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Let me try to answer your questions, Todd. .. Lengthy again confused smiley

October 18, 2018 08:13AM avatar
I'll start by posting the two comments you referred to from my post. They read:

#6..Now, take that very same detector and coil and use about three or four 3" to 4" iron nails and remove every-other coin and replace it with an iron nail. First, however, increase the detector's Discrimination to just reject the iron nail.

#5..Then, sweep down-the-line again and compare how that model's "recovery speed" might not be too impressive when it is also trying to reject (Discriminate or filter-out) the trash and still hit on the next desired target.

For those who haven't read that initial post, let me do a cut-and-paste of this particular point of interest addressing Recovery Speed:

"One problem a lot of detectors have had in recent years was their inability to recover quickly to be able to respond to a nearby desired target if they were busy Discriminating/rejecting some unwanted nearby piece of trash. There are some detectors that have a very impressive 'Recovery Time' if all they are doing is giving a positive signal from a series of not-rejected non-ferrous coins. An example would be placing 8 coins on the ground about 3" to 4" apart and slowly sweeping down-the row. Beep, Beep, Beep, Beep, Beep, Beep, Beep, Beep and the recovery seems to be impressive, signaling on each individual coin.

Now, take that very same detector and coil and use about three or four 3" to 4" iron nails and remove every-other coin and replace it with an iron nail. First, however, increase the detector's Discrimination to just reject the iron nail.

Then, sweep down-the-line again and compare how that model's "recovery speed" might not be too impressive when it is also trying to reject (Discriminate or filter-out) the trash and still hit on the next desired target.

I made bold the key points of interest I was addressing. ... Recovery Speed. The slow-motion and quick-response detectors that fit in the 2-Filter or 3-Filter category, also referred to as Double-Derivative or Triple-Derivative, are generally capable of producing a quick response and fast recovery or reset to be able to make a positive response and fast recovery on non-discriminated targets. The example was to use about 8 non-ferrous coins and place them in a row on the ground and space them about 3" to 4" apart. See how closely they can be spaced and be able to sweep a detector coil along that row and get a positive audio response from each individual coin with a slow but continuous sweep.

That is what we can see quite often in a You Tube video where someone is trying to dazzle the viewers with how fast the recovery speed is for a particular make or model detector or with some search coils. That is fine, but it is only part of the story. It is simply saying that if everything is good and favorable conditions, a particular detector and coil combination can give a good signal on these coins when positioned reasonably close since the Recovery Speed resets quick enough to respond to the next nearby target. Great!

But what they do not demonstrate is how well a particular make and models detector and using an assortment of search coils to find what works best, can Discriminate, or reject, common unwanted trash, especially the common iron nail. To be able to give a good hit on a coin, then handle a nearby Discriminated target and still be able to recover quickly enough to give a good hit on the next desired target after the trash and then also start rejecting the next piece of junk.

So to evaluate any detectors and coils YOU have in your arsenal, follow the self-demonstration instructions using 8 desired, non-ferrous coins and once you have placed the 8 coins close enough to sweep across them slowly and hear 8 good responses, you then challenge the detector and coil to hit on some of the coins if you are rejecting an iron nail that replaces every-other coin starting with the 2nd coin removed and so on.

Okay, now to Todd's questions:

My question is of a rhetorical nature : Why would that model's "recovery Speed" be impaired when in paragraph #6 the Disc. was increased to "just" reject the iron nail?
Let's start with: "What is Recovery Speed?" It is the time required for a detector to start at it's search setting base-point then respond to a target and recover back to the base-point reference..

Take a look at a Threshold-based All Metal mode. You hear the slight audio Threshold 'hum' and that would be the reference base-point. If you swept the coil towards a series of metal targets, and if the auto-tune was functional enough, the unit wowuld respond to a coin and then auto-tune or retune to the Threshold 'hum' base-point and then be able to respond to the next target because it had restored to the reference or starting base-point.

If the model did NOT have an auto-retune circuitry thin then operator would have to manually retune to restore the Threshold audio. If the detector wasn't back at the reference base-point from the first metal target quick enough, then it would be a loud audio response and would stay loud and you would not hear a separate response from another nearby target. It wowuld have to be quickly manually retuned or rely on a designed fast auto-tune process.

That is a simple look at Recovery Speed with a Threshold-based All Metal mode. Now, let's consider a conventional TR model prior to the addition of a Discriminate circuitry to reject problem trash. Those old and reliable TR's were also a Threshold-based design and the operator would listen to the Tuned Threshold audio 'hum.' Most of those TR's operated close to 100 kHz and have been recognized as usually being able to ignore common iron nails.

During a search if you encountered an iron nail, the detector tuned for a Threshold on the 'metal' side of the adjustment point and not the 'mineral' side (mineral referring to ferrous-based metals), would slightly null out and the Threshold would diminish and go silent. ... barely ... and then typically restore once the coil was past the ignored (in essence rejected or Discriminated) iron nail. You can lay one to several nails on or next to a US 1¢ coin and most traditional TR's will still respond quite well to the coin because there was very little 'rejection' and the detectors were still close to their reference base- point (the Threshold level) and could respond to the desired coin.

Move forward in our detector progression to TR-Discriminators, with most of the popular models still working in the 50 kHz to 100 kHz range as before, but now they had a variable or selectable Discrimination control to reject unwanted targets. That is based on an objects level of conductivity. The TR's relied on proper Tuner setting to adjust for a slight audio Threshold 'hum' on the 'metal' side as that is what most hobbyists were searching for ... better conductive, non-ferrous metals such as coins and jewelry, etc. Those TR's were reactive to the ground mineral and you had to adjust the Threshold setting and keep a uniform coil-to-ground relationship in order to maintain a functional Threshold 'hum'.

They were reactive to the ground mineralization and if a coil was lowered toward the ground the Threshold audio would null out. Neutral or very low mineralized ground like a good Florida beach would have very minimal effect on the Threshold reference-point, but as the negative ground mineral amount increased, it would have a greater negative effect on the Threshold and drive it deeper into the null, well below a functional Threshold setting and it would require the search coil to be moved away from the ground to reduce the negative influence or be retuned to the new setting to handle the more negative influence of the ground.

Common iron nails, with conventional, non-Discriminating TR's, were basically ignored. You would hear very little variance from the Threshold setting or perhaps only a subtle nulling effect. But other ferrous-based junk, such as a crimp-on bottle cap, pair of pliers, or other object would also usually respond with a positive audio. It was back then I learned about A.T.C., my term for Audio Target Classification, and I used and taught that technique where I coined the term 'E.P.R.' for Edge-Pass Rejection to help identify most problem ferrous trash. That was mainly iron nails back then. In my seminars and presentations I use my Compass TR units to demonstrate the behavior of a very basic TR circuitry.

Anyway, the conventional TR would pretty much ignore common nails which were lower on the conductivity scale than most man-made ferrous objects like hammer heads, bottle caps, pliers, roller skate keys, etc. But hobbyists were becoming annoyed by ferrous junk such as bottle caps, and lower conductive non-ferrous debris like small foil from gum wrappers and larger foil from wadded-up and discarded cigarette packs. So they designed Discrimination circuitry. The TR-Disc. models then, at least some of them, provided a switch to select a 'standard' non-discriminating TR function or a TR-Disc. mode with a variable Discriminate control.

A conventional TR wowuld respond to all sorts of positive conductive targets in close relationship as long as it had an auto-tune circuitry or a push-button nor toggle actuated manual retune ... as long as the coil-to-ground relationship didn't have a negative effect on a search. Then came the TR-Disc. models, still operating at the higher operating frequencies, but with an adjustment to Discriminate or reject unwanted trash. I use my trusty old, but functioning well, Garrett TR-Disc. models to demonstrate the behavior of the progression (was it ???) we made to TR-Disc. models.

They represent the challenges of using a TR-Discriminator because where a bobby pin or average nail might have no or very little influence on the conventional TR, those objects and other unwanted trash, such as bottle caps, were easily rejected with a TR-Discriminator. That means rejected and the audio driven way down below the operating reference base-point. Even at the minimum Discriminate setting you would usually be rejecting common iron nails, bottle caps and even larger-size iron stich as a screwdriver or pair of pliers. You would have to adjust up from the minimum Disc. setting to start to reject non-ferrous trash such as smaller then larger foil, etc.

If searching a site using Discrimination, you then encountered difficulty when hunting closely associated coins or other desirable finds with nails or other rejected iron nearby because the Discrimination then drove the signal well below the reference base-point and it would require a super-fast auto-tune circuitry to restore the detector to the base-point or the search coil would have to be moved farther from the negative object's influence in order to respond to a desired target. If a few coins were too close to each other and a rejected target was too close, the circuitry wouldn't recover from the negative response quick enough to a functional reference base-point to respond favorably to the desired targets. The higher the Discriminate setting to deal with more troublesome junk, the greater the time and effort required for the detector to restore to a base-point.

Back to the test I described and the question you asked. My test was to use 8 coins, sweep down the line and listen for a signal from each and see how close they could be positioned and still have functional Recovery Time. The next step in evaluating the detector and coil set-up would be to start with a coin, then remove the next and every other coin and replace it with a typical nail or let's say 3"-4" long. Then, with only the nails Discriminated, sweep along that route again and see if the detector had a fast enough Recovery Time, when rejecting the iron nails, to respond to the next close coin in a continuous sweep.

Reasonably easy for most old, outdated, conventional TR's, but a little more difficult with the progression to TR-Discriminating units. But my suggestion was to compare our modern motion-based Discriminators, and that gets tougher. Many of us have long appreciated the quick-response and fast-recovery of the Tesoro models (and remember, this discussion started on the Tesoro Forum) which gained a lot of attention early on after their release in mid-'83 because they were performing afield against mostly the so-called 4-filter or fast-motion detectors that had a longer ring-time, thus a longer recovery time and didn't perform as well in dense debris, especially with iron in the mix.

Since their start, roughly 35 years ago, we have also witnessed a continued progression to more highly featured detectors and over time most of them seemed to have gone the route of digital circuitry design and away fro simple analog circuitry. Often we read about a new detector's Recovery Speed and perhaps they are quick to recover from a nearby positive response from an accepted target. Often that is demonstrated by laying out 8 coins and sweeping down-the-row and hearing an impressive beep-after-beep.

But I don't hunt in a lot of places anymore with lined-up coins, or a lot of coins in close relationship without also encountering all sorts of unwanted junk. I also know that perhaps the vast majority of Hobbyists who are Coin Hunters do NOT like to listen to or recover iron junk. To deal with it they often set their Discrimination high enough to reject common nails, or even higher to simply knock out most iron (except the problem pieces like bottle caps) so they use a higher Disc. level at the ferrous/non-ferrous break point, and maybe even higher so that it is set just below US 5¢ coin rejection so they might still find those Nickels.

It's just like the early days, and at times even worse, because with the use of Discrimination, Recovery Time of many detectors is drastically impaired and they can not recover to the base-point fast enough to signal on closely associated keepers in iron infestation. And as a reminder here, it isn't only the ability of the metal detector itself, but the performance provided using different search coils. Naturally, the smaller-size coils will provide some enhanced performance in dense trash environments over a standard or larger-size search coil. Then, too, some detectors just do not recover well even with a smaller-size search coil, and in that case it is all in the circuitry design.

I guess this is another instance of how we interpret the word "just", similar to our past President's statement ""It depends upon what the meaning of the word is... is."
When I used the word 'just' it was simply to ask for a Discriminate level adjustment to 'just barely reject' the iron nails in this test scenario and not use a higher Discrimination level setting. Only knock out the offending nail trash targets.

If the Disc. had been set a smidgen or more farther up the arc, I assume the model's recovery speed would be operating at it's designed (unimpaired) speed and the coil would have passed over the nails with no response and signaled the coins in the normal manner......Agree?
Nope, I disagree. I can use my Tesoro Bandido II µMAX which had ED-120 Disc. and at that minimum setting it rejects nails and most common iron junk. It will work pretty well and respond to many targets in an iron-masking environment. That is close to what I was sugging people try in the test scenarios but with a broader Discrimination only increased to reject nails, not higher.

I can use my modified White's Classic ID that has a little more acceptance than the Tesoro and adjust it to just barely reject the nails. With a 5½" Concentric coil it can handle the test scenario pretty well. Or use my White's MX-7 and increase the Discrimination to knock out all the ferrous range and using a 6½" Concentric coil it might come close but might not do quite as well as the Bandido II µMAX or Classic ID in the text example I gave.

I could also grab my quick-response Teknetics Omega 8000 and increase the Disc. to just knock out nails and it will not do as well as the Tesoro, Classic ID or probably not the MX-7. And if they all had their Discrimination set to reject similar to the minimum Disc. level of a Bandido II µMAX using a 6" Concentric coil, and I used either the 7" Concentric I usually hunt with or switch to the 5" DD on my Omega 8K, I think the edge' would probably favor the Tesoro and my Classic ID would be right there with it, and the Omega 8000 would leave people wondering why I have it. Well, I actually have two of them for urban Coin Hunting, but they are not my detector pick for Relic Hunting or working any site with a dense amount of iron. I'll grab better detectors for those conditions.

With any detector, if you just barely reject nails then you're only adjusted a limited range from the detector's reference base-point. If you increase the Discrimination more, and the nails make no sound because they are rejected, the detector will not perform any better. The more Disc. used, the greater the impairment in Recovery Speed performance because it takes the detector longer to recover from the increased rejection signal.

I do these kinds of comparison tests in many of my seminars, as well as use my Nail Board; lay a 3½" nail on a penny to nickel sized non-ferrous object; use a small stack of large silver coins; and a few other test scenarios that are designed to help folks learn more and know the strengths and weakness of the detectors and coils they use. To get a better understanding of how detectors do or don't work for different scenarios.

It helps me, too, because I have always wanted to learn more, and when I do seminars I get to see models that others own and I don't so it gives me a chance to learn more about them as well as what I use.

Pardon the lengthy reply. I just felt using different models and detector designs along the way might help point out what I was getting at regarding Recovery Speed. I hope it helped, and if I find a site to host a seminar over your direction I'll let you know.

Time to go find a place to hunt today,


"Your EYES ... the only 100% accurate form of Discrimination!"

Stinkwater Wells Trading Post
Metal Detector Evaluations and Product Reviews
monte@ahrps.org ... or ... monte@stinkwaterwells.com
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Subject Author Views Posted

Does the number of filters in a metal detector affect recommended sweep speed ?

Anonymous User 488 October 12, 2018 08:34AM

The confusion of 'filtering' --- Required Vs Allowable Sweep Speed --- Recovery Time from Rejection.

Monte 446 October 12, 2018 09:20PM

Re: The confusion of 'filtering' --- Required Vs Allowable Sweep Speed --- Recovery Time from Rejection.

Anonymous User 320 October 16, 2018 06:45PM

ToddB64, pardon my tardy reply regarding your three models and 'filter' questions..

Monte 311 November 03, 2018 07:49PM

Re: The confusion of 'filtering' --- Required Vs Allowable Sweep Speed --- Recovery Time from Rejection.

Anonymous User 321 October 16, 2018 09:52AM

Let me try to answer your questions, Todd. .. Lengthy again confused smiley

Monte 392 October 18, 2018 08:13AM

Re: The confusion of 'filtering' --- Required Vs Allowable Sweep Speed --- Recovery Time from Rejection.

jmaryt 289 October 15, 2018 09:56PM

Re: The confusion of 'filtering' --- Required Vs Allowable Sweep Speed --- Recovery Time from Rejection.

Anonymous User 313 October 16, 2018 10:55AM

Re: Does the number of filters in a metal detector affect recommended sweep speed ?

Hombre 383 October 12, 2018 12:05PM

Re: Does the number of filters in a metal detector affect recommended sweep speed ?

Anonymous User 358 October 12, 2018 12:53PM

Just a quick tip on sweep speedthumbs up

Hombre 326 October 12, 2018 02:56PM

Re: Just a quick tip on sweep speedthumbs up

Anonymous User 310 October 12, 2018 04:01PM

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