The air continually exerts pressure on the body, and although you may not notice it on a regular day, if you have walked against a strong wind or ran full speed against the wind direction, you have noticed it. The air pressure which surrounds you is simply the air’s weight, and you don’t notice it because your body is primarily liquid and distributes pressure equally throughout your body. Just as air pressure exerts pressure on you, so does water when you submerge. However, because water is much denser and heavier than air, it changes much more significantly with depth.
At sea level, the air pressure is relatively constant and is a standard known as one atmosphere (ata) because this is the pressure of the atmosphere. The atmosphere is also known as a bar. For every 10 meter/33 feet depth under water, there is an increase in pressure by 1ata/bar.
Density and volume, two crucial factors which affect a dive change in relation with pressure. For every descent of 10m/33ft, you double the pressure and halve the volume. At a depth of 20m/66ft (three ata/bar) you have one-third the volume, and so on.
Also, as pressure changes, density changes. When you double the pressure and halve the air volume, the density doubles. When the pressure is tripled (20m/66ft) density gets tripled. And so on and so forth. To maintain air volume on a descent, you need to add air to match the volume reduction. This is the idea behind equalization; adding air in proportion with an increase in pressure.
As you may have figured out already, air expands in proportion to an increase in height and the pressure decreases. For example, air volume at 30m/99ft (four ata/bar) it compresses to one-quarter its surface volume. On return to the surface, it expands to its original volume.
If you add air to the space to maintain its volume, the air expands on reducing the pressure. If the air is in an open container, the expanding air bubbles out to the surrounding water. If in a closed and flexible container like a balloon or plastic bag inflated at depth, the air grows in proportion to the increasing pressure. If you inflate the at 30m/99ft, it will be four times its size on the surface as long as it can stretch that much. Otherwise, it will burst during ascent, and this has important implications as regards your body’s air spaces.
The air spaces in your ears are most sensitive to increasing pressure but assuming you are in a perfect state of health (no allergy congestion or head cold) you can easy equalize them. To do this, you pinch your nose and blow it gently with your mouth closed; this forces air from your throat into your ear and sinus air spaces. An alternate technique involves swallowing and wiggling the jaw from side to side. A third method includes both- swallow and wiggle your jaw while blowing air gently against your pinched nose.
Every few meters/feet before you feel discomfort because if you wait until you feel discomfort, you may not be able to equalize because the water pressure may close air passages shut. Rather, if you feel discomfort in an air space, ascend it eases, equalize and continue a gradual descent, equalizing more frequently. This will make the equalization experience easier with time.
If you find equalization impossible, discontinue the dive as continuing to descend under such conditions may result in a ruptured ear drum. Similarly, never attempt a forced equalization as this will lead to serious ear injuries and should this occur, abort the dive. If for any reason you are experiencing difficulties in equalization, cancel the dive immediately.
Congestions (due to allergies or cold) can block air passages, making equalization difficult or impossible. Sprays and decongestants can clear the openings but you shouldn’t take them and dive because their side effects may include drowsiness and their effects may wear off while you are diving, leading to equalization problems when you try to ascend.
One way persons create an unequalizable space in their ear canals are by wearing too-tight wetsuit hoods which seal your ear or by wearing ear plugs. Either way, you end up with an air space between your ear drum and the hood/plug which you can’t equalize. To avoid this, pull your hood away from your ears temporarily to allow the pressure equalize. and never put on ear plugs while diving. Although you may wear ear plugs made especially for scuba diving that allows for pressure equalization.
To equalize the air space in your mask, simply exhale into it through your nose. If you fail to do this, you will have a mask squeeze which is a pulling sensation on your eyes and face. However, we must take note that since your nose has to be inside your nose to equalize it, scuba goggles can’t be used while diving as they don’t close your nose and can’t be equalized.