TARGET TIME CONSTANT

[baum7154]

Yes this looks reasonably repeatable . Thanks for your good work!

Regards,

Dan

[Tinkerer]

With our tests we see that the relative size, considering the surface of the target, generates the largest part of the target signal amplitude response.

This size of the area is relative to the area of the coil.

I found this old test in my notes.

All targets are of the same diameter but different thickness.

The first target is a cutout from a single side PCB, I don't know the thickness of the copper.

All the copper lugs were cut from the same rod.

The last 3 targets are, a Nickel, a lead disk and a alu foil disk as references.

From the signal amplitude of the alu foil disk, we can see that the sensitivity as well as the speed of the setup is sufficient .

[baum7154]

I just performed a test using a 1976 US penny and an identical diameter disk of pure copper shielding foil. Copper is good because it eliminates the problem of the short TC of small aluminum targets. Any way the penny was detected by my CHANCE PI at virtually the same distance as the .001 thick copper foil. The penny is about 50 times thicker than the foil. The flat faces of the targets were presented to the coil in this test.

Dan

[Waikiki_Sweep]

I just performed a test using a 1976 US penny and an identical diameter disk of pure copper shielding foil. Copper is good because it eliminates the problem of the short TC of small aluminum targets. Any way the penny was detected by my CHANCE PI at virtually the same distance as the .001 thick copper foil. The penny is about 50 times thicker than the foil. The flat faces of the targets were presented to the coil in this test.

Dan

Cool! Probable explanation that target acting like a transformer loop. Eddy currents traveling on edge so more diameter of target makes stronger signal.
Rings mostly give stronger signal in comparison with heavier coins at least with my detector.
I remember that aluminum coca can bottoms made from thin aluminum but creating unbelievably strong signal because of big diameter.

[baum7154]

What is the T/C of an aluminum soda can?

[green]

What is the T/C of an aluminum soda can?

The T.C. is the line. The curves aren't quite straight.

[baum7154]

Green, Thanks for the quick response! 42us is a bit more than I would have guessed.

Regards,

Dan

[green]

I've been using some small coils trying to plot the time constant of a hypodermic needle. Wasn't sure if the coil has an effect on time constant. Plotted some data comparing three different coils with two different targets. The nickel plotted 10 usec the same as I've gotten in the past with other coils. The aluminum foil plotted 2.3 usec. The coil slopes are the same for each target. I didn't get the amplitudes the same except for the 5/8 round and the small rectangle with aluminum foil, they plot the same.

[baum7154]

Interesting.. . I read somewhere that target orientation to the coil has an effect on t/c.

Dan

[green]

Interesting.. . I read somewhere that target orientation to the coil has an effect on t/c.

Dan

Thought I would try with the targets rotated 90 degrees, edge not flat. The 3/8 x 9/8 rectangle coil was still hooked up. Rotated the 1.5 x 1.5 inch aluminum foil inline with the 9/8 side. Watching amplifier out there was no visible target signal with the foil centered. The signal time constant changed as I moved it to either side, getting longer as I moved it away from center. The nickel was similar except I couldn't get a zero target signal with it centered. Didn't try the other coils. If target position makes a difference I think it would be hard to compare time constants with different coils with the targets not flat to the coils.

[baum7154]

I agree. It is natural to lay targets flat, especially small ones, for your tests as it gives better signal strength as we'll.

Dan