Null found when pinpointing???

[Anonymous]

Hello All
I was doing some tests with coins just on the surface of the grass ( no metals below) and noticed that when I select pinpoint and then move the coil towards the coin the tone starts up before the coil edge reaches the coin and as it passes over the coin I get a completely quiet patch- if I move the coil in a circle at this point so as to keep the coin in the same part of the coil (same distance from centre of coil) it stays quiet. When I move the coil closer to the coin the tone comes back and the depth reading decreases until I am over the coin.
why this strange null? - the depth reading is not 12 at this point. so some metal is being detected...
thanks
John

 

[Anonymous]

I'll probably catch it from the pro's but in "my mind" I believe its because of the coil winding arangement and using the TR (transmit/recieve) mode. Depending on your coil's hight from the target you will get "dead" spots. The closer to the target the bigger the dead spot will be. Ok now I'm meat for the lions! Come get me!
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[Anonymous]

Especially when you are using a non-ferrous target sample. It is a little difficult for it to be heard with quite a few detectors on the market because they have a very fast autotune in All Metal and/or pinpoint function, or just due to various conditions, but they all will do it to some degree. It's a matter of understanding how different types of metals affect the electromagnetic field. Here's what's happening:
Basically you have two types or classes of metal that you are dealing with. Iron and Non-Iron, or Ferrous and Non-Ferrous.
When specimens of those two targets are detected, they have a different effect on the generated electromagnetic field. While a man-shaped piece of iron, such as a nail, does have some conductive properties to it, the iron lacks the higher eddy-current generating abilities of a higher-condutive metal, such as copper, brass, silver, foil, aluminum, etc., etc.
If you are in an All Metal mode and slowly move the coil toward an iron object about the size and shape of a 5

 

[Anonymous]

My spectrum would do that if the coin touched the coil while pinpointing. Perhaps the MXT will also. Check it out.

 

[Anonymous]

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[Anonymous]

OOPS, hit post before I entered my text.
Another beginner's observation. Sometimes it seems to be because I squeezed the trigger to pinpoint while the coil was over (or too close to) the target or another unknown target.
Most of the time if I relocate my target then move the coil to a different location it works fine.
Have you ever used a stud finder? I thought it was similar to when you squeeze the stud finder button while already over a stud or similar object behind the plaster. Then, when you move the stud finder off the stud (to an area less dense than when you pressed the button) it blinks and goes crazy.
How about it Monte? Any correlation?

 

[Anonymous]

Lee, I can relate to what you are saying. Sometimes, I think when I'm pinpointing, the MXT locks onto the VDI it last saw while passing the coil over the target; and so if I have multiple targets in the same area, it seems to me that it doesn't always lock onto the high VDI number, until I release the trigger and try it again. Guys who have been doing this a while....am I wrong???

 

[Anonymous]

Monte,
Thanks for your detailed reply.
I am an electronics engineer but am somewhat rusty on electromagnetism etc.
From what I do recall the receive coil normally is situated such that no voltage is induced in it when no metallic items are present in the transmit field. As soon as an items enters this field it is distorted and the balance is disturbed. A small voltage is then induced in the receive coil.
As you say for non ferrous items it is the eddy currents induced which give rise to small magnetic fields being produced in the target. As the conductivity increases (or resistance decreases) the eddy currents are stronger so the field is stronger.
the ratio of the resistance and inductance in the target determines he phase of the target field, and this info helps the detector determine the conductivity of the item.
For ferrous items little or no eddy current flows, but it is the ferromagnetic properties of the metal that allows it to be detected - ie it behaves like a magnet when exposed to a magnetic field and its magnetic field ADDS TO the field from the transmit coil. This field tends to "hang on" to the transmit field as the coil is swept across, explaining why iron etc has such a wide response.
I seem to be rambling.
Now - what I think what is happening when i get the null when the coin is close to and under the coil is as follows :
The magnetic field from the transmit coil travels down from the coil and then turns around and travels up around the outside of the coil remember the pattern around a bar magnet).
When the coin is right under the transmit coil it has some flux lines running down thru its left hand side, AND SOME FLUX LINES RUNNING UP thru its right hand side. These are equal and opposite in strength and cancel each other out - hence no eddy current and no magnetic field induce in target!!
How does my theory sound?
The target would need to be close to the coil (so that the flux lines were perpendicular to it) and right in the centre of he transmit coil.
What say you experts out there?
thanks
John

 

[Anonymous]

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