Lower depth when discrimination used in VLF m.d.?

[Kerimov]

Hi all,

I don't understand why VLF metal detector when using discrimination is having a lower depth than metal detector with discrimination off (all metal mode). I know from theory that discrimination calculates phase shift and tells detector whether to give audio allert or not.

Keri

[Davor]

It is easy to understand that all metal channel is indicating mostly metals, and it's greatest sensitivity is in the middle of the range, e.g. between Fe and non-Fe metals, or better say salts. Now, the discrimination allows only signals created by all metal to pass through if they are in its range and strong enough to trigger it. Because discrimination channel has low sensitivity in the salts region you'll surely lose sensitivity for small gold as well.

[dfbowers]

The problem is compounded when you add in all the junk that gets rejected anywhere near what you are looking for. Go find yourself a clay brick, ground balance your detector to the brick, hide a coin under the brick and then try to find the coin. No problem in all metal mode but very tough in discriminate!

[Waikiki_Sweep]

To avoid such problems it is better to use VLF detector in "All metal" mode but to build or to buy detector with Target ID.
In many VLF detectors you can hear different tones for different conductivity targets even with using "All metal" mode so it lets you to detect at maximum depth.
You can clearly hear where it is coin or gold ring (or aluminum pull-tab) or steel hairpin. If target is too deep you will have some overage sound until you get closer to it.
One of the good and cheap VLF with target identification that I have used for several years with great success is ACE 250.
To build VLF with sound tone target ID may be "Quasar" should be interesting: http://fandy.hut2.ru/Quasar/Quasar.html

[Kerimov]

May the problem is I was talking about difference between discriminiation on and zero discrimination (http://www.detectorprospector.com/go...ixed-modes.htm). I didn't know there is a difference between true zero discrimination and true all metal mode. So the solution is to choose md with mixed up mode or all metal mode with multitone.

Dou you have any magazine article describing schematics of discrimination mode and all metal mode and the difference please?

[Kerimov]

Is all metal mode using ground ballance or not?

[Koala]

I have never understood this also?

I use a homemade IDX and tested with a copper 2p and a 8" CC

If disc was set to just eliminate a small nail then the depth difference was 0 as long as the coin passed over the center of the coil.
There is a difference if the coin only passed the edge of the coil then the threshold volume rises but no tone is heard in disc mode.

[Kerimov]

dfbowers: did you mean that there is a one big difference between zero discrimination mode and all metal mode - making ground balance? So the all metal mode is without ground balance and is with highest possible sensitivity (maybe lowered by tresholding) and zero discrimination mode sensitivity is lowered by ground balance (and with?)? Thanks

[dfbowers]

May the problem is I was talking about difference between discriminiation on and zero discrimination (http://www.detectorprospector.com/go...ixed-modes.htm). I didn't know there is a difference between true zero discrimination and true all metal mode. So the solution is to choose md with mixed up mode or all metal mode with multitone.

Dou you have any magazine article describing schematics of discrimination mode and all metal mode and the difference please?

Your best bet is to buy Carl and George's book - Inside the Metal Detector. After reading the book you will get a pretty good idea on this concept.
Don.

[kt315]

standart decision and architecture (wow) of disc channel is a schematic on two comparators (fisher's design uses 1 comparator and opamp).
this is some kinda 'logical binary function AND', only in analog form. any reaction on unferrous target is AT FIRST a phase shift, no amplitude rising, then later amplitude rising is going. while the both comparators react only ON AMPLITUDE of signal, not on the phase shift.
ie the target has to be sounded, the target is near anywhere to coil, but the sound is absent.
in result you see more less depth in DISC MODE and that is layed principle in the schematic base.