To Eric....A question re. target time constants.

[TheMarshall]

Hello Eric.

I've just been reading the closed thread/lesson re-FLY-back.

Have you personally done any theoretical and/or measurement work on target time constants?

I'm sure I've read during occasional visits to this forum, you interest in TC of ores.....

What specifically is your approach to discrete objects...like coins...and their TC as related to your 'impulse' methodology.

Formulated or 'suck-it-and-see' ?

I would have joined in the debate on shielding but I assume readers appetite is satiated on the matter......Matt

 

[mikebg]

Matt, there is information in WEB for TCs of different targets but it is represented in frequency domain. Search WEB for "targets frequency response". There is such thread in this forum and in Carl's forum. My email is open for questions.
Mike-BG

 

[IBGold]

Hi Matt,
I don't think the shielding issue is finished with I still have many unanswered questions I am just waiting on a response from Eric,as I have a coil ready to shield waiting for some direction.
Regards, Ian.

 

[TheMarshall]

My thanks to you both..Mike and Ian, for your replies.

I found this tantalising info whilst web searching::-

******************************************

Nelson, C.V. Cooperman, C.B. Schneider, W. Wenstrand, D.S. Smith, D.G.
Appl. Phys. Lab., Johns Hopkins Univ., Laurel, MD
This paper appears in: Geoscience and Remote Sensing, IEEE Transactions on
Publication Date: Jun 2001
Volume: 39 Issue: 6
On page(s): 1129 - 1138
ISSN: 0196-2892 0196-2892
INSPEC Accession Number: 6974085
Digital Object Identifier: 10.1109/36.927425
Date of Current Version: 07 August 2002


ABSTRACT
A portable, wide bandwidth, time-domain electromagnetic sensor system has been developed and used extensively to measure the eddy current time-decay response of a wide variety of metal targets. The sensor has demonstrated the ability to measure metal target decay times starting approximately 3 to 5 us after the transmitter current is turned off and target decay time constants as short as 1.4

 

[Eric Foster]

Hi Matt,

I think I have that paper by Nelson. However, I have a unit that can work at delay times down to 1uS for investigating very small or poorly conductive metal targets. Calculating decay times is not too difficult for spherical or cylindrical targets, but in practice it is easier to measure them. This is particularly so for irregular objects such as gold nuggets. Other metals such as copper and silver, which comprise a small and variable percentage of most nuggets can have a major effect on the decay time. Ten percent silver can halve the decay time for a similar size nugget. This may seem counter intuitive as silver is a better conductor than gold, but that is what happens.

Now back to my workshop shaped hammock.

Eric.

 

[Eric Foster]

I have just powered up my fast PI and looked at some screening tapes. Using a ferrite cored probe held right on the tape the signal had decayed to zero round about the 5uS time. The tapes tested were Scotch 24, 3M 3190 woven nickel plated copper, and 3M 1190 woven copper. By comparison, a piece of 1in wide lead tape showed a decay of 15uS total. I practice, when the tapes are wound helically on the coil winding, the coil field is coaxial to the tape surface and even the lead tape could be used on a coil sampling at 5uS. Lead is now banned as a hazardous material, so can't be used anyway. 1190 has been discontinued, so the choice is Scotch 24 or the 3190. Both are quite easy to solder a ground wire to.

Eric.

 

[TheMarshall]

Good morning Eric, and thanks for your reply.

If you should ever discover that 'paper', I would be very interested in reading it.

(A) Reading your post on the lead tape, and it being banned. I realise the Health and safety risks/skin contact etc., but if it's used in a concealed application, does that still apply?

+++++++++++++++++++++++​

[size=large]Back to time constants, in metal things....[/size]

(B) QUOTING Eric:- Hi Matt,

I think I have that paper by Nelson. However, I have a unit that can work at delay times down to 1uS for investigating very small or poorly conductive metal targets. Calculating decay times is not too difficult for spherical or cylindrical targets, but in practice it is easier to measure them. This is particularly so for irregular objects such as gold nuggets. Other metals such as copper and silver, which comprise a small and variable percentage of most nuggets can have a major effect on the decay time. Ten percent silver can halve the decay time for a similar size nugget. This may seem counter intuitive as silver is a better conductor than gold, but that is what happens.

Eric...with ref. to the underlined passage.....Your comment and observations triggers my present interest in that very fact, of how 'alloying of two metals, markedly alters their conductivity.

I am endeavouring to derive a formulae that will predict the resulting conductivity of any such alloying of two different metals. I suspect that the answer is based on the exponential of the ratios.

To date, in my limited search for data, it appears that the resulting effect of homogeneous alloying is to produce a conductivity of a lesser value than the lowest individually involved.

That at least is what you appear to be seeing in 10% silver / 90% gold.. That would result in a shorter time constant, despite the ratios involved.

Can it possibly be that the 10%-90% is simply an analytical 'bulk-ratio' figure, rather than an indicator of the possible non-homogeneity of the total atomic mix?

If I accept your findings, then I predict that the unexpected TC effect is indeed due to the gold / silver 'mix' in a naturally occurring nugget being striated rather than a infused melt .

Hope that statement makes sense Eric.....If not, then please put a couple of extra hooks in your workshop's walls, and sling an extra hammock for this sleepy headed guy.

It's now 2 am Friday morning and the cocoa is taking effect.....Time for bed.

Best wishes to all on station.............Matt.

Now back to my workshop shaped hammock.
End QUOTE.

 

[Eric Foster]

Hi Matt,

I first observed this phenomenon back in 1968 when I was doing some measurements on gold/silver and gold/copper standards at Oxford University. These were artificially made and ranged from 99% pure gold to either 99% pure silver, or copper, in 5% steps. These were carefully controlled uniform alloys with no striation and even 5% of the other metal caused a marked change in decay time.

Eric.

 

[mikebg]

atents of professor Nelson; Carl V. (Derwood, MD)
The Johns Hopkins University
7,227,466 Nelson June 5, 2007 Expendable metal detector
7,176,691 Switched coil receiver antenna for metal detector
7,157,913 Re-configurable induction coil for metal detection
7,148,691 Step current inductive antenna for pulse inductive metal detector
7,132,943 Moving belt sensor
7,124,689 Method and apparatus for autonomous detonation delay in munitions
7,106,194 Method for metal object identification using a three-dimensional steerable magnetic field antenna
7,079,976 Dense object detection system and method
7,078,906 Simultaneous time-domain and frequency-domain metal detector
7,075,304 Variable damping induction coil for metal detection

 

[mikebg]

There is more early patents of Carl Nelson:
7,030,759 Steerable three-dimensional magnetic field sensor system for detection and classification of metal targets
6,970,086 Wide area metal detection (WAMD) system and method for security screening crowds
6,967,574 Multi-mode electromagnetic target discriminator sensor system and method of operation thereof
6,927,577 Digital nulling pulse inductive metal detector
6,853,194 Electromagnetic target discriminator sensor system and method for detecting and identifying metal targets
6,809,520 Compact, autonomous robotic detection and identification sensor system of unexploded ordnance site remediation
6,791,329 Portable metal detection and classification system
6,789,043 Magnetic sensor system for fast-response, high resolution, high accuracy, three-dimensional position measurements

 

[TheMarshall]

Many thanks Mike for your list of patents.

Apologies for not responding more quickly.

You have given me plenty to read.

......Best regards. Matt