Water pressure as a function of depth - The Planted Tank Forum
 
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post #1 of 9 (permalink) Old 06-26-2011, 04:02 AM Thread Starter
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Water pressure as a function of depth

It's my understanding that water pressure is dependant on the depth of the water column, and not the total volume of the tank. Am I wrong here?

The reason I ask is because I'm in the beginning stages of building a paludarium that will be a water fall with a cascade pool. I intend to make the front, viewable portion a solid pane of glass with the access doors in the sides. The viewing area will be just shy of 36" across and I have no intention of bracing the glass in the center. The depth at the front of the tank will be no more than 12-14", but the front pane will be 54" tall, so bracing at the top would not be terribly effective. There will be glass running from front to back maybe 6" in from each side, where a land area will be situated, unless I do the lowest pool riparium style. The glass will be retained by 1/2" or 5/8" plywood (I'd rather go lighter if possible as I will likely have to move this thing at some point.). In any case, there will probably be a 24-30" section without reinforcement.

The thicker I go with the glass, the heavier this thing will become, so I'm hoping to go with 3/8" or less. I'm hoping to get away with 1/2" glass (1/4" would be nice if doable.). My thought is that there won't be a tremendous amount of pressure pushing on the front glass, but at 36"x28"x12", there will be somewhere around 50 gallons of water in the lower section (A bit less due to land area, etc.).

Does anyone have any thoughts on this matter? Maybe someone less physics challenged than myself can set me straight?
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post #2 of 9 (permalink) Old 06-26-2011, 04:20 AM
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Yes, it's the depth that's the real issue when it comes to the glass. The way I think of it is if even 100 people lay next to me I feel no extra pressure, but if the lay on me I'll be crushed. I think that you will be absolutely fine with 3/8". You could probably go a little thinner, but I think 1/4" is a touch thin for peace of mind. My 54 breeder is 1/4", is 16" tall, but center braced. So 12"-14" with no brace might work, but personally I'd go 3/8".


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post #3 of 9 (permalink) Old 06-26-2011, 05:26 AM Thread Starter
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Thanks for the reply.

I just want to go thin to reduce weight. I'm afraid I'm going to end up with a 500 pound bohemoth (empty) that has to stay if I ever move.
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post #4 of 9 (permalink) Old 06-26-2011, 12:55 PM
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You're ok with the idea that its depth, not volume for liquid pressure.

The basic formula for pressure at depth is P=pgh; P=pressure, p= specify density of liquid, g=gravity, h=height or depth of liquid. Volume doesn't enter into this equation. The only point where volume is a factor would be the tank bottom; while the equation will result in pressure over surface (pounds per square inch, newtons per meter), the volume will get the total mass/weight that the bottom supports. Only important if bottom support could bow under the weight.

However, as I think about the problem you're presenting, a factor will be the wide viewing area of 36 inches. While the psi on the glass is the same as a 10 gallon tank (because of the same depth), thin glass is going to bow more and the long width will allow more bowing. So while 1/8(?) inch of glass would work for a 10 gallon, your unbraced width is so much greater and a lot of bowing will happen. A safe estimate for glass thickness is what would be required for a 14" deep tank of the same width, without a brace. I don't have any data or experience on this, but 1/4 inch seem workable.

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post #5 of 9 (permalink) Old 06-27-2011, 08:57 PM
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Here is a site that will calculate water pressure versus depth:

http://www.calctool.org/CALC/other/games/depth_press

If I remember correctly, each meter of depth adds roughly 1.25 psi, so at 12" you'll have about .3psi from the water plus 14.7psi from the atmosphere. Since both the inside and outside of the tank are exposed to the atmosphere you don't need to include atmospheric pressure in your force calculations. The total force on your front pane will be equal to pressure times area (keep track of your units). Stress and deflection calculations are a bit more complicated; especially with the glass anchored on three sides. For a worst case scenario you could do a simple beam analysis and ignore the bottom seam.

My gut instinct says 1/4" thick glass will handle the stress easily provided the seams are well made (my 55 gallon is made from 1/4" thick glass and it's 48" long with a H20 depth of roughly 20"). If you're worried you could move up to either 5/16" or 8mm (.315") thick glass.

I hope this helps and I didn't get too technical. PM me if you want help with the deflection analysis; I think I still have the appropriate formulas in a book somewhere.

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post #6 of 9 (permalink) Old 07-03-2011, 04:59 AM Thread Starter
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Looks like around 129 lbs of force spread out along the glass. That doesn't sound too impressive. I think I'll be OK with 3/8" glass.
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post #7 of 9 (permalink) Old 07-03-2011, 05:20 PM
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That's nothing. After your post I took a closer look at my 55 gallon. The front and back are 1/4" thick, but the ends are only 3/16". If you're using 3/8" thick glass you could probably fill it with mercury, provided your seams didn't leak.
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post #8 of 9 (permalink) Old 07-06-2011, 07:42 PM Thread Starter
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I don't think mercury is good for the fauna

It'll be a while before I get to the stage where I'm putting in glass, so I have some time to think about it.
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post #9 of 9 (permalink) Old 07-07-2011, 02:36 AM
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It's hard telling how much this will bow, as well. Even with thinner glass, the water is trying to bend a great deal of the stuff. The glass that is above the water would act like a fairly complicated brace, basically.
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