Accuracy of air tests?

[Digger_O'Dell]

Hi all,

I was just trying to figure out how accurate air testing really is when it comes to the real world. I would think that it's just an indicator of potential depth the machine could reach. I was thinking of this as I was watching some videos comparing different models of detectors and different coils. For example, one of the coils I am buying air tested a large cent size copper an incredible 20-22 inches! The same setup tested a quarter size coin 18-20 inches. The same machine with the Minelabs 10.5" DD coil tested the same coins far less than that. On the other hand, a CTX 3030 and E-Trak were tested with the stock coil in another video and both maxed out at barely 12" using a quarter, gold rings, silver, etc.

I also guess this opens the door to another question. What is the real advantage of the FBS machines over a single frequency machine that is set up correctly and has almost twice the depth capability? I know the FBS machines can even the field in depth between different metals such as gold or silver being at opposite ends of the conductivity spectrum. But, I would think the single frequency machine would detect more in the long run due to it's greater depth capability as it would give stronger signals on a target at depth over the FBS machines that are at the end of their range.

[nalc472]

Air tests on e-trac means nothing. The magic is when the coil is on the ground. If you own one you know what I am talking about

[OxShoeDrew]

:shocked03:wow LC at 22 inches? What's the coil and size? I've dug many coppers (I dug one today :lol:) and my record deepest was 11in.

[Digger_O'Dell]

:shocked03:wow LC at 22 inches? What's the coil and size? I've dug many coppers (I dug one today :lol:) and my record deepest was 11in.

If I'm not mistaken, it was the NEL 15" Attack coil on the X-Terra 705. Tested just over 55 CM in the video.

There were a few more tests on video with the pockets dug into the side of a trench for detecting through soil. Again, the X-Terra got some amazing depth. I did see one test video of the CTX, and again only got about 12" consistently. Here the advantage was getting the same depth with high and low conductivity metals.

[del]

Air tests on e-trac means nothing. The magic is when the coil is on the ground. If you own one you know what I am talking about


I tend to agree with nalc472 , I don't value air tests to much . especially for pulse machines they air test horribly shallow but put an item in the ground and they can see it very deep. test gardens are they way i compare machines or coils its a lot closer to "in the field" reality.

[Digger_O'Dell]

I tend to agree with that Del, that's why I put a little more credence in those tests where they test through the ground. Still likely not perfect, but a much better indicator I think.

[MangoAve]

Air tests can give a basis but in no way is it what you will see in the field. Even with a VDI number. BUT... if you hit a coin just right in the ground it should be the same VID as what you read in the air. I say that meaning if you get a VDI of 81 air test on a quarter and in the ground you get an 81 to 83, with a narrow pinpoint it's a really big probability it's a quarter. Speaking from the point of view from the physics and electromagnetics that I know, it is true that air test is not the same. As Dan stated, a pulse machine needs the soil to carry the pulse wave effectively. Think of how in the 50's kids using two cans on the rope to talk. The rope being the carrier to the sound wave...just having air in between won't work. I think the permeability of granite is ridiculously high. Idr the units but the number was ~6000 while air was 1. In the ground, the targets have a chance to halo so their readings can be a little stronger in the ground. The permeability of soil is much different than air for electromagnetic waves. So the waves will attenuate quicker through a medium like soil (and based upon frequency). Layman terms.... this is the same reason why your cell doesn't get reception inside buildings but can right outside the building.