MD Physics

[Skippy]

Quote:"It is a good idea to start object size and material oriented thread"

As you are new, you are unaware that we already have (several) threads on this topic, myself and Mr.Green being contributors. When I find time, and have a usefully functioning detector, I intend getting them going again. However, they did not specifically address finger rings, mainly being about general behaviour, and coins. Coins are 'universal', thanks to eBay etc, anyone can obtain a variety of coins. There is also a "large-target" discussion thread, for cache-hunting and large solid item hunting, eg. ingots, cannonballs, lead pots.

As hunting for finger rings is a real-world pursuit, it does deserve some special thread.

I will post up links to these threads later.

[green]

Quote:"Wondering why the smaller would detect 7cm more distance if both made of copper."


Not wanting to be repetitive, but CORROSION is the reason.

"I suggest that corrosion is one reason for the difference, especially if they are old and long-buried."

"Regarding Mikhal's rings: A gold plating will slow down the effects of corrosion on a copper item. It's not the plating itself that is responsible for the different resistivity, it's how it has stopped the good copper turning into rough-surfaced, porous high-resistivity copper."

Even if they were quite good quality copper, they would corrode a bit, and it's likely they will have some natural impurity in them, even if it's just 1%. Some of our British copper coins from the 1700's were made from natural unrefined copper, and they are usually very corroded, little detail remaining, often having lost 20% of their weight.
We find plenty of gilded buttons here, typically from the mid-1800's. It's pretty apparent when you've seen a few, that if the gilding is still well attached, the underlying metal ( brass or bronze in this case) must be uncorroded. Anywhere the gilding is absent has a rough surface, obviously heavily corroded.

Unless Mikhal mails his rings to us for testing, we're not going to make any progress. He could, however, post up some photographs of them.

Perhaps it's worth starting a new thread specifically on finger ring detection ? Tests on real rings, test rings made from known metals, mathematical analysis/modelling ? I'm not that interested myself, as I'm not a jewellery-hunter ... I've never done beach detecting.

Wondered if adding resistance in series would be the same as corrosion. The added resistance increased the signal at low delays. Would corrosion act the same?

[Skippy]

Mikhal: Here are some of the threads on modelling/simulation/testing of targets, coils:

An unfinished thread on target behaviour:
http://www.geotech1.com/forums/showt...6-test-targets

A thread on large targets/caches:
http://www.geotech1.com/forums/showt...PI-systems-%29

A long thread on target size and coil size:
http://www.geotech1.com/forums/showt...etection-depth

[Dave J.]

A target is an eddy current loop. For a given frequency, it can be described as an inductive reactance of the current path and the resistance of that path. (Describing it in the broadband pulse domain is a messier proposition, but begins with what happens at a frequency you regard as relevant.) The phase is the arctangent of the ratio of the conductivity of that current path to the reactance of that path.

For a given size of the current path, the inductive reactance is a constant. Maximum sensitivity to the resistive component is when the resistance equals that reactance. If the resistance is higher, the amplitude of the current is less and therefore the resistive phase signal is weaker. [The reactive signal is mostly iron minerals in the ground, and loop offsets if it's an induction balance searchcoil. So the metal detector is designed to ignore those as best possible.] High resistance = weak signal.

If you increase the conductance (decrease the resistance) to where the phase is 'way past 45 degrees, you can get up to 71% more current, but less of it is resistive phase. The result can be a large target with a surprisingly weak response. An example is the old Fisher Impulse. A US 5 cent nickel coin air tests about 11 inches and gives a fairly crisp response. The much larger silver dollar also air tests about 11 inches and sounds like a lump of mud.

[ifrenide]

Please help me to understand slopes and average of slopes related to R and X signals to use it in coding .thanx

[MTammet]

Hi,

Will add results in graph. Asked my friend K. Märtens. Unfortunately in Estonian. Horizontal unit 10 micro sec, vertical signal strength. 1. item -gold gilded baby copper ring 1,6 gram, 2. Bronze with nice patina 2 gram, 3. 2 euro cents for comparison, 4. Bronze with nasty Cu2O 4 gram, 5. Roughed copper ring 2,4 gram. Will post graph and pictures. When I made measurement with Vallon I also understood my mistake in interpretation. I measured those two rings at normal mode when changed to mineral the results are very similar with the graph. As I have understood Vallon is designed to find pcb tracs related to IEDś which are copper coated with.... It is probably because otf that internal firmware tuning that there is this enermous difference in normal mode.Metallobjektide decay-d.pdf

[ifrenide]

Hi.
I will make 2 leds vdi or maybe more. pic microcontroller based vdi with LEDs will be simple .
Thanx

[MTammet]

Thank you Skippy! Was interesting and educating reading.

Mihkel

[Skippy]

Mihkel:
There's also this thread, which is intended to be about making/modifying a PI detector to make it suitable for target testing. Mr. Green and Eric F. have both got some working setups.
But towards the end of it, I did a simple model of a square sheet of metal, by assuming it was a ring/loop. This is relevant to your finger ring testing. Posts 107 onwards are the significant ones:

http://www.geotech1.com/forums/showt...e-tester/page5

I forgot to mention in that thread what the inductance calculation was.
Here is an online calculator, showing the formula for inductance of a loop:

https://technick.net/tools/inductanc...circular-loop/

[MTammet]

I will try to get similar presentation. I will use your drilled zinc pennies for reference. As you noticed from my graph 2 eurocent is pretty close to zinc (it is steel) penny so I will drill it to.

[green]

Mihkel:
There's also this thread, which is intended to be about making/modifying a PI detector to make it suitable for target testing. Mr. Green and Eric F. have both got some working setups.
But towards the end of it, I did a simple model of a square sheet of metal, by assuming it was a ring/loop. This is relevant to your finger ring testing. Posts 107 onwards are the significant ones:

http://www.geotech1.com/forums/showt...e-tester/page5

I forgot to mention in that thread what the inductance calculation was.
Here is an online calculator, showing the formula for inductance of a loop:

https://technick.net/tools/inductanc...circular-loop/

What do I enter for relative permeability?

[Skippy]

Relative perm = 1.00 for air or a vacuum

[pustareka]

http://coil32.RU

[green]

Mihkel:
There's also this thread, which is intended to be about making/modifying a PI detector to make it suitable for target testing. Mr. Green and Eric F. have both got some working setups.
But towards the end of it, I did a simple model of a square sheet of metal, by assuming it was a ring/loop. This is relevant to your finger ring testing. Posts 107 onwards are the significant ones:

http://www.geotech1.com/forums/showt...e-tester/page5

I forgot to mention in that thread what the inductance calculation was.
Here is an online calculator, showing the formula for inductance of a loop:

https://technick.net/tools/inductanc...circular-loop/

Wondered how close to calculated inductance some wire loops would be to L=TC*R and how much the soldered joint would effect loop resistance. Tried technick calculation and one I've used in the past, LuH=r(inches)/9 for a single loop. Nothing close, wondering what I am doing wrong. Cut some AWG19 wire to length, wrapped in a circle and soldered. Recorded decay.

targets same location, centered on one of the 200mm fig8 coils.

[Skippy]

Gotta admit, I don't like the idea of soldering a loop of high-conductivity copper up with a random blob of low-conductivity lead alloy. Maybe it was not leaded solder, but lead-free? Was it a neat butt joint, with flat faces, or did you just chop the wire with cutters and leave that distinctive chopped taper etc? Did you file the solder down afterwards, or leave a spherical ball on the join ? Etc Etc?

Anyway: Case C , with the 10a + 9b bit in the formula is no use for single turns, ... it's for solenoids, I think?
You've obviously got L calculation problems, you're miles out, but your R value looks good.
Here's my maths for the 40mm coil:
Loop resistance = 3.25 mOhms ( using 1.68 x 10^-8 for resistivity )
Loop inductance = 97 nH = 0.097 microhenry
So Time-constant = L/R = 29.9 microsecs. Which fits nicely with the measured value of 33 usec.

The effect of the solder join could be investigated by making rings with 1, 2, 3, 4, 5 joints, and seeing how TC varies ( increases) ... the trouble is making a perfect cut without removing much metal.

I'm beginning to regret not starting the 'Ring target thread' now, we're going to have to copy stuff over.