PI coils- Shielded or Unshielded?

[dbanner]

I was under the impression that very short sample delay requires very good fast coil. For a fast coil it is inferred low self capacitative in coil and cable, and shielding. Yes, shielding is a must.

But what is the useable sample delay in undersea conditions? Or even at water's edge in wet sand.

If you intend to use the machine for inland detecting, then shielding is most wise move. Then you can do deep dive or scavenge around the inland parks.

[Polymer]

Am I correct in saying that undersea conditions, shielding is useless? (Pi machine)

Hmmm, a really interesting and logical assumption from my perspective.

Since seawater is very conductive, I would guess that the capacitive charge potential to the coil is "null".
So I would wager that your statement is most likely true. Have not tested this directly though.

BUT - yeah, always these buts ... gotta leave some wiggle room ...

One of the things my "PI building adventures" has tought me
is that there are more quirkies or devil in the details than I could have ever imagined.

Hope one of the more seasoned PI passionista give some input on this.

[Polymer]

Ahhh.. I would not have thought that it would make a difference but maybe it does.

GEB control is useful to eliminate those mineralised areas.
Before I added this feature the PI would respond to old red bricks found at a n 1800's farm.

Thanks for mentioning the red bricks - might need a PI with GEB for my situation.
It?s not red bricks, but fired red clay blobs and "rocks" from the last few millenia.
The MPP E reacts to these, not very strong, but enough to wonder if its a deeper ring.

[dbanner]

Am I correct in saying that undersea conditions, shielding is useless? (Pi machine)

[waltr]

@waltr

Thank you. How easy it is to get apples and oranges mixed up. You have quite a different PI setup to mine. A very refined version if I may say so.
I forgot to specify which PI my shielding trials and tribulations were made with.

@surfdetector
The PI I am using is the MPP version E.
Hope your beaches open soon again!
The sand here is mineralised, but not black sand.
There are also a helluva lot of old black iron flakes.

Ahhh.. I would not have thought that it would make a difference but maybe it does.

GEB control is useful to eliminate those mineralised areas.
Before I added this feature the PI would respond to old red bricks found at a n 1800's farm.

[Polymer]

@waltr

Thank you. How easy it is to get apples and oranges mixed up. You have quite a different PI setup to mine. A very refined version if I may say so.
I forgot to specify which PI my shielding trials and tribulations were made with.

@surfdetector
The PI I am using is the MPP version E.
Hope your beaches open soon again!
The sand here is mineralised, but not black sand.
There are also a helluva lot of old black iron flakes.

[waltr]

Yes, All three coils have been in use for nearly three years at Old farm sites, parks and the beach (not in the water but on dry & damp sand).
Remember that my HH2 uses the Three Sample GEB method that does cancel mineralzation. The one beach in RI I detected had areas of black sand that made my pin-pointer go nuts. The HH2 with the DD coil had no issues.

[surfdetector]

@surfdetector

I would agree with Qiaozhi?s info as a loose rule of thumb, BUT, it comes down to how it works in your ground, sample delays used and what you are looking for.

In my experiments shielding did not influence search depth negatively, it actually got better due to way more stability of the threshold.

I would definitely like to test the coil before and after shielding but as I do all of my detecting at the beach and 90% of that in the water it isn't possible right now due to the Covid19 closure of the beaches.
Cheers

[Polymer]

I have built three types of coils for my HH2 PI detector. A 10" Mono, a 10" DD and an 8" concentric. None of them have shielded coils and work well at 10-15 usec delay to first sample.
It is the pre-amp that limits this first sample delay not the coils.

Did you do the testing on real ground?

As some members have mentioned, testing in real conditions shows what?s up. I fully agree to this now.

I built a PI coil without a shield which worked fine on the test bench. Outside it was a squealy-piggy as soon as I lowered it within 1-2 feet of the ground.

I put an impromtu mesh shield around the outside of the coil for testing. This solved the squealy-piggy problem and showed that I definitely need a shield.

Hoping to get around implementing a shield is understandable, as it is quite a bit of extra work and may cause other (new) problems along the line.
It also opens up a new "rabbit hole" of testing and experimenting - which we do somehow like in weird way - but sometimes you just wanna finish off and go detecting.

@surfdetector

I would agree with Qiaozhi?s info as a loose rule of thumb, BUT, it comes down to how it works in your ground, sample delays used and what you are looking for.

In my experiments shielding did not influence search depth negatively, it actually got better due to way more stability of the threshold.

[waltr]

I have built three types of coils for my HH2 PI detector. A 10" Mono, a 10" DD and an 8" concentric. None of them have shielded coils and work well at 10-15 usec delay to first sample.
It is the pre-amp that limits this first sample delay not the coils.

[Qiaozhi]

I am reading this as with delays of >15us you do not necessarily need to shield your coil. Is that correct?
Cheers

In my personal experience, I've not always found it necessary to add an electrostatic shield, although it does help to eliminate external emi. At delays above 50us, a shield doesn't seem to add any benefit.
For a VLF, an electrostatic shield is mandatory.
I'm sure others may have some different views.

[surfdetector]

From my notes on sample delays:

8-10us: gold nuggets - coil must be shielded (eliminates electrostatic interference at low delays)
>15us: beach
30us to 50us: coin shooting
Coils for longer delays do not need a shield

I am reading this as with delays of >15us you do not necessarily need to shield your coil. Is that correct?
Cheers

[green]

Does a shield degrade the TX signal power?

I didn't see a drop in signal with the tests I tried.

[bklein]

Does a shield degrade the TX signal power?

[ZED]

G'day Elliot

Eddys build on a target during the TX period,after the TX period is terminated the current ramps down,now depending on the TX cct and coil the eddys created during
the ramp down will cancel eddys from the ontime and then build eddys again with the remainder of the coil current as it continues to ramps down,it all depends on the
TX cct as a whole and the target.
When the coil current stops increasing and flattens out the ontime eddys will decay away because there is no longer a rate of change with the coil current,its static "aka DC",so
there will be no eddys to cancel as the TX terminates and the current begins to ramp down,on the other hand if the TX period terminates while the coil current is still rising then the ontime eddys
will in part be canceled out by the eddys that are created as the coil current ramps down,and if your going to ask why it will be because the ontime eddys have a opposite polarity to the eddys that are
generated during the current ramp down period,and so there will be some cancellation of eddys during the ramp down period ,now this is just a basic description at best,there are a lot of other factors at play as well.
Its not uncommon to see many references to the coil current ramp down period as "the flyback",typically they both happen at the same time,i even use the phrase as well,its convenient,but !, the very loose
use of the phrase can be very misleading,and over a period of time it becomes the norm.

Now you said quote "how can you charge the target five times faster than the target's TC? "thats a slippery question,what does it mean,are we talking time,or amplitude,or both.A targets eddys will decay
at the same rate regardless of how strong the EMF and rate of change is,and why do you want to charge it faster,whats the gain ?
I dont see the point of setting a TX CCT (including coil) up for a particular target TC when like here in VIC targets can be as small as lead shot or as big as a can,its a science for sure but no one shoe
fits all.Coil current thats flat top at switch off has its place and coil current still climbing at switch off has its place as well,neither one dominates over the other.


I have attached 3 pics of scope shots showing the TX waveform and the target signal as seen by the receive coil,using a DD 11" coil,the TX winding is 0.4 ohms,on one scope shot i have used a 10 ohm series
resistor in circuit so the coil current flat tops before switch off.
If you have a keen eye you will notice the 1.5 ounce gold nugget still has on time eddys quit late in its decay

I hope im not being a nuisance,i kinda feel like im hijacking the thread.