DEEPER PI DETECTION DEPTH

[Carl-NC]

We know that thin aluminum foil is a good PI test target. We can cut a one inch square of foil and it will give a target with a TC of about 10us.

If we cut this piece of foil in half, we get 2 targets with a TC of 5us.

I think you have 2 targets still with a TC of 10us. The TC doesn't depend on surface area, surface area just affects the strength (amplitude) of the response.

If we take a thicker piece of aluminum, we get a target with a longer TC.

I have a set of 1" square targets made of household aluminum foil, with thicknesses of 1x, 2x, 4x, 8x, 16x, and 32x, each laminated with clear tape. Makes a good standard.

The skin depth means that the eddy currents generated in the target, only reach "skin deep" into the target.

Well, actually, eddy currents reach about 5 skin depths deep.

A slower switch off, will give a deeper skin depth. A deeper skin depth gives a longer TC for the same metal.

I don't understand the first half of this.

Lead has much less conductivity than aluminum or gold, so it has also a lesser skin depth and therefore a shorter TC.

While the characteristic skin depth of a metal affects TC, so does thickness. So saying "lesser skin depth" == "shorter TC" isn't necessarily true, unless the targets are otherwise identical. Ferinstance, a 58-cal lead Minie has a longer TC than a 1 grain pure gold nugget.

Therefore we want to design the TX pulse specifically for the size of nugget we are searching. We want a deep skin depth that generates a strong target response that lasts for a considerable amount of time.

"Skin depth" is a characteristic value that doesn't change with pulse width; I think what you want is a deep "penetration depth."

- Carl

[Midas]

Lower conductivity gives a shorter TC. Why? Does it have to do with the skin depth?

Lower conductivity = higher resistance = faster conversion of induced currents into heat. I'd say no, nothing to do with skin depth, in fact because your standard PI pulse can get currents to flow deeper in the lead (hence effectively through more conductor cross sectional area) in would counter the effect of its lower conductivity to some extent. Probably explains why people have no problems at all finding bullets.

[Tinkerer]

It's a DELL isn't it?

Their power supplies are illegal in the UK due to the floating voltages (called "Touch Voltages"). There is sufficient to kill someone with a weak heart. I believe they were forced to re-design them so all is well with current models - allegedly.

Hi Sean, thanks for coming back to this problem.

Yes it is a DELL. I suppose that the voltage is capacitivly coupled from the PSU. Whereas the alu housing is supposed to shield the EMI.

It is something to take a closer look at, so the same mistake does not creep in on a PI detector in an alu box.

Can anybody add some more information on this problem?

Tinkerer

[golfnut]

I like the idea of utilising the electronic used in Induction heaters.

High volts, some C, small L, Low R, big thump - lots of magnetism right there.


This site is as good as any..

http://www.richieburnett.co.uk/indheat.html

[Sean_Goddard]

Funny enough, my laptop computer gives me more electrical pricks than the TX connector with 800V Flyback, when I plug and unplug it under power.

It's a DELL isn't it?

Their power supplies are illegal in the UK due to the floating voltages (called "Touch Voltages"). There is sufficient to kill someone with a weak heart. I believe they were forced to re-design them so all is well with current models - allegedly.

[Tinkerer]

Hi Tinkerer,,

You raise some excellent points. People talk about maximizing dI/dT to increase performance but really what you want to do is maximize dI and optimize dT. Wiki has a really good write up on skin effect including some very useable formulas. Its even got an example calc on gold, 11.8mm at 50hz. Of course even though jamming energy in too fast is inefficient because of skin effect, too slow is just as bad since its constantly leaking out. There will have to be some happy medium in between. Oh btw according to the wiki formulas the reduced conductivity of lead actually results in a greater skin depth. I think its shorter TC is actually a direct effect of its reduced conductivity.

Midas

thanks for the feedback. You are right, lead has a greater skin depth.

Lower conductivity gives a shorter TC. Why? Does it have to do with the skin depth?

And yes, the TX time needs to be within precise limits. Tests have shown that the target response for large targets increases with a longer TX time, but only up to a certain limit, after that the response decreases again.

There is hope that, with digital technology, we may one day be able to adjust the TX pulses precisely for maximum target response, for our preferred targets.

Tinkerer

[Midas]

Hi Tinkerer,,

You raise some excellent points. People talk about maximizing dI/dT to increase performance but really what you want to do is maximize dI and optimize dT. Wiki has a really good write up on skin effect including some very useable formulas. Its even got an example calc on gold, 11.8mm at 50hz. Of course even though jamming energy in too fast is inefficient because of skin effect, too slow is just as bad since its constantly leaking out. There will have to be some happy medium in between. Oh btw according to the wiki formulas the reduced conductivity of lead actually results in a greater skin depth. I think its shorter TC is actually a direct effect of its reduced conductivity.

Midas

[Tinkerer]

Skin effect

Another matter to look at, for a deep search detector, is the skin effect.

How does the skin effect affect the target response?

We know that thin aluminum foil is a good PI test target. We can cut a one inch square of foil and it will give a target with a TC of about 10us.

If we cut this piece of foil in half, we get 2 targets with a TC of 5us. Only good PI detectors are capable to detect that at a reasonable distance.

If we take a thicker piece of aluminum, we get a target with a longer TC. In fact, the target response increases significantly up to the thickness of about 0.05mm to 0.1mm.

This is the skin depth of the TX switch off transient.

The skin depth means that the eddy currents generated in the target, only reach "skin deep" into the target.

A deeper skin depth of eddy currents will generate a target response with a longer TC or time constant. The target time constant is the amount of time it takes for the target response to diminish or decay to about one third of it's value.

A slower switch off, will give a deeper skin depth. A deeper skin depth gives a longer TC for the same metal.

Here we come to a new significant factor. The type of metal.

Aluminum and gold have nearly the same conductivity and therefore the same skin depth for the same frequency. However, most aluminum is alloyed and most gold is alloyed too.
Even virgin gold, like in nuggets often contains silver, platinum and other metals.

Nuggets also often contain stone fragments. This changes the TC of the nugget.

An uneven surface also greatly changes the TC of a target. It also changes the skin depth of the eddy currents.

Lead has much less conductivity than aluminum or gold, so it has also a lesser skin depth and therefore a shorter TC.

If we compare the density of aluminum with lead or gold, we see that if we have a sphere of gold of a certain weight, a sphere of lead of the same weight would be larger and a sphere of aluminum of the same weight, would be much larger.

The sphere of aluminum would therefore intercept more magnetic field line of the TX pulse and produce a larger response.

When we search deep, like 0.5m to 1m depth, for nuggets, we are not much interested in nuggets of milligram weight. We search for nuggets of at least 1 gram and upwards.

Therefore we want to design the TX pulse specifically for the size of nugget we are searching. We want a deep skin depth that generates a strong target response that lasts for a considerable amount of time.

We have seen many different examples of TX pulses above. Which type of TX pulse generates the strongest response in thick targets, like gold nuggets?

Tinkerer

[Tinkerer]

We have seen in the posts above, that it is well possible to pump a lot of power into a TX coil.

Why would we want a lot of power?

We are talking of detecting deep. We also want to detect small targets deep.

For deep detecting, a large coil gives best results. However, for a large coil to detect small targets, we need to boost the coil's magnetic field, to obtain a reasonable field density, needed for detecting small targets, deep.

A large coil, for example 1 meter x 2 meters covers a lot of ground. If we attach it to a vehicle, we could cover a lot of ground in a short time, for example in a dry salt lake.

How large are the average nuggets in that area?

Can we detect 1 gram nuggets with this coil? This is where the high power comes in. High magnetic field density.

A 1m x 2m coil would also make a good tow-able underwater detector, for example to search for artifacts like gold and silver coins, scattered over a large area by a sinking galleon, broken up in a hurricane.

Tinkerer

[Aziz]

May I add some comments to the induction heater principle coils?


With low operating frequency and very low TX coil inductance (5-25 µH), we would require a high tank capacitance.
Forget using only one capacitor. We would require a dozen or so and all of them need to be placed in the coil (or very close to it). Best operated with high current impulse capacitors like FKP-1 (or FKP-4, must be a metal foil capacitor). Would make the TX coil quite heavy.
Oh yes, the orientation of the capacitors does matter unless you do not want them to be melted (eddy current reduced orientation). And the connection of them does matter too.

Could be a challenge to realize it. Probably easier with big coils.

I think, it is much simpler to handle a high voltage rather than a high current.
What does matter again?
Oh yes, the TX coil current change (dI/dt).
Self induction voltage of the TX coil:
U = -L*dI/dt

Impedance matching: -> U gets smaller, I gets bigger, L is small
The standard variant: U is high, I is small, L is big

So where is the free lunch for me?

Aziz

PS: Merry Christmas and a happy new year to everyone.

[IBGold]

I have the same problem with small spiders and I thought I was the only one.

MERRY CHRISTMAS ALL.

Regards, Ian.

[Tinkerer]

So you get 33.33% more green lights, you gotta be happy with that!
FYI, It was a genuine question, I wasn't being mean. Its more common that you might think.

Midas

OK now I understand why every September when I have my eye test, to renew my drivers licence, they show me this funny polka dot picture with Father Christmas on it and ask what I see. Then they ask for a "generous Christmas gift", which I always have ready in cash, after which they give me a 20/20 for the eyes and full #5 grade (all vehicles) drivers licence renewal.

Unfortunately, when I populate the SMD board, the eye sight deteriorated. The TSOT23-5 parts sometimes walk away before I get the solder iron to it, because it turns out they have 6 legs, not five and are more commonly called Ticks.

Anyway, of course you are right, it is a light yellowish green. About like a green lemon.

Tinkerer

[Aziz]

Hi all,

I have "tried" (simulated) the induction heater principle to the TX circuit with impedance matching circuit. Even you can achieve a very high ampere-turns or a high current power factor (10 - 20x), it's efficiency isn't the best however.

It offers other benefits:
- very low inducitivity TX coil (5-25 µH !)
- low voltage everywhere (coil, TX circuit) so you can use the common low voltage mosfets with less on resistance
- simple to drive (H-bridge)
- coil leads and TX driver stage drives less current (less losses)
- low EMI emission during the switching

But the TX tank (LC, coil + caps) need a very low resistance/impedance parts. A few TX coil turns but you need a thick litz cable. Otherwise, almost all of the energy will be burnt in these parts (generating much heat due to very high current).

To sum up: It works!

Aziz

[Midas]

Sheeesh, now I know why the "green light" is always so short and the "other green" light and the red light take so long.

Tinkerer

So you get 33.33% more green lights, you gotta be happy with that!
FYI, It was a genuine question, I wasn't being mean. Its more common that you might think.

Midas

[Tinkerer]

Hi Tinkerer,

Your colour blind I assume? The coil current trace is actually green. From left to right you have: Green, Blue, Red, Turquoise, Magenta, Grey

Midas

Sheeesh, now I know why the "green light" is always so short and the "other green" light and the red light take so long.

Tinkerer