Why cant PI detectors discriminate?

I cant find the answer anywhere. Been telling my wife about the new Garrett PI detector coming out and this question come up. Need you experts to help me out here.

PI-based detectors are not very good at discrimination because the reflected pulse length of various metals are not easily separated. However, they are useful in many situations in which VLF-based metal detectors would have difficulty, such as in areas that have highly conductive material in the soil or general environment. A good example of such a situation is salt-water exploration. Also, PI-based systems can often detect metal much deeper in the ground than other systems.

P.I.'s cant discriminate 'YET' because it's a completely different principal than a Induction balance VLF detector you use....

A VLF detector transmits and recieves as a tuned antennna and when a piece of metal disrupts the field phase shift takes place and by having a 'KNOWN' base the obejct can be I.d.'d or discriminated out if you like....a VLF metal detector examines the amount of phase shift, using a pair of electronic circuits called phase demodulators, and compares it with the average for a particular type of metal then reports it..

A P.I. machine uses high volatage pulses fired into the ground then shuts off and fires again...it send's and recieves on the same coil winding... The P.i.s do have a pulse delay to slow or speed up the firing but its in microseconds...what happens in the simplest of terms is say you have a piece of foil and your detector fires on it then shuts off the fires again...well the foil is not very conductive so if your machine is at 10 micro sencond on the pulse delay on the next firing there wil still be some residual current left in the foil and it will casue a disruption and it will be signaled...but run it it up to say 15 microseconds and on the next firing it will be delayed enough so that when it fires again the extra amount of delay gave time for the foil to discharge and it gives no report......So now it seems like it has disc ......Ahh but it does not if the foil becomes bigger it would take mayeb 20 micro seconds delay.... but then iron can also hold a charge by the size of it..bigger metallic objects of any conductance takes longer to decay ....So size also messes things up...

but while the P.I. does not disc it can ignore ground mineral because the mineral in ground is weak and usually even the 10 micro can't report it....But wet salt can be reported at 10 micro at times so it takes maye 15 or 20 to ignore wet salt....So here lies a problem with looking for small gold now ...you have to advance your pulse delay so high it cant see small gold on a salt beach...

but anyway as you can see Pulse machine's are very powerful yet the design is so unlike a VLF it has no phase shift capability....And to be truthful detector's were made for years with no phase shift..George Payne discovered it and put it to good use...if phase shift was not discovered I would dare to say that you would still have a detector with no disc...

Alot of advancements have been made in pulse circuits on ground balance though, and it helps tremendously on allowing to keep your delay low....

Im not an expert but maybe it makes sense without getting it to be confusing...

See if this makes sense to you
[electronics.howstuffworks.com]

Keith



Edited 2 time(s). Last edit at 08/20/2013 02:49AM by Keith Southern.

The technical answer is that a PI detector produces a resistive (R) response, but not a reactive (X) response. You need both to get decent discrimination. It is possible to get the X response during the coil turn-on time if you use an IB coil. Quite a few people are working on the problem of disc PI and I believe it is just a matter of time before it is solved.

Good write-up Keith.

There was much discussion of the various Minelab patents that were granted near the launch date of the CTX. Some described the merging of both frequency domain based algorithms (VLF) and time based algorithms (PI) in a single platform. I for one thought that the new CTX was to be such a hybrid machine but it appears that ML was only attempting to corner a future market. I'll need to dig some of that stuff up again as I found it quite interesting.

Thanks guys. That clears things up a little better now. I still dont think i could explain it to someone though. Sounds like if someone could come up with a way to get the PI machines to discriminate than they would really be onto something.

Tim N Wrote:
-------------------------------------------------------
> Thanks guys. That clears things up a little better
> now. I still dont think i could explain it to
> someone though. Sounds like if someone could come
> up with a way to get the PI machines to
> discriminate than they would really be onto
> something.

Spot on. Imagine a PI Detector with a discriminator that nulls out iron targets but detects gold and silver.

Its just a matter of time I think until such technology becomes available.

Joe

go-rebels Wrote:
-------------------------------------------------------
> Good write-up Keith.
>
> There was much discussion of the various Minelab
> patents that were granted near the launch date of
> the CTX. Some described the merging of both
> frequency domain based algorithms (VLF) and time
> based algorithms (PI) in a single platform. I for
> one thought that the new CTX was to be such a
> hybrid machine but it appears that ML was only
> attempting to corner a future market. I'll need
> to dig some of that stuff up again as I found it
> quite interesting.

go-rebel, If minelab can solve the mergeing of the PI and VLF technology your speeking about could they do a software upgrade download for us CTX3030 owners at a fee that would be more lucrative than say a new model? I would pay big time for what sounds like a breakthru upgrade.

Im quite sure you would not see such a Download upgrade...

at least not on the CTX platform...

I think we will see a Disc circuit one day in a P.I.

I think the logical step might be Hybrid right now...I think its going to be an evolutionary process..

Keith

I think our biggest thing to watch out for is someone puts out a disc PI.... we all jump before we realize its just not any deeper. With computer tech i agree its a matter of time before someone does something with it. Then we will have a whole new ball game. I sure wouldnt want to be a dirt digger in a park with that machine. Because at that point.... the rules may change as well.

Dew

Not a PI hunter but understand some of them can to a limited degree. Just something I read and never used a PI unit...

if they did ,a valuable breakthru that would have buyers rushing to buy one