Detection distance for a US nickel and quarter

[Ferric Toes]

In the early days of PI development, detecting larger and highly conductive targets was not a problem. Tx pulse widths were longer, as were sample delays and sample pulse widths. Detecting small and thin coins were a problem, and because the focus at the time was surveying archaeological sites in Greece where very small coins abounded, some development time was spent on improving performance in this area. My early PI's for the hobby market could detect an old penny at 15 inches, but struggled to find a cupro nickel sixpence, which is a little bit larger than a Dime. Initial delay times gradually came down from 100uS to 30uS and today 10uS or less is not too difficult. Tx on times have come down too, which has not helped in the detection of good conductors.

Tx width is a factor in energising high conductors, but also the Rx sampling is very important and this is worth exploring more fully. Because of the desire to detect ever smaller objects, both flyspeck size gold nuggets and broken off needle tips in fabrics, ever shorter delay times resulted. Short Tx pulses, and increased repetition rates were also the order of the day. Now we need to go the other way, but maintaining some small object sensitivity at the same time.

Back in the late 1970's, my company was involved in developing a detector for clearing land of large live munitions and one of my technical colleagues suggested using a very long sample pulse so that a large part of the return waveform would be sampled, rather than just a small portion, as would be the case with a short sample pulse. He called it 'total field recovery' (TFR). As far as I remember, it was never tried in practice, but I have often thought about it since. Certainly we used longer sample pulses but because there was, as now in many designs, a second sample pulse for earth's field cancellation, this would have meant that for long signal decays the later portion of the decay would partially cancel the first. This would be true in the case of coins such as US clad Quarters and even more so for silver ones.

The best way to test TFR would be to use a bipolar Tx so that a sample, or samples, could be taken after a short delay from the end of +Tx1 right to the start of -Tx2. This alternate polarity pulsing would also cancel earth's field without any cancellation of the wanted signal. What I would like to know is whether a number of short successive samples which are summed at the integrator would add up to a much larger voltage whose amplitude is proportional to the length of the target decay. Presumably a long single sample would just give an average value?

I don't have Spice to try these ideas, so this could be a little lock-down or self -isolation project to have a go at, for someone who has.

Eric.

[green]

Thanks for the replies. Think some of the statements are missing something.

Why is the decay start 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. reply #7

The area under the curves (energy dissipated) is the same for the two spheres. reply #18. Think for targets having the same diameter, area under the decay curve is the same. If target time constants are the same, slope and amplitude will be the same. If time constants are different the lower time constant target will have a higher starting voltage.

Signal amplitude is proportional to the cube of the diameter reply #26. Think average signal amplitude is proportional to cube of diameter. If larger target is twice the diameter the smaller, the average decay signal for the larger target will be 8 times the smaller. If decay slopes are different, the larger target will be 8 times the smaller at one sample time only.

BTW, it's not enough to flat-top the current; it has to remain flat-topped at least 5x longer than the longest target tau you are dealing with. That could be a very long time, by PI standards. reply #19

Think above is true for a PI, don't know for a VLF.

[Polymer]

Following this thread with great interest. Did a simple test with my MPP E Ferrite Pinpointer.
It does fit in with what you are delving into, green. Normal silver stuff only gets about half the distance of the gold.

For what its worth:
PI probe 340uH, 68 Ohm resistor in series. cable/probe/shielding @ 256pF.
PPS 2370Hz, Pulse width 31us, sample width 17us.

Distances for solid detection:
Sample delays: 10.5,11,12,15,20,36us
Nickel: 9,9,8,8,7,5cm
Quarter: 6,6,6,6,5,5cm
3.1g Au 9ct.: 9,9,8,8,7,6cm

[green]

You are right in that we are not measuring the voltage induced in the target, but the the rate of change of the current with time and the voltage this induces in the now RX coil (assuming a mono). This is explained in a paper I have somewhere, but still packed in a box after last August's move. From memory, two identical solid spheres are described with the only difference being the conductivity. The low conductivity sphere exhibits a high starting voltage as seen as a voltage across a Rx coil, with a fast decay; while the high conductivity sphere starts with a low voltage and a long decay. The area under the curves (energy dissipated) is the same for the two spheres.

This effect is also exhibited by the coil itself, which becomes its own target at switch off. Suddenly open circuiting the coil causes the voltage across it to rise to several hundred volts as it tries to maintain the magnetic field. This energy is partly dissipated in the damping resistor.

Eric.

Don't have two identical spheres different conductivity. Tested a 1inch square and a 2inch square cut from aluminum foil. Wanted to see if area under the decay curve is the same. Extended the curves on the measured chart. Used Excel to make decay curves(similar decay rate that crossed at 3.5us) that I could calculate area under the curve for both targets. Calculated close to same if 1us was the start time. Thinking about doing the nickel and quarter. Quarter doesn't have straight line decay linear X log Y so generating a quarter curve with Excel is slowing me down. Any thoughts appreciated.

Wrong, compared 1 inch square targets with 1 and 2 layers.

[green]

Here are some linear plots which show why the detection range is greater for a Nickel than for a Quarter, or even a Dime.
The test setup consists of a Tx with a HER208 series diode to minimise the capacitance seen by the search coil. Pulses are not constant current but are 350us duration and 2A peak. Search coil is 8 inches diameter with coins placed on a platform 1.5 inches from the base of the coil and on axis. Rx is a NE5534 and NE5532 giving a total gain of 500. Waveform is inverted with maximum signal at the -3.25V saturation level. X axis is 10uS - 100uS. Comparisons are taken on the 10uS line. Probe was set to x10. Hence the Y axis values should be multiplied by 10.
The first plot is the response with no coin. [ATTACH]50190[/ATTACH]

The next plot is for the GB cupro nickel Shilling. [ATTACH]50191[/ATTACH] This 1954 coin was set 1.5 inches from the coil centre so that the maximum signal just drove the amplifier near to saturation at the 10uS time and -3V. The TC of the decay is close to that measured by Skippy.

The third plot is the clad Quarter. [ATTACH]50187[/ATTACH] Here we see that the starting amplitude at 10uS is very much less than the Shilling, but continues for a longer time. On expanding the timebase, the waveform finally merges with the noise ripples on the 0V line at400uS, compared to the 65 - 70uS for the Shilling.

Fourthly, the Nickel. [ATTACH]50188[/ATTACH] This has the same metallic composition as the Shilling, but due to its smaller diameter, has less amplitude.

Last a Dime. [ATTACH]50189[/ATTACH] Again, a small coin similar to the Quarter in composition. It also displays a low starting amplitude but long TC.

Eric.

Thanks for the test. Looks like your nickel and quarter were about the same at 20us+ like I got reply 24 https://www.geotech1.com/forums/atta...1&d=1588627927

Tried another test with larger target 2times the smaller. The wire ring time constants are closer than the solid targets. 2in target is 8times higher than the 1in target at about 30us and doesn't vary a lot.

[Ferric Toes]

Eric, there's something amiss with two attachments. ( And your inches have become question-marks )

Thanks Skippy. Not sure why that happened on the first two pictures, although there was a glitch when my computer went off page briefly.

Eric.

[Skippy]

Eric, there's something amiss with two attachments. ( And your inches have become question-marks )

[Ferric Toes]

Here are some linear plots which show why the detection range is greater for a Nickel than for a Quarter, or even a Dime.
The test setup consists of a Tx with a HER208 series diode to minimise the capacitance seen by the search coil. Pulses are not constant current but are 350us duration and 2A peak. Search coil is 8 inches diameter with coins placed on a platform 1.5 inches from the base of the coil and on axis. Rx is a NE5534 and NE5532 giving a total gain of 500. Waveform is inverted with maximum signal at the -3.25V saturation level. X axis is 10uS - 100uS. Comparisons are taken on the 10uS line. Probe was set to x10. Hence the Y axis values should be multiplied by 10.
The first plot is the response with no coin.

The next plot is for the GB cupro nickel Shilling. This 1954 coin was set 1.5 inches from the coil centre so that the maximum signal just drove the amplifier near to saturation at the 10uS time and -3V. The TC of the decay is close to that measured by Skippy.

The third plot is the clad Quarter. Here we see that the starting amplitude at 10uS is very much less than the Shilling, but continues for a longer time. On expanding the timebase, the waveform finally merges with the noise ripples on the 0V line at400uS, compared to the 65 - 70uS for the Shilling.

Fourthly, the Nickel. This has the same metallic composition as the Shilling, but due to its smaller diameter, has less amplitude.

Last a Dime. Again, a small coin similar to the Quarter in composition. It also displays a low starting amplitude but long TC.

Eric.

[green]

I found a 1954 shilling and it has a IACS of 5.4. Diameter 23.53mm, thickness 1.7mm.

A 1982 US quarter has IACS of 42.6, diameter 24.29, and thickness 1.74mm.

By comparison a 1971 nickel has IACS of 5.3, diameter 21.24mm, thickness 1.87mm.

A shilling is certainly a good match for the size of a quarter as Skippy stated.

This different diameter of the shilling and nickel should give an even greater initial signal for the shilling when placed in the same geometrical position in the coil. Signal amplitude is proportional to the cube of the diameter of the coin so we should get 1.36 x the nickel amplitude; if I worked it out right.

Hope to plot some decays shortly.

Eric.

A test with 1inch square vs 2inch square aluminum foil targets. 1 layer targets have a different decay rate so the 2inch square target is 8 times(9 divisions on the chart) the 1inch square target at around 6us only. The 1inch 2layer target has about the same TC as the 2inch 1 layer square target so the difference is consistent(about 8 divisions)6.4 times not 8 times. Tried with a round target this morning. 1inch and 2inch diameter targets have a different TC so they would be 8times different at one time only the same as the square targets. What is the reason or math why signal amplitude is proportional to the cube of the diameter? Wondering if I'm doing something wrong.

[Skippy]

I've done some tests on my 13 kHz VLF setup ( Fisher F75 based ). I used the three different versions of our British one shilling coin; 925 Sterling silver, 500 fine silver, and Cupro-nickel [ 75% Cu / 25% Ni, the same as US 5 cent ]. The coin is bigger than a 5c, smaller than a 25c US.
I placed the coins 8cm above the sweetspot of the coil, in the same place each test. The demodulated voltages were measured, and a bit of Pythagorus used to calculate the total signal strength.
The results are:
CuproNickel : 275 mV
500 silver : 442 mV
925 silver : 469 mV
As a percentage of the strongest, these values are: 58.6% ; 94.2% ; 100%

The measured time constants and corner freqs for them are:
CuproNickel : 9.5 usec / 16.8 kHz
500 silver : 35 usec / 4.55 kHz
925 Silver : 62 usec / 2.57 kHz

I think it's highly likely there's some skin effect visible on the two silver coins, the 1 : 6.5 ratio of the time-constants doesn't reflect the 1 :15 (very approx) ratio of conductivities.
This is just fer interest, it's something I've not tried measuring before.

[Ferric Toes]

I found a 1954 shilling and it has a IACS of 5.4. Diameter 23.53mm, thickness 1.7mm.

A 1982 US quarter has IACS of 42.6, diameter 24.29, and thickness 1.74mm.

By comparison a 1971 nickel has IACS of 5.3, diameter 21.24mm, thickness 1.87mm.

A shilling is certainly a good match for the size of a quarter as Skippy stated.

This different diameter of the shilling and nickel should give an even greater initial signal for the shilling when placed in the same geometrical position in the coil. Signal amplitude is proportional to the cube of the diameter of the coin so we should get 1.36 x the nickel amplitude; if I worked it out right.

Hope to plot some decays shortly.

Eric.

[green]

You are right in that we are not measuring the voltage induced in the target, but the the rate of change of the current with time and the voltage this induces in the now RX coil (assuming a mono). This is explained in a paper I have somewhere, but still packed in a box after last August's move. From memory, two identical solid spheres are described with the only difference being the conductivity. The low conductivity sphere exhibits a high starting voltage as seen as a voltage across a Rx coil, with a fast decay; while the high conductivity sphere starts with a low voltage and a long decay. The area under the curves (energy dissipated) is the same for the two spheres.

This effect is also exhibited by the coil itself, which becomes its own target at switch off. Suddenly open circuiting the coil causes the voltage across it to rise to several hundred volts as it tries to maintain the magnetic field. This energy is partly dissipated in the damping resistor.

Eric.

I have wondered about area under the curve. If I double peak Tx current target Rx signal doubles. Tx energy increases 4 times, does signal double because Tx out Rx back. I recorded amplifier out, no target and target. Subtracted no target from target and averaged signal from 8us to 108us. Quarter(.239V) nickel(.133V). Maybe my method isn't the best, wonder what you get if you try to measure average.

[green]

I can see that I will have to fire up one of my later test units where I can vary the flat topped pulse width from 100 - 1000uS and plot the decay for nickel and quarter. Ideally I will look for a low conductivity coin the same size and thickness as the nickel to remove the size variable.

Eric.

Interested in what you get. Added targets would be interesting. If you have a nickel and clad quarter I would be interested if your data is similar to mine. Still not sure why the smaller nickel has a larger signal than the larger quarter.
Charted a nickel and quarter again to see if I get similar results. Tx on time, 75us, 150us, 300us and 600us.

I'm guessing the nickel would have a larger signal if it was 1/2 the diameter of the quarter

[Skippy]

Eric: The 5c nickel IS a low conductivity coin, being Cupronickel. Our British 1 shilling ( or the identical large 5p ) is actually a decent match for the nickel, and if you can find 50% silver and 92.5% silver shillings, you have an experiment in the making.

[Carl-NC]

https://www.geotech1.com/forums/atta...4&d=1588518934 chart from reply #7. Tx is 20us ramp to .5A with constant .5A for 4980us. Should be long enough? Maybe I'm doing something wrong?

Oh, I see. Yes, 5ms should be long enough. Time to rethink.