Tests of big depth metal detectors ( TR and PI systems )

[green]

I did suggest using copper (pre-1982-ish) US 1 cents for several reasons. One was for consistency, if someone else is to reproduce a test, they don't want to do so with a random mix of zinc-cored and copper coins. Secondly, the copper coins are the close match to our U.K bronze 1 pence coin (we went to a copper-plated-steel coin in the 80's, for the same economic reasons as the US went to zinc-core). Also, I understand 'zinc' cents corrode badly in the ground, this makes them less suitable as a test target, im my opinion.

Re: the lead disc... the size/weight I've started off with (60 x 5, 160gm) was an engineering guess, based on a number of ideas. Clearly it's significantly larger than any coin-size object, so that puts in in the range where it should be detectable deeper than any coin. If you're searching in a way that reduces pickup of small coin-size items (eg. coil clear of the ground), then it will still be visible. During my own real-world deep-searching trials, I did detect a golf-ball sized lump of lead weighing 150 grams... it was only about 10 inches (30cm) down, so would probably have been detected with normal search methods. I had the search-coil about 40cm above the ground when searching.

It may well turn out to be a bit too large and easy to detect, but that's the point of experimentation, you've got to start somewhere.

I like the lead disc as a target because it could be representative of a real target - some coin hoards have been found buried in lead pots, which are often fragmented. Plus it could also be oriented vertically or at an angle to give that 'coin-on-edge' effect (see Riss' first post).

Regarding my mould for the lead: My first attempt was made from thick aluminium foil (actually 0.07mm) as used for meat pies, pastries etc by food retailers. I found some jam/preserve which had a lid about the correct size, and shaped the foil around it, creasing up the edge. It was about 12mm deep and 58mm diameter when I'd finished, close enough. The correct weight of clean-ish lead was weighed out before melting, allowing 5 grams extra for losses, eg. dross, metal staying in the melting pot etc. The mould was placed on a ceramic tile for thermal reasons.
I suppose I should state: take precautions. Gloves, eye protection, well-ventilated area, water-soaked cloth to hand. etc etc.

What detector and size coil were you using to detect a golf ball size lump of lead at over 27inches?

[garyq]

[Both of these coins produced a similar result except when two of the 2 cent coins where stacked they produced 155mm whereas the 2 Nickels stacked was 102mm.]
Above 2 stacked is 280mm not 155mm?

The above 2 stacked at 280mm is with 2 Pennies whereas the 155mm was with 2 stacked 2 cent coins.

I only tested the 2 cent coins for a comparison of its response to a Nickel coin since they are of similar diameter and thickness but have different Copper composition.

[garyq]

Good example of a PI hole. Tried the same test again, 2 stacked was closer to the PI hole bottom with my detector also.

In regards to the hole in the response when 2 Nickel coins were stacked on top of each other than once I raised the detectors sample delay from 8uS up to 16uS the hole disappeared and the depth of the 2 stacked Nickel coins increased from 102mm to 200mm.

[Ferric Toes]

An interesting experiment I once did was to set the GB on a Goldscan 5 to balance out a piece of Australian ironstone. I the filed down a piece of lead until it's response fell into the GB hole and it was virtually undetectable. What was puzzling was that if I left the lead for say half an hour and then retested it was detectable again. I thought at first that the detector setting had drifted, but eventually found it was a temperature effect of the lead. The act of filing had heated the lead which had changed its conductivity very slightly so that when it cooled the decay shifted just enough to start pulling it out of the hole. I subsequently found that holding the lead with the fingers while testing would do the same thing. With analog filtering, the hole must have a wide mouth but tapering down to a very sharp point at maximum rejection. I would suspect that digital filtering would give a narrower mouth and steeper sides which would be beneficial. Perhaps a brickwall filter with a variable width to adjust for ground variations such as kingswood reported on the Vallon thread.

Eric.

[Skippy]

Quote:"What detector and size coil were you using to detect a golf ball size lump of lead at over 27inches?"

A Fisher F75, modified to have double the front-end gain, with the stock 11" x 7" bi-axial search-coil. Operated in non-motion mode ( = permanent pinpoint).
The size searchcoil I was contemplating using for this kind of hunting would be 15" - 18". I'll likely home-build one, though it's possible a Fisher 15" coil may appear on eBay at an attractive price.

Edit: I'll try and find the actual lead lump...it's no doubt mixed in with 10kg of other lead, but I may recognise it.

Also.... just for test purposes, I also have a dug-up 3-bore musket ball. This is a monster, weighing 150g (1/3rd of a pound) and 31mm diameter.

[green]

The above 2 stacked at 280mm is with 2 Pennies whereas the 155mm was with 2 stacked 2 cent coins.

I only tested the 2 cent coins for a comparison of its response to a Nickel coin since they are of similar diameter and thickness but have different Copper composition.

Thanks, think I'm missing the US, British coin thing.

[green]

Quote:"What detector and size coil were you using to detect a golf ball size lump of lead at over 27inches?"

A Fisher F75, modified to have double the front-end gain, with the stock 11" x 7" bi-axial search-coil. Operated in non-motion mode ( = permanent pinpoint).
The size searchcoil I was contemplating using for this kind of hunting would be 15" - 18". I'll likely home-build one, though it's possible a Fisher 15" coil may appear on eBay at an attractive price.

Edit: I'll try and find the actual lead lump...it's no doubt mixed in with 10kg of other lead, but I may recognise it.

Also.... just for test purposes, I also have a dug-up 3-bore musket ball. This is a monster, weighing 150g (1/3rd of a pound) and 31mm diameter.

Thanks, If you do anymore testing I would be interested in the distance you can detect a single US penny and the 5mm disk you poured.

[garyq]

Thanks, think I'm missing the US, British coin thing.

Actually the 1919 Pennies I used for my tests were Australian which are the same in dimension and composition to the British penny from that era.
The 2 cent coins I used were Australian with the same composition as the Pennies and as I said similar in dimensions to the US Nickel.
I have US Nickels including Cents, Dimes and Quarter dollar but these are different in composition to the Nickel.

[Skippy]

Oh dear, I'm getting that deja vu feeling all over again......

If we're going to do multi-national collaborations and comparisons, we need to stop comparing grapes with melons.
British pennies from 1919 were great big bronze coins about 31mm diameter and nearly 10g in weight, presumably the Aussie ones matched these figures, but it's possible the alloys changed slightly, the British version certainly had it's 'recipe' slighly changed a couple of times.
The British pennies I was describing were Decimal ones (1971-on) weighing 3.56g and made of a high copper content bronze. These are reasonably comparable to the bronze US 1 cent (pre-1983?) weighing 3.11g. I note, however, that the US bronze alloy changed in the 1960's, technically becoming 'brass'.

This comparison issue is made more confusing due to the Yanks calling their Cent a 'penny'.

[Phil S]

And not forgetting that from late 1992 British pennies have been Copper plated Steel and from 1982 US pennies (Cents) have been Copper plated Zinc,, that's why I was chuntering on about the devising / specification of a standard target, my current thoughts are of standard sized circular punched / or cut squares of Lead (of the type used for roofing, Lead Flashing), I mean Lead is mostly / basically Lead wherever it is, one just needs to decide the circumference / length and the thickness

[Skippy]

"And not forgetting that ...etc..."
..these details have already been pointed out several times, earlier in this thread, eg.

"I did suggest using copper (pre-1982-ish) US 1 cents for several reasons. One was for consistency, if someone else is to reproduce a test, they don't want to do so with a random mix of zinc-cored and copper coins. Secondly, the copper coins are the close match to our U.K bronze 1 pence coin (we went to a copper-plated-steel coin in the 80's, for the same economic reasons as the US went to zinc-core). Also, I understand 'zinc' cents corrode badly in the ground, this makes them less suitable as a test target, in my opinion."

The use of "off-the-shelf" lead sheet has some merit if we want to replicate a 'low-conductor' coin hoard, for which I was earlier proposing using US 5 cent 'nickel' coins, (or their British close-equivalent 1 shilling/pre-1991 decimal 5 pence). I suspect the stumbling point is the non-universal thickness. SWG, AWG, metric, Imperial .... though a likely common thickness could be 1.6mm, which is 1/16th inch, 16 SWG, and close enough to 1.5mm to not matter. It's certainly easy to cut a sheet up into lots of 20mm squares.

Replicating a high-conductor coin hoard without using actual coins is a bit trickier. The only common source of copper sheet I can think of is copper water pipe (also used for gas supply here in the UK). But again, the thickness is going to vary, I'm sure. I've no idea what thickness our pipes are in U.K, plumbing seems to be non-standard in all ways...I've never found a spanner that properly fits any plumbing fixture, a mystery.
Aluminium sheet might suffice, the 1.6mm thickness figure is a possibile good choice. But bear in mind it's not copper, it's resistivity is about 35-40% of copper (IACS) and the particular aluminium alloy used can be relevant, so best to avoid anything fancy.

Also...sorry I've been really busy, and have been away from home a lot, so I've had zero chance of doing anything testing/detecting-wise.

[green]

"And not forgetting that ...etc..."
..these details have already been pointed out several times, earlier in this thread, eg.

"I did suggest using copper (pre-1982-ish) US 1 cents for several reasons. One was for consistency, if someone else is to reproduce a test, they don't want to do so with a random mix of zinc-cored and copper coins. Secondly, the copper coins are the close match to our U.K bronze 1 pence coin (we went to a copper-plated-steel coin in the 80's, for the same economic reasons as the US went to zinc-core). Also, I understand 'zinc' cents corrode badly in the ground, this makes them less suitable as a test target, in my opinion."

The use of "off-the-shelf" lead sheet has some merit if we want to replicate a 'low-conductor' coin hoard, for which I was earlier proposing using US 5 cent 'nickel' coins, (or their British close-equivalent 1 shilling/pre-1991 decimal 5 pence). I suspect the stumbling point is the non-universal thickness. SWG, AWG, metric, Imperial .... though a likely common thickness could be 1.6mm, which is 1/16th inch, 16 SWG, and close enough to 1.5mm to not matter. It's certainly easy to cut a sheet up into lots of 20mm squares.

Replicating a high-conductor coin hoard without using actual coins is a bit trickier. The only common source of copper sheet I can think of is copper water pipe (also used for gas supply here in the UK). But again, the thickness is going to vary, I'm sure. I've no idea what thickness our pipes are in U.K, plumbing seems to be non-standard in all ways...I've never found a spanner that properly fits any plumbing fixture, a mystery.
Aluminium sheet might suffice, the 1.6mm thickness figure is a possibile good choice. But bear in mind it's not copper, it's resistivity is about 35-40% of copper (IACS) and the particular aluminium alloy used can be relevant, so best to avoid anything fancy.

Also...sorry I've been really busy, and have been away from home a lot, so I've had zero chance of doing anything testing/detecting-wise.

More tests. Copper squares cut from 1/2 inch tubing, about .8mm thick. Was going to test 10, 20 and 40mm squares. TC should be linear with length 10mm(40usec) 40mm(165uec), didn't test 20mm yet. Thinking about 5, 10 and 20mm thick poured lead targets. The form I wrapped the foil around was 64mm instead of 60mm as suggested. Needed a little more lead to get 20mm thick. A 1oz copper coin has a TC close to 500usec, maybe need a 40mm thick lead disk for large copper or silver coins. I still think the poured lead would make a more repeatable target than a number of coins. How the coins are stacked can effect detection distance. Just something to think about.

[Skippy]

The cast lead targets were intended to be representative of 'large targets' , not copper/silver coins. Such targets might be lead /bronze vessels, bronze axe-heads, cannon balls, weapons, and doubtless plenty of other stuff. They're not necessarily going to have low freqs / high TC values, but they should give a medium-strong signal.
The coin-cache modelling is a particularly difficult target to mimic, though we have made a good start, I think.

Some technical stuff:
I noticed you stating that a 1 ounce copper coin has a TC of 500 usecs (f_corner = 318 Hz ). This is quite a bit different to what I would expect. Unfortunately, I don't have a good non-dugup example of our British 1 oz copper coin, only many corroded ones. But I do have a US Peace dollar. This is just under 1 oz, and 0.900 silver, so not exactly the same, but... I measured it having F_c = 1080 Hz (TC = 147 usec) [measured at 13kHz] and the 'George Payne' quoted f_c = 800Hz ( TC = 200 usec) [measured at 6.6kHz]. So I'm surprised you're seeing 500 usec for a 1 oz coin. Do you have a US silver Dollar (Morgan/Peace) in your collection? It would be good to see how it measures up on your rig.

I'm going to rummage and see what copper pipes I can find, just to see how they compare to your sample.

[green]

The cast lead targets were intended to be representative of 'large targets' , not copper/silver coins. Such targets might be lead /bronze vessels, bronze axe-heads, cannon balls, weapons, and doubtless plenty of other stuff. They're not necessarily going to have low freqs / high TC values, but they should give a medium-strong signal.
The coin-cache modelling is a particularly difficult target to mimic, though we have made a good start, I think.

Some technical stuff:
I noticed you stating that a 1 ounce copper coin has a TC of 500 usecs (f_corner = 318 Hz ). This is quite a bit different to what I would expect. Unfortunately, I don't have a good non-dugup example of our British 1 oz copper coin, only many corroded ones. But I do have a US Peace dollar. This is just under 1 oz, and 0.900 silver, so not exactly the same, but... I measured it having F_c = 1080 Hz (TC = 147 usec) [measured at 13kHz] and the 'George Payne' quoted f_c = 800Hz ( TC = 200 usec) [measured at 6.6kHz]. So I'm surprised you're seeing 500 usec for a 1 oz coin. Do you have a US silver Dollar (Morgan/Peace) in your collection? It would be good to see how it measures up on your rig.

I'm going to rummage and see what copper pipes I can find, just to see how they compare to your sample.

Couple tests. The 16msec Tx constant current I did awhile back with a 1troy oz 99.9%pure copper coin and a silver dollar I borrowed. Remembered 500usec TC, after expanding chart looks closer to 450usec. Just did the 160usec Tx constant rate, 400usec TC.

[Mega B]

Seems a very interesting thread,may put some additional targets in my test garden and carry out a few experiments over the coming weeks,have some interesting machines that i can/could use ie original Crossbow PI,TDI Pro,Nexus MP,Nexus SE,Fisher TW-5 twin box and a substantial amount of coils for these machines upto 20'' so may well,so over the next few weeks and various targets will try and come up with some test results.

The technical side of machines are way over my head,but for my style and my permission of deep pasture and targets that i am after then i use the best tools for the job.