Vallon VMH3CS Mine Detector

[bbsailor]

[QUOTE=Ferric Toes;218330]For the first test I wound the 4in x 3/4in rod with as many turns as I could get on and achieved 1.8mH straight off. Resistance was 3.5 ohms so, we are in the right ballpark straight away. I left off the outer shielding at this stage, but had the core grounded with a drain wire running its length. Plugged it into the shaft connector of the Vallon so that it had all the cable capacitance and additional resistance. It ran through all the startup checks and the detector settled down with no problem. The difference I noticed was that it seemed quieter than the other probes I have made. My home workshop is quite noisy electrically, much of it from surrounding shops and light industry. I'll have to check it out away from the premises to be sure, but I definitely gained a couple of inches on a nickel. 10in instead of 8in. Over the weekend I will add the outer shielding and house it in a spare Vallon shaft tube; which happens to be just the right diameter.

Eric,

From looking at the photo of your probe and doing some fast back of the envelope calculations I guess that you are using AWG 30 Teflon insulated wire that is .024 inches OD?

thanks

Joseph J. Rogowski

[Chet]

For the first test I wound the 4in x 3/4in rod with as many turns as I could get on and achieved 1.8mH straight off. Resistance was 3.5 ohms so, we are in the right ballpark straight away. I left off the outer shielding at this stage, but had the core grounded with a drain wire running its length. Plugged it into the shaft connector of the Vallon so that it had all the cable capacitance and additional resistance. It ran through all the startup checks and the detector settled down with no problem. The difference I noticed was that it seemed quieter than the other probes I have made. My home workshop is quite noisy electrically, much of it from surrounding shops and light industry. I'll have to check it out away from the premises to be sure, but I definitely gained a couple of inches on a nickel. 10in instead of 8in. Over the weekend I will add the outer shielding and house it in a spare Vallon shaft tube; which happens to be just the right diameter. [ATTACH]36403[/ATTACH]

Eric.

Hi Eric


Thank you for the information in the previous post. Your latest coil (10” on a nickel) wow! That is something to be proud of. What does the field around the side of the coil look like with this kind of probe? Does it still pick up the nickel at several inches?


Have a good day,
Chet

[Ferric Toes]

By the way , magnetic permeability cannot be "large than needed" in this kind of sensors . Even if we have a ferrite with mu=10000 - it isn't too high ... You see , the overall performance of the MD with ferrite rod coil directly depends on 3 parameters - ferrite rod length , its diameter and magnetic permeability . When I was experimenting with my constant-current PI with ferrite sensor , I had compared 2 coils - one core was made with a bunch of rectangular ferrite rods made for AM radio antennas ( mu=600 ) , and another was glued together 3 pieces of EPCOS ferrite sticks for a SMPS transformers ( mu=2700 ) . And although the first search coil was slightly longer - the second one did have noticeably better sensitivity .... so , as I think , if I find a ferrite with even higher permeability ( 10000 or more ) , I should achieve much more performance . Maybe it's a good idea to try not a ferrite but a kind of amorphous alloy materials , something like this - http://www.nanoamor.com/cat/catalog_amor.pdf - because they can have permeability of 100000 or even more ...

Coincidentally, I only read about amorphous alloys this morning in the following article about a new ferrite. I like the huge pot core - great for searching under wire fences. Only problem would be the weight.

Eric.
http://www.fdk.com/cyber-e/technical/pi_technical06.html

[Ferric Toes]

4in x 3/4in probe core test.

For the first test I wound the 4in x 3/4in rod with as many turns as I could get on and achieved 1.8mH straight off. Resistance was 3.5 ohms so, we are in the right ballpark straight away. I left off the outer shielding at this stage, but had the core grounded with a drain wire running its length. Plugged it into the shaft connector of the Vallon so that it had all the cable capacitance and additional resistance. It ran through all the startup checks and the detector settled down with no problem. The difference I noticed was that it seemed quieter than the other probes I have made. My home workshop is quite noisy electrically, much of it from surrounding shops and light industry. I'll have to check it out away from the premises to be sure, but I definitely gained a couple of inches on a nickel. 10in instead of 8in. Over the weekend I will add the outer shielding and house it in a spare Vallon shaft tube; which happens to be just the right diameter.

Eric.

[deemon]

But according to Ferric's notices; those are still having permeability larger than needed in this case.

By the way , magnetic permeability cannot be "large than needed" in this kind of sensors . Even if we have a ferrite with mu=10000 - it isn't too high ... You see , the overall performance of the MD with ferrite rod coil directly depends on 3 parameters - ferrite rod length , its diameter and magnetic permeability . When I was experimenting with my constant-current PI with ferrite sensor , I had compared 2 coils - one core was made with a bunch of rectangular ferrite rods made for AM radio antennas ( mu=600 ) , and another was glued together 3 pieces of EPCOS ferrite sticks for a SMPS transformers ( mu=2700 ) . And although the first search coil was slightly longer - the second one did have noticeably better sensitivity .... so , as I think , if I find a ferrite with even higher permeability ( 10000 or more ) , I should achieve much more performance . Maybe it's a good idea to try not a ferrite but a kind of amorphous alloy materials , something like this - http://www.nanoamor.com/cat/catalog_amor.pdf - because they can have permeability of 100000 or even more ...

[Ferric Toes]

Hi Eric


There are three numbered types of soft ferrite rods on eBay plus some Russian types. There is a type 33, type 61 and a type 77. The specifications are listed here; http://www.cwsbytemark.com/CatalogSh...06/FR_MATL.pdf
The types 33 and 77 are Manganese-zinc ferrite (MnZn), appear to have higher permeability and lower resistivity than the type 61 which is Nickel-zinc ferrite (NiZn). In some other references it states that NiZn ferrites exhibit higher resistivity than MnZn, and are therefore more suitable for frequencies above 1 MHz. For applications below 5 MHz, MnZn ferrites are used; above that, NiZn is the usual choice.


NiZn has a resistivity of 1x〖10〗^8 ohms per cm versus MnZn has a low resistivity of 1x〖10〗^2 ohms per cm. Could you do a rough ohm meter check of one of your cores to see whether it has a high or low resistivity?


What are your thoughts on these three types?


Have a good day,
Chet

Hi Chet,
All of the rods that worked for me are MnZn type so I think you could discount type 61. That leaves 33 and 77; however, 78 looks interesting for higher power projects.

The first rod that I used was FX1040 and was 3/4in diameter and maybe 8in long, but I can't find any data on that now. I seem to remember using F4 ferrite when I used Neosid UK rods. That is what I have most of now, but in 3in x 1/2in size. I did find a 6in rod and a 4in x 3/4in one which I am going to wind and compare performance. Also to try are 2 x 3in x 1/2in superglued together and also adhesive heat shrink sleeve over the two rods. Past experience has shown that the slightest movement, or crack, that develops, makes the probe useless.
My various rods measure 10k ohms up to nearly a Megohm. The ones I know work are nearer the lower end of the resistance range.

I have also successfully used pot core halves as PI sensors, usually for industrial use when you want the detection field to be only on one face of the coil.

I'll report more when I have done more tests.

Eric.

[Chet]

Hi Eric


There are three numbered types of soft ferrite rods on eBay plus some Russian types. There is a type 33, type 61 and a type 77. The specifications are listed here; http://www.cwsbytemark.com/CatalogSh...06/FR_MATL.pdf
The types 33 and 77 are Manganese-zinc ferrite (MnZn), appear to have higher permeability and lower resistivity than the type 61 which is Nickel-zinc ferrite (NiZn). In some other references it states that NiZn ferrites exhibit higher resistivity than MnZn, and are therefore more suitable for frequencies above 1 MHz. For applications below 5 MHz, MnZn ferrites are used; above that, NiZn is the usual choice.


NiZn has a resistivity of 1x〖10〗^8 ohms per cm versus MnZn has a low resistivity of 1x〖10〗^2 ohms per cm. Could you do a rough ohm meter check of one of your cores to see whether it has a high or low resistivity?


What are your thoughts on these three types?


Have a good day,
Chet

[Chet]

My Ferrite bible was 'Soft Ferrites - Properties and Application' by E C Snelling. It is still available; up to £500 for a new and revised addition. I don't have my original any more as it was company property of Pulse Technology when I worked there. I am certainly not buying it again, even at £50 for a used unrevised one. There is good information on the internet such as 'Soft Ferrites, a user's guide' as a .pdf download.
I find it easiest and quickest to test rods in the way that bbsailor suggested, which is what my MVM does. That was designed to test the ferrite properties of soils. Like you, I have boxes of rods of different lengths bought over the years, and some from companies that no longer exist. My first rods were from Neosid and I let them do the work of sorting out the technical stuff as a metal detector probe was then a new application in the 1970's.

Eric.

The Soft-Ferrites Properties and Applications book can be downloaded for free at the address below. I found the PDF WITH TEXT to be the better download choice. It allows you to open it with Adobe reader and use the FIND option.

Have a good day,
Chet

https://archive.org/details/SNELLING__SOFT-FERRITES__1969

and another free book;

https://archive.org/details/PHILIPS1959SmitWijnFerrites

[Ferric Toes]

Ideal for our enthusiastic LRL friends.

That's a great idea. I was wondering how I could off-load of some of my rejects.

[WM6]

3. Some rods respond to the earth's field even when the detector has earth's field cancellation. This is not just swinging the rod but the baseline zero changes if the rod is positioned NS and then changed to EW;

Eric.

Ideal for our enthusiastic LRL friends.

[Ferric Toes]

"...I find it easiest and quickest to test rods in the way that bbsailor suggested, which is what my MVM does..."

I noticed that, i will keep that in mind.

Some problems I have come across with certain ferrite rods that cannot be determined by reading specifications are -
1. Some rods ring mechanically and cannot be damped out.
2. Some rods are microphonic such that if you hit a rock they give a false signal.
3. Some rods respond to the earth's field even when the detector has earth's field cancellation. This is not just swinging the rod but the baseline zero changes if the rod is positioned NS and then changed to EW; acting a bit like a fluxgate sensor.
4. Rods from the same batch respond slightly different.

Right now I have two seemingly identical rods bought at the same time, wound and shielded the same and tested on the Vallon. One is noisier than the other.

Eric.

[ivconic]

"...I find it easiest and quickest to test rods in the way that bbsailor suggested, which is what my MVM does..."

I noticed that, i will keep that in mind.

[Ferric Toes]

"...The core increases the magnetic field in proportion to its relative permiability (uR). Unfortunately I bought the cores many years ago and have mislaid the spec on the core material.
The uR can be as much as 10,000 for ferrites, but the field can be limited by magnetic saturation before you reach that multiplying figure..."

"...This material does not hold the magnetism after the current turns off..."

Obviously those rods are made from compounds with lower permeability. How much the uR can be than?
It is desirable to establish the "reference"; if you know what i mean?
Common medium-wave antenna rods are not having high permeability... comparing to mumetal stripes used in fluxgate sensors.
But according to Ferric's notices; those are still having permeability larger than needed in this case.
So? How to distinguish that and to find proper rod? A " reference"?
Otherwise; whole story further will not make much sense, because each one will face quite different results.
Other solution; to prevent saturation of the core at right moment. How?

My Ferrite bible was 'Soft Ferrites - Properties and Application' by E C Snelling. It is still available; up to £500 for a new and revised addition. I don't have my original any more as it was company property of Pulse Technology when I worked there. I am certainly not buying it again, even at £50 for a used unrevised one. There is good information on the internet such as 'Soft Ferrites, a user's guide' as a .pdf download.
I find it easiest and quickest to test rods in the way that bbsailor suggested, which is what my MVM does. That was designed to test the ferrite properties of soils. Like you, I have boxes of rods of different lengths bought over the years, and some from companies that no longer exist. My first rods were from Neosid and I let them do the work of sorting out the technical stuff as a metal detector probe was then a new application in the 1970's.

Eric.

[Ferric Toes]

Eric,
Try cutting the Scotch24 into a single layer and put a layer of tape over the long end to prevent the wire mesh from shorting out around the probe circumference. I have found that this method reduces the detection of the wire mesh while still providing a good shield.


Joseph J. Rogowski

Hi Joe, yes, I will try that as I have a whole reel of Scotch 24. I see it is readily available at low cost on ebay, whereas the copper fabric tape is no longer available and I have a limited amount. It was expensive too.
I did try a core with a lead foil tape (1/2in wide) helically wound and that worked OK.

Eric.

[bbsailor]

Thanks for the link!
Splendid data book!
...
During the years i collaterally collected several hundreds of various cores.
On a pile.
This could be interesting series of experiments.

My spelling error" The statement in my previous post should read: "The soft ones give the least response at low delays". Sorry for my mistake!

Joseph J. Rogowski