Zooming in on Boyle's law and dancing oxygen molecules
Just as scuba divers, freedivers need to take some extra precautions when returning from the depth of the big blue. But, we use very different safety procedures.
The reasons to these procedures derive form the same chemical and physical laws, since all divers need to consider and are affected by both Boyle's (gas pressure & volume) and Henry's law (gas dissolved in liquid).
Since freedivers hold their breath during a dive we don't do safety stops as scuba divers, but we plan our dives and are relying on dive buddies to be safe.
Let's zoom in on mister Boyle
- Boyle's law: the pressure and volume of a gas have an inverse relationship
For a descending freediver the increase of pressure surrounding us means that our lungs are compressed, already at 10 m/2 bar our lungs have half of the volume! See the picture below:
Picture 1 - Boyle's law described (source: Molchanovs.com )
This means that we are, if weighted correctly, neutral at 10-12 m and negatively buoyant after approximately 15 m. depth. After 15 m we usually start falling down without having to use force to go deeper, this part is called the "free fall".
It doesn't matter if you dive to 20 m or 100 m, we work our way down to 15 m using fins, kicks and/or breaststrokes and then we fall (or fly!) down by the line that ends at the bottom plate at our designated depth.
The mental game
After being negatively buoyant and free falling for most of your descent, your ascent however means hard work. The same physical law that "helped" us to go down now means that we're negatively buoyant all the way up to 10-12 m again.
This is where the mental game begins! Since, you don’t "want to" ascend, and you definitely shouldn’t look up towards the surface since it risks over stretching your trachea (air-tube that connects your airways in the throat to your lungs), since that’s pretty much deflated due to the surrounding pressure.
The only thing you want and should do is to kick with your fins or feet, may be counting your kicks - but, without following that thought with “how many more?”
Every thought created, specially the negative ones, consumes oxygen. The only thing you have and are is your present - that kick or stroke that’s slowly taking you somewhere to a “next” present.
Oxygen are the dancers and your lungs the dance floor
Just before reaching the surface your body will transform from having the same density as the water surrounding you, to become buoyant again since your lung volume is doubled (back to normal), i.e. going from 2 bar (10 m) to 1 bar (0 m and surface).
That’s when you know if you dove too deep, if you consumed too much oxygen during the dive making your oxygen saturation too low to return to where you first started.
What does that mean?
Back to Boyle again - imagine your lungs being a dance floor and the dancers oxygen molecules: at depth the dance floor (lungs) have a smaller volume that gives the dancers (oxygen) less space, making it feel crowded and groovy. However, near the surface when the pressure is decreasing that area (lungs) increases in volume and gives the dancers (oxygen) more space - making the dance floor somewhat empty and un-groovy.
Just as a bad song can cause an empty floor and take the away the groove, a badly planned dive can take away your possibility of reaching the surface since the amount of oxygen is too low to continue the dancing, I mean diving!
That's why one of the most basic and important security procedures in freediving is to have a dive buddy meeting you when ascending at approximately 10-15 m, and following you all the way up.
Your dive buddy can be seen as a guardian angel that has a similar capacity as you, and someone who knows your capacities and limitations. Also, a someone who has a camera or not to immortalise your strained face!
Finally, when breaking the surface you get the highest reward there is - to breathe fresh air, and to see all of your buoy hanging buddies again of course!
PC: Damir Zurub
To dive even deeper into what it's like to freedive, watch the inspiring and informative Ted Talk by world record holder William Trubridge from 2018 - "This is why i freedive"