Air is massive. All around and above us we are "swimming" in air and the weight of the air above us exerts pressure on everything in it. We do not notice or feel the air pressure around us because our bodies are compressed and exert an equal force back.
The weight of the atmosphere exerts a pressure of 14.7 psi at sea level. In other words, a 1 inch column of air as tall as the atmosphere (about 50 miles) weighs 14.7 pounds. This unit is called 1 Atmosphere of pressure or 1 ATM.
Scuba divers are concerned with pressures in tanks, in heads, in masks and under the water. We know that water is much heavier than air, so it exerts higher pressures with less volume. A cubic foot of water weighs 62.5 lbs, whereas a cubic foot of air weighs 1/12 lb. Sea water is even heavier - a cubic foot of sea water weighs 64 lbs. To exert 14.7 psi (1 ATM) it takes 50 miles of air, whereas it only takes 33 feet of water to exert the same pressure.
The point to remember is that at the surface we are not at zero. We start descending with 1 ATM working on our bodies. At 33 feet underwater we are at 2 ATM and at 66 feet deep we reach 3 ATM.
Let's suppose you fill a balloon with 24L of air. Let's take it scuba diving.
|sea level||1 ATM||12||1x|
You see that the volume of the air changed the most between 0 feet and 33 feet, where it was squished to half of its previous size; a loss of 6L. Descending from 33 feet to 66 feet changed the volume by only 2L. As our air goes deeper the relative compression decreases. You will experience this effect firsthand when scuba diving; the first 15 feet is when you'll feel "the squeeze" as all air spaces (lungs, sinuses, ear canals, the air in your mask, intestines and even bits of air trapped in your teeth) are compressed to half of their sea-level size. When descending you need to compensate for that loss of volume and "reinflate" your body by equalizing your ears, exhaling into your mask (which will literally suction itself to your face). You also need to add air to your BCD to maintain buoyancy. Once you're past 15 feet it gets easier and once you're below 40 feet you might hardly feel the difference in pressure at all.
The reverse holds true when ascending to the surface. All the air spaces will double in size in the last 33 feet; this means you need to leak out air as you ascend, bubbles will escape from your mask and you need to release air from your BCD; lest it inflate like a balloon and send you bobbing quickly to the surface (and probably give you the bends while you're at it).
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