Sweep Speed and Length of Sweep

[Cody]

I often read a post about sweep speed and have done some testing to see how much difference it makes how fast we sweep the coil. These tests were done with a couple of things in mind. The lowest frequency of operation is about 3.5khz, I say about as engineers round these figures off, and the highest is 100khz. This means in electronic terms the target is essentially standing still no matter how fast we humans can swing the coil. At 3,500 pulses per second we are not going to swing the coil so fast that we outrun the electronics. This was a given so the other consideration was that the detector needs motion so how much is needed. One last concern was will a new user try to move the coil so slow that they actually miss targets.

I decided to test actual targets in the soil as they were located. It takes some time to locate targets at different depths and types then go through a testing routine to see how the different functions perform but that is how I spend a lot of detecting time. I could never swing the detector so fast that a target could not be detected as long as I was consistent with the sweep. Once the target was located and pinpointed so I knew the center of the target there was no way to sweep the coil so fast that the target was not detected.

I could move the coil so slow that the target was not detected at a level that was sufficient enough that I would recognize the target from ground noise. Since motion is required there is a point where the motion is so slow that targets break up or they are not detected at all. If you are like I am to say to move the coil so it covers so much soil in a second just will not work for me in that I cannot get a good feel for that speed. It is difficult to come up with an example that we all relate to when it comes to sweep speed. I would describe it as about the speed a buffer is moved across a hallway if you don

 

[dsurcy]

Cody,
I like this post... mainly because.. in the beginning
I couldn't relate to the term "slow" in relation to a sweep
speed.. "slow" is relative.. but you examples to sweeping
a buffer and/or trim painting I can relate to.. this gives
a point of reference to "slow".. and helps..

Of course.. you KNOW I've got questions..

While doing some "testing" on my own.. to try to get
familiar with sounds.. I placed a nail, nickel and quarter
on the ground.. (three tone setup procedure).. while doing
this.. well.. it was very interesting the combination of
"sounds" when the nail and quarter were close enough to the
outer edges of the coil to "influence" the reading/sounds of
the nickel.. that's about all I can say... it was interesting..
but this made me start thinking (and helped me visualize what
I was hearing for "unseen/in the ground" targets).. so.. I placed
the old rusty nail on the ground (length wise.. head of nail
out in front of me and point towards my feet) and a dime
about 1-1.5 inches to the right of the nail.. when I swept..
too fast.. I didn't hear the dime.. when I slowed it down..
I did hear the dime & nail separately.. ?!.. by the way..
I only hear the dime when I swept from right to left.. never
when I swept left to right.. no matter what the sweep speed.
Did I do something wrong?! Is there something else I could do
to help hear the dime from both directions?

When you are saying that you could never sweep fast enough
to "not" hear the target.. where you specifically talking
about a single target?! and not referring to separation of
targets?

Seems I have to slow my sweep speed down.. the more sensitivity
I have set.. does this sound reasonable?! Or am I "in left field"
again.. as usual?!

Dan

 

[Cody]

Yes, I was talking about a single target.

The nail and dime are interesting and what I call null discrimination. When a target is detected and to be rejected the threshold goes null. It takes time for the threshold to recover. If the recover is too fast we miss deep targets. There needs to be a recover so the threshold nulls for discrimination and recovers so we can detect a co-located target such as you describe.

Here is the problem with thinking a one way hit is trash. If we sweep the coil so we hit the nail first the threshold will null and we miss the dime unless the threshold has recovered. However, if we hit the dime first we hear the hit then a null for the nail. So, we have a hit in one direction but not in the other.

The relationship of targets that are to be accepted and rejected presents a complex target volume to the electronics when co-located. Nails or iron in the shape of a nail present a particular problem. Eddy current flows around the axis of the nail and if we could see them they would be in the shape of a dumbbell. The eddy currents in a coin flow in a circular way around the coin at the surface. It turns out that the time constant generated by nail eddy current around the axis and in circular fashion for a coin are pretty much the same which means a nail will look like a silver coin.

I avoid most of this by using ferrous sounds then the coins sound a noticeable higher pitch than a low sound for a nail.

 

[Ralph Bryant]

This type of situation is also a good example of why smaller search coils work better in target separation in very trashy areas. A smaller coil field simply sees a smaller surface area, and allows for better separation of closely co-located targets. I've been a big advocate of smaller coils for years, especially so after realizing that there are so many detectorists out there these days, that most of the "easy pickins" are long gone, BUT that there is still alot to be found amoungst the trash in areas that most avoid with their detectors and larger coils. I would venture to say that there is much more missed due to target masking than there ever is due to unreachable depth, and that is advantageous for those of us who like to "trash pick" with a basic little lower sensitivity machine with a small coil attached, preferrably of 2 filter design for better recovery speed and sweep speed "tolerance". This kind of hunting set-up is obviously not meant to go after the deeper targets, but it is deadly within the top 4 to 6 inches of the surface, even in the worst of trash or close to sidewalk rebar, playground equipment, or metal fences. Even around old churches, it's amazing how many old coins are within the first few inches of the surface and mixed amongst alot of non-ferrous trash. Here is an example of some 2004 finds using nothing but a screwdriver and an inexpensive $150 detector.

Ralph

 

[Cody]

n/t

 

[dsurcy]

I do not use any discrimation... although I would love to be able
to have an area and "swing away" until I hit a target I "know" I want to dig.. seems most places I go are infested with nails, etc.. I can't stand to think because of nulling I wouldn't hear a target..
even with no discrim.. you don't hear the dime next to the nail when sweeing over the nail first.. just kinda made the point to me.. that
once you hear a target.. even if it sounds like a nail.. to stop..
and examine from all directions.. SLOWLY.. I noticed that if I sweep
from left to right along the nail.. which would mean that the dime would be below the nail (close to my feet).. I didn't hear the dime at all either (I believe I am remembering this correctly?!). Guess that hammers home the statement.. "sweep from all directions".. I really don't have the expertise to making any judgements on these observations.. just observations really.. just things that a "newbie" to the explorer observed... and let me "see"
the sounds that may be occurring beneath the ground.. definitely made me see the importance of "slow" and sweeping from different directions.

I guess now what I need to learn is "the sound of a deep coin".. ?!
Anyone got a ... hmmm.. what's the word.. anyone got a description such as the "buffing" or "trim painting" example?!

I hear these sounds like a small "chirp".. and I wonder.. hmmm.. is that the sound of a "deep" coin/target?! Then I think.. oh no.. I ain't falling for that one.. dig a hole to china and find nothing!!!
then I always wonder.. ARE those the sounds of the "deep" ones?! I probably shouldn't even be tempted to listen for those deep ones at this point.. but I can't help to wonder..

Thanks for the posts.. I am making progress.. believe it or not..
and.. one of these days!!!

PS: and your mention of shortening the sweep... very important to me.. actually learned this one on my own.. after realizing the importance of "slow"..

Dan

 

[dsurcy]

Wow!! Must be nice.. and.. what size coil is that?!
I'm using the 8" presently.. but.. that looks like
a 5" coil.. or less?!

Dan

 

[Cody]

You should be able to adjust the other settings so you hear both the nail and dime in all metal from both directions. You do need to use ferrous sounds and will hear high low if you sweep the dime then the nail and low high in the other direction. This is what they call a warbling sound.

 

[Ralph Bryant]

That's a little 4 inch Tesoro coil. (I hope the mention of other brands is not a "forbidden" here..... just classroom chat, not pushing any particular brand). Something that small causes you to instinctively slow down and cover things a little more thoroughly than you might otherwise with a larger coil. Navigation in between the junk targets is great. I actually did a little experiment back during the time period that all of those coins in the photo were being found, using different machines along the side of a school between the sidewalk and the building, an area of about 30 feet long by 3 feet wide. It was absolutely like a trash can with all the junk littering the area. I started with a popular high-end unit with 8 inch coil, followed up by the same machine with a smaller coil, and then finally with the little machine you see in that photo. First pass resulted in about 20 coins, second pass same machine with smaller coil an additional 12 coins, and then the final pass with the little 4 inch coil shown netted another 38 coins from the same strip of ground. I'm sure had I just started with that last machine, I would have found all 70 coins as the deepest was only about 4 inches. I modified one of these for my Dad a couple of years ago too, and last year he hunted under some bleechers at the local stock car race track and found well over 500 quarters and many more dimes and pennies over the period of a week or so. He actually paid for the machine in about a week, and hasn't used another machine since. haha... And that's the beauty of determining what kind of hunting you like to do and matching the right machine to the task. We see so much about "depth" and "high sensitivity" that it's sometimes easy to forget that there is an opposite side of that scale where tuning things down becomes a big asset in our results. I've found this especially true in coinshooting trashy conditions, though relic hunting and other types of hunting can easily be at the other end of that same scale. For relic hunting many areas, "depth" surely is the name of the game, while electronic gold nugget prospecting requires a high gain for the best sensitivity to small gold nuggets. It just remains a matter of matching the right tool to the task at hand, and having a good understanding (through experience and experimentation) of what works best in those particular circumstances. Every once in awhile we stumble onto things that just seem to "click", and it sometimes requires thinking outside the box a little bit to reach that point. The first time I took that little Compadre out after modifying it with the tiny 4 inch coil, I ran across a couple of guys at the local school who were using a couple of $1000+ top-end detectors who just couldn't seem to resist making a few "comments" and snickering under their breath at my "new little toy". They had been detecting for a couple of hours when I arrived, and after that afternoon when we stopped to compare finds, they've stopped laughing at my "little toy". I think they were both actually a little embarassed that I had found more coins in half the time than BOTH of them combined that day. haha

Ralph

 

[dsurcy]

Cody,
Okey-Dokey.. I'm up for another testing..
I'm using ferrous.. I believe I had gain at 8,
variability at 8, and limits at 10 the time I tested..

What other things would I change?

I believe currently I have gain at 5, variability at 7 or 8,
limits at 8 (audio 1).

I seem to remember getting that barbling sound when I had the
targets separated about the width of the mid bar to outer edge..
(8 inch coil)..but not when I had the targets withing 1-1.5 inches apart..but.. I have been wrong before..

I'll give it another try..

Thanks,
Dan

 

[dsurcy]

Ralph,
Whatever works!!
Now you've got me thinking about a 5" coil for the Explorer!!

Do you think that would help?! or would it take another unit?!

Yeah.. I keep thinking.. I must get past the trash and deeper.. maybe not?!

Dan

 

[Dok-Ath]

I noticed that iffy sounds with IM-12 ( coin on edge ) will be missed at high sweep speed. I am usually concentrating on iffy sounds and useing medium speed like "painting", Gain 10, Sens 27-30, Var 10 and most interesting - Threshold-0, this helps to avoid noise at so high sensitivity in any conditions.

HH

 

[dsurcy]

That is interesting.. I've noticed even at lower sensitivity
in a high trash area that I spend LOTS of time investigating
suspect sounds.. most time to only find (by going into pinpoint mode)
that the suspect sound was trash being given off at the edge of
the coil...

and.. if I bump the sensitivity.. I spend even more time investigating
"suspect" sounds..

Funny, because, the idea of going into a "silent search" mode has crossed my mind and might save time.. (probably miss some targets?!).. but.. might be worth a try.. you are saying..
"threshold = 0" is silent search mode.. right?!

So.. with this setting you avoid hearing those "pesky" nulls..
right?! (I can't stand nulling).. but.. how do you make yourself
keep a slow sweep speed.. seems to me that when I'm hearing no
responses.. I tend to speed things up (ain't nothin' here.. I'll
speed up to cover more ground til I find some targets.. type thing)..
have a hard time staying focused and staying slow. But, then,
if it is a high trash area... I guess that wouldn't happen..
errr... wait a minute... are you using discrimination.. or the IM -12 setting?!

I guess my idea of using the silent search mode was to set up some
discrimination pattern(s).. probably set for something like silver coins.. and swing until I got a hit.. but with
the silent search.. I wouldn't hear the nulling.. so I would cruise
along (slow). until I got a "hit".. ?!

When you say "iffy" targets.. are you talking about a good sounding
strong signal... or.. something like what I hear sometimes.. like
a short, faint "chirp"?!

Man.. I do have questions.. and do tend to ramble..don't I?! Sorry.. I get carried away..

Thanks for sharing the technique.. something I will be interested
in trying.

Dan

 

[Ace]

In short, the needed sweep speed is one in which you are comfortable, not too slow that you will actually miss targets, and not too fast, too fast that you will get tired ina short time, a pace in which you feel is comfortable and that you're confident you cover lots of ground if not all.

Also a good way to swing your coil is watching the coil itself so that you know how much ground you're covering and if you're missing spots, etc. And also good to look which way you're going, watching the screen all the time is a big mistake, you just need to sweep until you get a good sound, THEN you can look at your screen to see where the target is falling.

Here's a diagram on how the sweep should be and at what you should look at while detecting, sorry I was REALLY lazy while doing that drawing so I drew a stick-guy or whatever they're called, just remember the semi-circle or arch is smaller, you don't have to stretch that much, it just has to be a nice, comfortable 180 degree angle sweep:

 

[Ace]

You know guys, Andy Sabisch explains to us that when a lot of threshold nulling occurs we should switch to Fast Recovery so that the Explorer recovers faster makin the Explorer pick up targets really close to trash.

 

[Ace]

A 5" coil will get you better target separation as well as an increase in sensitivity to smaller, thinner targets like goldchains or 3 cent pieces, it will shed off weight from your Explorer. I have the Detecht 'Excelerator' 5" coil and it is great, it goes DEEP. Sunray also makes a 5" coil called the Sunray Stealth X-5 and I believe is of better quality than the EXcelerator (It's a Sunray what did you expect? ), as of depth and performance I don't know the difference since I haven't used a Sunray, but if the Detecht is good you should expect the same if not more from the X-5.

 

[Cody]

Andy has written an excellent book but he makes no attempt to go past the basics or does he try to answer all questions. I consider the book to be very good for the new user. My gues is experienced users or others consider the work to be very basic. However, if we want to go further than it is the forums that seem to do that for me.

 

[dsurcy]

I went out and checked the 5" coils and about feel
out of my chair when I saw the prices!!!

 

[dsurcy]

I agree. If you are using a lot of discrimination and
the learn/edit techniques there are a lot of good techniques
discussed and a lot of good info to be aware of..
but.. If you can't stand the nulling... as I can't..
you will most probably look for other ways..

I am still flip-flopping and looking for what works
best for me.. and.. I like to hear what others have
tried or are trying, etc.. I get distracted with too
many noises.. but.. can't stand the nulling.. danged if
I do and danged if I don't..

Dan

 

[Dok-Ath]

I was out from my office and may reply now. The main point in silent mode is to set Gain 10, otherwise you miss some iffy sounds ( I mean weak and short both directions repeatable slightly higher than ferrous and more higher sound ) And the speed as to my knowledge is to be medium, because I found out while testing that smaller targets will be missed at high or low speed and this is logical because they have weak back signal and in trashy areas the processor needs time to reply and in the lack of time the sound will be simply cut, when the motion is very slow than there is no enough "charge" of the target since it is motion MD. I am hunting in IM 10 or 12 and should go semi-slow because of nulling. I found out that with IM-14 I still have to dig big rusty debris, which disappear at -12. Variability - 7, if lower I could miss old bronze coins which sound like almost ferrous, higher setting is for the guys with musical hearing. Try to bury medium coin at max depth where it becomes iffy and play with above settings, I think you will find your point.

HH