The Nail Board Test And Sensitivity

[EL NINO77]

A very important element in various detector separation  tests.. / type Steve .., Monte .., Carl .., Calabash .., Abenson .., David TnS., Sube /, ,,, is to find out how fast we can sweep the coil over the tested target. to successfully detect the tested target ....

This is perhaps one of the main findings of such testing ... It is a very valuable lesson .. which you gain by testing such detectors.

 Because some detectors prove the same test even with faster sweeping the coil .... and other detectors you need to slow down the sweeping reasonably to achieve the best results..that you can get from the brand of the detector ..

 Of course, there are detectors that have different technical limits .. and so they get poor results in tests ... even when slowly sweeping the coil ... and here we can say that the detector certainly has shortcomings in the type of separation ...

And that's why, in a given terrain, you will also find other valuable targets with a separation-better detector or coil ... which have so far been masked by iron when using a separation-less good detector ...

Adjust the style and speed of sweeping to achieve the best results in the separation .. ,, in the iron-polluted terrain where you expect still well items.. found ...

[nickelNdimed]

20 hours ago, Geotech said:

When I test separation I use a 1 meter sweep at 1m/s set by a metronome. 

I’ve been wanting to use this test to compare a few detectors I own and thought this variable needed some control.  I thought it was odd this was not used in video comparisons and I was such a nerd for thinking that a metronome and some sort of markings for sweep length/distance for reference might be useful....  Guess I’m not the only one so that makes me feel better. 

And I am now wondering if the cadence foot sensor from my Garmin might be a useful tool in monitoring this if I attach it to detector shaft far enough away from the coil or my arm so it does not interfere. I really only need a point of reference from unit to unit that I am testing 🤔 

[Geotech]

Dimed, I use a metronome app on my phone sand and just listen to the ticks, it's pretty easy to synchronize the sweep reversals on the ticks and still listen to the target responses.

Steve, you are right that a mechanical sweeper would be best but that's quite a project. I use a much simpler jig made of 2x4's:

I make it ~6 feet long and glide the coil right on the top board to the beat of the metronome. Targets are placed on the 3" tier* (nail-quarter-nail) which is marked in 1/2" spacings** so now I have precisely repeatable depth, target spacing, and sweep speed. There is also a 6" tier so I can do a deeper test or a staggered-depth test. I can also place Monte's nail board on a tier and test that.

I don't think it's necessary to take the audio out of the equation and, in fact, it could be really tricky to do that. I'm listening for 3 distinct audio responses and as the targets get really close together this becomes a very rapid-fire sound that might be tough to "meter." With everything else tightly controlled, it's not too hard to hear when those responses start to blend.

I came up with this back at White's and did a bunch of testing on all sorts of detectors, new & old. At that time the Garrett AT-Gold was the best at about 1-1/2" minimum spacing and the Tejon was second at 2". Most all other detectors were around 4" including the Deus which had 1st-gen firmware before they got serious with recovery speed. The old analog XL-Pro was around 8".

* not 3-1/2" as I said in the prior post

** I cut shallow v-notches for the nails so they don't roll around

[Steve Herschbach]

On 12/25/2021 at 10:48 AM, Geotech said:

I don't think it's necessary to take the audio out of the equation and, in fact, it could be really tricky to do that. I'm listening for 3 distinct audio responses and as the targets get really close together this becomes a very rapid-fire sound that might be tough to "meter." With everything else tightly controlled, it's not too hard to hear when those responses start to blend.

I agree for separation type tests. I was thinking max depth tests where people are straining to hear a signal at all. A meter response would mean they would have to hit the same mark on the meter to be considered a signal. Think like old TR deflection meter days.

[Geotech]

That's exactly how I measure the responses of various coils. I take the demod outputs straight to an oscope and measure real signal strength. That would not work for measuring the overall response of a real detector because it bypasses all the filtering & DSP, but you could take the signal off the headphone jack and put it on some kind of meter.

[mcjtom]

On 12/25/2021 at 2:51 AM, Geotech said:

A lot of people test by placing the nails longitudenally but this becomes a test for a co-located composite response instead of a separation response. If you understand why nails give a double-beep when swept longitudenally then you will understand the mechanics of this test.

Does 'longitudinally' mean the nail length being oriented parallel to the direction of the sweep?  If yes, is the double-beep on a single nail because the speed of the sweep is low enough, and the recovery speed high enough, and the nail long enough to appear as a target in the two adjacent response recovery cycles?  If this is true, doesn't it indicate separation ability: with the swing an recovery speed fixed, there will be a nail short enough to beep as one target?

[Geotech]

Yes, that's what I mean by longitudinal.

A nail creates a double beep because it has a maximum return signal when the coil's magnetic field aligns with it. Here is an illustration

The arrows show the direction of the coil sweep. The magnetic field bends outward and the nail will have a maximum response at points (a) & (c), while minimum at (b). A coin is the opposite; it will have a maximum response at (b). So imagine a nail at (a) and a coin at (b); as the coil sweeps over they will both have a maximum response at the same point in the sweep. This is independent of recovery speed, it's just geometry.

[mcjtom]

Awesome explanation!  I think I understand why longitudinal nails have the strongest response at the edges (the induced currents will be swirling in the nail around the nail's axis) and the flat coin in the middle (for the same reason, the coin axis is vertical there just like the magnetic field lines).

Which bring me to a probably related question: with double-D coil, if a flat coin is close enough to the coil there are 3 beeps in one sweep.  When you stand up the same coin (the plane of the coin perpendicular to sweep direction now) it beeps twice, just like longitudinal nails. 

I can explain the latter with the nail theory above, but what happens in the first case with the flat coin?  The edge signals are weaker than the middle but still strong enough to be classified as a target?

 

p.s. Or the double flat coin beeps occur when the 'returning' magnetic field is perpendicular to the coin again, beyond the far side edges of the coil?  The field is weaker there than in the middle, but flat coin is a strong enough target to register even there if the coil is close enough?

[Jeff McClendon]

This entire topic is one of those that should be saved FOREVER in an easy to find place in the Testing section of the How To Metal Detect Hall Of Fame.

[JCR]

And continued to be expounded upon.