insulation pad or not?

[Kurt_Nelson]

Kurt, You need to read a book on physics or go back to school. ...

Trust me... I've read a few. I have a pretty successful career of "not understanding physics", so I'm not really too concerned about your suggestion or the fact that you think I'm personally driving tank prices up!

No need to lock the thread in my opinion, An t-iasg - I'm done with it!

 

[dskidmore]

Dskidmore...

You mention the foam putting force on things like it's creating the force.

Technically the only real forces in the equasions are gravity, the strong and weak forces holding the atoms together and the electromagnetic forces holding the molecules/mateials together. I have no motivation to attempt to individually sum up the basic forces of the foam, the stand, the house, and the earth itself that make them able to hold up the aquarium when it is obvious by the fact that the tank is not accelerating that this force is exactly equal to that of gravity, which is much simpler to compute. So the foam is exerting a force up on the tank equal to the load the tank is exerting on the foam. This also happens to be equal to the amount of force, minus the weight of the foam, that the stand is exerting on the foam, and the foam is exerting on the stand. (And so on through the stand to the floor, to the rafters, to the foundation, to the ground, to the bedrock...) But that's all semantics.

I don't get your point at all. Physics problems are regularly simplified as much as possible to obtain the rough numbers. Like my treating the foam as a series of infinately small theorretical springs. We all know that foam is not a perfect spring, but the comparision will help us understand what is happening even if it is not a perfect model. If we required perfect models for everything, the science of physics would not exist, for it is too complex. Instead we build approximate models, and over-engineer in a safety factor.

 

[dskidmore]

No need to lock the thread in my opinion, An t-iasg - I'm done with it!

Ah, but it was so interesting. I talked with my mother in law (a math teacher) about it, and the next evening I have reading time I'm going to pull out hubby's statics and dynamics book. I never got that far in engineering before switching majors, but I think with the textbook in front of me I could puzzle out in a few weeks what the adept student should be able to handle in their evening homework.

 

[dskidmore]

Ok, here's the best I can do without the statics and dynamics book:

Fx = function describing the curve of the stand
k = spring constant
t = thickness of the foam
d = t-Fx = distortion of the spring at point x(assuming foam is compressed everywhere enough to touch the load everywhere. This is a minimal case for usefulness, a real case would have an additional constant compression, and therefore constant additonal force, across the whole contact surface. Because this other force is constant, we will ignore it for now.)
Fs = kd = spring force
Fs = k(t-Fx) = spring force at point x

W = total weight of load supported by foam

W = ∫k(t-Fx) dx

I'm not going to solve it with real numbers right now as Fx is realistically complex, I have no real data for a sample k, and it's been a long time since I took calculus, but we can look at the relationships and make some conclusions.

We'd want W to be less than the weight of the tank, as if it was more than the weight of the tank then that means our tank would not compress the foam enough to provide support across the whole tank. We would still have an unfilled gap between the tank and stand.

As the tank weight begins to dwarf W, the tank is lowered closer to the stand, and will compress the foam by the same additional amount at all points, and therefore the spring force will also increase constantly across all points. The more the weight of the tank dwarfs W, the greater the percentage of the load is evenly distributed. The softer the foam, the more the weight of the tank will dwarf W.

There is a limit to how much the tank weight can dwarf W, at the point where the foam is compressed enough to deviate significantly from the ideal spring, reaching maximum useful compression. At this point it's just like setting the tank down without foam. A more resistant foam must be used if you reach this state.

There is a second limit in that the maximum (used) deflection of the foam should be less than the diffrence between the bottom of the tank trim and the bottom of the glass, to avoid the earlier mentioned situation where the foam starts transfering some load to the aquarium glass. This is probably the situaton that tank manufacturers worry about when warning against foam. A more resistant foam must be used if you reach this state.


The thickness of the foam should have it's diffrence between minimum and maximum compression minimally equal to the diffrence between minimum and max height of the supporting surface. Thicker is better in this case, but given the same material, a thicker foam is softer, and could bring you to one of the above limits.

So, in conclusion:

You want a foam that will compress at least a little bit everywhere, but not touch the bottom of the aquarium glass, or get completely squished. Within those limits, a softer foam will redistribute more weight than a firmer one.

I enjoy being a geek.

 

[vfc]

dskidmore,
Good work; the world needs geeks like you to take theories and separate them into facts or myths.

So, in conclusion:

You want a foam that will compress at least a little bit everywhere, but not touch the bottom of the aquarium glass, or get completely squished. Within those limits, a softer foam will redistribute more weight than a firmer one.

That what I have been saying: foam selection is most important.

The perfect foam for a 75G tank is the 1/2" interlocking black foam exercise mat that Target sells. It comes with six 24" pieces that interlock like puzzle pieces. The foam compresses between 10-20% when you initially fill the tank and can offer stress relief for stand level variations of up to ~ 3/8". You can stack two pieces of 1/2" foam to handle larger variations.

If the foam is too stiff, it offers no stress relief. I have that situation on a tank now. I built the stand, and because I had an extra piece of 1" pink foam from a home project, I placed that under the tank. When I filled it up I had three spots along the frame where I could slide a plastic card between the tank and foam. After 8 months, the gaps are still there; the foam did not compress at all.

 

[dskidmore]

I have a white foam with blue backing under my tank right now. It's slightly softer than the pink foam, but not much, and it has no rebound after the load has been lifted. My stand is hollow in the center, so there is little risk of pressure on the glass if I use a foam too soft, so I think when I next move, I will replace it with a softer foam.

I do need to use some sort of foam, as my stand is made from interlocking 4x4s and there is a little bump where the join is. The good news though is that they can hold the weight with no center brace and no noticable bowing.

 

[vfc]

dskidmore,
Here's a better picture of the entire front of the tank. I put three small pieces of uncompressed foam on the stand for reference.

Here is a rough representation of the frame and force distribution with and without foam.

 

[dskidmore]

I'm not going to check your exact numbers, I don't have the data, but assuming the foam is not totally crushed on the ends, and there are no gaps in the middle, that could be right. Of course the blue area is itself not evenly distributed weight either, the point load varies with the amount of compression at that point, which varies by the origional gap size.

 

[SpeedEuphoria]

vcf the no foam pic, is that how the tank sits full of water on the stand, or just empty?

 

[vfc]

SpeedEuphoria,
The first picture I took a few hours ago. The tank has been set up for close to 3 years.

If I did not have the foam under the tank, there would be a gap between the tank and stand in all the blue areas.

When I first put the empty tank on the foam/stand, there were no gaps. When I filled it up, the foam compressed evenly.

 

[BillD]

putting soft foam under a tank, is in my view of little use. It will fill the gap, but i don't believe it will offer much in the way of support. The original question, if I read correctly is about the use of rigid foam, one of the various types of styrofoam. Styro will absorb discrepencies in flatness on a stand, whether it be open or solid top. It will not help with leveling. The popular metal stands rarely if ever support the entire bottom edge of the tank. I have yet to see one that doesn't have a gap in the middle somewhwere, including ones I have built, taking great care to use the straightest steel I could find (both angle iron and square tubing). The reality is that if you took a tank and supported the ends only, it would still hold water. The tank would be under a lot of stress, but it would be strong enough to hold together, as long as the 2 supports were in the same plane (ie,level). No one in their right mind would do this of course.
The other extreme is to support the entire bottom. If done correctly, it takes the bottom pane of the aquarium out of the equation as it no longer carries any weight, only holds in the sides. The first all glass aquariums were built locally, when a guy who was using silicone in his work came up with the idea of gluing glass together to make a tank. He was one of the principals who started Crystal aquarium. They established the thicknesses needed for the various sizes of tanks. The first tanks were built without frames and broken bottoms were a problem. This was not because of any inherent weakness in the design, but caused by people setting the tank on a piece of gravel and having the point load created by filling the tank, causing the bottom to fracture. This problem was solved by adding a bottom frame (wooden ones at first) to lift the bottom off the stand or whatever the tank was sitting on. Keep in mind that in the late fifities, there were not the choices of stand types we see today. Most custom tanks are built without any plastic frame on the bottom, and are set on styro tosupport the bottom and provide a cushion to protect the bottom. The styro will compress where there are high spots and the bottom of the tank ends up supported over its entire area if a solid base is used. If the bottom is supported, it is less likely to break if something is dropped on it since glass is very hard and if supported so it can't flex, it won't break. A tank with an unsupported/suspended bottom can be broken with a point load of a rock pressing down on a piece of gravel. Our club president lost 2 tanks to this, and now puts styro under all his rocks.
The often repeated as gospel, "the manufacturer says", the tank should not have the bottom supported is for their protection. They have no control of how we set up our tanks, so they build a stand that eliminates some of the dangers caused by improper tank placement. It may not be the best, but is a compromise that is in their best interest. The suspended bottom that fits inside the sides rather than beneath them that some manufacturers use addresses the liklihood of the tank being placed on an eneven surface, or a stand that sags over time. This design allows for a little more flex, but isn't as strong as side on bottom. The silicone is expected to carry the weight of the load in the tank, as well as hold the sides in. Would you build a stand this way where only the fasteners carried all the weight? Not likely, as most will put the legs under the weight rather than beside it.
So, I will continue to use styro (white) under my tanks, particularily the 5 I built that have no bottom trim. I have seen the difference it makes, and it is the way industry deals with machinery, where grout is used underneath a machine to support the weight once it is leveled.

 

[dskidmore]

putting soft foam under a tank, is in my view of little use. It will fill the gap, but i don't believe it will offer much in the way of support.

If the foam is completly squished, you are right. But the softest foam that does not get competely squished at the highest load point will redistribute the most weight.

A really hard foam that doesn't squish enough is only marinally better than putting the tank on plywood.

Need a happy medium. I think the ideal foam should vary by tank size.

 

[BillD]

I use the white styro because it is softer than the coloured ones. If the stand is flat there is virtually no squish, with the frame barely indenting the styro. I have 4 48" x 12" x 12" x 3/16"tanks I made, that sit on steel stands. In each case the centre sat 1/8" below the bottom of the tank. The styro, while covering the entire tank bottom only makes contact with the top frame of 1 1/4" angle or tubing. The ends of the styro compressed about 1/8" so the contact is uniform. These tanks have held water now for about 15 years without issue. I think when dealing with styro, thickness is less an issue than the type used. Hopefully, any stand used would have only minor discrepencies, so that any compression would be barely noticeable. Styro will only absorb pressure once, hence it's use in helmets.

 

[vfc]

BillD,

I think when dealing with styro, thickness is less an issue than the type used....Styro will only absorb pressure once, hence it's use in helmets.

That's why the 1/2" exercise mat is perfect for under the tank; it's designed to take hard impacts (compression) and then restore it's original shape.

 

[BillD]

BillD,
That's why the 1/2" exercise mat is perfect for under the tank; it's designed to take hard impacts (compression) and then restore it's original shape.

It doesn't do the job the same way styro does. It gives too much. In a helmet that would kill you.

 

[vfc]

dskidmore,
Here is some information a Nerd would enjoy. I created an Excel spread sheet that calculates force per frame inch for some popular tanks. I also tested three different foams I had available to see how they match up to the various tanks.

Pic 1 - The spread sheet
Pic 2 - Foam test materials
Pic 3 - Foam test procedure
Pic 4 - Picture of how the pink & blue foam retained the compression dents and how the exercise foam was not affected.

The exercise foam is the way to go....

 

[dskidmore]

dskidmore,
Here is some information a Nerd would enjoy.

Very cool.

Looks like the Lowe's foam might be good for larger tanks though. The really big ones that are in the most danger of cracking?

I'm looking forward to moving for one more reason now. I can look at the indentations in my foam.

 

[vfc]

dskidmore,
I would think twice about using the blue Lowes foam. I needed 15 lbs to get the test frame to make a small dent in it. My 150G Tall has the highest force/inch frame of any of the tanks on the list @ 10 lbs.; yet after 2 years, the blue foam under my tank is not compressed at all. It may be reducing stress on any sharp high spots, but would do nothing if the stand had a raised corner. I may be okay though because I used high-grade dried 2x6 wood and the stand is on concrete.

 

[dskidmore]

No, I agree, no Lowe's foam for any of the tanks on your list. But 210 or larger? Your pounds per inch chart is going up almost linearly.

 

[vfc]

dskidmore,
I added the 210G to the chart (I used an educated guess on the weight). The frame force is nearly the same as my 150G Tall and the blue foam is too stiff for that. I need to look for resilient foam in the 10lb range.