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Summary

There are a few major freedive certification agencies:

• PFI (Performance Freedivers International)
• FII (Freediving Instructors International)
• PADI
• AIDA
• Molchanovs

For whatever reason, partial pressures are something that just did not click with me the first, second…or third time I learned it. I understood why they are important as well as the impact they have on the necessary skills for freediving, but the way they affect us just didn’t make sense to me.

So I did some re-reading, some extracurricular research, and some soul searching to understand it. What follows are the components of partial pressures as I understand them. Hopefully this helps them make sense for you.

What are Partial Pressures?

Let’s start with some really basic principles about air and gases. Partial pressures are useful to understand near sea-level, high altitudes, and at depth for freediving.

The air you breathe is made of many gases

Generally speaking air is made of the following gases.

For the purposes of this article, we don’t care about the other trace gases.

These percentages remain the same at different pressures. But the mass of each gas changes for the same volume. This is what Boyle’s Law refers to.

Meaning, at sea-level one liter of air contains the same percentage of oxygen as one liter of air would contain at the top of Mount Everest. In fact, at the top of Everest, there is 1/3 the amount of oxygen in one liter than in the same volume at sea level.

This is why it’s harder to breathe at high altitudes. Your lungs hold the same volume of air, but there is less oxygen (and other gases) within that same volume.

< IMAGE HERE TO SHOW DIFFERENCE IN VOLUME >

Each gas in air has a different measurement of pressure.

This section to be filled out

How does Oxygen get into our blood?

Lungs are filled with little sacks called alveoli. These sacks of air have capilaries in their membranes. When blood passes through these capilaries, O₂ is absobed into that blood. This absorption requires pressure. I like to think of the pressure in our lungs pushing O₂ into our blood.

The amount of O₂ that gets pushed into our blood is determined by the partial pressure of O₂ in our lungs.

…Make sure you read that a few times. Because this is the piece of the picture that I couldn’t wrap my head around. This means that if there is more pressure in your lungs, then your blood will absorb O₂ at a faster rate. This is referred to as Henry’s Law.

Why does this matter for freediving?

Before I get to the answer, let’s revisit the basic principles:

1. The deeper we travel underwater, the more pressure our bodies experience.
2. As air pressure increases, three things happen to the air inside our lungs:
   • The volume of air decreases (Boyle’s Law).
   • The mass of gases in that our lungs remain the same.
   • The partial pressures of the gases increase (Dalton’s Law).
3. The rate at which gases are absorbed into blood is determined by the partial pressure of oxygen in our lungs (Henry’s Law).

When applied to freediving, this means…

1. The deeper you go, the faster your blood pulls oxygen out of your lungs.

This is probably the easiest to understand. At depth, you’re lungs are pushing more O₂ into your blood than on the surface. If too much of the O₂ in your lungs was pushed into your blood at depth, then you might not have enough O₂ in your lungs as you get closer to the surface.

2. The partial pressure in your lungs decreases as you ascend.

This is a major contributor to shallow water LMC’s and blackouts. As you get within 10 meters of the surface on your ascent, there is less O₂ in your lungs, but there is also less pressure to push the remaining O₂ into your blood. If you used up too much O₂ at depth, the partial-pressure of O₂ might be too weak to push that remaining O₂ into your blood.

3. Nitrogen Narcosis (aka The Bends)

This is more common in scuba diving, but it also affects freedivers who frequently dive below 25 meters. Nitrogen also gets absorbed from our lungs into our blood. As we ascend, the partial pressure of that gas in our lungs also decreases.

If freedivers continually dive and don’t take long enough surface intervals, the nitrogen in our blood can remain and expand, which causes narcosis when those bubbles travel to our brain.

Location: Lake Washington off of Mercer Island

Noteworthy Conditions: Water temps in low 50s, poor visibility.

What was the plan for the dive?

There were 4 divers involved:

1. Diver A - This person was doing a dive to 36 meters FIM.
2. Diver B (author) - This person was following Diver A with the intention of filming their entire dive. Diver B’s target depth was 30 meters with the intention of filming the Diver A’s bottom 5 meters from there, and then following Diver A to the surface. Diver B was using bifins as propulsion. Note: Since Diver B is the author, I may switch between referring to them as Diver B and first-person pronouns.
3. Safety A - Safety for Diver A. Meet Diver A at 15 meters.
4. Safety B - Safety for Diver B. Meet Diver B at 15 meters.

This is considered an easy dive for all the divers involved. We have all done many dives to similar or deeper depths, in the same conditions, and in the same location. The intention of filming was to document the dive for a skills evaluation.

It’s also worth noting that the visbility in this lake is pretty poor, and it gets pitch black at around 20 meters. We always have a light on the bottom weight, and Diver B was carrying a flashlight to keep Diver A lit up for the camera.

What happened?

Diver A made it to their target depth. Diver B followed Diver A and stopped at around 28 meters. While waiting for Diver A to come back up, Diver B felt anxious and decided to start their ascent. On the way up, Diver B started to feel tired and more anxious.

Diver B met Safety B at 15 meters, and immediately signaled for help. The signal was a wave, but more wrist rotation than a side-to-side wave. Safety B continued swimming up with Diver B, but did not help Diver B as requested.

Thinking there was a miscommunication in sybmols, Diver B started using a more side-to-side and erratic wave. Safety B still continued swimming but did not help Diver B.

Getting more nervous, Diver B started shaking his head, and waving more frantically. After a second or two of this, Safety B grabbed Diver B and helped them to the surface.

It’s hard to know exactly how long Diver B was signalling to Safety B before Safety B started their assist, but it was clearly long enough to be a problem for Diver B. It’s fair to say it was at least 5 seconds, and at most 10 seconds.

At the surface, Safety B held Diver B up high out of the water and kept their air ways overtly clear of the water. Diver B had a light LMC and tunnel vision, but did not black out. After a round of recovery breaths, Diver B gave the okay signal and was considered good.

What went wrong?

It’s easy to understand that Safety B did not react quickly enough, but I think there are a few other things that went wrong with this dive that put Diver B in the position to need help in the first place. I’ll break down each one.

Why didn’t Safety B help at the first signal?

Safety B said they saw the signal, but thought it meant something different. They thought Diver B was trying to tell them to move out of frame for the video. Only after Diver B gave a more sustained and frantic signal did Safety B realize they were asking for help.

It’s also worth noting that this was Safety B’s first “real” save. Since they’ve never been in a scenario where they’ve actually needed to help someone, it was a bit of a shock for them to realize what was actually happening.

Why did Diver B need help?

I’d like to make it clear that 30 meters is considered a relatively easy dive for Diver B. I’ve hit greaters depths many times prior at the same location, and even did a successful 33 meter dive during the same morning prior to this dive.

So what went wrong? A few things:

Diver B is not familar with camera work.

This is textbook task loading. During the breathe-up I was focused on filming Diver A. In retrospect, I definitely did not relax during the breathe-up the same way I would have if not being the camera operator.

During the dive, I was also constantly making sure I had Diver A in frame, and that my flashlight was shining on them. This is a lot more work than I am used to doing for dives this deep.

Diver B was already tired

This was not my first dive of the day to this depth. I did a 33m dive 15 minutes prior to this dive, and felt my legs burning much of the way up.

This does not usually happen for me during 30-35 meter dives. This should have been an indicator that I should have taken the rest of the day easy and not targeted deep dives, let alone film them.

Rushed Implementation

This one is a bit harder to explain, but there was a bit of a rushed feel. Once we determined our roles and what we were going to do, we just sort of did it. We didn’t do a practice run, and didn’t take it very slowly.

What did we do right?

Despite the things that went wrong, there were a few decisions we made in the planning process that made it so things didn’t go worse.

1. It was a great idea to make sure both divers had a safety. We talked before the dive wondering if it was necessary, or if 4 divers on the same line would be too crowded. I am very glad we made the decision to have one safety for each diver.
2. Since I was feeling tired, I took my ascent very slow and continued the same slow pace the entire time I was with the safety. Even though my safety did not react immediately, I did not panic, and made a very concious effort to stay calm. Once the safety started their assist, I relaxed completely and stopped kicking. This probably prevented me from having a blackout.
3. Except for an immediate response, Safety B did everything else correctly. They met me at the right depth, they assisted me correctly - once they realized what was happening, and they kept me elevated out of the water once at the surface.

What are the lessons learned?

There are a few takeaways from this to make sure the same mistakes don’t happen again.

1. Any signal should be presumed as a bad signal! This is the obvious one. There is no reason for a diver to communicate anything except “I need help!” Even if I was telling my safety to move out of frame, they should make the assumption that I need help. A good camera shot is not worth the risk of blacking out. Hawaii HammaHs has a great video of a very similar incident and comes up with the same rule. I understand this rule doesn’t apply for competition dives as a safety touching a diver can ruin a white-card…but this was not a competition dive.
2. Plan your dive and dive your plan. Borrowing this one from the scuba world. This was a complicated dive. If we didn’t plan ahead, it could have been worse.
3. Practice Complicated Scenarios First. Even though we all knew the plan, it would have been a good idea to have done a practice run of the same dive but at shallower depths. That way everyone is comfortable with their roles and we have a better idea of what to expect when doing it deeper. Ideally, this would have helped me realize that I am not relaxed when filming.
4. Practice safety drills more often. There was a unanimous agreement after this incident that we should run through safety drills more often.
5. Be concious of your limitations. I should have known after my first dive of the day that I was not in the best shape to do more 30 meter dives. I also should have practiced filming dives more often and at shallower depths so that those skills are refined, and come naturally.

So…uh…how’s the footage?

Here’s the video recorded by Diver B. It was not my camera, and isn’t hosted on my channel. https://www.youtube.com/watch?v=EC35LUD2roY

You can hear me in discomfort at 28 meters before turning around. Unfortunately the incident with the safety was not caught on camera. Thankfully, it still qualifies Diver A for the skill evaluation they were shooting for!

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