Pond Boss Forums AERATION Systems questions Diffuser area of influence

Something that many people may not think about or know (but SCUBA divers have to be well aware of) is how air released at depth expands.

A little background first. The distance from outter space to sea level is our atmosphere. At sea level the "weight" of this air is just under 15# per square inch. When we measure tire pressures or any other air pressure we do not count this pressure. Instead we start the pressure scale at zero, but if it was measured in outter space the pressure we measure at sea level would be almost 15 psi more at sea level than outter space (when measured at ground level we measure negative pressure on a scale as vacuum). So at sea level we are breathing air and the surrounding air is at slightly less than 15 PSI.

Water is denser than air. It weighs more than air. So as we decend in water the pressure rises more rapidly than as we decend in air. In fact decending about 33 feet in water has the same effect as decending from outter space to sea level in air. So one atmosphere of air pressure is the equivelant to a depth of 33 feet in water, or one atmosphere of water for every 33' of depth. This is why it takes a compressor that will develop about 15 psi to overcome the head pressure of water if a hose or diffuser is submerged to a depth of 33 feet. If your compressor will only develop 10 psi, it will stop flowing air as the hose decends down to about 22 feet.

Ok, that was long winded. Why does it matter concerning diffusers? The bubbles themselves do not infuse that much O2 into the water. The bubbles push the water to the surface so the air/water interface at the surface can do the majority of the work of putting O2 into the water.

Here is where all the long winded part explained above comes into play. If you were to fill a toy balloon fully with air at a depth of 33 feet, tie it off and release it, it would burst before it reached the surface. The one atmosphere of water pressure (almost 15 psi) it took to fill the balloon at 33 feet (two atmospheres of pressure) would expand to two atmospheres in size and burst the balloon as it rises to meet the surface pressure of one atmosphere.

So bubbles released at depth expand as the water head pressure decreases as the bubble rises. The bubbles get bigger. Put a tiny it of air in a balloon at 100 feet depth and it will be fully inflated when it reaches the surface. This is why divers never hold their breath when accending. The compressed air we are breathing at depth expands in the tiny air passages in our lungs and bursts. With a full breath and rising holding ones breath it can happen in as little as a four feet and is called air embolism. It is a bad and often leathal thing.

So how does this apply to pond aeration? Well first off it tells us we need a compressor that will operate at adequate pressure for the depth we place the diffusers. But it also tells us that the deeper the diffuser, the greater the expansion of the bubbles as they rise to the surface, so with greater surface area the bubbles will push more water. That is why a diffuser placed at a deeper depth (assuming the same cfm of air delivery) will move more gallons per hour than a shallower placement. The bubbles are released under greater pressure therefore have a greater expansive capability therefore a greater water movement capability.

WHEW! Anyone still awake.

I'm no expert on this, but I am a scuba diver so have to understand what happens when breathing air at depth. Same applies to air being released from a diffuser at depth.