Learning to scuba dive is fun, but it does require a little bit of math.
Don't let the basic physics of understanding atmospheres underwater confuse you, it's simple and easy!
When teaching beginner divers, it's common to see a few blank stares when talking about atmospheres and how the relationship between pressure, density and volume changes the deeper you dive. It's very simple to remember. The deeper you dive, pressure and density increases and volume decreases.
This relationship is consistent as you diver deeper and the quantities change every 33' of depth, because every 33' of depth puts you in a higher atmosphere (ATM).
- At 33' you're at 2 ATM
- At 66' you're at 3 ATM
- At 99' you're at 4 ATM
- And finally at 132' you're at 5 ATM – the deepest you're trained to dive as a recreational diver who holds a PADI Advanced Open Water Certification.
Watch the 3 minute animation below to learn more about understanding atmospheres while scuba diving.
Think about it. Your lungs are balloons.
When you breathe in, your lungs inflate, and when you breath out, your lungs deflate.
So what would happen if you held your breath while scuba diving?
If you held your breath while descending, your lungs will become smaller – creating dense pressure inside your full lungs. If you held your breath as you ascended, the air in your lungs would increase in size under pressure. In either scenario, eventually your lungs would fail. This is why the number 1 rule of scuba diving is NEVER HOLD YOUR BREATH!
So, you might be thinking – why does pressure change underwater? It’s simple. The deeper you dive, the higher atmosphere you’re in. When diving into a higher atmosphere, the volume of air spaces will get smaller, and the molecules in those spaces will become more compact.
The term atmosphere is simply a unit to measure ambient pressure. Since water is denser than air, greater changes in ambient pressure occur underwater. Every 33 feet of depth adds another atmosphere to the ambient pressure. Once you understand atmospheres you can calculate your own air consumption at a given pressure, helping you plan your dives efficiently and accurately.
As a recreational scuba diver, you are trained to dive safely between 1 and 5 atmospheres. At sea level you start your dive at 1 and as you descend deeper underwater, the pressure and density increases – and the volume decreases.
This relationship between pressure, density and volume stays consistent as you dive. At 33 feet deep you’re at 2 atmospheres. At this depth, volume is 1/2 of what it was on the surface and the density is 2 times denser.
This relationship continues as you dive deeper – all the way down to 132 feet where at 5 atmospheres the volume is 1/5 of what it was on the surface and the density is 5 times denser.
Make sense? Think about it this way. If you had a balloon at the surface with a volume of 10 units, taking it down to 5 atmospheres, would reduce the balloon’s volume to 2 units – because at 132 feet, the volume of an object is 1/5 of what it was on the surface.