A technical question about one PI

[michael]

Thank you Carl, then what are the main indices or distinctive features for: 1- Penetration Power to depth 2- Accuracy and precision of results in one PI MD ?

[Seeker]

Hi Seeker, although I'm not electrician, but know one thing( have read in a book) that VLF is abbreviation of; "Very Low Frequency" it can be each one of; BFOs, VCOs, TRs & PIs.
VLF is for Freqs lower than 30 Khz. e.g. White's Spectrum XLT is an VLF BFO which the operation freq is 300 Hz.
or from TRs; Discovery TF900 works in 2 freqs; 12 & 72-5 Khz. with 12 Khz is much reliable for metal detecting whereas in 72-5 Khz is more cavity finder, whereas Fisher gemini3 works only with 85 Khz and you hit predominantly the mineral piles or cavities.
In PIs you see nearly all of them are VLFs but in different frequencies.( some 2-4 Khz, some 300-400 HZ and some other in 400-500 Hz) all of them are VLF. but based on the book the less frequency (longer wave length) the more reliability and precise for metal detecting. when one PI MD works with freq lower than 1 Khz, it's cute (as DTI Genesis works). If I am wrong, you or others lead me please, I will be grateful.

Hi Michael, look at the picture. The differences are output power:VLF up to 2W ,PI~ 400 - 700W ; waveforms VLF - sinusoid ,PI - short pulses, etc.
So, if you compare two types of MD, do not use electrical parameters, but check the mains -depth, discrimination of metals,low ground effect,etc.

[michael]

Thank you Seeker,useful. I repeat my question; then what are the main indices or distinctive features for:
1- Penetration Power to depth
2- accuracy and precision of results in one PI MD ?
it's understood from Carl's note that frequency in PIs is not as important as other kind of MDs, and using VLF phrase for PIs is mistake. yes? no discussion here, I want to know DTI guys comments are true or not? they introduce pulse Peak Height(900 V),Peak Power(600 - 700 W) and coil current(10A) as the factors for penetration power . Can it be really so ?

[Carl-NC]

PI transmit depth is determined by the slew rate of the magnetic field, dB/dt. This depends on the initial strength of the field (B) and the speed at which it is shut down (t).

For a skinny donut coil, B is proportional to the current I, the number of coil turns N, and inversely proportional to the coil radius R. At the center of a coil, B = µ*N*I/(2R)... it falls off as you move away from the coil. However, the reduction-vs-distance also depends on radius, so even though a larger coil will have a weaker field at its center compared to a smaller coil (everything else equal), at a distance it can be stronger than the smaller coil.

The other parameter is dt, the speed at which the current is turned off. Faster is better. And this translates into a higher flyback voltage, where V=L*dI/dt. So, in general, a higher flyback voltage translates into a higher I or a lower dt. However, I believe you can clip the the flyback voltage without any negative effects, so a lower peak flyback that is clipped might still have a high I and low dt. This is on my experimentation schedule.

- Carl

[Unregistered]

Michael,
You might want to go to the PI detector forum where a couple years ago there was a very long thread/debate on all this with the top PI designers in detail. Lots of this kind of information already there. Besides the power pumped through the coil, you have to get noise down as much as possible.

I built a detector with even higher pulse parameters than what you mention. I detect a paint can at 11 feet away with the large coil. It's big and heavy but I haven't found anything to beat that test yet. I was thinking about selling it but it sounds like JC1 is saying I might be causing health issues if I do. Guess I hadn't thought too much about that. I've got a lot of hours of detecting time on it. Haz'nt afekted mee seau fur.
;0)
Boattow

[michael]

Carl, Thank you for your useful info. If you have any arranged experimentation schedule, would you please put it here or mail for me?
Unregistered, Thank you, I used to search in such PI forums, but saw no useful technical info, or they predominantly write about some ordinary PIs, e.g. I never saw to discuss about digital PIs like as DTI Genesis or other kinds that I know.
You have stated about the PI with even higher pulse parameters, and the power was cute(a paint can at 11 feet). As you know this is not enough for a reliable detection.
what about its' accuracy and precision? Did you make complete tests with various things?:
- Naturals; every kind of stone, minerals or viscous magnetised.
- Manmade objects; bricks, ceramics, cements, plaster).
If yes, what was your results?

[michael]

Unregistered,
As it's impossible to put PM for you or e-mail to you, I have to repeat my latest above questions in here;
You have stated about the PI with even higher pulse parameters, and the power was cute(a paint can at 11 feet). As you know this is not enough for a reliable detection.
what about its' accuracy and precision? Did you make complete tests with various things?:
- Naturals; every kind of stone, minerals or viscous magnetised.
- Manmade objects; bricks, ceramics, cements, plaster).
If yes, what was your results?
Oh, by the way;What was its' operating frequency? and what was your searching loop size? Thanks a lot.

[Unregistered]

Hi,
I saw your question after you first posted it but didn't reply because it was unclear to me. I'm sure it makes complete sense in your mind but I don't know what you mean by accuracy and precision? I've stated it doesn't have discrimination so I don't understand that. It was built just for my use in order to solve a problem which is maximum depth...nothing else. As people have found out about it through word of mouth, I've built a few more and made changes based on feedback. The reason I chose a paint can to test was that I could have someone else, anywhere else test their detector against that object and have an apples to apples comparison. I have no need or desire or capability to test it on bricks, cement, and plaster and not really sure why I would since I mostly look for iron objects and it does a fine job on iron. I've been using it for 3 years now and I'm happy with the performance.

It's operating frequency is variable but it centers around 100hz. I am primarily using a 50", 20", and 14" coils.

Boattow

[Unregistered]

JC1

Yo Botatow,

I cn't say if tehere is haelth rsik or not

I is not a Docotor.

Good Detectter thouhg and taht'w what matatters.

[michael]

... I don't know what you mean by accuracy and precision? I've stated it doesn't have discrimination so I don't understand that.... I have no need or desire or capability to test it on bricks, cement, and plaster and not really sure why I would since I mostly look for iron objects and it does a fine job on iron...

Thank you Unregistered. But why no need to test on other manmade objects?!!
My discussion is not about discrimination as I don't believe in a satisfiable discrimination in every kind of detectors (at least for deep objects).
If a MD detects only (emphasize; only) metals, it's accurate and precise.
for this reason I asked from ceramics, bricks ,...
I have sen so many detectors that detects these kind of objects as easily as metals.
every zone you step in encounter to manmades; ancient ceramics, bricks,...( at least in Middle east is so) then It's essential to know does it detect these objects & minerals or no is an utter metal detector.
When one detector beeps for every kind of object as metal is one of them it's not accurate or precise, then you can't trust to signals for digging.
In every judgment via trial & error most important indicators are;
Consistency/ Repeatability, Accuracy & precision. If you make these tests and inform me, I will be much grateful to you, as I think are not hard tests. Thank you again.

[Boattow]

check out this site

Here you'll see a list of detectors on a one gallon object:

http://www.detection.com/Deep/howdeep.html

I've had a couple people test their 104B and others on a paint can and 7 feet is the best any of them did. Mine goes 11 feet on a paint can. I tested several paint cans...years old and brand new with the same result to try and verify that they are all the same material. I know this isn't what you want to hear but I figured it was a simple test of apples to apples to give me an idea on how my detector did against the big boys...that's all. I can tell you that my detector does detect orange clay using both the 14" coil and the large loop at 10us. I'm very glad it does.
Boattow

PS-I tested the detector they sell, the pulse 2000, as you would expect from a pulse detector that runs on a 9v battery, I couldn't get it to perform as well as the claims made for it.

[Darren in NC]

boatow's depth

Hello boattow,

So how about your depth on the smaller coils? I read about your 11 feet with the paint can (can you imagine digging that far? ) but what about your smaller coils. What kind of depth did you get? I would think the guys at the beach who dig everything would love a deeper machine like that.

Best,
Darren

[Boattow]

Sorry

Darren,
I haven't tested it with smaller coils since I don't hunt coins or rings. I'll post results when I do run some tests.
Thanks,
Boattow

[Darren in NC]

Thanks

Thanks, Boattow. Is this a closely held project or do you mind posting how you built this unit? Do you have a schematic and pcb layout? Better yet, you mentioned you might sell yours. How much did you have in mind. I'm willing to take a health risk

[Tim Williams]

Hey Darren how have you been? Boattow that does sound like you would be getting a heavey dose there. I just found out that I have been bathing in 60hz field from a nearby power line for 3 years. So I'm moving my office to one of the back rooms!

take care,

Tim