisher F75 Information Page

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Yup, same deal with T2 as a predecessor of the F75 but the ferrous range runs from 1 - 40 instead of 1 - 15. Going to two tones on the T2 will automatically make anything under 40 go ferrous low tone. I agree that as low as 30 is worth investigation. In fact trough trial and error people have determined that a disc setting of 21 on the T2 achieves about the same result as a disc setting of 6 on the F75.

Remember you will dig more trash so this is last ditch type stuff. The Gold Bug Pro has an inherent tones advantage since as a nugget detector it was recognized how important this is, and as a newer model the V-Break variable ferrous tone break was created for the Gold Bug and Gold Bug Pro. The Gold Bugs have ferrous starting defaults starting at 40 but I found gold regularly down to 35.

And you are right again Simon - hunting all metal only works with sparse targets. Monotone and dialed in disc is better for anywhere the target density gets excessive.

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Steve, basically a good article.   However a couple of misses ....

a) ferrous in contact with non-ferrous tends to draw down the identification and can bring it down into the ferrous range of 1-15.   The disc level of six works because the ferrous/non-ferrous combined TID is greater than the nail TID by itself.   Tom explains that well in his posts.   Its basically a post about iron masking and its affects.

b) the newer F75 DST models also include a tone mode called 1N.   This is a VCO tone mode that incorporated preset 'nail only' iron rejection.   



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Yes Mike, but non-ferrous can read into the ferrous range for many reasons. That’s the entire reason I wrote the article. It is just as valid for a non-ferrous item in mineralized ground with no other items nearby as it is for a non-ferrous item surrounded by ferrous. I run into this constantly as a nugget hunter where no ferrous at all is involved, just gold nuggets in mineralized soil. Same issue with a gold ring on a black sand beach. You don’t need nails around for non-ferrous to get pulled down into the ferrous range. I almost never hunt the nail beds, but deal with this every time I detect. My ground here will make a dime read ferrous at depth with no other items nearby. I have seen the F75 identify a .22 shell casing as ferrous when sitting in plain sight on mineralized ground.

I was only referencing Tom's original as I figured people would think he "discovered" this and then I write up this article later without crediting him - well, you know how people can be. I am glad you posted because t made me do some looking and I found that Tom did an official article on this for Fisher - 2009 Fisher Labs World Treasure News on page 11. I added the link to my original post above.

Did not know (actually forgot) about the 1N mode but being preset - well, it would have been better to offer an adjustable tone break. Unfortunately I have been told the T2 / F75 architecture does not allow this as an easy change.

Anyway, I am not trying to knock the F75 - it is one of my favorite all-time detectors. Just trying to pass on some info that might help somebody out there. The bottom line is simple. The tone break settings on the F75 (and T2) are factory preset for all tone modes except mono tone, and cannot be adjusted. The break between ferrous and non-ferrous is too aggressive, and will misidentify many items, especially gold, as ferrous when the signals are weak. This is most often seen in highly mineralized ground.

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