which coil for which application ?

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Bfg
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which coil for which application ?

Post by Bfg » Tue Jan 16, 2018 11:10 pm

.
My apologies if this has been asked by newbies a thousand times over. I had hoped to find a general introduction here - to what are different coils particularly suited to ?
And when do you use one but not the other. ?

Would you take a selection with you to every permission, perhaps to change over if it starts raining or perhaps you find yourself working over a different soil type ..like a sand bank or silt deposit in a predominantly clay area ?

Of course I'd be glad to just be pointed to a topic where it's all been answered before. Thanks indeed ::g

I'm thinking it ought not to matter, but if it is pertinent then I'm asking about those for a Minelab x-terra, where changing the coil also changes the frequency.

What I've (sort of) gathered so far is that ;

> Big diameter gives greater search coverage per sweep. But despite the width diameter there is a sensing blade oriented to 12 and 6 (analogue clock).

But do the side rims of the open style act like a dish reflector ?

> Small diameter gives ease of moving through tighter confines like brambles, or getting into corners like inbetween tree trunks, and even for wading (assuming it's waterproof) where the small size will be less pushed around by waves or current.

> Small is also more localised / focused signal, which may be useful when there's a lot of other stuff in the ground. ?

I can only guess that this might also be useful in high mineral soils and salt water wet sands. ?

Aside from that I haven't yet quite worked out why the different frequencies 3, 7. 18.75, 28 or whatever. ?

Does one end of the frequency range have a faster recovery time ?

I hazard a guess that some penetrate deeper than others, but perhaps the others give better definition nearer the surface.

Possibly one frequency range is better for fresh water streams and other for salt water shallows ?

..or is that different metals are more easily found at different frequencies, with gold for example at the 18.75khz end of the range, with pig iron at 3khz ?

I'll leave the questions at that for now, but would appreciate someone in pointing me in the direction of a cheat sheet.

Many thanks,
Bfg ::g


. . . . . . . . . . . . There is more worth in a kindness than in gold. Introduce yourself : old giffer taking fist baby steps in md'g Minelab E-trac

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Re: which coil for which application ?

Post by Koala » Wed Jan 17, 2018 12:05 am

First frequency. For coins and larger it makes very little difference. It does for sub gram nuggets when prospecting but for general detecting it's just marketing hype.

Now there two terms that get mixed up
Recovery speed and separation.

Recovery speed is purely due to your machine. It is the time after large
iron before your machine resets and will detect a target again. The slower the recovery the slower you have to swing. (forget about those YouTube demistrations most are bogus the nails to thin and stainless and the coin too big. Any machine will work under these conditions)

Separation this is purely down to the coil. Regardless as to whether it's a DD or concentric the gap between say half a horseshoe and a silver sixpence is half your coil diameter.

So as an example iron is equally spaced 18" apart all over a hypothetical field
You are using an 18" coil. As you clear one iron target the coil is affected by the next iron target

Or put it another way out of the 18" its detecting for zero of the distance

Now if we swap to a 9" coil. After 4.5" you will clear the first bit of iron and you detector starts working again. It will continue to work until the other edge of the coil is over the next iron.

Or put it another way out of the 18" its detecting for 9" of the distance

On are hypothetical field the large coil will be equivalent to being switch off 100% of the time
Where as the smaller coil is only off for 50% of the time.

The more iron in a area the smaller the coil you need.

Most machines the stock coil is a nice compromise between coverage and sepetation.

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Re: which coil for which application ?

Post by Twit » Wed Jan 17, 2018 1:06 am

Very roughly as stated by both. For coils it is a trade off on

Weight
Coverage
Sensitivity (not obvious and depends on various factors which size is better)
Depth ( usually larger coil deeper, but not always in practical terms)
Ability to discriminate ( not always obvious either)
Emi ( larger usually more if present)

Different size coils have a different overall feel to them also...which also changes under different conditions... though they can usually all be tamed down... but just the change in weight for example will change how fast and how you swing.

Usually the stock coil ( @ 10" usually) is a good compromise, but it is worth trying different coils to get an idea (not necessarily by buying them) ...and it definitely makes for a new experience if you are tired of a standard coil. Usually people find a coil that suits them and their conditions ok and they don't change around too much... so on my machine a 9" coil has become standard just because , but will change up or down if I think there is an advantage or for something new.

Frequencies are a different topic and won't start on that.

Either way don't be put off by it all, there isn't usually so much difference that any cannot still work ok, the rest is just learning and experience... and even that is particular to a person and so there isn't really a standard reply, just various details that are part of the balance.

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Re: which coil for which application ?

Post by fred » Wed Jan 17, 2018 1:30 am

I usually detect my sites to death with the stock coil and then do them again with a larger one to get a little extra depth. I might use a smaller one on realy junky sites. There is little point in worrying about coil sizes as a beginner though. :D
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Re: which coil for which application ?

Post by Easylife » Wed Jan 17, 2018 2:00 am

Bfg wrote:
Tue Jan 16, 2018 11:10 pm
.
My apologies if this has been asked by newbies a thousand times over. I had hoped to find a general introduction here - to what are different coils particularly suited to ?
And when do you use one but not the other. ?
I can understand your questioning, everybody will no doubt answer them differently but with the same ultimate meaning. Best thing is to just get out there and learn your machine from experience in standard set up.
Bfg wrote:
Tue Jan 16, 2018 11:10 pm
Would you take a selection with you to every permission, perhaps to change over if it starts raining or perhaps you find yourself working over a different soil type ..like a sand bank or silt deposit in a predominantly clay area ?
Some might take along other coils if they had them, but most will just use the one coil whether they have a single frequency machine or not. Rain is not an issue and you may just need to make minor adjustments to suit slightly higher mineralization.
Bfg wrote:
Tue Jan 16, 2018 11:10 pm
I'm thinking it ought not to matter, but if it is pertinent then I'm asking about those for a Minelab x-terra, where changing the coil also changes the frequency.
Using just the one frequency will be fine, anything from say 6khz to 19khz will get most targets.
Bfg wrote:
Tue Jan 16, 2018 11:10 pm

What I've (sort of) gathered so far is that ;
> Big diameter gives greater search coverage per sweep. But despite the width diameter there is a sensing blade oriented to 12 and 6 (analogue clock).
Think of it more like an inverted cone covering all the area within or just like a narrow blade depending on coil type.
Bfg wrote:
Tue Jan 16, 2018 11:10 pm

> Small diameter gives ease of moving through tighter confines like brambles, or getting into corners like inbetween tree trunks, and even for wading (assuming it's waterproof) where the small size will be less pushed around by waves or current.
> Small is also more localised / focused signal, which may be useful when there's a lot of other stuff in the ground. ?
You are correct.
Bfg wrote:
Tue Jan 16, 2018 11:10 pm

I can only guess that this might also be useful in high mineral soils and salt water wet sands ?
Well that might depend on the machines ability to handle such highly mineralized ground.
Bfg wrote:
Tue Jan 16, 2018 11:10 pm

Aside from that I haven't yet quite worked out why the different frequencies 3, 7. 18.75, 28 or whatever ?
Generally different frequencies respond to different metals better depending upon their size. eg. 3khz will do deeper on large silver and iron, 18khz will be better on small silver and gold. 7 or 12 khz will be a good allrounder, will get most targets but just not hit the above quite as well.
Bfg wrote:
Tue Jan 16, 2018 11:10 pm

Does one end of the frequency range have a faster recovery time ?
No, that is purely down to the detector processing.
Bfg wrote:
Tue Jan 16, 2018 11:10 pm

I hazard a guess that some penetrate deeper than others, but perhaps the others give better definition nearer the surface.
Lower frequencies go deeper but smaller targets generally respond better to higher frequencies.
Bfg wrote:
Tue Jan 16, 2018 11:10 pm

Possibly one frequency range is better for fresh water streams and other for salt water shallows ?
Yes, any single frequency is okay for low mineralization whereas for higher mineralization multi frequencies together would be best but a single high(est) frequency would also be desirable.
Bfg wrote:
Tue Jan 16, 2018 11:10 pm

..or is that different metals are more easily found at different frequencies, with gold for example at the 18.75khz end of the range, with pig iron at 3khz ?

That's right.
Bfg wrote:
Tue Jan 16, 2018 11:10 pm

I'll leave the questions at that for now, but would appreciate someone in pointing me in the direction of a cheat sheet.
Many thanks,
Bfg ::g

Hope this helps. ;)
Nox 800, 15" coil & Garrett carrot.
White's TDI SL

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Re: which coil for which application ?

Post by Koala » Wed Jan 17, 2018 11:12 am

Don't believe the marketing hype by MineLab when first produced a DD

Often see it reproduced.

It isn't correct and alters with the shape and size of the target

It's not and never has been a blade verses a cone for any size shape or orientation of a target

Test it for yourself. If yourself. Get a sixpence and stick it in the ground verticaly and facing you
Get a DD and sweep over it. There will hardly any depth in the centre of the coil. Yes I know this is an extreme example.

For most targets the concencentric is like Xmas pudding
The DD is similar but the sides are squashed in and a bit lob sided and off centre.

If you have a no motion machine it's easy to plot with piece of card and pencil.

Size for size the concentric is always deeper. Not by much. Not enough to worry about.

A DD is surpose to be better in mineralized ground. I have not found any difference in the ground myself but have no reason to doubt this. The science seems to make sense. The squashed in sides means it seeing less of the ground. But any difference is too small for me to tell in my fields.

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