"treasure" hutning magnet questions

[daveh55et]

I apologize if these are trivial questions. I have several books on order including Moskowitz.

I live in a tourist town with rocky beaches. I'm interested in using magnet assemblies for "treasure hunting". I expect to attach a line to the assemblies and drag them along a beach or from a pier.

I have the following questions:

I know that pole pieces attached to the magnetic faces of a block magnet can focus the field so that a 4 pound pull on each face becomes something like a 16 pound pull on the pole piece edges. Assuming "hardware store" cold rolled flat stock for pole pieces and I was considering making my own assemblies with ceramic or Neod magnets. Is there a very rough calculation that could give me pull values with pole pieces?

Should I enclose all but one side of the assembly for completion of the magnetic circuits and maximum pull?

Some assemblies like the one from Edmund Scientific seems to be design for a straight down, straight up process. In dragging or dredging it does not seem well designed as the line attachment point would leave the magnetic pull area sideways to the search surface. I think a cylindrical shape of some kind with an line attachment on the side would work better. Any suggestions for configurations to reasonably maximize surface area and dragging characteristics?

Thanks,
Dave

 

[IRstuff]

The reason most such hunters stroll the beaches is that unless you've got an auto wrecking yard electromagnet, you're not going to have a magnet strong enough to excavate the items you want from the sand

TTFN

Eng-Tips Policies FAQ731-376

 

[MJR2]

Plus the Neo magnets will not last too long immersed. And then you be getting fur balls from all of the magnetic sands. You can do this but you will probably be disappointed.

Mike

 

[daveh55et]

hmmm, Ok let me say the following:

- I'm not after engine blocks, just various trinkets so I don't think I'll need a crane.

- Regarding what I assume is a concern about corrosion, I likely will Plasti-dip the whole thing before using.

- Furballs, yep, already thought of that. I'll probably tape a 3 mil trash bag over it to keep the sand out.

 

[IRstuff]

That wasn't the point. In order to pull something buried under sand, you need to lift the sand above the object with the object.

Given the density of wet sand, you'd be looking at lifting around 1 lb of sand for a 2-in diameter object buried 3 inches down

TTFN

Eng-Tips Policies FAQ731-376

 

[daveh55et]

A 2 inch object 3 inches down would be a pretty good representative case. I would not think though the sand would lift as one cohesive unit, I would expect the item to sift through to at least some extent.

Which brings me to a slightly different question from my first post, would pole pieces allow field penetration through 3 inches of sand? What strength would my magnet need to be to reach?

 

[MJR2]

Dave,
This is the problem I was thinking of as I got to your last post. The magnets you are talking about would probably not reach down for a two inch object under three inches of sand. They are mostly contact types.

Mike

 

[daveh55et]

Right, which is why I was asking about pole pieces on the faces, and if that would focus the field and allow a greater penetration.

 

[IRstuff]

There would be minimal "sifting" even with dry sand, since you still need to vacate to get the sand to move behind the object. Sand can be considered to be an extremely dense fluid with ultrahigh drag resistance and very low viscosity. Damp sand would be worse.

The more the poles are physically separated, the weaker the field.

Frankly, I see this as a pointless exercise. I would think that you'd need a magnet with a 6-inch ot 8-inch face to even come close to pulling this off, which means dragging something that'll weigh close to 10 pounds through the sand.

TTFN

Eng-Tips Policies FAQ731-376

 

[MJR2]

The pole pieces do focus the field but the opposite of what you are desiring. They concentrate the flux lines to travel thru a metal object they come in contact with.

You would need to space the poles well apart to reach out three inches. Flux wants to travel the easiest shortest distance.

Why don't you toss out some ideas of the magnets and size of your pole pieces. We might be able to comment. I cannot really picture your cylindrical shape idea.

Mike

 

[daveh55et]

Ok, I see what you're saying now. I'll see what I can do with regards to providing an image. Thanks.