An e-acquaintance writes:
"...on my gravity fed water filter system, when the water level is high in the filtered water [steel tank], my magnets are unable to retain their strength...the farther down [the outside surface] I slide them so that there are more inches of water above them, the weaker their magnetism becomes until near the bottom, they sometimes fall completely off, esp. if nudged very slightly. So I just duct tape them on. But I can't see why just having more water in the tank and above them would have that effect. I have used regular fridge magnets, 3950 gauss therapeutic magnets, and some thin 12,000 super-neos and they all act the same--they lose strength when more water is above them..."
Any BABBling explanations/experiences available?

If this is a narrow tank and the magnets are on opposite sides, I wonder if they may be attracting each other. Then perhaps the water blocks that attraction to some degree. I don't know if water will actually do this, though. Just taking a stab.

Water is diamagnetic, that is, when exposed to a magnetic field, it creates an opposing magnetic field.

You may have seen those photos of magnetic levitation of a small frog--I'm prettty sure the frog was levitated by the diamagnetism of the water in its body.

In the case of the water tank, the diamagnetic effect is being amplified by the induction of a magnetic field in the steel, by the magnet attached to it. I think the thin steel is expanding the magnetic field along a large surface area, and the internal surface of the magnetized steel is being opposed by the corresponding diamagnetic effect of the water against it, which partly cancels the field attraction between the steel and the magnet. That'd be my guess anyway. Here's a link to an easy demonstration that reveals the diamagnetism of water: link

Stainless steel? Is it guaranteed uniform makeup throughout?

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Wait, I thought stainless steel hasn't enough ferris in it to let magnets stick!

I found out about this when shown that magnets wouldn't stick to a stainless fridge.

Edit: I re-read and see what your saying now.

Maybe this proves that surface tension is a magnetic phenomenon. :^o

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"Oh no no no I'm a rocket man Rocket man burning out his fuse up here alone." -- Sir Elton John

J Pax

Why would you put magnets on your water tank in the first place?

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'The eye can only see what the mind is prepared to accept'

I was wondering if it was (partly) stainless because the magnets don't stick to at least part of it. I'm not a materials person, but I've heard that stainless steel, depending on formula, has different magnetic permeability.

Also I was just reading that working -- hammering, drawing, stressing -- stainless can change its permeability. That's why I was wondering if the tank was uniform material throughout, and I wonder if all the material has been been treated the same at all times. Being a water tank, perhaps the weight of water has stressed the lower portion more than the top -- though I think that might cause the reverse of what was described.

The description seems to indicate something is different about the permeability of top and bottom. Besides the water's weak diamagnetic effect, I'm just wondering what else might be different about the top and bottom of the tank.

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Yes, the filtered-water holding tank is stainless steel.

Simple test: empty the tank, do you get the same effect?

Also I would be surprised if a super rare earth magnet could be prised off a sheet of normal 18/8 stainless (they are great fun to play with but don't get them near your hard drive!). Fridge magnets on the other hand are pretty pathetic.

You will get a flatter surface on an empty tank whereas there will be some bulging when the tank is full leading to an incomplete point contact which may or may not be at the point of maximum flux. I wouldn't go getting woo about wierd effects of the water without thouroghly investigating the tank materials and its geometry first. If you must have magnets attached to the tank why not use a toriod speaker magnet? It will be much less susceptible to effects of the tank bulging when full.

Truely though, like Captain Swoop, if the water is properly filtered I don't see why you need a magnet. Unless you are on the pointy end of a sword a little iron never did anyone any harm.

Unlike silver. But that's a whole different case load of quack medicine.

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By asking questions we sometimes get the wrong answers, from wrong answers we learn to ask the right questions.

In fact the water from our local reservoirs on the northern edge of the NY Moors has quite a high Iron content. It filters through tw inronstone seams that were quite extrensively mined up until the 196os.

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'The eye can only see what the mind is prepared to accept'

Maybe she's a poet?
I've e-mailed her to find out---and about the size and shape of the tank and whether she's checked her magnets' abilities when the tank is empty.
So far, scourge's observations sound pretty good, thanks, have passed it on, too.

From link #-o I have a real dictionary with 1347 pages 8pt, 2 col./page. It cost £1.00 ($1.92 at the current rate) from a second hand bookshop. I love it because it contains older uses, spellings and meanings as well as more modern ones. Ideal for poetry.

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By asking questions we sometimes get the wrong answers, from wrong answers we learn to ask the right questions.

Martensitic stainless steels are uniformly magnetic.

Austenitic stainless steels (the 300 alloy range) are non-magnetic. Some can become magnetic through cold working, notably 303 and 304 SS, whereas 310 and 316 do not.

A SS water tank may be made from 304 SS which means the degree of magnetism of the tank walls may vary locally depending on the amount of cold working done.

I'm guessing the tank is only mildly magnetic and the magnets are just holding on. When the tank fills with water a trace of condensation forms and the magnets slide off due to the lubricating property of the water.

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When I am done here I think I will go create something from metal.

You might like this...

http://www.bibliomania.com/2/3/255/frameset.html

I have a hardcopy of this as published in 1894.

Thanks for the link, saved in fav. I also got some interesting ad headers there including ones to Bible interpretation and others to Wicca spells

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By asking questions we sometimes get the wrong answers, from wrong answers we learn to ask the right questions.

You can impress your friends at parties by referring to Xerxes' Tears and Diderick of the Heldenbuch.

You got me curious so I did a little investigation round the house. With one exception, all the stainless steel cutlery I have is quite strongly magnetic. Some items even have a permanant magetic field of their own (partiularly a bread knife). The kitchen sink (deep drawn) is non magnetic and of the 2 sheets of stainless I have in the workspace 1 is magnetic, the other not (which came as a surprise!)

Checking the web 304 and 316 grade stainless are austenitic and should be non magnetic. Matensitic grades will be strongly magnetic and ferritic grades weakly magnetic.

So I stand corrected, 'normal' 18/8 is non magnetic but not everything marked as stainless is what you think it is As you've also pointed out, cold working can make a difference to its properties.

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By asking questions we sometimes get the wrong answers, from wrong answers we learn to ask the right questions.

That caught quite a few of us off guard one day. We were plowing through a bunch of piping trying to find out how much carbon steel was left in the system. (Upgrading to SS.)

Unfortunately, this piping wasn't exactly high on the importance list so the drawings were badly out of date, and a significant number of valve drawings were missing.

So we're out there and somebody gets the bright idea to do the magnet test. Worked fine until we ended up staring at a valve with an obviously 304 cast into it and yet the magnet was hanging happy as could be from the bottom of it.

That quickly killed the magent idea.

Shirley responds:
"...I use an Aqua Rain set up. It is about 12 in. diameter and maybe a little taller in the holding tank. It is circular. I don't think it ever bulged, but it has split on one side and is currently being helped with duct tape. Haven't decided what to do yet. Maybe get someone to weld it, but that raises other concerns. Still gets out the chlorine so that's good. I haven't tried the magnets on an empty tank. That's a good idea...Actually, I will go now and empty it and try it with no water in it...Well, that didn't take long!! The magnets act the same way as before. They still lose strength going down to the bottom. I had thought that maybe the magnetism was being dispersed over the whole metal thing, but can't see why the dispersal would increase when the magnets went lower down on the metal...I was aware that not all stainless is magnetic, but hadn't really put that together to mean anything. There is no heating element anywhere on the unit. I do not discern any difference in appearance. If I remember correctly, it is supposed to be made of a single piece of metal and it has no seams, but I forget how they formed it--pressure I guess...P.S. I use the magnets because the n pole is supposed to energize the water in a beneficial way..."

That pretty well settles it. The top of the tank has been cold worked more than the bottom and is therefore more magnetic. It most likely is 304SS. If it is welded the magnetism will disappear in the heat affected zone.

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When I am done here I think I will go create something from metal.