test targets

[green]

[I was also going to point out that the 1/64th / inverse-6th power effect is really applicable when the target is a minimum of 1.5 coil-diameters away from the coil, it's less severe when close to the coil, maybe even as low as inverse 4th power.]

Charted some data for three different coils, log amplitude and log distance. Added a line for each coil(double distance signal reduces to 1/64). Look like the lines are close to matching the target slopes past the coil diameters. Using a larger target to allow charting at a greater distance would make it better.

[Skippy]

Correct value? There is NO correct value, we've measured different cans, with different metal thicknesses, and possibly slightly different alloys that have somewhat different resistivities. Unless we use the SAME target samples, or IDENTICAL cans, eg. two from the same 6-pack and I post one to you, we're going nowhere. This is why drinks-can metal, (and aluminium foil) are not universal targets. I have some other cans that are thicker (by about 15%) and targets cut from them definitely read lower freq / longer tc value. I think I still have some of this can material in my 'bike stuff' box, (I use it to shim rear gear cassettes on freehubs so they don't move about) , if I get the opportunity, I'll pick up a few random cans off the street, to see if there's any particularly thin/thick ones amongs them. Experience shows they are near-identical across Europe, so even if I get some Polish lager can (my country is littered with Tyskie/Lech/Zubr/Zywiek thanks to migrant workers) it will be from the same Scandinavian factory.

And re:"What power is double the current double the distance?" - I don't understand, and I know little about PI's either, so I can't answer.

[green]

It has a good 00 - 99 target ID display, which I have calibrated so target frequency / time-constant can be simply measured. I can also probe internally and do direct measurements, but the point of the calibration was so measurements can be done easily and quickly. Internal probing is a bit laborious, and for tiny targets is also less accurate.

I was also going to point out that the 1/64th / inverse-6th power effect is really applicable when the target is a minimum of 1.5 coil-diameters away from the coil, it's less severe when close to the coil, maybe even as low as inverse 4th power.

25mm/20mm*4.6(TC)=5.75 not 7 what you stated for the 25mm square. Recorded TC data with 1.5 in. diameter Rx coils(think I used to measure the TC data above)and the 8 in. diameter Rx coils(distance data above). Charted fairly close today but different than what I charted before. Still wondering why my TC data doesn't repeat better. Some of it can be drawing the line thru the charted curve. Was hoping another method of determining TC might help explain why. I used the same targets except for the added 25mm square that were used before. Maybe some of our difference is using different targets, our methods for measuring TC or something else? Maybe the difference we are seeing isn't a problem but I would like to know what the correct TC is. Anyone have any ideas?

What power is double the current double the distance?

[Skippy]

It has a good 00 - 99 target ID display, which I have calibrated so target frequency / time-constant can be simply measured. I can also probe internally and do direct measurements, but the point of the calibration was so measurements can be done easily and quickly. Internal probing is a bit laborious, and for tiny targets is also less accurate.

I was also going to point out that the 1/64th / inverse-6th power effect is really applicable when the target is a minimum of 1.5 coil-diameters away from the coil, it's less severe when close to the coil, maybe even as low as inverse 4th power.

[green]

I've done similar experiments, using a 13KHz VLF machine, with good results. The time-constant is proportional to the linear size of a square 'drinks can' target.
I haven't investigated the variation with metal thickness, it's difficult getting enough significantly different samples to test. I'll have another try.

I'll post up my test results later when I've dug them out...but a summary is:

Sample size 25mm x 25mm has time-contant = 7 microsecs. All other delays are in absolute proportion.

How do you measure time constant with a 13KHz VLF detector.

[Skippy]

I've done similar experiments, using a 13KHz VLF machine, with good results. The time-constant is proportional to the linear size of a square 'drinks can' target.
I haven't investigated the variation with metal thickness, it's difficult getting enough significantly different samples to test. I'll have another try.

I'll post up my test results later when I've dug them out...but a summary is:

Sample size 25mm x 25mm has time-contant = 7 microsecs. All other delays are in absolute proportion.

[green]

Was reading some other posts the other day. If I read it right, stated time constant proportional to thickness and size didn't effect time constant. I made some test targets awhile back where time constant is proportional to thickness and length of square targets cut from the side of an aluminum coke can. Charted some data. ITMD states signal strength is proportional to diameter cubed. For the square targets with the same time constant signal strength increased from 8 to 10 times when doubling length, the 10 could be measurement error. Tried to get the target lengths as close as I could. I read, need to increase the coil current 64 times to double the detection distance. I find that to be true for targets that can be just detected at distances greater than coil diameter assuming delay and sample time could remain the same. For a nugget that can just be detected at 1 inch with a 8 inch coil doubling the current should double the distance.