Tests of big depth metal detectors ( TR and PI systems )

[Davor]

I'd say a small amount of borax as a flux for clean smelting could go a long way.
Have you watched this? https://www.youtube.com/watch?v=yTOCAd2QhGg

[green]

Bought a 1x.25 inch aluminum bar for a repair job today. Cut two pieces 1 inch long and two pieces 4 inches long. Recorded the time constant to see what kind of target they would make. The 1x1x.25 piece and 2 pieces stacked had a TC=400usec. The 1x4x.25 inch piece and 2 pieces stacked had a TC=465usec. If I stack 2 pieces of aluminum foil 1x1 inch the time constant doubles or double the length(2x2 inches)the time constant is double the 1x1 inch piece. I think the time constant not changing very much with increased length over width is correct. Should stacking the .25 inch pieces cause a bigger change, maybe double the single piece time constant? Would a piece 1x1x.5 inch have a time constant twice the 1x1x.25 inch piece? Would a piece 2x2x.25 inch have a time constant twice the 1x1x.25 inch piece? Why doesn't stacking the .25 inch thick pieces double the time constant?

[green]

Bought a 1x.25 inch aluminum bar for a repair job today. Cut two pieces 1 inch long and two pieces 4 inches long. Recorded the time constant to see what kind of target they would make. The 1x1x.25 piece and 2 pieces stacked had a TC=400usec. The 1x4x.25 inch piece and 2 pieces stacked had a TC=465usec. If I stack 2 pieces of aluminum foil 1x1 inch the time constant doubles or double the length(2x2 inches)the time constant is double the 1x1 inch piece. I think the time constant not changing very much with increased length over width is correct. Should stacking the .25 inch pieces cause a bigger change, maybe double the single piece time constant? Would a piece 1x1x.5 inch have a time constant twice the 1x1x.25 inch piece? Would a piece 2x2x.25 inch have a time constant twice the 1x1x.25 inch piece? Why doesn't stacking the .25 inch thick pieces double the time constant?

Maybe someone could reference a paper or link that would answer the questions or better yet how to calculate the time constant for the targets. Thanks

[Davor]

The time constant limit as experienced by a PI detector will depend on the sample duration. If for example your sample duration is, say 50us, you won't notice increase in apparent time constant past, say 100us. Time limit will impose a frequency limit, which will in turn impose a time constant limit.
In case of PI, the time constant depends on a PI device.

[Tinkerer]

Bought a 1x.25 inch aluminum bar for a repair job today. Cut two pieces 1 inch long and two pieces 4 inches long. Recorded the time constant to see what kind of target they would make. The 1x1x.25 piece and 2 pieces stacked had a TC=400usec. The 1x4x.25 inch piece and 2 pieces stacked had a TC=465usec. If I stack 2 pieces of aluminum foil 1x1 inch the time constant doubles or double the length(2x2 inches)the time constant is double the 1x1 inch piece. I think the time constant not changing very much with increased length over width is correct. Should stacking the .25 inch pieces cause a bigger change, maybe double the single piece time constant? Would a piece 1x1x.5 inch have a time constant twice the 1x1x.25 inch piece? Would a piece 2x2x.25 inch have a time constant twice the 1x1x.25 inch piece? Why doesn't stacking the .25 inch thick pieces double the time constant?

The skin effect plays a significant role with thin targets.

[garyq]

Did a test comparing 2 stacked coins vs single coin vs 2 coins side by side. Used a PI detector, wondering if a VLF detector would give similar results.

Your test inspired me to do my own air test with a PI and 6" mono coil using US Nickel coins for a clear strong signal response.

Target - US Nickel
GB - On
Single - 203mm
2 Stacked - 102mm
2 Side by Side - 229mm
2 Overlap by Half - 178mm

[green]

Your test inspired me to do my own air test with a PI and 6" mono coil using US Nickel coins for a clear strong signal response.

Target - US Nickel
GB - On
Single - 203mm
2 Stacked - 102mm
2 Side by Side - 229mm
2 Overlap by Half - 178mm

Good example of a PI hole. Tried the same test again, 2 stacked was closer to the PI hole bottom with my detector also.

[Wahyu]

Your test inspired me to do my own air test with a PI and 6" mono coil using US Nickel coins for a clear strong signal response.

Target - US Nickel
GB - On
Single - 203mm
2 Stacked - 102mm
2 Side by Side - 229mm
2 Overlap by Half - 178mm

Thank for info

[Ferric Toes]

IACS conductivity of coins.

I tried some coins for conductivity on a Hocking AutoSigma 2000 conductivity meter. The reading is a percentage conductivity compared to annealed copper which is 100.

US Cents up to 1982.......53
US Cent 1989................28
Dime............................59
Quarter........................43
Nickel...........................5
UK Penny 1971..............50
Lead disc.......................8 (30mmD x 2.5mm)
2kg lead weight..............8

Silver $ 1889................80.7
Silver 1/2 $ 1964..........84.4
1/2 Crown (UK) 1845.....62.6
.999 pure silver...........101

Eric.

[Ferric Toes]

I have just located a test disc that I had back in the good 'ol 1960's. It is 56% Cu and 44% Ni. Diameter 42mm and thickness 4.7mm. The conductivity reading is 3.6 and at the time was considered to be a target with a fast decay.

If I were making a PI for big depth on a coin hoard, I would be hoping to find silver or gold coins. A hoard of over 600 Roman silver denarii were found recently in a field close by, with many still in the base of an earthenware pot. I would choose test coins to be somewhere close in conductivity to those that I would like to find and of a similar size. Copper US or UK pennies would be a good choice. Nickels and lead are too far down the conductivity scale for my liking.

Be prepared though to use wide TX pulses i.e. at least 500uS and 10A or more or so peak current. The repetition rate can be lower at a few hundred pulses per second and there is no need for short delays before sampling. 25 - 50uS would be OK with sample pulses the same. I have a PI board that is suitable to do this, so I shall be doing a few tests and will report back.

Eric.

[green]

Tried making a lead disk. Mostly an exercise in pouring one, ended up being twice as thick as Skippy's suggestion. I have some 1/2 inch copper tubing used to plumb propane gas. Easy to cut to different sizes with tin snips. Might be a alternate for the pennies. I have 6 or 7 pennies 1982 or higher for every one less than 1982. Charted the disk, one penny and 100 pennies. 100 pennies and disk charted close to same first 75usec. Wondering if 300 or 400 pennies would chart closer to the disk a longer time or maybe a 5mm thick disk would chart closer to 100 pennies. The target sample is usually taken in the first 75usec, does it matter if they chart the same after? Does anything on the chart matter when comparing targets? If not what would be a good way to compare different targets?

Just reread Eric's reply #70. [25 - 50usec delay would be OK] so maybe what happens after 75usec does matter. Does GEB effect detection distance more when sampling at say 10usec vs 50usec?

[green]

Searched Carl's cache test to see what he used. Didn't remember as many coins on edge. From my testing that would make it harder to detect. Reply #3 http://www.geotech1.com/forums/showt...4062#post44062

Another reason a solid lead disk would make comparison testing possible.

[green]

Searched Carl's cache test to see what he used. Didn't remember as many coins on edge. From my testing that would make it harder to detect. Reply #3 http://www.geotech1.com/forums/showt...4062#post44062

Another reason a solid lead disk would make comparison testing possible.

A test comparing US quarters mostly flat and some on edge. Plastic sour cream container about 80mm diameter at bottom. 81 quarters>1964. Don't have silver quarters but should be similar result except for time constant. Coins swinging from a pendulum, mostly flat GB off(21inches)GB on(23inches). Some on edge GB off(21inches)GB on(18inches). GB off no difference, GB on lost 5inches detection with some coins on edge. The lead disk charted close to quarters flat.

Think would loose more distance if coins on edge didn't start detecting when crossing coil edge?

[garyq]

Another air test with the PI and 6" mono coil this time using Pennies (dated 1919) which are 97% Copper 2.5% Zinc 0.5% Tin.

Target – 1919 Penny
GB - On
Single - 280mm
2 Stacked - 280mm
2 Side by Side - 330mm
Single on Edge - 155mm
2 Stacked on Edge - 200mm
2 Side by Side on Edge - 200mm

The US Nickel in my previous test is 75% Copper 25% Nickel.
Have now tested a 2 cent Coin 97% Copper 2.5% Zinc 0.5% tin of similar diameter and thickness to the Nickel.
Both of these coins produced a similar result except when two of the 2 cent coins where stacked they produced 155mm whereas the 2 Nickels stacked was 102mm.

[green]

Another air test with the PI and 6" mono coil this time using Pennies (dated 1919) which are 97% Copper 2.5% Zinc 0.5% Tin.

Target – 1919 Penny
GB - On
Single - 280mm
2 Stacked - 280mm
2 Side by Side - 330mm
Single on Edge - 155mm
2 Stacked on Edge - 200mm
2 Side by Side on Edge - 200mm

The US Nickel in my previous test is 75% Copper 25% Nickel.
Have now tested a 2 cent Coin 97% Copper 2.5% Zinc 0.5% tin of similar diameter and thickness to the Nickel.
Both of these coins produced a similar result except when two of the 2 cent coins where stacked they produced 155mm whereas the 2 Nickels stacked was 102mm.

[Both of these coins produced a similar result except when two of the 2 cent coins where stacked they produced 155mm whereas the 2 Nickels stacked was 102mm.]
Above 2 stacked is 280mm not 155mm?