I didn't need another project -

[Carl-NC]

Is there a way to test sea water effect on delay say in a bucket of water?
Something like adding a graphite powder /something else - higher conductive is going to be needed I guess/ step by step will give similar result as sea water /just for testing purposes of course/.

I have a JW Fishers 8X underwater PI and I want to see how it performs if conditions are not ideal /air - regular ground etc/.
Or just go with a bucket full of dirt and black sand /Magnetite/.

Salt water effect varies based on the volume of the seawater. As you go from hunting wet salt sand to fully submersed the volume changes considerably. A bucket of seawater won't replicate that no matter what you do to it.

[green]

Salt water effect varies based on the volume of the seawater. As you go from hunting wet salt sand to fully submersed the volume changes considerably. A bucket of seawater won't replicate that no matter what you do to it.

Does saltwater decay straight line linear time log amplitude, if yes any idea what range of time constants?

Does salt water act the same as a smaller target? With a small target and a 200mm coil, increasing the the distance between a coin and coil from 200mm to 225mm reduces the signal to about half. Is there a ball park distance to move a 200mm coil in or near saltwater to get a reduction in signal to 1/2?

A figure8 Rx coil reduces ground signal, would it be a better at cancelling saltwater?

[Qiaozhi]

And it was right here on Geotech the whole time:

https://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=info/lancaster/index.dat

That's just like when you've rented or bought an expensive item for a one-off job and someone says "I've got one of those that you could have borrowed for free".
I never even thought to look there.

[Carl-NC]

Does saltwater decay straight line linear time log amplitude, if yes any idea what range of time constants?

Does salt water act the same as a smaller target? With a small target and a 200mm coil, increasing the the distance between a coin and coil from 200mm to 225mm reduces the signal to about half. Is there a ball park distance to move a 200mm coil in or near saltwater to get a reduction in signal to 1/2?

A figure8 Rx coil reduces ground signal, would it be a better at cancelling saltwater?

In wet salt sand I believe the decay is reasonably exponential. I assume the tau depends on salinity but probably on the order of a couple of microseconds. In full submersion it may behave differently, I've never investigated. I would also assume that moving the coil away from wet salt sand will require more distance than a coin to cut the signal in half because the salt "target" is so much larger.

Yes, a Figure-8 will cancel salt effect.

[ivconic]

That's just like when you've rented or bought an expensive item for a one-off job and someone says "I've got one of those that you could have borrowed for free".
I never even thought to look there.

Neither than i.
Last place on Earth to look up for such material!

[WM6]

Attached is a description of the workings of the UHF detector. [ATTACH]53357[/ATTACH]
This is from an article in the Electronics World magazine for December 1966. There is no indication of how small a metal
object can be detected and I don't know of any actual devices that work in this way. You would think that developments of
this technique had progressed since 1966
The resonance method might be worth investigating and, as you say, a frequency sweep method and a spectrum analyser
for a Rx display. Objects at resonance should appear as a spike on the display.

Eric.

It is not best possible copy, yet good enough for overview.
If it was of a grater value for me; i would pass it through the OCR and make proper
fine document... but is not.
OCR is not perfect and sometimes demands additional labor.
That's why i processed only part of the last page, which interests WM6 mostly.

Electronic Metal Locators
(Continued from page 42)
U.H.F. Type
This metal locator has several unique operating features. Fig, 6 shows the block diagram. The u.h.f. locator
is capable of detecting either metallic or non-metallic objects and is able to discriminate between these objects
and the normal clutter of rock discontinuities. Operation is somewhat similar to the induction-balance locator,
except that the operating frequency is 400MHz and the loops are replaced in a search array consisting of
inductively loaded dipole antennas.
Two transmitting dipoles are used with a receiver dipole between them. A figure-eight pattern is produced in the absence
of any target, resulting in balanced voltages that nearly cancel in the receiver dipole. The presence of any object of uniformly
different conductivity and dielectric constant from the surrounding medium upsets
the balance and produces an output signal. There are two modes of operation, the "Search" mode and the "Point" mode.
In the "Search" mode, all of the spurious return is averaged out by a long time constant a.g.e. loop, while any sudden changes
in the field patterns are greatly amplified by an expander circuit, indicating the edge of a target direct below the search array.
In the "Point" mode, the receiver output is amplitude-sensitive and the instrument may be used to outline
the buried object.
The u.h.f. locator is principally used by the military for the detection of metallic and non-metallic mines.

Thanks both.

Despite (needle) resonance way is possible, as we know from from resonant small pole WiFi antennas,
we are here with about 3 - 5cm needle length in couple of GHz band to get resonance , so searching dipole
antennas become very small and unpractical to search some school yards, cause antenna should be put as
close as possible to hidden object (to exclude as much amount as possible of disturbing surrounding medium).
UHF dipole antenna can be increased in stacked antenna way (this add more questions and a lot of research)

Differentiating way (as described in present article) is in fact based on eddy current principle. At very high
frequencies different medium (soil par example) act as ferrite core load with very different characteristic
(par example eddy currents free or absence substances act as hole in surrounding medium). Main problem
here is amount of detecting medium (needle) which we are trying to differentiate from surrounding medium.
This amount is very small, usually to small to get useful distinct for indication.

Regular sewing needles are easily to grab with magnetic broom but hypodermic needles are made from
high grade medical steel and this way immune to magnet.

Probably we need here another approach and start to produce ferromagnetic medical injection needles.

[Ferric Toes]

I seem to remember that in the '40's -50's, hypodermic needles had a larger metal fitting at the end to connect it to the syringe. Re-introducing that would solve the detection problem.

[WM6]

I seem to remember that in the '40's -50's, hypodermic needles had a larger
metal fitting at the end to connect it to the syringe. Re-introducing that would solve the detection problem.

Yes, nice idea and probably most simple solution, Ferromagnetic ring or bead at syringe connection part
of hypodermic needle.

[Carl-NC]

I seem to remember that in the '40's -50's, hypodermic needles had a larger metal fitting at the end to connect it to the syringe. Re-introducing that would solve the detection problem.

I recently consulted for a company who was designing a medical metal detector. As you can imagine, they wanted to detect all sorts of difficult targets including titanium which was impressively hard to detect. Mostly instruments though, and the topic came up of tagging everything with RFID chips. They are super-tiny and completely passive. The needles I see lately have a stainless shaft but a plastic base. An RFID chip embedded in the base solves the problem.

[6666]

Here in Australia we currently have some maniac putting sewing pins about one inch long, into strawberry's and people are finding them after they have bought them in super markets.
Also happened in 2018 but they caught that nut case.

[kt315]

like a maniac. have to inform. winter theme again? we had have huge thread on needles & pins. just reminder to you.

https://www.geotech1.com/forums/show...dermic-needles

[WM6]

An RFID chip embedded in the base solves the problem.

Don't say, someone will realize this post-mechanical approach,
and then we face with a lot of interference with human RFID.

[dbanner]

Ask a bunch of nuclear physicists for a bread recipe, they might include a particle collider on the list, just after wheat flour and before yeast.

How about glow in the dark.

[ivconic]

Thanks both.
Despite (needle) resonance way is possible, as we know from from resonant small pole WiFi antennas,
we are here with about 3 - 5cm needle length in couple of GHz band to get resonance , so searching dipole
antennas become very small and unpractical to search some school yards, cause antenna should be put as
close as possible to hidden object (to exclude as much amount as possible of disturbing surrounding medium).
UHF dipole antenna can be increased in stacked antenna way (this add more questions and a lot of research)
Differentiating way (as described in present article) is in fact based on eddy current principle. At very high
frequencies different medium (soil par example) act as ferrite core load with very different characteristic
(par example eddy currents free or absence substances act as hole in surrounding medium). Main problem
here is amount of detecting medium (needle) which we are trying to differentiate from surrounding medium.
This amount is very small, usually to small to get useful distinct for indication.
Regular sewing needles are easily to grab with magnetic broom but hypodermic needles are made from
high grade medical steel and this way immune to magnet.
Probably we need here another approach and start to produce ferromagnetic medical injection needles.

You don't need to worry much on antenna design. Been there. Done that.
You'll have to figure out "only" the rest of circuit.

[kt315]

Ask a bunch of nuclear physicists for a bread recipe, they might include a particle collider on the list, just after wheat flour and before yeast.

How about glow in the dark.

nothing new. such technology exists 200 years. we call that nuclear magnetic resonance (NMR).
NMR is an abbreviation for Nuclear Magnetic Resonance. An NMR instrument allows the molecular structure of a material to be analyzed by observing and measuring the interaction of nuclear spins when placed in a powerful magnetic field.

For the analysis of molecular structure at the atomic level, electron microscopes and X-ray diffraction instruments can also be used, but the advantages of NMR are that sample measurements are non-destructive and there is less sample preparation required.

Fields of application include bio, foods, and chemistry, as well as new fields such as battery films and organic EL, which are improving and developing at remarkable speed. NMR has become an indispensable analysis tool in cutting-edge science and technology fields.