Can You Use a Rubber Bracelet as a Buoyancy Reference for Freediving?

[Bearing]

TL;DR Summary

How will the velocity of an object change as it rises to the surface of the ocean?

First of all, I'm new to the forum, and rather uneducated, so I apologize this is likely in the wrong section.

I've recently (like last night lol) started looking into freediving. I haven't taken any classes or anything yet, so all of this might be common and answered there. However, one of the safety guidelines sent me down a curious trail of wondering. You're supposed to limit your ascent to about 1m/s (I think is what I remember reading), which I believe generally scuba divers use their air bubbles as a reference for. Since you shouldn't breath out at all as a free diver, it made me curious if you could bring something to use as a reference with you. For example, if you wore one of those old school rubber bracelets that used to be super popular, and it was buoyant enough to rise to the surface at about that rate, you could release it when starting to ascend and follow it up. Whenever I first starting googling stuff I figured out that it will accelerate to some extent, but is it enough to make it totally unrealistic?
Anyway I hope all of that makes sense, if not I can try to clarify whatever.

 

[sophiecentaur]

Summary:: How will the velocity of an object change as it rises to the surface of the ocean?

looking into freediving.

With free diving you have a fixed amount of air in your lungs (unless you breathe out, of course) and that will be compressed (Boyle's Law etc) as your depth increases. In only a matter of five or six metres, that will compress enough to make your body neutrally buoyant. If you are free diving, at any depth below that you will have no buoyancy and the air in your lungs will continue to collapse so your buoyancy will be increasingly negative.

You have to swim upwards to get to neutral buoyancy again.
You could work out the approximate buoyant force, after you're neutral by knowing your lung capacity on the surface and nothing else in your body compresses significantly.

That's taken care of the buoyant force but your acceleration will depend on that force minus the drag force and that will depend on your body shape and attitude and the velocity. Having done a certain amount of snorkelling in the past, I can report that the increase in upward speed and pressure on my lungs were both very detectable near the surface but nonnumerical results I'm afraid.

Decompression sickness used to be thought of as a problem only for scuba diving (and others) but multiple free descents to great depths is now thought to be a potential safety issue. Google "Freediving accidents" if you want to be put off from the whole idea!

 

[Bearing]

Let me clarify something first. When I say free diving, I'm not talking about competitively forcing my body as far as I can into the crazy stupid depths people are doing now. I'm just interested as an extension of snorkeling, for places shallow enough it makes more sense than worrying about a whole bunch of scuba gear. I'm guessing depths less than 20 or 30 meters. I'm also brand new to all of it, so this is just ignorant curiosity and I plan on taking classes in both disciplines before actually doing anything.

I was just curious what it would take to take the guesswork out of it, if that makes sense. I know your buoyancy will change, but I assumed something without air that isn't changing like I mentioned would be consistent in buoyancy at least.

 

[hutchphd]

When I say free diving, I'm not talking about competitively forcing my body as far as I can into the crazy stupid depths people are doing now.

At any reasonable depth just free diving (with a snorkel) one need not worry about decompression sickness. You can pass out by improper hyperventilation: that is a thing to worry about.
This is in contrast to using SCUBA where you can easilly generate a lethal embolus in a few meters of water by holding a big lungful of air while you surface. And you can go deep enough on even one tank to suggest a decompression stop on the way up.

 

[Baluncore]

Human lungs and rib cage anatomy did not evolve for deep diving. The rib cage and lungs need to collapse while the stagnant air that was not breathed out is quickly absorbed into the blood stream. On surfacing the lungs must be able to partially reinflate from that dissolved air without tissue damage, or the bends, before breathing fresh air in. It seems that a whale is negatively buoyant while diving so it can descend, but it must expend energy to swim back up. I wonder how it senses which way is up from such a dark place?
https://scubadiverlife.com/top-five-deepest-diving-animals/

 

[berkeman]

At any reasonable depth just free diving (with a snorkel) one need not worry about decompression sickness. You can pass out by improper hyperventilation: that is a thing to worry about.

I agree with this. I've been free diving for 40+ years, and never had to worry about my ascent speed. I do tend to hyperventilate a bit before each surface dive, so indeed I have to be careful about when I turn around and head for the surface (especially when diving alone -- none of y'all do that, okay?).

I'm just interested as an extension of snorkeling, for places shallow enough it makes more sense than worrying about a whole bunch of scuba gear. I'm guessing depths less than 20 or 30 meters.

Unless you are an Olympic swimmer, you probably won't be diving much to 30 meters (plus there's not much to see down that deep). Most of my free diving was between 10-15m, and I'm a very good swimmer with good lung capacity. One key is to relax and slow down your body rhythms, to use as little oxygen as possible while you are down gliding around. I usually will get into a rhythm of a minute on the surface and a minute under water -- if I stay down longer, it takes me longer to recover on the surface.

It's best if you can find another person who is already an experienced skin diver (preferably a certified scuba diver), who can tutor you a bit in the subtleties of free diving. If you are going to be wearing a wet suit (what is the water temperature where you plan to dive?), you will need to also wear a weight belt with the appropriate ballast weight, and it's a good idea to wear a "buoyancy compensator" when you are wearing a weight belt. If you are free diving in warm water, all you need is your swim suit and mask/fins/snorkel (no wetsuit or the other stuff).

 

[berkeman]

One key is to relax and slow down your body rhythms, to use as little oxygen as possible while you are down gliding around. I usually will get into a rhythm of a minute on the surface and a minute under water -- if I stay down longer, it takes me longer to recover on the surface.

BTW, you know you are doing this right if you feel super relaxed and mellow when you get out of the water after free diving for a while. Your heart rate will likely be very low, and you will be fairly buzzed on endorphins. Wonderful feeling...

https://nextishawaii.com/best-snorkeling-in-maui/

 

[Bearing]

Sorry it's been a minute, been a crazy couple days. Honestly the op was more a theoretical manifestation of a curiosity, than actually trying to solve a problem I needed to figure out, you know? When I got the idea for a reference object, I realized I didn't even know if the object would accelerate or not as it rose. I've more so just been following that thought train in hopes of learning a little something if that makes sense.

 

[Bearing]

Human lungs and rib cage anatomy did not evolve for deep diving. The rib cage and lungs need to collapse while the stagnant air that was not breathed out is quickly absorbed into the blood stream. On surfacing the lungs must be able to partially reinflate from that dissolved air without tissue damage, or the bends, before breathing fresh air in. It seems that a whale is negatively buoyant while diving so it can descend, but it must expend energy to swim back up. I wonder how it senses which way is up from such a dark place?
https://scubadiverlife.com/top-five-deepest-diving-animals/

From what I found, it looks like it might have to do with the magnetic field. That last whale exhaling before diving was interesting. I mean, have you ever tried to hold your breath after exhaling without taking another breath? Much less for 2 hours!

 

[Baluncore]

From what I found, it looks like it might have to do with the magnetic field.

Many whales and seals have been tracked diving in the Southern Ocean. It does not depend on any magnetic field.

You have a sternum that ties your ribs together at the front, whales do not, so they have almost empty lungs when they dive.

Breathing is a habit we get into. It takes practice to calm yourself, so as to reduce the CO₂ production that triggers you breathing again.

 

[Bearing]

I agree with this. I've been free diving for 40+ years, and never had to worry about my ascent speed. I do tend to hyperventilate a bit before each surface dive, so indeed I have to be careful about when I turn around and head for the surface (especially when diving alone -- none of y'all do that, okay?).Unless you are an Olympic swimmer, you probably won't be diving much to 30 meters (plus there's not much to see down that deep). Most of my free diving was between 10-15m, and I'm a very good swimmer with good lung capacity. One key is to relax and slow down your body rhythms, to use as little oxygen as possible while you are down gliding around. I usually will get into a rhythm of a minute on the surface and a minute under water -- if I stay down longer, it takes me longer to recover on the surface.

It's best if you can find another person who is already an experienced skin diver (preferably a certified scuba diver), who can tutor you a bit in the subtleties of free diving. If you are going to be wearing a wet suit (what is the water temperature where you plan to dive?), you will need to also wear a weight belt with the appropriate ballast weight, and it's a good idea to wear a "buoyancy compensator" when you are wearing a weight belt. If you are free diving in warm water, all you need is your swim suit and mask/fins/snorkel (no wetsuit or the other stuff).

Yeah, 30 meters seemed crazy deep, but everything I was reading made it seem like nothing. All I care about is having some fun and seeing some cool dang.
In the 80s during the summer, gets down to about 50 or so in the winter. I've got a buddy that runs a dive shop, I've been planning on getting my scuba through anyway, so I'll probably also be able to find some good contacts throughout that as well.

 

[Bearing]

Many whales and seals have been tracked diving in the Southern Ocean. It does not depend on any magnetic field.

Huh, okay *shrug* I just skimmed a couple articles with some big words I didn't understand lol. Might do some more research on it later.

 

[sophiecentaur]

No apparent mention of 'Terminal Velocity' in this thread and surely it 's got to be one of the most relevant effects in this situation.

The OP asks how the velocity changes when rising through the water. If the volume of the object doesn't change then the buoyant force is constant. This means that, very soon on the upward journey of a buoyant object, the drag will become equal to the buoyant force and the velocity will approach a constant value or Terminal Velocity. Water is much more viscous than air so terminal velocity will always be pretty low (whether floating up or falling down). Compare this value of a metre or two per second for a diver with the terminal velocity of a freefalling loony / pharachutist which can be more than 80m/s. At terminal velocity, the resistance force is equal and opposite to the accelerating force.

 

[Bearing]

No apparent mention of 'Terminal Velocity' in this thread and surely it 's got to be one of the most relevant effects in this situation.

The OP asks how the velocity changes when rising through the water. If the volume of the object doesn't change then the buoyant force is constant. This means that, very soon on the upward journey of a buoyant object, the drag will become equal to the buoyant force and the velocity will approach a constant value or Terminal Velocity. Water is much more viscous than air so terminal velocity will always be pretty low (whether floating up or falling down). Compare this value of a metre or two per second for a diver with the terminal velocity of a freefalling loony / pharachutist which can be more than 80m/s. At terminal velocity, the resistance force is equal and opposite to the accelerating force.

Thank you! How would you calculate that for a given object? If that's pretty complicated, I would think it would be rather easy to just try it out and time an object coming up from different depths.

 

[berkeman]

I've got a buddy that runs a dive shop, I've been planning on getting my scuba through anyway

Great! Yeah, getting your scuba certification will make you a much more competent and comfortable free diver. Plus, you'll appreciate free diving more when you contrast it with all the gear you have to lug around and wear for scuba diving.

 

[Baluncore]

How would you calculate that for a given object?

That is something you must adjust by experiment since your mass, weightbelt, wetsuit and body muscle to fat ratio are not predictable.
To descend faster you carry more weight. To rise faster, use a bigger airbag. The airbag is open at the bottom so it will vent as it approaches the surface, which gives it a constant volume, buoyancy and rate of rise.

 

[sophiecentaur]

Escaping from a sunken submarine used to be done using an inverted bucket over the escapee’s head. They are placed in a chamber and water (from the sea outside) admitted into the chamber until the pressure is high enough to open the hatch at the top. He breathes the air in the bucket (regularly without holding breath) as the bucket lifts him to the surface. Air constantly spills from the bucket.
Exposure to high pressure is short and compression sickness is often avoided.
You have to keep breathing!
Escape chamber is pumped out ready for the next guy. Etc. etc..

 

[Bearing]

Great! Yeah, getting your scuba certification will make you a much more competent and comfortable free diver. Plus, you'll appreciate free diving more when you contrast it with all the gear you have to lug around and wear for scuba diving.

Especially if I were going out of town. I wouldn't want to pack all that crap and lug it across the country.

 

[Bearing]

That is something you must adjust by experiment since your mass, weightbelt, wetsuit and body muscle to fat ratio are not predictable.
To descend faster you carry more weight. To rise faster, use a bigger airbag. The airbag is open at the bottom so it will vent as it approaches the surface, which gives it a constant volume, buoyancy and rate of rise.

I meant calculating terminal velocity of an object rising to the surface. I was reading another thread before I posted and they were talking about a sphere rising from 300 meters. Post in thread 'Time it takes for submerged object to rise to surface' https://www.physicsforums.com/threa...object-to-rise-to-surface.627681/post-4031595
Made me curious what terminal velocity would be for it.

 

[Bearing]

Escaping from a sunken submarine used to be done using an inverted bucket over the escapee’s head. They are placed in a chamber and water (from the sea outside) admitted into the chamber until the pressure is high enough to open the hatch at the top. He breathes the air in the bucket (regularly without holding breath) as the bucket lifts him to the surface. Air constantly spills from the bucket.
Exposure to high pressure is short and compression sickness is often avoided.
You have to keep breathing!
Escape chamber is pumped out ready for the next guy. Etc. etc..

Thought submarines were supposed to sink...? :P Man I bet that woulda been a trip.

 

[Keith Koenig]

Summary:: How will the velocity of an object change as it rises to the surface of the ocean?

First of all, I'm new to the forum, and rather uneducated, so I apologize this is likely in the wrong section.

I've recently (like last night lol) started looking into freediving. I haven't taken any classes or anything yet, so all of this might be common and answered there. However, one of the safety guidelines sent me down a curious trail of wondering. You're supposed to limit your ascent to about 1m/s (I think is what I remember reading), which I believe generally scuba divers use their air bubbles as a reference for. Since you shouldn't breath out at all as a free diver, it made me curious if you could bring something to use as a reference with you. For example, if you wore one of those old school rubber bracelets that used to be super popular, and it was buoyant enough to rise to the surface at about that rate, you could release it when starting to ascend and follow it up. Whenever I first starting googling stuff I figured out that it will accelerate to some extent, but is it enough to make it totally unrealistic?
Anyway I hope all of that makes sense, if not I can try to clarify whatever.

To answer your question, and a very good one at that, just watch an air bubble rise. It quickly reaches a terminal velocity, where the force from friction equals the accelerative force. The same is true on the opposite extreme - free fall. Skydivers quickly reach terminal velocity, after which they stop accelerating. There probably is a perfect material to rise with, but what it is I cannot say. I suspect that if it was in common use you could easily google it, so ixnay on that idea. Great question though.
Regards.

 

[sophiecentaur]

Thought submarines were supposed to sink...? :P Man I bet that woulda been a trip.

But they don’t always come up again.

 

[Bearing]

To answer your question, and a very good one at that, just watch an air bubble rise. It quickly reaches a terminal velocity, where the force from friction equals the accelerative force. The same is true on the opposite extreme - free fall. Skydivers quickly reach terminal velocity, after which they stop accelerating. There probably is a perfect material to rise with, but what it is I cannot say. I suspect that if it was in common use you could easily google it, so ixnay on that idea. Great question though.
Regards.

Thank you, figured if there was a perfect sauce it'd be common practice. Just made me curious to learn something about the physics of it all.

 

[Bearing]

But they don’t always come up again.

Well that's an extra feature, only their deluxe models offer it