Question , relation between frequency and depth of a metal detector

[Mr.Jaick]

hi all
from my observations, experience and research i can tell the lower the frequency the higher the depth(particularly for bigger objects), why?
i have seen this on my pi machines and also have read many times about VLF apparatus that they seem to behave
the same as PI, what's the limiting factor?
for example:
can i have the frequency of 1kHz with the same pulse width as i have for 100Hz and still get the same depth?

[waltr]

hi all
from my observations, experience and research i can tell the lower the frequency the higher the depth(particularly for bigger objects), why?
i have seen this on my pi machines and also have read many times about VLF apparatus that they seem to behave
the same as PI, what's the limiting factor?
for example:
can i have the frequency of 1kHz with the same pulse width as i have for 100Hz and still get the same depth?

It is complicated.
One reason for VLF detectors is the TC of the Target. High conductive targets have longer TC so for a VLF a lower frequency give a better response. Like wise a lower conductive target has a shorter TC so a higher frequency gives better response.

Similar for a PI detector but it is the Sampling width that makes to difference between low verse high conductive targets.
Higher sampling (cycle) rate will improve response to 'small' targets if the sampling periods and integrator design is opamized.

[Carl-NC]

If you are only talking PI, then pulse rate has no effect on the per-pulse depth. That is, if you have a 100us TX pulse you will get the same target response with 100Hz vs 1000Hz, assuming a target tau under, say, 500us. However, a faster pulse rate allows more demod integration which improves SNR (and therefore depth). PI detectors that are set up for large deep targets typically run a lower pulse rate only because they also run a wider TX pulse and either have timing constraints or need to keep power consumption under control.

[Mr.Jaick]

It is complicated.
One reason for VLF detectors is the TC of the Target. High conductive targets have longer TC so for a VLF a lower frequency give a better response. Like wise a lower conductive target has a shorter TC so a higher frequency gives better response.

wait a minute... i thought higher conductive material has shorter TC so that's why they're dying faster, right?
they charge faster they die faster(eddy currents) because they are so conductive, isn't it true?!

Similar for a PI detector but it is the Sampling width that makes to difference between low verse high conductive targets.
Higher sampling (cycle) rate will improve response to 'small' targets if the sampling periods and integrator design is opamized.

thank you that was also a good point
because they are so small and their eddies die super fast, we need to bombard them with pulses so they stay alive for a second for us to measure them

[Mr.Jaick]

If you are only talking PI, then pulse rate has no effect on the per-pulse depth.

I was exactly thinking the same based on the information on forum and also Jeorge's and your book
but wondering why I can't get the same depth on my pi with different frequencies
probably I need to adjust my integration TC accordingly

That is, if you have a 100us TX pulse you will get the same target response with 100Hz vs 1000Hz, assuming a target tau under, say, 500us. However, a faster pulse rate allows more demod integration which improves SNR (and therefore depth). PI detectors that are set up for large deep targets typically run a lower pulse rate only because they also run a wider TX pulse and either have timing constraints or need to keep power consumption under control.

I'm not really sure about demod integration and what SNR stands for sorry i'm sort of a noob compared to pros here

but thank you very much for your time, that was a privilege right there talking to you Carl for the first time
you're one of my favorite celebrities along Dave Jones.
happy new year by the way

[AK48]

is M.F good to resolve the problem ? simultaneoiusly or one by one sendind a low fr?quency ( for exemple 7 KHz ) and a high one ( 20 or more KHz ) ?

[Carl-NC]

wait a minute... i thought higher conductive material has shorter TC so that's why they're dying faster, right?
they charge faster they die faster(eddy currents) because they are so conductive, isn't it true?!

No, low conductors die faster. A perfect superconductor would never die, the eddies would continue forever.

[Mr.Jaick]

No, low conductors die faster. A perfect superconductor would never die, the eddies would continue forever.

oh wow I certainly have not learned my lessons, gotta read the book again
now i think more thoroughly that makes more sense.
so if conductors die slower why we need to sample the early stages of our pre-Amp output curve to detect gold?
shouldn't they carry along the curve longer so we can have more sample delay and ignore low conductive?

[Carl-NC]

so if conductors die slower why we need to sample the early stages of our pre-Amp output curve to detect gold?
shouldn't they carry along the curve longer so we can have more sample delay and ignore low conductive?

Most gold is low conductive. Gold nuggets and small jewelry. Low/high conductive has less to do with the metal type and more to do with size & thickness.

[Mr.Jaick]

are we talking electrical conductivity or some sort of eddy current conductivity and permeability stuff?
because electrically pure gold is the third place in metals conductivity
but of course most golds have huge impurities
and about shape i have some experiences
Pi machines DON't like chains AT ALL!

I think it has something to do with that old " Ohm per Square " Physics law

[Skippy]

"Electrically pure gold is the third place in metals conductivity"
Yes, but gold alloys are MUCH lower conductivity. Even 0.900 fine alloy ( US gold coins ) is only 15% IACS, very similar to that of Tin , just one fifth of that of pure gold. And of course many jewellery items are 10 / 14 / 18 ct , with rubbish metals like nickel included in their mix.
Some data for you:
http://eddy-current.com/conductivity...y-resistivity/

[Carl-NC]

Eddy current conductivity. Look up "skin effect."

[Mr.Jaick]

thanks everyone for replies
i read some articles about skin effect
i think my question on frequency and depth makes sense to some extent

higher the transmit frequency = higher the AC resistance of the metal target due to skin effect => so lower the eddy currents penetration into the metal target
and possibly lower the returned voltage from the charged target for us to measure

correct me if i'm wrong.

[Carl-NC]

That's correct.