Detection distance for a US nickel and quarter

[green]

Have been thinking comparing detection distance in ground is difficult because grounds are different. Was thinking in air would be better. Detection distance is controlled by signal to noise ratio. Testing in air in a faraday cage vs next to a AM radio station would probably give different results. Thinking measuring signal at amplifier out might be another way. Coil signal=amplifier out/amplifier gain. Tested my 8inch figure8 coil and a 8inch flat spiral. With Tx_160us,cr, 1A peak the figure8 calculated a little less than 2mV@10us after Tx off, the 330uH flat spiral a little over 2mV for the nickel. N, Q_4 is calculated coil volts out at 300, 400 and 500mm distance using distance vs amplitude_15 with coin centered on coil. Target delay_1 are some measurements I made awhile back with target swinging on a pendulum at about 1meter/second. Target was lowered to coil until it was detected every time. Have read where a GPX4500 can detect a quarter at 24 inches, a Garrett ATX 19 inches. Think the peak current is higher and a larger coil than 8 inches. Hoping to learn what I need to do to detect the quarter near 20 inches. Looks like going to a 12 inch coil and doubling the peak current might get the quarter close to 19 inches.

Does 2mV at coil out 10us after Tx off with 1 A peak current make sense?
Just some thoughts, any ideas or suggestions appreciated.

[waltr]

Remember that a US Quarter is a high conductor (longer TC) verse the Nickle as a low conductor (shorter TC).

I found that a high conductor needs a longer TX pulse to fully excite and have it 'return' a strong signal.
The typical rule of thumb is the TX pulse time should be 3-5 times the target's TC.

I found that increasing the TX pulse time from 100us to 180us noticeably increased the output signal of high conductors (Quarters) and increased the detection distance. This is part of the logic behind some of the PI detectors that do two or more TX pulse times. For example the SD2000 TX pulses are 240us then 4 pulses at 60us. Of course the Sampling timing is also later for the long TX pulse and sooner for the short pulses.The long pulse detect high conductors whereas the short pulse detect low (gold nuggets) conductors.
Try increasing the TX pulse time to over 200us and see if the Quarter's signal increases.

Another thing to look at is the higher conductor's signal is still pretty good for a longer time. What I do in my HH2 is have a longer sampling time on the second sample that drives the integrated output in the opposite polarity. This creates a larger (negative) output signal for high conductors than sampling using only the first sample.
This brings up the idea of the integration which adds up a signal over a period of time (the sampling periods). Looking at the Voltage at an instant of time is not the best way.

[waltr]

Read through this thread and the links in it that go over the issues of detection distance, signal and various target conductivity.

https://www.geotech1.com/forums/show...pth-not-really

[green]

Remember that a US Quarter is a high conductor (longer TC) verse the Nickle as a low conductor (shorter TC).

I found that a high conductor needs a longer TX pulse to fully excite and have it 'return' a strong signal.
The typical rule of thumb is the TX pulse time should be 3-5 times the target's TC.

I found that increasing the TX pulse time from 100us to 180us noticeably increased the output signal of high conductors (Quarters) and increased the detection distance. This is part of the logic behind some of the PI detectors that do two or more TX pulse times. For example the SD2000 TX pulses are 240us then 4 pulses at 60us. Of course the Sampling timing is also later for the long TX pulse and sooner for the short pulses.The long pulse detect high conductors whereas the short pulse detect low (gold nuggets) conductors.
Try increasing the TX pulse time to over 200us and see if the Quarter's signal increases.

Another thing to look at is the higher conductor's signal is still pretty good for a longer time. What I do in my HH2 is have a longer sampling time on the second sample that drives the integrated output in the opposite polarity. This creates a larger (negative) output signal for high conductors than sampling using only the first sample.
This brings up the idea of the integration which adds up a signal over a period of time (the sampling periods). Looking at the Voltage at an instant of time is not the best way.

Thanks for the replies. Recorded constant current because I couldn't get 70us constant rate. 20us ramp to .5A, then constant .5A. Charted log Y scale, easy to compare change in detection distance with included calculated distance chart(at 400mm 1division is about 20mm change in detection distance). About 1division increase in amplitude from 1TC to 2TC on time. Not much increase after 2TC increase in on time. Test controls constant current, If current wasn't constant, increasing on time could increase peak current and on times greater than 2TC would cause a higher increase in signal.

[green]

This brings up the idea of the integration which adds up a signal over a period of time (the sampling periods). Looking at the Voltage at an instant of time is not the best way.

Have wondered which is best. Interested in detection distance for a quarter with King JL's detector he is working on. Think he is sampling at a delay time not integrating. Sample at start of sample time is greater than average. Integrator out=average volts in during sample time*integrator gain[integrator gain=R feedback/R in*sample time*sample rate]. Wondering if one method is better for short TC targets(1us) and maybe the other is better for long TC targets.

[green]

Read through this thread and the links in it that go over the issues of detection distance, signal and various target conductivity.

https://www.geotech1.com/forums/show...pth-not-really

Statement at end of test reply#1 Decreasing the background noise in the electronics by 50% gives the same depth improvement as increasing the power by 10 times (or there abouts)

Been thinking decreasing noise by 50% gives the same depth improvement as increasing signal by 2(4 times the power).

[green]

Was looking at some charts I've posted in the past. Why is the signal for the nickel greater than the quarter or the Ike dollar? The nickel is smaller, I could guess but then it would be a guess.

[waltr]

Was looking at some charts I've posted in the past. Why is the signal for the nickel greater than the quarter or the Ike dollar? The nickel is smaller, I could guess but then it would be a guess.

The Nickle signal (Ta) is larger ONLY at a much shorter time since it has a short TC (low conductivity). After 10-30 usec the Nickle signal is LESS than the higher TC targets.

Possibly in a low conductive target the the magnetic field is stronger, for a shorter time, due to Eddy currents collapsing much faster.
Another thing to play with is the TX ON time verse various targets.

The nickle and other low TC target only require a 50-80usec TX pulse (3-5 TC Tau) to obtain largest signal whereas, high TC targets, like a Quarter, requires a 180-200 us or more TX pulse.

[green]

The Nickle signal (Ta) is larger ONLY at a much shorter time since it has a short TC (low conductivity). After 10-30 usec the Nickle signal is LESS than the higher TC targets.

Possibly in a low conductive target the the magnetic field is stronger, for a shorter time, due to Eddy currents collapsing much faster.
Another thing to play with is the TX ON time verse various targets.

The nickle and other low TC target only require a 50-80usec TX pulse (3-5 TC Tau) to obtain largest signal whereas, high TC targets, like a Quarter, requires a 180-200 us or more TX pulse.

Thanks for the reply. Trying to understand why the nickel has a higher signal at start of decay. TRT_58 used a constant current Tx 5000us on time so all targets should have decayed before turn off.

Tried another test to see what happens during Tx on with constant rate Tx at 16700A/sec. With the same Tx signal the quarter has a higher signal during Tx on, the nickel has a higher signal at beginning of Tx off. Decay for the nickel is noisy during Tx on but probably good enough to see the decay slope.

IDMD discusses a coin and a coin with a hole using a VLF to compare signal strength. My PI tester shows the coin with a hole has a higher signal at start of decay TRT_58 reply #7, not much higher. Don't know if coin with hole relates to the nickel quarter comparison.

[Koala]

There's two composions of the Eisenhower dollar and two for the nickel.

Are we sure that it just a size difference

[Skippy]

Skin depth for the 5c nickel will be roughly 4 times deeper than for high-conductor coins like a 25c. Remember skin depth was inversely proportional to the square-root of conductivity of the metal. And Cu-Ni as used in the 5c has IACS conductivity about 5% ( 4.8% from memory?), so its skin depth is 4.5 times that of pure 100% IACS copper.
I assume if more metal has currents flowing in it, it will give a stronger response ? Negated somewhat by the smaller diameter/thickness of the 5c.

Obviously you would need a selection of coins the same size but different conductivities to really do any scientific analysis. Using test blanks would be one way, but using real coins, there is some choice with British coinage. A number of our pre-decimal coins were struck in 925 silver, 500 silver and Cu-Ni ( same alloy as the US 5c ). If you include the New Zealand CuNi threepence, you can obtain a weight-proportional family of coins: 3d , 6d, 1shilling (=12d), 2shilling(=24d) and 2s6d and 5s if needed.

[Ferric Toes]

Another way of looking at it, is that the Tx sets up a given value of magnetic field which we can take as a step from a plus value down to zero. The current path in the nickel is a high resistance relative to the low resistance in the quarter. There is a basic law (Maxwell's?) that states that current will circulate in a conductive target to try and maintain the magnetic field at the value it was just prior to switch off. For either target the field is the same, with the result that the voltage has to rise to a higher value for the high resistance target than it does for the low resistance one to try and maintain the same current and field. The end result is that the induced voltage in the Rx mode also starts at a higher value for the low conductive (nickel) than for the quarter.

Eric.

[Skippy]

I meant to add: When I stated the skin depth in Cu-Ni was greater than copper / silver , this would be for a large sample of the metals. Coins have a finite thickness, which obviously puts a limit on the depth currents can flow. They can't flow 3mm deep in a 5c coin, because it's not that thick.

[waltr]

Another way of looking at it, is that the Tx sets up a given value of magnetic field which we can take as a step from a plus value down to zero. The current path in the nickel is a high resistance relative to the low resistance in the quarter. There is a basic law (Maxwell's?) that states that current will circulate in a conductive target to try and maintain the magnetic field at the value it was just prior to switch off. For either target the field is the same, with the result that the voltage has to rise to a higher value for the high resistance target than it does for the low resistance one to try and maintain the same current and field. The end result is that the induced voltage in the Rx mode also starts at a higher value for the low conductive (nickel) than for the quarter.

Eric.

That is what I was trying to say. Well stated Eric.

[Skippy]

I'm struggling with this statement of Eric's:
"with the result that the voltage has to rise to a higher value for the high resistance target than it does for the low resistance one to try and maintain the same current and field"
It is the current that creates the magnetic field, which is measured by the detector coil. So the different voltage on the target is an interesting side-effect, but not one that is being measured. ?