Why does your ear go numb and your tank drain faster at depth? Boyle's Law explains pressure, volume, and diving's number one rule: never hold your breath.
## Introduction Why does your ear suddenly go numb on the way down, and why does your tank empty almost twice as fast at 20 meters as it does on the surface? Here's the honest answer: water presses on you brutally - at just 10 meters you're already under twice the pressure you feel on land, and that pressure changes every pocket of air in and around your body.
The law behind it is called Boyle's Law (described independently by physicists Robert Boyle and Edme Mariotte in the 17th century), and it's the single most important piece of physics you need as a diver. It explains why you have to equalize constantly, why your tank drains faster at depth - and why you must never hold your breath. This DOMISPORTS Academy guide breaks the physics down so it actually clicks, not just something you memorize.
Four pieces of gear help you handle the rising pressure safely:
On land, air presses on you at 1 bar - you don't notice because your body is used to it. Underwater, you add another bar every 10 meters: 2 bar at 10 meters, 3 bar at 20 meters, 4 bar at 30 meters. For comparison: at 4 bar, every square centimeter of your body carries roughly the weight of a full crate of water bottles. That steady climb is the trigger for everything else in this guide.
The formula sounds dry, but the idea is simple: double the pressure, and a gas's volume halves. Picture a balloon with 6 liters of air that you carry from the surface down to 10 meters (2 bar) - it shrinks to 3 liters. At 20 meters (3 bar), only 2 liters are left. The reverse is exactly as true: on the way up, the gas expands right back to its original volume.
Your body has several air-filled cavities: middle ear, sinuses, lungs - plus the artificial pocket created by your mask. As the volume in these spaces shrinks with rising pressure, a vacuum forms that painfully pulls tissue and blood vessels inward (a barotrauma). That's why you equalize early, for example with the Valsalva maneuver (pinch your nose, gently blow against it or swallow). Into your mask, you exhale gently through your nose on descent, or it'll suction tight.
Your regulator delivers air at exactly the pressure surrounding you. Compressed air is denser - so you inhale more air molecules per breath even though your lung volume stays the same. At 20 meters (3 bar), the air is three times as dense, and you drain your tank roughly three times as fast. The deeper you go, the shorter your available bottom time - that's just physics, not bad luck.
As you ascend, pressure drops and the compressed air in your body expands again. If you hold your breath out of fright, the expanding air in your lungs has nowhere to go - and tears the delicate lung tissue. In the worst case, air bubbles enter your bloodstream and block vessels in the brain (arterial gas embolism). That's why "never hold your breath" is rule number one in every single dive course.
Breathe slowly and evenly, always - and never, under any circumstances, hold your breath. Surface in time, before your supply hits reserve, and ascend at a maximum of 10 meters per minute - about the speed of your own smallest exhaled bubbles. If you feel a sharp pain in your ear on the way down, stop immediately, go up a meter or two, and try equalizing again without force.
After a few dives, calculate your SAC rate (Surface Air Consumption, also called RMV) - how many liters of air you use per minute, normalized to the surface. Knowing this number lets you calculate exactly how much air you'll need for a planned depth before every dive. A relaxed, horizontal body position (trim) and deep belly breathing save even more air - shallow, rushed breathing wastes it.
### Why does my ear hurt when I descend? Because rising water pressure shrinks the volume of air in your middle ear, creating a vacuum that pulls on your eardrum. Equalize early enough - a gentle blow against your pinched nose usually does it - and the pressure disappears instantly. Wait too long, and equalizing gets harder and more painful.
### How much faster does my tank empty at 20 meters? About three times as fast as on the surface, because the air is three times as dense at 3 bar. That means: plan your bottom time at depth more conservatively, and keep an eye on your pressure gauge throughout the dive.
### What happens if I hold my breath while ascending? The compressed air in your lungs expands as you rise and has nowhere to escape. In the worst case, lung tissue tears and air bubbles enter your bloodstream - potentially fatal. That's why you always keep breathing, no matter how the situation feels.
### How fast am I allowed to ascend at maximum? No more than 10 meters per minute - roughly as slow as your own exhaled bubbles rise. That gives your body time to release excess nitrogen in a controlled way, and gives the air in your cavities time to expand gradually.
### Do I need to memorize the formula P1·V1=P2·V2 as a beginner? No, you won't need the formula itself underwater. What matters is the principle: pressure up, volume down - and vice versa. Once you understand why that's true, the right calls on equalizing, air consumption, and ascent speed follow naturally.
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