To What Extent Does Iron Shield Target Detection?

[GB_Amateur]

On 4/18/2025 at 3:41 PM, @Geotech said:

Some detectors used to include a full static AM mode but for any that still do it's implemented as a static pinpoint mode for which you have to continuously hold down the pinpoint button.

Not sure if you're considering the F75 as no longer an actively marketed and sold detector, but it has a static all-metal mode.

From the User Manual F75MBLK_REV5_11.06.14.pdf:

[Fredneck]

That's why I love my ole White's and they stay in the line up. Designated non-motion All Metal modes, especially Mixed Mode which would be perfect for the archeologist, works for this amateur archy.  😁

[Geotech]

4 hours ago, GB_Amateur said:

Not sure if you're considering the F75 as no longer an actively marketed and sold detector...

???

[Steve Herschbach]

Tom's article is a standard refence on this website but it is not like he discovered anything at all. What he described is simple target masking that has always existed with discriminating metal detectors. It is the basic reason why many people prefer to use PI detectors and dig everything. Even then ground balancing/ground elimination can cause some targets to be missed. The staple bit was overdone but he was trying to make a point.

There are thousands of YouTube videos doing various nail masking tests on every modern model ever made right up to the very latest and greatest. This is not even Detecting 101, more like Detecting 001.

 

[JCR]

And, there are very few Static mode Non Motion All Metal capable units available on today’s market.

[CliveHamy]

On 4/18/2025 at 8:41 PM, Geotech said:

Tom's experiment was valid but his conclusion -- that the staple altered the TX field so much that it could not energize the dime -- was not. He even more-or-less proved it by using the SD2200 to find more targets in a "cleaned-out" area. If iron was suppressing the TX field, it would still be happening with the SD2200. Also, his comment that an 8" Double D coil will generate a signal (hypothetically) 8" long by 1" wide is also not true; the TX field of a DD looks about the same as that of a concentric.

The culprit that causes the masking is the motion filters used for ground balance. It's not good enough to hunt in all-metal or with Disc set to zero because, for many detectors, the motion filters are still fully engaged. Instead, you need to use a static all-metal mode. Some detectors used to include a full static AM mode but for any that still do it's implemented as a static pinpoint mode for which you have to continuously hold down the pinpoint button. Anyway, when you test a dime with a staple above it you will then find that the staple has practically no effect on how deep the dime can be detected.

If your detector doesn't have a static AM mode then the next best thing is an AM mode with a single derivative filter. This is commonly how VLF gold detectors operate or MF detectors with a prospecting mode. Not as good as a true static AM mode but better than a double-derivative VLF-motion mode. The SD2200 effectively operates this way and is even closer to a static AM operation than a VLF gold detector.

 

Hi Geotech, great feedback. Your first paragraph strongly undermines the concern I had that there could be "silent iron" hiding non-ferrous and itself being undetected. - Great, sigh of relief.

Could you explain more about "motion filters" mentioned in your second paragraph? The machines I've used - the old Garrett Ace, the Nox 800 and currently the Manticore all have what I think as the pinpoint mode you mention, but there is very little in forums about how they work and what they are really doing/showing. When working in a trench I still use the "motion mode" to detect targets, and then pinpoint (pinpoint mode) them to isolate them. Because the trenches are quite small I've often not bothered at all with the "normal" mode but just used pinpoint, but I've never been sure what the detector is actually doing.

Cheers Clive

 

[Geotech]

The motion filters are high-pass filters that remove the slowly changing ground signal. This leaves you with a supposedly ground-free target signal, which should improve target ID. But the motion filters (or the subsequent DSP) can combine co-mingled target responses in a way that causes masking. Whenever I hear a motion-mode response that is not clean (has an audio truncation, glitch, or other artifact) I switch to static AM mode and usually find there is a second target.

[GB_Amateur]

On 4/20/2025 at 2:35 PM, GB_Amateur said:

On 4/18/2025 at 3:41 PM, @Geotech said:

Some detectors used to include a full static AM mode but for any that still do it's implemented as a static pinpoint mode for which you have to continuously hold down the pinpoint button.

Not sure if you're considering the F75 as no longer an actively marketed and sold detector, but it has a static all-metal mode.

22 hours ago, Geotech said:

???

Hopefully my comment (and this reply) wasn't interpreted as being contentious or flippant; it was intended as neither.  What I quoted (reshown above) sounded to me as being in the past tense.  I just wanted to point out that a currently available detector *does* have a static all-metal mode that doesn't require putting the detector into pinpoint mode.

Maybe this is the last detector designed with that feature, and it is a 15+ year old single frequency detector which in its heyday was state-of-the-art.  Now that we're fully into the multi-frequency era, though, it understandably takes a back seat to multiple newer models which don't share that one feature in particular.

[Geotech]

Oh, I read it wrong. I thought you were saying the F75 is no longer available. Never mind [in Emily Litella's voice].