ABOUT BIG SEARCHING COILS

[shila]

Dear friends,

Here is my question if you appreciate! may be very simple.

Why pnetration depth in soil increases when you are using
big searching coils in, PI metal detectors ?

YOURS. SHILA

[chemelec]

Mainly because the Bigger the coil, The Larger the magnetic field. This also includes its depth. But bigger coils usually only detect LARGER Objects, missing the smaller ones.

[shila]

PLEASE AGAIN

Hi everyboby!

I have to repeat my question in other words.
suppose, you have two big&small searching coils with same characters. I mean same inductance ,same resistance,...
When you are using big searching coil ,you can detect at longer depthes.


You answered, this is due to stronger magnetic field ,It means
more current must be passing through the coil. But current is
same ,because we are using coils with same inductances.

If so, why stronger magnetic field for bigger searching coil??

Thanks alot.

[Carl-NC]

For a single circular loop, the magnetic field strength at the center of the loop is B = uI/2r, where u=permeability, I=current, and r=radius. So at the center of the loop, a larger radius gives a proportionally weaker field.

But as you move away from the loop, the field strength drops as a function of 1/d^3. At some distance (depth), there will be a cross-over point where the larger loop has a stronger field.

- Carl

[shila]

hi

Dear Carl!

I'm still confused!
Both equations you wrote me are dropping by distance.
They have no intersection or any cross_over at a point.
I have to think about it more!:confused:

[Carl-NC]

The equation I posted was simplified for the field strength right in the middle of the coil. The general equation for the center axis of a single loop is:


B = (u*I*r^2) / (2*(r^2 + d^2)^(3/2))

u=permeability, r=radius, d=distance

So let's take 3 points of interest. In the very center of the loop (d=0) we have

B = u*I/2*r

At a distance equal to the radius (d=r) we have

B = 0.35 * u*I/2*r

At a distance equal to the diameter (d=2r) we have

B = 0.09 * u*I/2*r

This shows just how fast the magnetic field dies off versus depth. For a loop with N turns of wire, the B field is simply N times as strong.

We can also consider field strength vs loop radius. Let's take a second coil with twice the radius (r' = 2r) but compare the B field at the exact same depths (d=0, d=r, and d=2r). At d=0 we get

B = 0.5 * u*I/2*r

At a distance equal to the smaller coil's radius (d=r) we have

B = 0.36 * u*I/2*r

At a distance equal to the smaller coil's diameter (d=2r) we have

B = 0.18 * u*I/2*r

So, ferinstance, a 10-inch loop and a 20-inch loop will have about the same field strength at a depth of 5 inches; beyond that, the 20-inch loop wins.

Keep in mind this all pertains to transmitted field strength. There is still the issue of the target's return signal, and how loop size affects sensitivity. I have not yet worked out the math on that.

- Carl

[shila]

hi

Dear Carl,

I think this is a good introduction for me to start a theoretical study.
As soon as I found some results ,I send it to you. I believe that always
a good job is based on a strong theory.

Thanks alot for your help!

[shila]

about permeability

As you know detection depth in soil decreases drastically
compared to free space test.
This is due to difference between magnetic permeability
of soil compare to air.
But when you copmare numerical deta ,you dont see any
valuable changes?
According to a handbook ratio of permeability compared to the free space is about 1.0005 for dry sand and 1.0084 for wet sand.
It means theoretically ,if you can detect an object at 1meter in free space ,its possible to detect same object at about same depth in soil.

But in practice ,penetration depth reduces awfully!


WHY???

[chemelec]

Just a Wild Guess, Dirt is More than Just Sand.

Many assorted Minerals other than silicon, lots of bacteria and other living oranisms. Aluminum and Iron are usually both in abundance.