Excalibur and small gold

[bklein]

Calabash posted videos of the Excalibur not seeing small gold. The gold wasn't really that small. I was wondering if the high EMI he had at the site of his videos hurt the Excalibur's performance but others have said they also see poor detection of gold items. Any idea why this is so or any improvements that might be made? Some have said it is because of a mismatch to the larger coil. What have you seen for Excalibur detection of smaller gold? Would some kind of coil booster circuit help things?

[Carl-NC]

Excal uses BBS which is 3.125kHz & 25kHz. However, 3kHz is very much the dominant frequency so BBS does very well on deep silver but not so good on small gold. Sovereign & Explorer has the same issue. ETrac had improved small gold response.

[bklein]

So I should put my EQ600 in single frequency mode and see how they compare? Is the salt water a general deterrent much like mineralized ground or only at 25 Khz (higher frequencies). Wondering why small gold performance wasn’t more important in the Excal design or was this the best they could do? How does the Beach hunter TDI compare?

[Skippy]

"Wondering why small gold performance was not more important in the Excal design"
You probably have the US detectorists to blame, there, with their obsession with milled silver coins buried deep in their 'pounded parks'. But as 3k/25k also works well in saltwater, it was an obvious choice to not 'clean-sheet design' the Excal, simply transfer known reliable tech into a new housing.

[Carl-NC]

But as 3k/25k also works well in saltwater, it was an obvious choice to not 'clean-sheet design' the Excal, simply transfer known reliable tech into a new housing.

That'd be my guess.

Keep in mind that any detector that does salt subtraction is going to have reduced sensitivity to small gold. Salt water has a tau of maybe 2-4us so any conductor with that tau will also get notched out. And it's not a perfect notch so nearby taus get attenuated.

[green]

That'd be my guess.

Keep in mind that any detector that does salt subtraction is going to have reduced sensitivity to small gold. Salt water has a tau of maybe 2-4us so any conductor with that tau will also get notched out. And it's not a perfect notch so nearby taus get attenuated.

Not understanding salt signal. Maybe someone could correct my thinking. Charted decay for 3layers of 15mm sq foil at 1.5inches. Doubling target size increases signal strength 8 times. The ocean is a lot bigger than 15mm sq. Why is the salt signal so low?

[Skippy]

A flaw in your thinking is this:
saltwater is not one homogenous solid target, so its signal does not get disproportionally bigger as its expanse increases. It's probably more comparable to a lump of ferrite ( or powdered iron ). So each micro-droplet of water has a magnetic ( diamagnetic ) response, but the bulk response is like millions of droplets, not one massive lake.
In addition: you are intentionally placing your metal target at the coil's sweet-spot, where you know it gives maximum response. In other locations, for example 3 inches 'outside' your coil, the response will not be so strong.

[Carl-NC]

Saltwater is a weak ionic conductor. If you take, say, a pint/0.5L of seawater you might not be able to detect it. When calibrating an MF detector to salt I use at least a gallon container and even that is not very strong. So, yes, volume matters, up to a point. Seawater has a typical conductivity of around 4 S/m so at 10kHz the skin depth is 2.5m. That's well beyond the detection reach of a normal metal detector. Also the seawater target size expands laterally well beyond the edge of the coil so it also sees the horizontal and reverse field (around the outside of the TX coil) which gives rise to partial signal cancelation. In other words, eddy currents are oriented all over the place; horizontal, vertical, backwards, etc. You can't compare it to a usual metal target.

[green]

A flaw in your thinking is this:
saltwater is not one homogenous solid target, so its signal does not get disproportionally bigger as its expanse increases. It's probably more comparable to a lump of ferrite ( or powdered iron ). So each micro-droplet of water has a magnetic ( diamagnetic ) response, but the bulk response is like millions of droplets, not one massive lake.
In addition: you are intentionally placing your metal target at the coil's sweet-spot, where you know it gives maximum response. In other locations, for example 3 inches 'outside' your coil, the response will not be so strong.

Thanks for the reply. I have no idea what the salt response should look like. I have tried measuring salt response in the past. Tried again this morning. I get a response but don't see a TC decay. Tx off response is similar to what I see with a ferrite core. I see a response when Tx on with the ferrite, not the saltwater bag. Most of the salt was absorbed.

Carl, thanks for the reply. Missed it while I was replying.

[bklein]

Sorry to be off topic a bit but hey it’s my thread, Calibash was testing the Excal at his home for some of his videos and he had a ton of EMI. Does EMI affect a detectors depth? It’s hard to make out targets in the EMI but you can sometimes. I suggested he get a doughboy pool to test depth in - would this kill the EMI? Should it be saltwater?

[Carl-NC]

Water will attenuate high frequency EMI but not so much 60Hz. Depending on the design, yes, EMI can reduce depth.

[SkyeHi]

Thanks, good educational thread.
bklein,
Have a look at the build Chance Pi thread. Aim, the adaptation, modification of the ChancePi and coil to enable small gold detecting. They achieved very good results.

[Hamid]

SG

I'm going to paint few lines if my memory is remember right, well, Minelab built Excalibur based on Sovereign first model, Gary Storm and his talented friend from UK, they gave the design and still in production from many years ago.
The powerful of Excalibur is very sensitive to gold, why excalibur ignores iron and still feels small gold....
Right coil and right setting what it makes difference. The first model is killer of gold jewelry, Right Users.
flawess design did pretty well at deep water.
Beach metal detecting needs many metal detectors.
Here's some pictures.
Hamid

[Hamid]

TG

Used Excalibur and Sovereign.
If you want to find very tiny gold, Equinox 600 then 800 and Vanquish 540. The last technology did pretty awesome.
Hamid

[ionut_mtb]

I'm going to paint few lines if my memory is remember right, well, Minelab built Excalibur based on Sovereign first model, Gary Storm and his talented friend from UK, they gave the design and still in production from many years ago.
The powerful of Excalibur is very sensitive to gold, why excalibur ignores iron and still feels small gold....
Right coil and right setting what it makes difference. The first model is killer of gold jewelry, Right Users.
flawess design did pretty well at deep water.
Beach metal detecting needs many metal detectors.
Here's some pictures.
Hamid

Hello, what settings did you use for those small gold targets? Did you hunt salt water? 8” coil? I noticed with my sov gt that is not so deep with 10” tornado and cannot use sens at max because of false signals. Unfortunately i don’t live close to the sea to test and hunt more. This guy have some interesting videos:
https://youtu.be/6B9Eg6AZHhQ