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Old 08-15-2008, 05:57 PM   #31
vfc
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dskidmore,
I agree that you should use a fresh piece of foam when moving the tank. The same tank/stand combo will have a new floor structure to conform to.

I even had a case of moving a tank from one spot in my finished basement to another. I thought the concrete floor would be uniform, but it turned out that the new position of the tank was on a high spot, where behind the wall, the builder had tapered the cement up around a support pole to prevent water from collecting around the base and rusting the metal. In a short time, I noticed the foam had compressed considerably in the back right corner.

Again, without the foam, that tank would be under a very high twisting force.
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Old 08-16-2008, 12:54 AM   #32
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OK... I've tried sitting on my hands about this thread, but it's just not working!

While I think the original poster's question has been beat to death, it sure seems like there's some misunderstandings as to what foam will and won't do. I apologize in advance for the length of this, but hang with me here for a minute!

Maybe it's easier to think of the foam as a bunch of little springs under your tank, because really... that's all foam is - a different type of spring. And like any spring, foam has a spring constant which tells you how many pounds it takes to compress it an inch. For a given spring rate, the more the spring (foam) is compressed, the more load is transferred through the foam.

Now lets think about our tank that is sitting on a perfectly level stand, evenly compressing the foam all the way around the frame. The compression is even, so the force transmitted from the tank through the foam is the same all the way around. That's perfect!

Now over the years, the stand sags for whatever reason, and you notice that the gap is wider between the stand and the tank frame in some spots versus others. In other words, the foam has rebounded and is now compressed less in some areas than others. Remember... the load transferred through the foam is directly proportional to the amount of compression.

So, the area of the frame where the foam is now compressed less does NOT have the same load going through it compared to other areas under the frame that have compressed more. The foam does NOT even out the load - it transmits a load that's proportional to the amount of compression. The bigger the gap (less compression), the less the load going down to the stand in that area. And if the load is less in that area, then it's MORE in another area.

To add another wrinkle, most foams have some amount of permanent compression set - over time, it will NOT rebound to 100% of its original thickness. This compression set varies between foam types, and even foam manufacturers. So what this means is that even if you measured your foam before you put it under your tank, and years later as things start sagging and the foam "expands" and you think it still has some compression left in it... it might not. In reality, that foam under the sag might not be carrying any load at all. And that puts you right back in the situation that you were trying to avoid - having no support under the frame of the tank.

My long-winded point it that having foam UNDER OF TANK FRAME ONLY really doesn't hurt anything, but it also doesn't guarantee one bit that your load distribution is equal along the length of the frame. Depending on the amount of gap, and the type/strength of foam you use, it could be the exact same thing as having nothing under the frame. Foam can not transmit constant force and compress less, at the same time - it's just beyond the current laws of physics!

And if you've read this far... thanks!
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Old 08-16-2008, 11:55 AM   #33
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Kurt,
I generally agree with your statement. Foam will not completely even out the force (only a sealed fluid bed would do that) but it will lower the extremely high forces, especially on a raised corner.

On my 75G tank with the sagging center; without the foam the entire weight of the tank would be supported by just 36" of frame (both 18" ends). So instead of 148" distributing ~ 800lbs of force (@ an even 5.4lbs per frame inch); the frame would have 4X the pressure @ 22.2lbs/inch on the ends. With the foam, and doing a rough calculation with the assumption the sag is linear, the ends might have a force of ~ 10lbs/inch and the center ~ 2lbs/inch. That is not perfect, but way less stress, and with that, better odds the tank won't develop a leak or split a seam.

Try dealing with that center gap on a 6 ft tank. As an example, I had a 6 ft tank set up for a few years with no problems. I gave the tank to my brother who put the tank on a stand with no foam and on a wooden floor. Within a year, the center brace broke and the glass panels were bowing outwards.

I have another 75G tank that my sister gave me. She had it set up ~10 years on a DIY stand that had the same poor design that Ziggy's tank has (i.e., not enough center support). I was amassed that the tank held up and was starting to rethink my foam requirement. I looked at the silicon seams and they seemed solid. However, when I filled up the tank (new DIY stand and foam BTW) with cold water I noticed condensation forming about half way between the glass panels. Turns out that the silicon had separated about 50% from the two glass panels; so instead of a 1/4" bead of silicon, there was a 1/8" bead. My theory is that the stand was twisting the tank and the force was pushing the front glass panel in causing the silicon seal to slowly peel away.

The most important thing to remember is selecting the correct foam. The pink foam from HD does absolutely nothing for your tank. It is so stiff that it takes >25lbs/per to make it compress slightly. Even the 200G or larger tanks only have a little over 10lbs/frame inch of force. For an acrylic flat bottom tank, the force per square inch is only around 1lb. So the foam has to be the light weight white foam that they use to cushion large computer cards when they ship them.
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Old 08-16-2008, 03:09 PM   #34
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Quote:
Originally Posted by vfc View Post
Kurt,
I generally agree with your statement. Foam will not completely even out the force (only a sealed fluid bed would do that) but it will lower the extremely high forces, especially on a raised corner....
Foam can't "lower" forces - closed cell foam only transmits the forces from one side to another. If you have "extremely high" forces in one area of the foam, then the foam will compress more in that area. The force on one side of the foam is exactly the same as the other. Foam doesn't change physics.

Now if you're talking really stiff foam, more like styrofoam (but you aren't), then the foam can distribute that load out to a bigger area. But that type of foam doesn't have any rebound so once it's crushed, it's crushed.

I'm not looking for debate... I'm just trying to clear up some things that have been said in this thread that just plain aren't true. I'm in no way doubting your experiences, but I think you're attributing way too much of your success to the foam.

If you want to use foam, great... use foam! But understand what it will, and will NOT do for you.
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Old 08-16-2008, 07:23 PM   #35
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Kurt,

Quote:
Foam can't "lower" forces - closed cell foam only transmits the forces from one side to another. If you have "extremely high" forces in one area of the foam, then the foam will compress more in that area. The force on one side of the foam is exactly the same as the other. Foam doesn't change physics.
Your theory, if I understand the above statement, says that after the foam on the sides (36") compresses (50%), that no other weight is being supported by the remaining 112" of foam that is compressed anywhere from 49% to 10%.

I don't think so; in my calculation:

Without foam, each of 18" side frame are supporting half the weight of the tank (400lbs x 2) and the center is supporting zero.

With foam, the ends are supporting 180lbs each and the center is supporting 440lbs.

The foam I'm using under the 75G tanks is not "stiff" like the pink foam, but an ideal density to support ~ 5lbs/frame inch. During my foam selection process, I balanced a 10lbs weight over a 1" piece of wood (to simulate the plastic frame) and placed it on a piece of foam. The foam compressed ~ 40%. A 5lb weight compressed the foam ~ 20%.
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Old 08-16-2008, 11:01 PM   #36
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I'm afraid it's been too long since I took differential equations to do out the full math, but I agree the foam does the most good where the two firm surfaces normally don't meet. You'd have to know the spring constant and thickness of the foam and the formula describing the curve of the stand to really figure out the weight distribution change.

The foam fills the void between the two to a varying degree, and puts force on the tank anywhere it is compressed. The total load on the tank bottom has not changed by putting the foam in place, so if the foam is providing any force in areas that previously had no force applied, it must logically be putting less force on the areas previously bearing all the weight. No, it does not completely redistribute all the force, but it redistributes some of it.

Arg. I'm feeling anal now, want to run the numbers. I think I might have a calc book here somewhere, but I'm not going to do the math tonight.
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Old 08-17-2008, 01:11 AM   #37
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Kurt,

Your theory, if I understand the above statement, says that after the foam on the sides (36") compresses (50%), that no other weight is being supported by the remaining 112" of foam that is compressed anywhere from 49% to 10%.
#1... it's not "my" theory - it's basic physics.

#2... that's not what I'm saying at all. I'm saying the weight the foam is transferring is proportional to the amount it is compressed. The foam doesn't "support" anything - it just transfers load from the bottom of the tank to the top of your stand.


Quote:
...During my foam selection process, I balanced a 10lbs weight over a 1" piece of wood (to simulate the plastic frame) and placed it on a piece of foam. The foam compressed ~ 40%. A 5lb weight compressed the foam ~ 20%.
Thanks for proving my point. Think about what you just wrote... less weight compressed the foam less. 10 lbs = 40% compression. 5 lbs = 20% compression.

Now think about that sag in the middle of the tank where the foam is at its thickest. That is also where it is least compressed. Less compression means less load above. Since the tank still weighs the same (I hope!), that means that other parts of the frame are taking more load. And that would imply you are getting the dreaded 'twisted tank' that you are trying to avoid - with or without foam.

Dskidmore...

Quote:
The foam fills the void between the two to a varying degree, and puts force on the tank anywhere it is compressed. The total load on the tank bottom has not changed by putting the foam in place, so if the foam is providing any force in areas that previously had no force applied, it must logically be putting less force on the areas previously bearing all the weight. No, it does not completely redistribute all the force, but it redistributes some of it.
You mention the foam putting force on things like it's creating the force. I think that's where everyone is getting messed up. Foam is just "there" - it's no different than air, except the spring constant is really really different! The foam just transfers the load from one side to another, and in the process compresses an amount that is proportional to the load going through it.

OK... I'm going back to sitting on my hands now because obviously people will think whatever they want to think about how foam behaves. And y'know... as long as it's not hurting anyone, folks can think what they want!
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Old 08-17-2008, 10:10 AM   #38
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Kurt, You need to read a book on physics or go back to school. The same force that is compressing the foam down and transferring the force to the stand is also pushing up on the tank. In relationship to gravity; when the top force is higher, an objects sinks; when the up force is higher, the object rises; when they are equal, the object is stationary. My tank has be vertically stationary after the first year as it slowly sank part way into the foam.

In my foam test procedure, it took 10lbs to compress the foam ~ 40%. The table under the foam and weight had a downward pressure of 10lbs/inch. Likewise, the bottom of the weight had an upward force of 10lbs/inch. That's the force that held the weight above the table and that's the same force that is holding the tank up off the stand. Regarding tank physics, that upward force of the center foam (foam not under the two 18" side frames) that is compressed anywhere from ~10% to ~49% is exerting an upward force of ~ 440lbs in my rough calculation. That would be like two men standing on both sides of the tank with their hands under the center tank frame and each guy supporting 220lbs.

BTW - I just read the last page of Ziggy's post and he said his 120G tank also cracked. I only read the beginning of the post where his 150G cracked and saw the picture of the stand. I knew his new 120G tank would also be under stress and I am not surprised that it cracked. What upsets me more is that the manufacturer replaced it as a manufacturing defect. I'm paying higher tank prices for people like you and Ziggy who don't know physics. Ziggy is solely responsible for the tank cracking. I wonder how the tank would have fared if he had put 1/2" black exercise foam pad under it? I'm sure his foam would have looked similar to the foam under my 75G (i.e., less compressed in the middle from a sagging center that lacked proper support).
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Old 08-17-2008, 11:20 AM   #39
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the more foam compresses the harder it is to compress meaning more upward force is transferred to the tank. the less it is compressed the less force is transferred. so you are still getting less support in the middle of the tank where the foam is not compressed as much as the ends.

Quote:
BTW - I just read the last page of Ziggy's post and he said his 120G tank also cracked. I only read the beginning of the post where his 150G cracked and saw the picture of the stand. I knew his new 120G tank would also be under stress and I am not surprised that it cracked. What upsets me more is that the manufacturer replaced it as a manufacturing defect. I'm paying higher tank prices for people like you and Ziggy who don't know physics. Ziggy is solely responsible for the tank cracking. I wonder how the tank would have fared if he had put 1/2" black exercise foam pad under it? I'm sure his foam would have looked similar to the foam under my 75G (i.e., less compressed in the middle from a sagging center that lacked proper support).
To say this is just plain ignorant. there is no way to tell what actually caused the breakage. it is more likely from an impact due to shipping than a slightly uneven stand that caused the tank to crack. to blame "higher tank prices" on people that dont know physics would be more far fetched then me blaming you for higher gas prices.

i think this topic has been beat to death enough and ppl are becoming less civilized so hopefully a moderator will lock this thread.
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Old 08-17-2008, 11:32 AM   #40
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This thread contains some very useful, and lively, but polite discussion - up until the post where Ziggy's tank failures are brought up. We will never know what happened there exactly, and Ziggy feels quite bad enough about it. There is also no reason to insinuate that people who "don't know physics" are to blame for higher tank prices.

I do believe the OP's question has been answered. I will leave this thread open but I'll be watching closely.
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