eddy currents mass vs surface area ?

[wyndham]

Carl I read your introduction into induction and have a brief question for any who wish to help.
For a moment I would like to ask if a non ferrous object has a mass of say a flattened bb(maybe a 1/10 gram) and that same shape for the surface area does the surface area rule over the mass in the resulting induced eddy current so that the mass is not a factor. In another way of seeing it. We flatten a aluminum, copper, lead, gold bb's to the same surface area but with different mass, which factor, mass, conductivity or surface area would produce the greatest induced eddy effect. Would the time induction decay be a factor in mass and or conductivity Or is this the wrong way to see induction?
Thanks for your time Wyndham

[Carl-NC]

For targets of the same shape and size, in the same magnetic field, mass becomes unimportant once the thickness has exceeded 2 or 3 skin depths. It is conductivity that matters. Beyond that, shape, surface area, and thickness play strong roles.

In fact, I think I mentioned an experiment where I drilled out the center of a silver quarter, and I could then detect it deeper than a solid quarter, even though it has less mass and surface area. In this case, shape matters more.

- Carl

[wyndham]

Thanks Carl, I can see that a target below a certain size would drop off the radar because of diminishing returns. Part of what I have been wondering is the min target size that an eddy current non ferrous trash separator could be effective. It looks like small flour gold would be too small because of surface area and small time constant/decay and the long interval between mechanical polarity switching.Wyndham

[Max]

Thanks Carl, I can see that a target below a certain size would drop off the radar because of diminishing returns. Part of what I have been wondering is the min target size that an eddy current non ferrous trash separator could be effective. It looks like small flour gold would be too small because of surface area and small time constant/decay and the long interval between mechanical polarity switching.Wyndham

Hi,
eddy current absolute values depends on the shape of object when they are small...like Carl explained. One simple description in in the old (1980) Corbyn's article you can find in the archive/projects section.

He used a cylinder to make calculations. When you have a small conductive object the "path" currents may flow in is really important and depends on conductivity, shape, dimensions. I think that eddy current / decay disc is a wrong method to give discrimination infos...reliable I mean. Cause there are situations (e.g. tilting the target or interaction with surrounding matrix) that virtually make unuseful that kind of informations.

Anyway, is the common way most best seller PIs do their disc (very approssimative).

Best regards,
Max

[okantex]

as you say decay times discrimination only effected when object is small.
that is okey.for coin shooters you are right.
but what about cache hunters.
if they use big coils.I means bigger than D60cm (30inch) they can omit small objects.so that they wont have problem withsmall items.
and discrimination would be posibble for them.
Am I right

when they omit smal items.their signal volumes became allmost similer.so that means they could differ items from deceay times and help of osciloscop grafic.
this osciloscop need can be omitted by using right programs on PICs.

what do you think about my thoughts.
btw if it was not a lie.someone told me that he found gold coin.diameter something like5cm or more.it was at 135cm dept.parallel to ground surface.no mineral in ground.machine was deepmaxx3 .you must know last version.
it is PI with discrimination as I claim.

now I want your assistance to see osciloscop views and write suitable software.
please welcome to my test thread and do it together.
or continue here.