Zero Discrimination/Ground Chatter???

Hi Everyone,

Here’s a topic that might be interesting...

We often see that F-75 owners and others seem inclined to operate in “0” discrimination at times, in an attempt to maximize depth and hear all signals. I wonder if using zero discrimination leaves the unit vulnerable to increased ground noise or “chatter” (maybe a further source of instability), a condition perhaps enhanced as the soils magnetic susceptibility increases.

I was flipping through my old White’s Spectrum XLT manual, reviewing their example illustrating the relationship between ground phase, target ID and discrimination. For clarity’s sake, White’s have produced VDI scales for both ground balance and target ID/discrimination that are easy to follow:

The GB scale runs from VDI –95 (ferrite) up to VDI +10 (wet salt beach).
Target ID/discrimination scale runs from VDI –95 (ferrite) to VDI +95 (a final classification for questionable targets, immediately above the silver dollar VDI range) Other examples, small foil would VDI into lower single digits, while iron range falls below VDI “0”on down to VDI –95.

Here’s an example provided by White’s engineers from a subsection about phase measurement…

“If the ground phase measures at a VDI of –90, then accepting VDI –90 in the discrimination circuit (in other words…“0” discrimination…my words) will produce operational problems, because the detector will see the ground as a target."

I have to presume this statement is made with the understanding the unit is also ground balanced to the –90 VDI. With default autotracking in all programs, this seems a reasonable presumption.

“With the same logic, if a target measures VDI –90, and the ground measures VDI –90, and the detector is ground balanced to this VDI –90 ground, such a target will be virtually invisible to the detector.”

Any comment from anyone re: additional ground noise “pick-up” as described in the uppermost example when using zero discrimination? Thanks,

Jim.

Confirmed and confirmed. I have a deliberately 'undug' target (unfortunately about 45 miles from home) that provides unique/ideal/textbook circumstances for converting 'theorem' into 'proof'. This target is barely detectable. On days where local EMI is near zero........ it is then (and only then) the target can be detected with a Disc setting of 0, 1, and VERY scratchy at 2..... (with the best non-EMI day/conditions being/authorizing a Disc setting of '4' (((only happened once)))... and the target could be ascertained). Interestingly, on the day where EMI was at minimum absolute.......... is the day where critical datum occured; hence, collected. The non-ferrous target was detectable (VERY VERY scratchy) at a Disc setting of '4'. It was weak, but VERY clear with Disc on '3'. Suddenly, some external power source must have been incrementally activated (may have been two different/independent external EMI sources/generators). With the first EMI source activated.... a Disc setting of '2' became the required setting to detect the target. A higher Disc setting... and the target was not detectable. The sudden minute' increase in EMI was ALMOST completely audibly undetectable. Then..... just a few moments later.... the EMI became slightly more discernable....and confirmed. Raising the Disc to '4'.... and the audible EMI could not be heard. Dropping the Disc to '0'.... and a mild amount of asymmetrical EMI could then be audibly detected. IN EITHER CASE......... NO MATTER WHAT DISC SETTING..... the target was COMPLETELY undetectable.
Summation: What ever the current local EMI conditions......If Disc is to be utilized to mitigate audible EMI.... a inherent 'prescribed' amount of depth-performance is lost with this increased Disc setting. If a Disc setting of '0' is chosen....(understanding that the operator must tolerate audible EMI).... the EXACT same amount of depth is STILL at a loss.

What is interesting is the fact that the CZ operates in the EXACT same fashion; with one exception. On days with a slight amount of EMI..... the CZ will NOT detect this target. On days where EMI is near absolute zero..... the CZ WILL detect the target. There's one exception; in BOTH cases.... the CZ is AUDIBLY STABLE! Quiet operation. Zero fatigue. You just simply do not know that the CZ is operating at a reduced capacity during the days of said EMI.

Ground balance/ground phase & target phase operate in the exact same fashion.

Tom

Thanks Tom,

The F-75's chief concern among users/potential purchasers has been operational instability. Instability caused from the units inherently high sensitivity to the effects primarily of EMI, and soil minerals. We can manipulate our settings wrt to disc/gain and sensitivity, use of smaller coils or flipping over to motion all-metal...to deal with these "instability" factors.

I question as to whether we sometimes unnecessarily, and perhaps unknowingly create/induce an additional source of instability in terms of additional "ground chatter" by using the zero discrimination setting. I suspect that higher magnetic susceptibility soils would enhance this effect. It certainly would not help.

I suppose ultimately it comes down to how overall conditions will allow one to operate the F-75. In short, if one can satisfactorily use zero discrimination, and is so inclined, then hunt away. Appreciate your advice and example Tom. Thankyou.

Jim.

WOW great read and Info...

People ask what they want of the next F75... how about industry leading EMI protection maybe an order of magnitude improved over what is currently available? That would require a coordinated engineering effort involving material shielding and circuit design. This would be a worthy undertaking that, if successful, would warrant a new model designation even if all functions were left unchanged.

The best solution for this type of problem is to switch to motion all metal mode if the ground is not to trashy. Just set the threshold to about -1, sensitivity 80, ground balance and listen carefully. Short story:
Yesterday I went hunting in an area where in the past all the waste coal cinders were spread out on the school lawn. This material cause my F75 to chatter only when the coil is in motion. Reducing sensitivity and increasing discrimination doesn't seem to help. Changing frequencies had no effect on the amount of chatter. But changing to the AM mode mentioned above allowed me to pull 5 rings and several coins from this area. I did some test each time I found a good target by switching to disc mode and adjusting down to 0 with other setting combinations. Most of the time in discrimination mode the target either disappeared or became erratic in the audio response. Some of the targets could be heard if I would lower the discrimination down to 0, but there were two times where even at 0 discrimination the target didn't give a good enough response to prompt me to dig.
Hope this helps.

I do indeed find 'some' reduction in A.M. mode when there is mild-to-moderate EMI; yet, I absolutely can not hunt the specific sites that I hunt....whilst in AM mode. The tremendous volume of iron targets...... my face continuously glued to the VDI..... and an extreme-rapid-fire of rolling numbers on the VDI will fatigue me in a matter of minutes.