Air bladder basics?

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Air bladder basics?

Postby sacharlie » 22 Aug 2019, 01:13

In reference to air cushions.
If the bladder is 12x12=144 square inches.
At 1psi it will support 144lbs without deflection, right?
At 0.5psi it will support 72lbs without deflection, right?
For the above it makes no difference if the bladder is 1", 2" or 3" thick or cubic inches don't come in to play, right?

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Re: Air bladder basics?

Postby shirley_hkg » 22 Aug 2019, 03:20

Very true , provided that everything is static and evenly distributed.
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Re: Air bladder basics?

Postby Burgerman » 22 Aug 2019, 08:22

There will always be deflection. No matter what the initial pressure or depth. Because your weight will increase the pressure and compress the air. Abd because your rear is not a flat shape. So the bits that stick out will be pushing harder than the bits that dont depending on the stretchability of the material that the bladder is made from. Thats why roho use lots of smaller air sacs. Each can be depressed individualy while keeping the same pressure as the rest on your bony bits.
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Re: Air bladder basics?

Postby Irving » 22 Aug 2019, 09:32

sacharlie wrote:In reference to air cushions.
If the bladder is 12x12=144 square inches.
At 1psi it will support 144lbs without deflection, right?
At 0.5psi it will support 72lbs without deflection, right?
For the above it makes no difference if the bladder is 1", 2" or 3" thick or cubic inches don't come in to play, right?

I ask cause I don't know what i don't know. :worship
shirley_hkg wrote: Very true , provided that everything is static and evenly distributed.


As Shirley says, but that's not the case because air is compressible (and so, by definition, is your bladder). As you increase the pressure on the top surface the pressure inside will increase and therefore the volume will decrease. This relationship (at a constant temperature) is called the adiabatic relationship and is given by the formula:

PSX_20190822_064628.jpg
PSX_20190822_064628.jpg (13.96 KiB) Viewed 371 times


Where P is pressure, V is volume and gamma (the little y thing) is the adiabatic index, about 1.4 for air.

What this saying then is if pressure increases, volume decreases (at a constant temperature) given by:

p1 v1^1.4 = p2 v2^1.4

Or, rearranging

v2 = ((p1 v1^1.4)/p2)^(1/1.4)

You fill it with air at 1psi above atmospheric and seal it so no air can escape. Let's suppose your bladder is a box 12 x 12 x 2, a volume of 288cu in, at that pressure. So every surface of your bladder experiences an internal pressure of ~15.7psi (this would be so much easier in sensible SI units). You now place your weight on top applying an extra 1psi to the gas which increases to 16.7psi. The volume of the air will then decrease from 288 to 275.6cu in.

Calculating the dimensional change will depend on the material & construction of the bladder and is too complex for now, but intuitively since top and bottom are dimensionally constrained the top surface drops and the side walls expand. But let's assume for now that the side walls are elastic vertically but rigid horizontally (treating the top as an unconstrained piston). Then the top surface drops to 275.6/144 = 1.9". Unless your side wall material expands significantly this will be a good approximation.
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Re: Air bladder basics?

Postby ex-Gooserider » 27 Aug 2019, 04:46

As Irving said in much more detail, the volume in an air-filled bladder will change when you sit on it because air compresses....

If you had the same bladder filled w/ liquid it would not change volume because liquids don't compress (at least not to normal measurement - they do compress a tiny amount, but you won't be able to measure it outside of a VERY good lab....) This is the reason why hydraulic cylinders work....

However as long as the volume of the bladder once you've sat on it and compressed it is enough that you don't bottom out (which also requires that the material be sufficiently non-stretch that it can't bulge like a balloon) it doesn't make a great deal of practical difference how much extra thickness / volume it has...

However as practical, application, a single large chamber will be VERY unstable as any unevenness in the weight distribution will cause the air to move from the heavy point (higher pressure) to the lighter, lower pressure area, probably bottoming the high weight point and pushing the low point way up.....

Dividing the bladder up into a bunch of small linked pockets will slow (but not stop) this air shift, and (more importantly) tend to limit how much any pocket can expand, and thus limiting the shift.... Obviously isolating the pockets from each other completely would totally prevent the air shift, but then the pockets would not all have the same pressure, so the math gets more complex as each pocket would need to be treated separately...

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