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Comment by plq | Hacker News Reader

Actually, you can "fall home" in the ocean. I have no idea what procedures exist in missions that go to extreme depths like these, but if you are a recreational diver (who is not allowed to go below 30m) and you have an emergency down there, the last resort is to get rid of all your additional weights* as fast as possible so that you get pushed to the surface with zero effort.

Of course, you need to deal with fast changes in pressure during the short journey towards the surface (protect your eardrums, lungs etc) and you may get decompression sickness depending on the dive time and avg depth at the moment of emergency, but hey, at least you're alive to deal with all those problems!

*: Generally, a scuba diver takes additional weights to counter the positive buoyancy effect of wetsuits, fat etc. These are mounted on belts or similar accessories that are designed to be very easy to get rid of in case of emergency.

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> so that you get pushed to the surface with zero effort

This is not always true.

Your buoyancy decreases as you go deeper (and have more water above you, causing more downward pressure).

At a depth of 30 meters (100 feet), the pressure is 4 atmospheres, 4 times that of sea level.

Furthermore, not everyone is positively buoyant. The less fat one has, especially if not wearing diving suit, the more likely staying afloat/level to become harder.

As the depth increases, lungs and all bodily cavities shrink, further adding to one's density.

So, at 30 meters, it is actually quite a struggle to swim back up, even more so if you've lost your fins.

As for weights, I used to work out and exercise daily. During that time, I also dived few times a week spanning various sea conditions throughout the year and never have once taken weights as I was already negative.

Fast forward 2 years to today, I put on 15kg (33 lbs) and I need 10kg weight to enjoy my dive. But on the upside, I'd probably skyrocket to the surface if needed, lol :D

>(and have more water above you, causing more downward pressure).

But also higher pressure pushing you upwards too! Buoyant force is equal to weight of displaced fluid, and so is independent of depth. (edit: given same fluid and temperature) (edit2: and well, buoyant force will be slightly stronger because fluid will be more compressed at the bottom)

No, higher pressure does not push you upwards. That's all about buoyancy and a liter of water weighs just as much at the bottom of the ocean as it does at the surface. That higher pressure works on all sides of your body which means it cancels out.

I reckon that water under high pressure does have a somewhat higher density. However water is pretty much the textbook example of an incompressible fluid so the difference will probably be rather small.

Edit: Found a direct quote on wikipedia https://en.wikipedia.org/wiki/Properties_of_water#Compressib...: >The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.[40]

Water is tremendously stable, cheap and commonly available, that's exactly why it - and nowadays oil - was used since time immemorial as a working fluid to transfer force from one place to another. In essence the limiting factor is the burst strength of the tubing.

Yes, I just wanted to correct OP statement that "and have more water above you, causing more downward pressure" by pointing that increasing pressure exerts force on bottom side, so in effect it cancels out increasing pressure pushing at top, and so buoyant force is constant. (with added trivia about minuscule changes due to fluid compressibility)

Hmm, I think have gotten my physics backwards with regards to the correlation of more pressure to more downward force.

Thank you for correcting this.

Water density increases with depth. Though only 4.96% at the bottom of the Mariana Trench.

Near the surface it’s almost never an issue, but for ultra deep dives it’s important.

> Your buoyancy decreases as you go deeper (and have more water above you, causing more downward pressure).

This statement is wrong on many levels. I'll try to correct just one part:

Your buoyancy must NOT decrease as you go deeper. That's one of the fundamental rules you need to follow if you want to dive more than once :) If you don't watch your depth and take necessary steps to maintain neutral buoyancy, you'd sink like a stone (or rise like a balloon). Nobody wants that.

What you mean is: you need more air in you buoyancy compensator (BC) to maintain neutral buoyancy (= keep your volume constant) as you go deeper.

If you don't take weights, you need to inflate your bc to lift you. In an emergency, you generally don't have time to swim up anyway.

All that said, I was just trying to point out that it was possible to "fall home" from underwater, assuming you are within the limits of your equipment.

The difference is critical.

A buoyancy compensator, filled with a fixed quantity (not volume) of air, does decrease in volume as you descend and the pressure increases. You'd sink faster and faster if you did not add air as required. Therefore, you must actively manage a BCD!

On the way up, the opposite happens. The quantity of air in your buoyancy compensator has an increasing volume of air as you rise and the pressure decreases, providing you with increasing buoyancy. Uncontrolled, this would rocket you towards the surface. Therefore, you must actively manage a BCD!

Lungs do not change in volume in SCUBA as they do when free (breath hold) diving, because your regulator is allowing you to constantly breathe air which is at the same pressure as the water around you.

When free diving, you carry weights to be neutrally buoyant at around 30 feet. Above the neutral buoyancy depth, you float towards the surface, and must swim down to descend. Below the neutral buoyancy depth, you become negatively buoyant, sink increasingly quickly, and must swim up to ascend. This is also important to be aware of; you don't want to strap weights to yourself that you can't drop: You may think you're capable of swimming up against your negative buoyancy, but if you only test at 10 feet under you may not be capable when you get deeper!

> What you mean is: you need more air in you buoyancy compensator (BC) to maintain neutral buoyancy (= keep your volume constant) as you go deeper.

I did indeed neglect to explain what needs to happen. Thank you for clarifying. Let's all hope to enjoy many more dives :)

> Furthermore, not everyone is positively buoyant. The less fat one has, especially if not wearing diving suit, the more likely staying afloat/level to become harder.

I'm not sure if it is just me, but I could not understand this at all. Do you mind rephrasing it? Which type of person has a harder time stating afloat?

Higher body fat makes you float more easily. I think.

Fat has a specific gravity < 1.0, floating in water.

Bone + Muscle have specific gravities > 1.0.

Generally, the more fat on a person, the greater that person's buoyancy.

Muscle is denser than fat per volume.

Fat can naturally float on water due to its density being less than that of water.

The more fat one has, the less overall average density becomes, making it harder to sink. I have a close friend who is very overwheight and she can practically sit upright floating. I must admit I'd usually envy her when I struggled staying afloat while she enjoye a cool refreshment like it were her couch :)

So in short, less fat, more muscle mass, more bone mass harder to stay afloat.

Also the diving suite (which is made from a foam-like material) and some of the hallow diving gear make it harder to sink, hence the need for weights.

At the depths we're talking about that would not be possible: "Decompression from these depths takes approximately one day per 100 feet of seawater plus a day. A dive to 650 feet would take approximately eight days of decompression."

This is why saturation divers spend significant amounts of time in a pressure vessel after a period of diving to decompress.

The article is very light on technicalities but as far as I can tell, this expedition is WAY outside the sat territory. Sat divers do perform "seawalk"s during their missions, so they do need to perform the necessary steps to adjust to pressure changes.

This can't be the case here, they were inside their submarines the whole time.

ps: 100 feet = ~30m, 650 feet=~200m

Good point. You could maintain a more reasonable pressure inside a submarine.

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