I’m watching an old Nova and they were talking about native divers who could stay down for five minutes at a depth of 80 feet. They claimed that at this depth, they no longer float, but could walk on the ocean bottom.
It seems to me that if anything, the water is denser at that depth so it should be more buoyant. I suppose the body is compressed some as well.
My thought was that the man they showed was so thin he would tend not to float at the surface either.
What’s the straight dope?
the density of water at 80 ft is not significantly different from the surface. For most buoyancy calculations the compressibility of water is not worth factoring in. The change in density due to temperature is more significant.
As for the free divers at 80 ft, their lungs and tissues compress reducing their buoyancy. It is a problem for scuba divers wearing wetsuits as well. At the surface the suits are relatively thick, at depth they compress and the buoyancy is significantly reduced. If one were to add air to their buoyancy compensator (ie life vest), that can cause a positive feedback problem during ascent. Better to work harder near the surface and carry only enough weight to be neutral at depth.
Yes, the significant distinction between liquid and gas is that liquid is essentially not compressible. It retains pretty much the same volume no matter what pressure it’s under; whereas gas, by Boyle’s law, volume is inversely proportional to pressure all other things being equal.
At 80 feet deep, the pressure is approximately 2 and a half atmospheres higher than sea level. So the air in the divers’ lungs compresses to a much smaller volume, but is still only buoyant by the volume of water it displaces, which is now smaller.
So a cubic foot of air, for example - at sea level, is under about 14.7psi; at 80 feet, (80/33)x14.7 = 35.6 psi more. (total 35.6+14.7=50.3psi) 1x14.7=Vx50.3, V=0.29cubic feet.
Since water an any depth weighs about the same per unit volume, that means whatever internal buoyancy the diver is experiencing on the surface, is only .29 as much at 80 feet.
I think.
Just to complete the story, water is almost 1000 times more dense than air at sea level.
Buoyancy is the the mass of water displaced minus the mass of whatever is displacing it. Since the mass of the air is negligible relative to water, all that matters here is the volume of the air that is displacing water, as described above.
I don’t really see that the compression of the air in the divers lungs matters too much. The diver clearly weighs the same so all that matters is how much his body is compress. It’s certainly not compressed to less than 30% of its volume at the surface.
The water will be a bit denser at depth than at the surface not due so much to the weight of the water above, but due to a difference in temperature. Colder water is denser down to about 4 C.
Yes, water is not penetrating inside his body, so it’s a question of how much his body overall compresses, i.e. how much the volume of water displaced by his body as a whole changes. But that overall compression is essentially the air within the tissues and within the body cavity and the lungs compressing. Feel what happens to your chest when you breathe in and out. The ribcage is not a stiff immobile shell, it allows for a lot of expansion and contraction according to the volume held within the lungs.
A human body, especially that of a lean, muscular person, is not buoyant except by virtue of the air contained in the lungs - it’s only the lung volume that needs to be reduced (either by compression or exhaling) in order to tip the balance and allow the person to sink.
There isn’t any way the diver’s body can prevent the compression of air in the lungs.
At 80 feet under water, the pressure is about 35 PSI. if I’ve done the math right, that means the air in the diver’s lungs is now about 40% of its original volume.
ETA: and if there’s any doubt that pressure can reduce the volume of gas in an object so as to make it sink, it’s easy to prove experimentally: Pressure Diver Fish - Atomic Shrimp
This is correct. Your math is spot on based on the chart on page 11 of this PDF document. The only thing wrong here is that 80 feet is roughly 3.5 atm, not 2.5 atm. I recently mis-stated the numbers on this in another thread based on what I think was a simplistic PADI rule that pressure doubles and volume halves every 10 meters. It’s also possible that I just recalled my coursework wrong.
The mass of the body is not compressed to 30% at 80 feet but the gas contained in the body is. Volume of gas in the body has an enormous affect on buoyancy.
Also stated by rbroome in the first response:
This is mostly correct, but a BC is not a life vest. It’s an inflatable jacket used to compensate for buoyancy. This means you can empty air from it during ascent. I would also add that your tank becomes more positively buoyant as you empty it over the course of a dive, so you do need to be slightly more weighted than neutral at depth when starting a dive.
Which brings me to my final point that scuba and free diving are very different in this conversation because a scuba regulator supplies gas based on ambient pressure so when scuba diving at 80 feet, your lungs are at normal capacity and not 30% as they would be while free diving. This is why holding your breath while ascending as a scuba diver can be fatal. It’s also why you can get the bends while scuba diving and not while free diving.
Anyway, I’m a recreational scuba diver and far from an expert on any of this. As I mentioned, I got some things wrong the last time I participated in a similar thread, but I learned from it and I learned from this one. I’m not trying to be a know it all and I hope none of this comes across as condescending. If I’m mistaken on anything, I welcome correction.
Buoyancy of a human body can be strange. I can easily float on my back, barely moving at all and even in a freshwater pool. My wife can’t do it, even in the Mediterranean, although she managed it in The Dead Sea (no great feat really). I do have to take a deep breath and keep my lungs full though, or I will sink.
Adult human male lung volume is about 6 liters, which provides about 6 kilograms (13.2 pounds) of buoyancy in seawater. So if your lungs are fully inflated at sea level and you then descend to 80 feet, you’ve lost about 9.2 pounds of buoyancy. That’s easily enough to make the difference between floating and sinking.
Note also that it’s not just gas in the lungs that gets compressed. Every gas volume you have in your body compresses just as much: the space in your middle ears, your sinuses, your mouth, your throat, and every pocket of gas in your GI tract. So the loss of buoyancy at 80 feet is likely somewhat more than 9.2 pounds.
Been years since I went scooby-diving, but as I recall, the general idea is to be close to neutrally buoyant at whatever depth you’re at. Part of your job during ascents is to manage your buoyancy, releasing air from your buoyancy compensator as needed to maintain a safe ascent rate. Obviously if you’re fiddling with it a lot throughout the dive then you end up wasting air, but the opposite extreme requires you to maintain depth either by kicking a lot (which is aerobic activity that requires additional breathing), or by trying to keep your lungs mostly full or mostly empty (which gets uncomfortable after a while).
Yes.
Assuming uniform seawater, back-of-the-envelope calculations suggest that the buoyancy difference due to density for a human body would be on the order of a few grams.
But water is rarely uniform with depth, and (as noted) changes in temperature and salinity could have a much bigger effect.
It doesn’t double every 10 meters. It goes up by 1 atmosphere every 10 meters. So it’s 1 atm at the surface, 2 atm at 10 meters deep, 3 atm at 20 meters, 4 atm at 30 meters, and so on. Gas volume is the reciprocal of that, so at 10 meters, volume is half of the surface, at 20 meters it’s a third, at 30 meters it’s a quarter, and so on.
The effect of temperature is small, at least as regards water density. The thermal expansion coefficient for liquid water is 0.0214% per degree C; drop the water temperature by 20C, and its density increases by 0.43 percent, increasing buoyancy by as much.
Note however that if you’re utilizing external sources of buoyancy such as a wetsuit and buoyancy-compensating vest, the air in those devices will cool and thus shrink by a much larger percentage; the combined effect on buoyancy by transitioning from warm surface water to cold water at depth may actually be negative.
At recreational SCUBA-dive depths (up to ~33m), ocean salinity is pretty constant. The free-dive depth record is 245 meters (and the SCUBA depth record is only a little deeper); the chart in the previous link suggests salinity is pretty constant even to that depth. Basically you (as a diver) won’t encounter any major changes in ocean salinity (and therefore negligible changes in seawater density).
This is pretty easy to demonstrate in a swimming pool. Take a large breath, and you’ll float. Exhale as much as possible, and you’ll sink right down to the bottom and settle.
I understood that every 10 meters increased one atmosphere. I misunderstood the affect on volume, probably my own fault. Whatever the cause of my misunderstanding, the part of your post that I highlighted is very intuitive to me and I didn’t even see the relationship by looking at the chart I linked. I appreciate you explaining it in those terms. Thank you.
This only applies to some people. Others can’t float at all, others will still float after exhaling. I’m one of the latter. bob++ mentions upthread his wife is one of the former.
It works for the vast majority.
I buy that some people float even after exhaling. Fat is less dense than water, so some combination of more fat, smaller bones, less muscle, etc. will make you positively buoyant even with lungs as empty as you can get them.
Are there really people who will actually sink in water with a lungful of air? Maybe if they have, say, lots of metal or other dense materials surgically implanted. Even thin muscular bodies just aren’t that dense. Maybe they have a very small lung capacity? Anyone have a cite for this, as I am unconvinced.
bob++'s account of his wife isn’t necessarily such a cite. You might be positively buoyant, but unable to easily float on your back due to weight distribution. Probably, such a person still does float, they just float in an upright position with their head and nose below the waterline. Which I will admit is not a very useful skill for a human, but which is still positively buoyant.
I’m not certain that most people can actually sink to the bottom by exhaling, either. I suspect that for most, after they exhale it’s just a matter of not floating high enough to breathe, like you describe. Though there are certainly some highly-muscular people who can do it.
Interesting.
I’ve never been particularly muscular, and I’ve always been able to sink to the bottom of a pool by just exhaling.
I’m not claiming that’s proof of much, just that it would surprise me if most people couldn’t do that.
Searching around for human body densityI found this study (pdf) done by the military in the 1960s of males from age 17 to 69, with a variety of body fat percentages, and every individual had body density higher than that of water. None of the individuals studied were very large (body weight topped out around 200 lbs for the heaviest), so that might not hold for obese individuals (the relative density of bone will have less of an impact, and that of fat will have more).
Also might not hold for women, who tend to have higher body fat percentages. On the other hand, the guys had body fat percentages up to 40%, which is not exactly rail thin.
I don’t think you need to be particularly muscular - just low in body fat. If you’re healthy, then your bones are much more dense than water, and skeletal muscle is about as dense as water. Basically, if you don’t have a lot of fat on you, then your bones will readily drag you down if you exhale.
Since most Americans are carrying excess fat, they may indeed be likely to float even if they exhale - but I don’t think it’s related to how much muscle they have or don’t have.
