Eric Foster quote on how to "air test" PI detectors and his warning about applying the same test to induction balanced detectors.

Vlad....There is an old maxim in Stereo that to double your audio volume you are increasing power output by 8X. The detector still sees the small deep target-you just don't hear it.

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hobbit said:

Here is Mr. Foster's quote, (a copy and paste), from another forum of which he is the moderator:



Re: Do PI detectors "air test" poorly??????

Posted by: Eric Foster
Date: October 19, 2010 02:19AM

Moderator
Registered: 6 years ago
Posts: 301


Hi Reg and all,

I totally agree. There is no reason an air test should be worse than an "in ground" test, except for noise. When I do tests in my garden, which is in quite a noisy location, it is very noticeable that the noise diminishes as the coil is lowered toward the ground, even from 6 inches down to 1inch height. For an air test, always have the coil horizontal. Noise signals are polarized so that a vertical coil will always pick up far more noise than a horizontal one. For a realistic air test, lay the coil on a piece of 1in thick wood, MDF, or plastic on the ground surface, then wave a target over the top of the coil. The range obtained will not be measurably different to that if the target was buried. The above is true for PI detectors, but not necessarily so for induction balance types, where the operating frequency can make a very noticeable difference.

Eric.


I am personally looking forward to seeing the results of the tests you perform with your PI detectors. With different coils and a wide variety of targets, we should all be able to learn a lot.

http://www.findmall.com/read.php?34,1312558,1312558#msg-1312558

http://www.findmall.com/read.php?34,1312558,1315862#msg-1315862

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Terry Soloman said:

http://www.youtube.com/watch?v=yfUm05ob0z0

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Rudy said:

The only valid reason for air testing detectors is that testing in air provides a way to accurately reproduce results just about anywhere in the world between two detectors of the same type or different designs.

In the real world of detecting, the ground the target is buried in has a significant effect in detection depth, so comparing test results obtained from two different locations is difficult at best. However, the air present at two different geographic locations can be depended upon to have almost exactly the same magnetic permeability and electrical permittivity, so the effects of different ground conditions on test results can be totally eliminated.

Ignoring, for the moment, the possibility of a halo, air testing will, in general, give you the upper bound on how far the detector can detect in the ground. The actual in-ground depth that one can achieve can be a lot less depending on how well the design matches the ground conditions. A prime example of this is the BeachHunter ID detector. This detector works extremely well on east coast beaches, but BeachHunter ID pre-amp gain is fixed by design and is quite hot. Too hot for most of the highly mineralized beaches in California. As a result, it can only be used at very low gains here in the west coast and is therefore not a deep machine here. Of course, air testing would not tell you that character flaw.

Now, almost all knowledgeable people will acknowledge that, everything else being equal, a PI capable of using a shorter pulse delay will be more sensitive to small, low conductors, compared to a PI detector with longer pulse delay. Does that mean the longer delay PI will not find gold? No, of course not. But it won't find gold as small or as deep as the shorter delay PI, everything else being equal.

What do I mean by "everything else being equal"? Things like coil diameter; the peak amps of current pumped into the coil times the number of wire turns in the coil; Gain; etc.

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Raphis said:

I think halo has very little effect on a saltwater beach as it does in the dirt. That is because of sand/target movement. Tides, swell, storm surge, etc. create sand movement/erosion which makes ferrous/non-ferrous targets move frequently, along with the sand around them, not allowing much of a halo to build up on metallic targets. Obviously, with an inner harbor/protected beach, the halo would have a little more impact on detection depth.

The #1 reason you get better depth at the beach is because of saltwater. Saline water conducts much better than air.

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Raphis said:

I think halo has very little effect on a saltwater beach as it does in the dirt. That is because of sand/target movement. Tides, swell, storm surge, etc. create sand movement/erosion which makes ferrous/non-ferrous targets move frequently, along with the sand around them, not allowing much of a halo to build up on metallic targets. Obviously, with an inner harbor/protected beach, the halo would have a little more impact on detection depth.

The #1 reason you get better depth at the beach is because of saltwater. Saline water conducts much better than air.

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Lineweed said:

I feel that this isn't true. Although i still consider myself a newbie, I've dug my fair share of stuff, and can safely say that halo does play a part in detection depth.

Say, e.g a badly oxidised cupronickel coin (that's what most are made of over here) v.s a recently dropped coin, both resting at 10 inches in the sand. The old coin will produce a nicer "model" blare vs. the new one, which could be very tinny and almost "unhearable".

I haven't dug enough silver to check whether on that has turned black (silver sulfide) produces a better signal than a freshly dropped one, but have a sneaking suspicion it most certainly does.

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Lineweed said:

I feel that this isn't true. Although i still consider myself a newbie, I've dug my fair share of stuff, and can safely say that halo does play a part in detection depth.

Say, e.g a badly oxidised cupronickel coin (that's what most are made of over here) v.s a recently dropped coin, both resting at 10 inches in the sand. The old coin will produce a nicer "model" blare vs. the new one, which could be very tinny and almost "unhearable".

I haven't dug enough silver to confirm whether one that has turned black (silver sulfide) produces a better signal than a freshly dropped one, but have a sneaking suspicion it most certainly does.

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Hotrod Tom said:

If someone wants a lesser machine GOOD for them and me and a few others.

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