Air testing metal detectors, Does it work?

Charlie P. (NY)

Gold Member
Feb 3, 2006
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South Central Upstate NY in the foothills of the h
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Minelab Musketeer Advantage Pro w/8" & 10" DD coils/Fisher F75se(Upgraded to LTD2) w/11" DD, 6.5" concentric & 9.5" NEL Sharpshooter DD coils/Sunray FX-1 Probe & F-Point/Black Widows/Rattler headphone
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Metal Detecting

SouthFLdigger

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Mar 16, 2014
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Beach:Fisher CZ-20, Beach Hunter ID 9.5" Whites DFX, Minelab Safari and Excalibur 2.
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Loaners:ACE-250 9x12 and 7x9.
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Nugs Bunny

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Mar 13, 2013
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Ohio
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White's MXT Pro
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Air Test Guitars... Not Detectors!

Air tests can be inconclusive, here is a method that will provide more accurate results.

Dig a hole about a foot deep, stick a coin in the undisturbed dirt in the side of the hole, push it in as far as possible. Backfill the hole and tamp the dirt, now you have a target in undisturbed soil.

Here is a diagram I made for an example.

hole.png

How Metal Detectors Work - HowStuffWorks

Metal detectors use one of three technologies:

Very low frequency (VLF)
Pulse induction (PI)
Beat-frequency oscillation (BFO)

In the following sections, we will look at each of these technologies in detail to see how they work


VLF Metal Detector.gif

VLF technology
The receiver coil is completely shielded from the magnetic field generated by the transmitter coil. However, it is not shielded from magnetic fields coming from objects in the ground. Therefore, when the receiver coil passes over an object giving off a magnetic field, a small electric current travels through the coil. This current oscillates at the same frequency as the object's magnetic field. The coil amplifies the frequency and sends it to the control box of the metal detector, where sensors analyze the signal.
The metal detector can determine approximately how deep the object is buried based on the strength of the magnetic field it generates. The closer to the surface an object is, the stronger the magnetic field picked up by the receiver coil and the stronger the electric current generated. The farther below the surface, the weaker the field. Beyond a certain depth, the object's field is so weak at the surface that it is undetectable by the receiver coil.

PI Metal Detector.gif

PI Technology

The sampling circuit sends the tiny, weak signals that it monitors to a device call an integrator. The integrator reads the signals from the sampling circuit, amplifying and converting them to direct current (DC). The direct current's voltage is connected to an audio circuit, where it is changed into a tone that the metal detector uses to indicate that a target object has been found.
PI-based detectors are not very good at discrimination because the reflected pulse length of various metals are not easily separated. However, they are useful in many situations in which VLF-based metal detectors would have difficulty, such as in areas that have highly conductive material in the soil or general environment. A good example of such a situation is salt-water exploration. Also, PI-based systems can often detect metal much deeper in the ground than other systems.

BFO Metal Detector.gif

BFO Technology

The most basic way to detect metal uses a technology called beat-frequency oscillator (BFO). In a BFO system, there are two coils of wire. One large coil is in the search head, and a smaller coil is located inside the control box. Each coil is connected to an oscillator that generates thousands of pulses of current per second. The frequency of these pulses is slightly offset between the two coils.
As the pulses travel through each coil, the coil generates radio waves. A tiny receiver within the control box picks up the radio waves and creates an audible series of tones (beats) based on the difference between the frequencies.
If the coil in the search head passes over a metal object, the magnetic field caused by the current flowing through the coil creates a magnetic field around the object. The object's magnetic field interferes with the frequency of the radio waves generated by the search-head coil. As the frequency deviates from the frequency of the coil in the control box, the audible beats change in duration and tone.

Buried Treasure

Metal detectors are great for finding buried objects. But typically, the object must be within a foot or so of the surface for the detector to find it. Most detectors have a normal maximum depth somewhere between 8 and 12 inches (20 and 30 centimeters). The exact depth varies based on a number of factors:
The type of metal detector - The technology used for detection is a major factor in the capability of the detector. Also, there are variations and additional features that differentiate detectors that use the same technology. For example, some VLF detectors use higher frequencies than others, while some provide larger or smaller coils. Plus, the sensor and amplification technology can vary between manufacturers and even between models offered by the same manufacturer.
The type of metal in the object - Some metals, such as iron, create stronger magnetic fields than others.The size of the object - A dime is much harder to detect at deep levels than a quarter.

The makeup of the soil - Certain minerals are natural conductors and can seriously interfere with the metal detector.

The object's halo - When certain types of metal objects have been in the ground for a long time, they can actually increase the conductivity of the soil around them.

Interference from other objects - This can be items in the ground, such as pipes or cables, or items above ground, like power lines.
 
 

Apr 17, 2014
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Tartarus Dorsa mountains
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I hung my metal detector horizontal from the ceiling and while air testing it the darn thing kept going off everytime I walked by it, an hour later I was completley nude and it was still going off everytime I walked by it. Then it dawned on me...."airhead!" :laughing7:

Well DAYUM you were funny back in the day :P

And how about this necro thread diggin?
 

rwd mo

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Jul 26, 2011
183
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SW Mo
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Minelab/whites/Tesoro/
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Cache Hunting
Air testing is just a general idea as to the depth of the machine you also have to factor in the personality of the detector such as 2 detectors have different personalities. as one might go an inch deeper due to components and adjustments of pots inside.But gives a general idea as to how deep or powerful it might be. 8in air might only get 5in. in the ground in real time. shortribs rwd mo
 

littlewheaty

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Dec 7, 2014
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missouri
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GARRETT ace 250, at-pro, pro pointer
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My observations are these:

Air testing will show a machine's depth potential. So if one machine air tests at 8", and one tests at 11", the 11" machine will always test deeper in the same soil. I've tested this a million times and it was true every time.

Your machine will not detect coin-size objects in the ground beyond its air-test depth. You might get improved depth due to halo effect on iron but in such cases you are actually detecting minute metal particles inside the original air-test depth. Not extra depth.

All machines lose depth in the ground compared to air tests. How much depth lost is dependent on soil makeup and density.

Last of all - nobody will agree with all above statements. This is a highly debated issue in metal detecting.

OT

I almost agree with this statement. Higher frequency machines air test better than lower frequency machines but lower frequency machines handle the the ground better so they dont lose as much depth in the ground. Now take a pi machine against a vlf machine and the vlf will air test better but vlf will not get the depth of the pi in mineralized soil. Most of the time you wont get deeper than an air test shows on regular machines.
 

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mikeraydj

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May 19, 2014
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Minelab E-Trac, Deteknix X-Pointer, Garrett Pro Pointer
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I think air tests are good to get familiar with the numbers that different targets give on your VDI. As far as hunting, those numbers are only good on shallow targets with no masking by other targets close to them. Otherwise steady VDI #'s are tougher the deeper the target and the trashier the area. How unsteady I am sure varies from soil conditions to the detector you are using. Thankfully the tones usually do not lie. So it is just as, or more important to learn them while air testing. If your machine is adjustable, it is always a good place to see what the adjustments do on known targets, so you can fine tune your machine for the best results. Play with the gain, threshold, and notch features.
I have never seen the depth that an air test gives me.
 

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Charlie P. (NY)

Gold Member
Feb 3, 2006
13,003
17,106
South Central Upstate NY in the foothills of the h
Detector(s) used
Minelab Musketeer Advantage Pro w/8" & 10" DD coils/Fisher F75se(Upgraded to LTD2) w/11" DD, 6.5" concentric & 9.5" NEL Sharpshooter DD coils/Sunray FX-1 Probe & F-Point/Black Widows/Rattler headphone
Primary Interest:
Metal Detecting
I bury a test garden in the yard - I have coins, silver and clad (all denominations), at depths from 6" to 12", some with nails or pulltabs over them, and a musket ball at 12", plus trash like bottlecaps and shotshell bases. All about 16" apart in a long row. This lets me get out (even with the ground frozen) and try variations with my tones and modes to see what I like.

Good practice or to test resets (I always reset to default settings before heading out). And I made a good map, with several copies, and marked each item with a plastic golf tee (four colors that I sequence through). Made sure the soil was well swept before I buried them. The garden has been in place 11 years now.

Neither of my detectors work very well indoors with all the EMI so if I am outside anyway I might as well be swinging.


If I want to try for better depth I choose a 12" deep quarter and lift the coil as I swing. I guess that's a hybrid air test. ;-)
 

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NeilinFR

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Oct 11, 2007
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Teknetics T2 ltd
Minelab Etrac
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Actually one or two of the minelab testers from years ago posted that they were getting better depth in ground than in the air and that when this info was presented to the minelab engineers they were told that the minelabs needed a ground matrix to compare against for both ID and recognition.
Personnally I know nothing about physics and such but would agree with some of Old Towns posts on most detectors but Ive seen over the years Sovs, Explorers and now Etracs detect an object deeper in the ground than air test it....and that's stuff like a gold ring or coin which I do not believe gold will give any halo but sometimes an old silver coin will have that black soil around it.
Again how/why I cannot say, I just know Ive dug some pretty deep stuff and while minelabs (the ones I mentioned) do airtest ok..........I say they will detect deeper than they airtest. [

Dont the laws of physics say that a bumble bee should not be able to fly based on its wing size to body size/weight.....but hey, Ive seen a few of those flying around:occasion14:UOTE=SouthFLdigger;4334145].

Agreed 100% and this has been my observation as well, confirmed by experts from Minelab.[/QUOTE]
 

Treasure_Hunter

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Staff member
Jul 27, 2006
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BBS and FBS tests better in the ground than in the air, I have yet to find a coin floating I the air too. I have a test garden with different coins buried at different depths. A real life scenario with undisturbed soil is always better test than air tests...

Signature Line .... Trump 2016!
 

Apr 17, 2014
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BBS and FBS tests better in the ground than in the air, I have yet to find a coin floating I the air too. I have a test garden with different coins buried at different depths. A real life scenario with undisturbed soil is always better test than air tests...

Signature Line .... Trump 2016!

Go to the bank drive up window and ask for change, those tubes float coins very nicely :D
 

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beerguy

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May 6, 2004
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Park Ranger and Pack Mule pouches by Freeloader!
I will say that my CTX air tests poorly but is extremely deep. It likes moist soil, and on several occasions I have gotten a deep signal only to dig a plug and lose the signal in motion mode. Pinpoint always works fine, but it seems that the machine needs the soil matrix to carry the signal to the coil.
This is only on very deep targets, and marginal signals.
 

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