F75 Lowest number Ground Grab can go to

Whats the lowest GB number that Ground Grab can go to ? on a pasture field the other week with my F75 LTD2 Nel Tornado coil and the Ground Grab would only go down to 42 it was still not ground balanced at that as it was giving positive signals when pumped so i had to ground balance manually and it came in at 27.

From operator's manual:

MANUAL GROUND BALANCING (continued)
The Fe3O4 bar graph on the LCD display indicates the amount of magnetic mineralization.
The searchcoil must be in motion to measure Fe3O4. The most accurate measurement is
obtained by pumping the searchcoil, as in the Ground Balancing procedure.
The two-digit G.B. Setting number displayed on the LCD indicates the type of ground
mineralization.
Some typical ground mineralization types are:
0 – 10 Wet salt and alkali
5 – 25 Metallic iron. Very few soils in this range. You are probably over metal.
26–39 Very few soils in this range -- occasionally some saltwater beaches
40–75 Red, yellow, and brown iron-bearing clay minerals
75–95 Magnetite and other black iron minerals
When manually ground balancing, try to “feel out” a spot on the ground to make sure
there is no metal present. In order to avoid locking onto metal, the computer will not
balance to ground where the GB setting is less than 40. Where the ground reads less than
40, manual ground balancing is required.

Yes your getting into salt range that low...seems the F75 T2 does not ground grab below 40...

Keith

“I don't care that they stole my idea . . I care that they don't have any of their own”
-Nikola Tesla

Keith / Sanjuro: You're actually nowhere near the salt range at the GB=40 value. It's a grossly non-linear scale, with 40 being a significant 'breakpoint'. "90" through to "40" represents a ground phase lag of probably about Zero degrees ( ferrite ) -> 15 degrees, and "40" through to "01" indicates 15 -> 90 degrees (=salt). I've not made many proper scientific measurements, I may have a go one day, but I believe what I've just stated is ballpark accurate.
I've detected on ground that GB's close to 40, I recall it fastgrabbing at 42 in some spots, but some spots needed manually setting, at about 37. I'm not actually sure whether 40 or 41 is the lowest it will set itself at.
I always felt confused when the machine fails to fast-grab, as it leaves the GB value at the last-used figure. It would be more obvious if it reset GB to 90, then you would realise it was not fast-grabbing. Or...possibly set it to 40, though that might fool you into thinking that's the correct value....spotting the CAN'T GRAB (or whatever it says) indication isn't easy.



Edited 1 time(s). Last edit at 04/29/2015 07:36PM by Pimento.

Pimento Wrote:
-------------------------------------------------------
> Keith / Sanjuro: You're actually nowhere near the
> salt range at the GB=40 value. It's a grossly
> non-linear scale, with 40 being a significant
> 'breakpoint'. "90" through to "40" represents a
> ground phase lag of probably about Zero degrees (
> ferrite ) -> 15 degrees, and "40" through to "01"
> indicates 15 -> 90 degrees (=salt). I've not made
> many proper scientific measurements, I may have a
> go one day, but I believe what I've just stated is
> ballpark accurate.
> I've detected on ground that GB's close to 40, I
> recall it fastgrabbing at 42 in some spots, but
> some spots needed manually setting, at about 37.
> I'm not actually sure whether 40 or 41 is the
> lowest it will set itself at.
> I always felt confused when the machine fails to
> fast-grab, as it leaves the GB value at the
> last-used figure. It would be more obvious if it
> reset GB to 90, then you would realise it was not
> fast-grabbing. Or...possibly set it to 40, though
> that might fool you into thinking that's the
> correct value....spotting the CAN'T GRAB (or
> whatever it says) indication isn't easy.

I noticed that when it cant ground grab it kept the last used figure you are right to say that it would be better to display 90 when it cant ground grab then at least you would know it had not ground balanced and you could proceed with a manual balance, the ground i was detecting on was old pasture dark soil FE meter 2/3 bars and it was very wet, 27 is the lowest GB figure i have come across so far.

He said it was at 27 ...I did not hopefully imply it was salt at 40 ....I have found salt to be single digits and or mid 20s......I said it would not ground grab below 40...

If I have made some sort of erronous info I'm sorry..



Keith

“I don't care that they stole my idea . . I care that they don't have any of their own”
-Nikola Tesla

Not criticism, Keith, just trying to fill in some missing information that "the user manual doesn't tell you". Ground balance is not an intuitive thing, I find, being a combined effect of minerals and 'rust'. And the number on the screen is a rather arbitrary figure, like the 1 - 10 on a hi-fi volume control.
I do have a few ideas related to "simulated ground", and when I find time I'll have a go at calibrating the GB figure on my F75. If I get any worthwhile results I'll post them.

Good info Pimento!


Keith

“I don't care that they stole my idea . . I care that they don't have any of their own”
-Nikola Tesla