Cylinder Coils?

[Carl-NC]

Since we have all the experts assembled, let's look at the possibilities of a coil for a CCPI. (round scramble-wound), diameter 300mm, 70 turns, 2 Amps current,
di/dt = 140/10us. At a distance of 600mm this gives us 0.0836 Gauss.
This is possible for a CCPI, but would be an unusable coil for a traditional PI.

This is where directly comparing CCPI with monopolar PI is tricky. It is dB/dt that determines the induced target kick and, in your example, that would be 8.37uT/10us = 0.837uT/us. Let's compare that with a monopolar design, where the coil is limited to, say, 20 turns but can turn off in 2us. The magnetic field at 600mm (for the same 2 amps) is now 2.39uT so dB/dt = 1.195uT/us, which is actually higher than the CCPI case. However, the CCPI can likely run a much higher N on the RX coil without compromising the timing. The bottom line is, it's hard to see all the gotchas involved so you almost have to build everything out and compare with real targets to get an accurate answer.

[Tinkerer]

The equation is

which states that total flux is proportional to inductance and current. This suggests that a higher inductance always wins. However, for a fixed power supply voltage, current is usually inversely proportional to inductance. In a traditional PI the turn-on slope is VTX/L where VTX is the TX power supply so peak current is VTX/L*t_on (assuming R_L = 0). In a sinusoidal TX the current is proportional to VTX/(2πf*L). This suggests that total flux is ideally independent of inductance regardless of the TX driver.

Even if it were, flux distribution can vary with winding geometry. For example, a scramble-wound coil vs a flat spiral. And then you have the RX side, where ε = N*A*dB/dt, so here you want more turns on the coil. In a split-coil design you have independent control of the TX and RX coils, but in a mono coil PI you don't. If you were to increase the turns on the coil then, yes, RX coupling would increase but it would also slow down the coil decay and push out the earliest sample delay. This reduces depth. All of this suggests that, for a given coil type (say, round scramble-wound), there is an optimal inductance that produces the deepest depth on a given target.

Since we have all the experts assembled, let's look at the possibilities of a coil for a CCPI. (round scramble-wound), diameter 300mm, 70 turns, 2 Amps current,
di/dt = 140/10us. At a distance of 600mm this gives us 0.0836 Gauss.
This is possible for a CCPI, but would be an unusable coil for a traditional PI.

[Detectorist#1]

In the words " on a given target " is cashed the dog. For small targets the fastest coil is important, for medium size targets medium speed coil with optimal inductance is preferred.

[Detectorist#1]

Thank you Carl for this " All of this suggests that, for a given coil type (say, round scramble-wound), there is an optimal inductance that produces the deepest depth on a given target.

[Carl-NC]

The equation is

which states that total flux is proportional to inductance and current. This suggests that a higher inductance always wins. However, for a fixed power supply voltage, current is usually inversely proportional to inductance. In a traditional PI the turn-on slope is VTX/L where VTX is the TX power supply so peak current is VTX/L*t_on (assuming R_L = 0). In a sinusoidal TX the current is proportional to VTX/(2πf*L). This suggests that total flux is ideally independent of inductance regardless of the TX driver.

Even if it were, flux distribution can vary with winding geometry. For example, a scramble-wound coil vs a flat spiral. And then you have the RX side, where ε = N*A*dB/dt, so here you want more turns on the coil. In a split-coil design you have independent control of the TX and RX coils, but in a mono coil PI you don't. If you were to increase the turns on the coil then, yes, RX coupling would increase but it would also slow down the coil decay and push out the earliest sample delay. This reduces depth. All of this suggests that, for a given coil type (say, round scramble-wound), there is an optimal inductance that produces the deepest depth on a given target.

[Detectorist#1]

No "magic coil". For every different purpose have optimal coil as shape, measures, inductivity, capacitance and soo further.

[Smirnov-Arta]

...

Sorry friend). I went through all this at school), not counting the universities. Why do I need your reference?). You have given one of the formulas. There are several of them. Any physical quantity can be expressed using different parameters, is this news to you? I pointed you to another one. They are equivalent if translated to the area of a specific coil. If you are not lying, then take textbooks and watch / read, measure the parameters and calculate. And then take the oscilloscope and look. And then take a spectograph and take a look. And everything will fall into place... Therefore, there is no need to "reinvent the wheel", everything has been open for a long time. You just need to take it and do it, based on really measured physical processes occurring both in the coil and in the ground, etc., and not hypothetical assumptions about some kind of "Magic Coil".

[Detectorist#1]

The real formula for the magnetic flux for the coil is "Ф(Webers) =B(Tesla) x S(meters ^2)"
This is different from multiplying inductivity by current!

[Smirnov-Arta]

..

Hi. Your joke is not appropriate. Everyone understood what it was about. It's about the magnetic flux formula. Yes, I made a typo in the designation of the magnetic flux "Ф". (Google translation). But this does not change the essence. So your reference to mathematics is... zero. It's about physical processes.

[Detectorist#1]

Hi Smirnov-Arta,
It is useful to know elementary mathematics. The expression "F=F x I" is possible to divide by F for both sides of expression and the result will be - I (current) always is equal to 1

[Smirnov-Arta]

The magnetic flux of any coil. F= FxI. F is the inductance, which will depend on the length of the coil, the method / shape of the winding, and the density of the coupling of the coils. I is the current strength, it will depend on the winding R, on R at a specific frequency and on the applied/supply voltage V (5, 6, 9, etc.). Therefore, it is absolutely not correct to say 325uh or 330 or 335, without specifying the MD scheme.

[Nova]

OzGold , mate "325uh is optimum for medium to large gold", again, have you done some serious testing against mass produced same size coils on buried targets to prove this?,
some people say , "i know it's better because i'm finding targets i couldn't find previously", even that is not proof as the temp of the ground could be different, those targets might of been missed previously etc....

[Nova]

MdToday, yeah mate , i have done my historical research, "Trove' brings up most of the info on his field, and yes the fork area produced the majority of the chunky nuggets from 2oz to upto 22oz that the founder of the gully was named after, there is very little mention of the other gullies except for one mention of nuggets upto 2grams found in some of them, so we know that if they were digging 2 gram nuggets at the bottom of the gully at 3 to 5ft deep then what chance do we have?, BUT people like Nenad who got in first cleaned up the known/found patches so that leaves the rest of us little to no chance of scoring any, but i might take your advice and hit those areas you mentioned as i haven't detected up that way yet, what i would like to see is a map and all the patches that have been found marked (by all the finders), since the place has been smashed it shouldn't be considered blasphemous to ask this, it's not like some new tech will come along and suddenly allow you to hit the area again and start digging up ounce chunks ( what do you think of this suggestion, still a bit too much to ask?)

[Mdtoday]

Hi Oz Gold, i was at Mt Crawford yesterday to test out my new Coiltek 11in Flatwound mono, i was hoping to hit Watts Gully but some bugger got there before me, and I got there at 7.10am (Bugger), so i hit Dead Horse Gully towards the very northern end (next to the house on the hill) and the usual turned up, (Bullets and rust), i'm not going to give it away but by a suggestion of my brother i tried something a couple of months ago, and by testing i can get targets anywhere from 4-6 inches deeper, BUT i still can't fing any Gold in this God Forsaken Gold field, i'm really starting to believe that there never was any Gold on this field (with the exception of the richest part in Watts Gully), have you detected in this field and actually found some Gold?, i know i'm looking in the right spots and i'm coming home with deep targets that no one else is getting, i'm just about at the point of cursing that place and never stepping foot on that bloody soil again ARRGGHHHH!!!!

Hi Nova, Watts Gully has been flogged over the years, the alluvial areas were frequented during the depression and worked down to the fine stuff.
Also the lower section of Watts Gully was used as a gravel wash and if you look carefully, you can see the packed layers where they have been spread out. The bigger gold came from about mid ways up the gully just below the fork and a lot of it was anywhere from 4 - 12ft down. The deeper leads were further up on both the left and right forks. There is gold there but to save sanity, I take my pan with me anytime I visit that area so I can at least break the detecting up if I get sick of digging buckshot. My advice if you haven’t already done so, is to purchase the yearly pass and get down into and around sailors gully and south of that, up through the pines and back towards the Forties road.
The problem with the whole of Mount Crawford is the surface work that has been done over the years for the forrest plantation. It’s altered a fair portion of the landscape to suit, so all those nice little gullies and original surface might be under as much as 3ft of soil in some places.
I live not far from this goldfield and been going out there for the best part of 40 years detecting/ panning and field testing detectors.
I have found gold but it’s a lesson in patience for sure. If you can find gold in Mount Crawford, you will shoe it in on the Vic or WA fields, well assuming the gold gods are with you….

Apologies for posting in this thread, I thought it was under general sorry guys.

cheers

[Tinkerer]

Smirnoff-Arta

I agree with your technical coil observation. There are a few things that could make a difference in detector or coil performance that needs to be kept in the back of the minds of detector and coil builders.

1. Coil wire size. Go to a wire chart and look up the skin effect frequency for your chosen wire diameter.
2. Build a spiral, single layer coil and a bundle wound coil and test them on different TC targets. The spiral, single layer coil will require more turns than a bundle wound coil to get the same inductance.
3. When looking for low TC gold targets, you need to minimize the coil seen capacitance to allow for a higher damping resistor value and earlier sampling and the integration of many sample RX pulses.
4. Shielding the coil will depend what type and size targets you are seeking and the temperature, and environment of where you are seeking your targets.
5. Coil shapes and sizes will depend on whether you are searching for maximum depth or maximum sweep area with a lesser depth.

Optimum coil design is a balancing act!

Joseph J. Rogowski

Optimum coil design is a balancing act!

I fully agree, even more so when we move to a Bipolar Continuous Current Pulse Induction design. Sweep area, depth, weight and ergonomics, power consumption, sensitivity, al have to be balanced. And then one even has to balance between objectivity and subjectivity of the testers and users.