Pulse Induction air test. Can someone help me?

[mts]

Hi, I'm currently building my own pulse induction metal detector based on a conglomeration of different schematics out there on the web. The circuit is very straightforward and I don't expect that I have any real issues in the circuit itself. I've been doing some basic air tests to see what kind of range I'm getting. This thing is still in the breadboard stage with wires hanging everywhere and it is attached to an oscilloscope so I can't really take it outside. Anyway, my latest tests show that I can detect a recent issue (non-silver) US quarter at 10cm (about 4 inches). This is worse than my cheapo Bounty Hunter detector. I would have expected a PI detector to go much deeper. I've heard that PI detectors generally don't air test well and go much deeper in the ground. I certainly hope so!

Anyway, here is where I could use some help. Can someone with a PI detector do an air test with a US quarter and let me know what kind of depth you are getting? I'd also be curious to know what model your detector is so that I will know if it is top of the line or a cheapo unit. I'm not expecting to contend with the top of the line models. Heck, I don't even expect to do that well against the cheapo units. But if a cheapo is 10 times deeper than what I'm building then there is something seriously wrong going on.

I've also been experimenting with all kinds of coils and I don't see a sizable difference in depths between the various designs.

Thanks for the help.

 

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[mts]

Just a quick update. I looked at one of the schematics and noticed that the author was using an additional amplification stage. I added one
(x100) which increased my quarter air test to 18cm (about 7"). That jives with what I'm seeing at this site: http://home.global.co.za/~trh/#Depths

I'd still like to see some test results from commercial units as well as hear your thoughts on how deep a PI unit can go in the ground compared with air tests.

Thanks!

 

[vondrewvious]

I am no expert but I have read many times on the forums that you can't get accurate distances air testing a PI unit. Has something to do with how it works.

 

[lamar]

Dear mts;
I'll start relying on air tests when gold nuggets start floating in the air, my friend. Air testing a PI detector is terribly inaccurate and I only air test my detector to confirm that it's working and the battery is charged. A US quarter, lying on it's side, should be easily detected at 16" in the ground using an 18" coil. The reason why PI detectors do not work well with air testing is because the signal pulse uses the earth as a conductor. Air, as we might know, is a very poor electrical conductor, therefore the signal pulse rapidly dissapates when traveling through air. This is why it's imperative to keep the coil as close to the ground as possible, for even a slight amount of air between the coil's lower surface and the earth acts as a resistive force and greatly affects the PIs performance.
Your friend;
LAMAR

 

[mts]

vondrewvious and lamar, thanks for the info. I had heard that PI detectors don't air test well but would still be interested in hearing what other owners are seeing with their PI MD's. I realize that it won't tell me what kind of depth I'm going to get in the ground. But if someone comes back and says that their commercial device air tests a quarter at 20" then that at least tells me how far off I am. It's just for my own general knowledge so that I have a basic idea of when to stop tweaking this circuit. At this point I really don't know what to expect and getting an air test of a quarter at 4" was a little disappointing. Now that I've added the pre-amp with a gain of 100 the results are more respectable.

For now it looks like my device is working well enough for my intended use.

Thanks!

 

[Treasure finder]

MTS,
Air tests with PI's are not at all indicative of in the ground performance.
I have a Garrett Infinium and I have detected dimes as deep as 14 inches in black sand
at the beach. Yesterday I found a 1952 half dollar at the same beach with a slight signal
at 7 inches. Turns out I was scanning too high above the sand when I got that signal.
I reburied it at 7 inches and got a real nice signal by keeping the coil close to the ground,
I had been moving too fast and too high. That little bit of air makes a huge difference to
a PI, I have heard that two inches too high above the ground can lose 4-5 inches of depth.
Also be careful of exaggerated claims for depth by detectorists that don't have a ruler with
them. I marked off my sand scoop with a ruler so I can check. I think the conductivity
of the ground has a lot to do with it, but I don't know what kind of ground is better or worse.
Also on coil size I think one of the more important issues is how much ground you can
cover with each sweep. On coin hunting at the beach I like my 10X14 DD for larger coverage.
Try a plastic container with a coin at the bottom and some fill dirt and check for depth.
Good luck on your project.
R

 

[Kentucky Kache]

Would it be a good idea to use a coil cover with the P.I. and scrape the ground as you swing?

 

[Treasure finder]

Yes, it would be a good idea, usually I just move slow and careful.
If you want to see another coil type look at the AccurateLocators.com
web site, they have a blanket coil that is dragged along the ground and
has no air space. It is used for larger targets and has quite a depth to it,
and is a PI.
Rich

 

[Treasure finder]

Lots of trash at my beach that would really slow you down, the other
companies usage is for large targets and the coil is wound such.
Good luck,
Rich