Ferrous metal detector

[sachindra]

Hello everyone.

I am a newbie in metal detector world and this is my first post here. I have been observing and learning from posts here to understand metal detector theories. I also started reading "Inside the Metal Detector" to understand basic principles and circuits in systematic way.

I plan to apply metal detector principle in different field than treasure hunting. I am a civil engineer and we use steel fibers or iron compounds (like Fe₃O₄ powder) in some concrete to achieve certain properties: such as tighter cracks and more load bearing. I want to detect whether the concrete has steel fiber or iron powder incorporated.

After reading some theories and experimenting with some cheap hobbyist pulse induction (PI) metal detector, I found the metal detectors either reject the ferrous metals, that I am interested in detecting, or the range is not so good. In some cases, such as iron powder sample, it does not detect the material at all even though the sample is very magnetic when tested with a permanent magnet.

Questions I have:

1. What type of metal detector would be best for detecting ferrous material -- theoretically as well as if there are some product available?
   Best => greater depth (for similar coil size) or more sensitivity to ferrous material than other; discrimination of other metals like gold, silver, aluminum (this sounds like opposite of what most metal detectors do but iron is like gold for me in this case )
2. With pulse induction metal detector, I could detect small concrete sample (1.5 inch x 1.5 inch x 18 inch) with some steel fiber ( 0.5% by volume -- which would be around ten to twelve 1 inch long steel fibers randomly distributed) up to distance of 6 inches. However, similar sample with iron filling (Fe₃O4) is not detected at all. I believe the eddy current generation is very small in iron fillings because of their physical dimension or perhaps due to random orientation they cancel each other. However, when I test with permanent magnet the iron filling sample is more attracted to it than concrete sample. Are there any other metal detection theories or devices that detects more magnetic material rather than conductive material that can generate eddy current?

Any help or suggestions would be highly appreciated.

[waltr]

Wow, interesting application. Hope some of the tech Gurus here can help.

However, similar sample with iron filling (Fe₃O4) is not detected at all. I believe the eddy current generation is very small in iron fillings because of their physical dimension or perhaps due to random orientation they cancel each other.

Yes, this happens with any quantity of very small particles. Gold chains with very small links are very hard to detect with any type of detector.

[Krzysztof]

Are ideal for magnetometers, only detects iron. I think that the application of the differential magnetometers (gradiometer) would give even better spacing of interference. The same touch of a single sensor slab with iron content, gives already a big jump in measurement, regardless of the moisture and porosity. The complication of the layout depends on the required sensitivity and accuracy.

[Koala]

if it to see if the correct amount of steel been added before a pour wouldn't a magnet work ?

[Koala]

resistance ?

[sachindra]

Wow, interesting application. Hope some of the tech Gurus here can help.


Yes, this happens with any quantity of very small particles. Gold chains with very small links are very hard to detect with any type of detector.

Thank you. Yes, it is an interesting application in a different field. Eventually I would like to expand more on this idea for application in civil engineering.

I hope some tech Gurus here can help me towards right direction.

[sachindra]

Are ideal for magnetometers, only detects iron. I think that the application of the differential magnetometers (gradiometer) would give even better spacing of interference. The same touch of a single sensor slab with iron content, gives already a big jump in measurement, regardless of the moisture and porosity. The complication of the layout depends on the required sensitivity and accuracy.

Thank you. I will check more into gradiometers. A quick search showed a youtube video (link) where a tool was detected at fair distance. I will check if there were some similar discussion in this form for gradiometers.

For my application, I cannot use the touch method unfortunately. It has to be at least 10 inches above the surface of concrete.

[Krzysztof]

What you've watched is an analog sensor with an ADC transducer; will be the equivalent of FGM-3, it works directly digitally.
At the distance between the sensors about 70 cm, you can quietly explore the content of iron in concrete.

[Ferric Toes]

Hi Sachindra,
A company that could be of great help is Bartington Instruments Ltd which are located in UK but appear to have a US agent in California.
Starting with The Fe3O4 material, which in its natural form is magnetite and on which PI metal detectors will give no substantial response. Any weak response will be due to magnetic lag, or viscosity as it is often called. This weak response has no relation to the quantity of magnetite present and would not be useful in your project. Induction balance continuous wave metal detectors do respond to magnetite but are not suitable for other reasons. What is needed is a magnetic susceptibility meter which is designed for measuring even very small percentages of such minerals within a non-magnetic matrix.

I have a Bartington MS2B susceptibility meter which I use to measure the susceptibility of soil and rock samples plus fired clays such as in house bricks and earthenware pottery. It is a very accurate instrument as you will see from reading their literature.

https://bartington.com/wp-content/uploads/pdfs/datasheets/Magnetic_Susceptibility_DS0333.pdf

The steel fibre is a different problem as much depends on the diameter of the fibres. Steel wool can be measured with the MS2 and the readings will generally be higher than for the magnetite, but will depend on the percentage of course. The fact that you can detect it with a PI metal detector is indicative of magnetic lag (viscosity) rather than an eddy current response. Bartington make probes for the MS2/MS3 sensors which may be good for your application. It all depend whether it is a small sample you intend to analyse, or a large one.

There is also a Magnetic Viscosity meter based on PI technology that will detect viscous iron minerals and also ferrous metal. This something I designed the accompany the MS2B for a fuller analysis of rock samples as it does not respond to susceptibility. Using a 1 inch ferrite cored probe it also makes a good rebar pinpointer in concrete structures.

Eric.

[Ferric Toes]

For my application, I cannot use the touch method unfortunately. It has to be at least 10 inches above the surface of concrete.

I missed the above, but the susceptibility meter approach may still be valid as an 8in coil is available from Bartington. Best discuss it with them.

Eric.

[waltr]

Just a thought:
Would the Iron filling respond like 'hot ground'?
If so then maybe the reverse of GEB may work.

[Koala]

we can't get closer than 10"

What environment are we trying to test the concrete in. Chernobyl ?

[Koala]

I think we need to know the exact situation.

Rebar, services and so on will affect result. Normal fibre reinforcement is for floor slabs. But doesn't fit with can't get within 10" ?

[kt315]

try go from simplest to difficult on your way of the learning. begin from Barracuda, then build Delta Pulse or Clone or Mirage PI.
you will get first experience and will be know where to run to.

[sachindra]

What you've watched is an analog sensor with an ADC transducer; will be the equivalent of FGM-3, it works directly digitally.
At the distance between the sensors about 70 cm, you can quietly explore the content of iron in concrete.

Thank you. I will look further into FGM-3. I found one website (link) that sells those sensors. One of their documents show that they could detect 530-mm pipeline buried 2m underground (figure 15 in this link). It might be good to test it in my application.
Does anyone know if we can buy FGM-3 sensor in the US?
I found this really good document (link) by Carl. However, the company in the US seems to have closed down?