Your posts about disc and sensitivity interelated F75se

I've read quite a few of your posts regarding your experience with the F75se. I believe I've seen you say that a disc above 5 will reduce the sensitivity. But the manual says just the opposite is true.
Also, on some other forums, I've mentioned this and they say I am wrong. What disturbs me is if it is true that increasing the disc above 5 reduces the overall sensitivity and thus ground depth then why should there be any relationship at all to discing out ferous objects with the penalty of decreasing the overall performance for all other non-ferous targets? Is this a design flaw?

Her is the paraagraph from the user's manual:

The F75 discrimination system is much improved over conventional discrimination systems,
and may function quite differently than other discriminators you may be accustomed to. In
older systems, the apparent sensitivity decreases as discrimination level increases. The F75s
sensitivity may increase as discrimination increases, depending on the process number and
notches being used. Therefore, do not use the discriminator to control sensitivity. First set the
discrimination level and notches to establish objects to detect or eliminate; then adjust
sensitivity downward to eliminate interference, or upward to work into the noise, if preferred.

This subject has bothered me, too. My standard F75SE shows the huge (3+ inches) loss of depth in DE mode, when going from Disc 0 to Disc 11, then returning to 'normal' by around Disc22. Yet some people report (eg. in the three oft-quoted threads) nothing like this severe effect. Also, there is no method I am aware of to determine software version, if that is relevant. I've done tests, plotted sensitivity curves, and have subsequently found online similar curves done by others.
Changing to JE mode seems to make things more logical. There's no 'black-hole' in that mode. But I wonder what the catch is? If it's better at low conductors, is it worse at high-conductors? It doesn't seem like it.
I've learnt how to use the machine effectively, I think, but I never felt I understood it.



Edited 1 time(s). Last edit at 04/16/2012 10:16PM by Pimento.

Perhaps its like the T2, in De mode, where disc affects gain. I see the value in this type of system....for coinshooting.......probably not for relics though. Why doesn't someone do tests like Mal & I did with the T2, on the F75/F75LTD.

I will find my test results and post them. When I can, I will do some more with other targets, such as US coins ( I used UK coins), and if possible do them in a lower EMI environment.



Edited 1 time(s). Last edit at 04/17/2012 09:07AM by Pimento.

In all of my real-world evaluation(s) of the F75 SE........... the most pronounced effects are with the 5" DD coil..... and in any of the 'boost process' modes (ie 'bp' or 'cl').

A additional sensitivity enhancement/boost program is employed when Disc is set at '4' (and lower). With a more rational Disc setting of '5' ...... the sensitivity is returned back to normal levels. When Disc is above '20' ..... due/because a lot of chatter is reduced......... this allows for the sensitivity enhancement/boost program to be employed again. There is justification for this engineering intent. Most folks would not understand this rationale ...... or even know it exists; yet, Dave Johnson is on top of engineering attributes that many would never have reason to think thereof. There's plenty of other attributes within the programming logic of the T2/F75 architect that are 'behind-the-scenes' ....... and most folks may never discover it. What's (underlying) is............ as you become more of a seasoned detectorist and delve into the 'professional treasure hunter' category..... then (and only then) do you start to become more 'demanding' of your unit...... with additional required performance/programming ........................................................ only to discover ............................. these things have already been 'thought' out................... AND are employed in the detector. This is called 'forward thinking'.

Most interesting...

Steve

I've found my test data, and have attached it to this post. These are airtests. The British 5p is a small cupronickel coin, like a '2c nickel', the 1/2p is a small copper/bronze coin, like a '1/2c wheat cent'. EMC meant the very low disc readings were not possible. the 5p ID's at 24/25, hence no good depth figure above disc=23. I did these tests because I was intrigued by persistent mention of 'switching' sensitivity at disc 5 and disc 20 on various forums etc. This seemed odd, so I did my own tests to find out. I was pleased there was no such 'switch',and it was actually a continuous smooth function, but perplexed by the nature of it. The manual makes no mention of it whatsoever (and still doesn't, in the latest versions). Sorry if the link gives too many ads, etc.
[imageshack.us]



Edited 2 time(s). Last edit at 04/19/2012 04:19PM by Pimento.

Hi Pimento….

From your graph... I think we can agree that there’s quite a reduction in detection depth over the upper-half of the iron range that can be attributed to decreased gain levels. Otherwise we’d see continual, more linear detection depth reductions right across the graph as discrimination increases. But that is not the case. Instead we see a dramatic decrease in detection depth heading towards the upper-half of the iron range followed by a similar (almost a mirror-reflection) increase in detection depth as we move away from the iron range. In short… the graph illustrates how both discrimination settings and gain interact to affect detection depths for these targets in the DE mode air tests.

By contrast, if one desires a uniformly high gain process mode, the JE mode is available. I occasionally use it for prospecting applications…for example in areas replete with iron debris… at sites unaffected by EMI. In areas where EMI is an issue, we soon learn why the designer of this unit adjusted the gain levels in the DE mode. Uniform high gain in such areas is simply not a practical or useable alternative.

The graph conveys useful information about operating this unit in the DE mode. It clearly indicates that discrimination settings can be used to control gain… aside from using the sensitivity control. It further indicates how we can achieve reduced gain while increasing iron discrimination… not a bad option at all since discrimination is most effective where gain is kept at a stable level… but the trade-off is reduced detection depth. Another obvious benefit…in conjunction with reducing the sensitivity control… can be derived when operating at sites where we need to deal with elevated EMI levels.

All hobbyists purchasing this unit are not dyed-in-the-wool relic hunters specializing in searching iron-infested sites. The design attributes of the DE mode as discussed above will appeal to many hobbyists who want stable performance with effective iron discrimination. But for advanced or specialized applications that include searching in layers of nails / iron junk…the choice of operating at reduced discrimination settings is available. Good examples for such applications include the options of searching at a discrimination setting of “6” in DE mode monotone / or in concert with iron tone (iron below “6” is gone while “good” iron signals between “7” and “15” inclusive produce low iron tones), or operating at a high gain “0” discrimination setting using multi-tones.

The manual does not address every possible nuance associated with this unit. For example the stat all-metal mode is mentioned in conjunction with searching for large deep targets. However for prospecting applications it is highly useful to evaluate questionable rock signals. Not attempting to push forward my reports on the subject at all, but for more information you may wish to refer to the links below that discuss my understanding of the unit and how I use it in prospecting applications…

[www.treasurenet.com]

[www.treasurenet.com]

[www.treasurenet.com]

Jim.

Thank you for your reply, Jim. Great links, too, beautiful surroundings to be in.
Here in the U.K, we are mostly relic hunters, and can have heavy iron sites. The F75/T2 and XP's GMP/Deus are the main weapons-of-choice here, and the high price tag of them (1200 US dollars for an F75) tend to make these machines regarded as for 'dyed in the wool relic hunters'. Hence why I tend to expect an 'expert-level' manual!
I find EMI issues to be uncommon here, very high sensitivity settings are normal, though whenever I do encounter interference, the DE mode disc setting is my means of controlling it. I generally like to hear the iron, and tend to use DE and disc of 0 - 2. When I want to disc out the iron, I use JE mode, to avoid the sensitivity-change issues.
I posted the graph mainly to see if it agreed with anyone else's experience. The fact that some report no such performance change, and some just see a change from Disc 4 to disc 5 concerned me. If it's relevant, my machine is an early one (2007). When I re-do these tests, I will include US coins to make comparisons a little easier. I'll test my 'fake dollar' too.

Hi Pimento....

Quoting you...

"I posted the graph mainly to see if it agreed with anyone else's experience. The fact that some report no such performance change, and some just see a change from Disc 4 to disc 5 concerned me. If it's relevant, my machine is an early one (2007)."

A couple of years back I did a plot of depth vs discrimination levels. I don't have that scrap of paper anymore but your graph looks very similar... my unit was one of the first produced too...although I doubt that makes any difference.

In prospecting applications I rarely have to deal with EMI issues either. Although I normally search in the all-metal motion mode, occasionally over severe iron infested sites I use the JE mode in monotone but with small iron discrimination settings. This set-up eliminates or breaks-up small iron signals but permits deeper non-ferrous targets that target ID as iron to produce a good signal. It also achieves maximum depth in "disturbed" ground... for example, mine tailings... depths that can only be duplicated in DE mode using a "0" discrimination setting.

Jim.

Thanks for all the GREAT info for sure!!!