# Buoyancy, incompressibility and G forces

• Tiran
That is not correct. Divers do not have neutral buoyancy: their buoyancy decreases with depth, as their lungs get compressed.The human body is not uniform in density, so when you apply a high g-force, the various parts of the body will try to re-arrange themselves according to the differences in density. I don't know what would kill the person, but my suspicion is the chest cavity would collapse due to the weight of the ribcage. Due to the difference in density, either the ribcage is insufficiently supported or the lungs are overpressurized

#### Tiran

Based on the Comex Hydra 10 tests, a diver breathing the correct mix can withstand a depth of 2300 ft, or the pressure of a column of water 2300 feet tall - nearly 1000 psi.

Since the diver is buoyant (being the same average density as the water), they are essentially weightless. Not in freefall, but weightless.

Given neutral buoyancy and a "preload" of 1000 psi, is there any reason a person couldn't endure the G force acceleration equivalent of the weight of that 2300 foot water column? In other words, if you put a pilot/astronaut in small vat that is under that high pressure, and then subject that vat to 67 Gs of sustained acceleration, would the pilot even feel it? (1000 psi / 14.7 psi = 67) I understand that the life support system may need to adjust the pressure as G load increases because the water above the pilot will get heavier.

First, does that make any sense? And am I applying the right basic formula in making High G equivalent to high pressure at 14.7 psi = 1G?

Thanks!

Tiran said:
is there any reason a person couldn't endure the G force acceleration equivalent of the weight of that 2300 foot water column?
There is no such thing. A pressure is not equivalent to an acceleration.

67 g can lead to serious damage to internal organs. If a human would have a constant density everywhere we would tolerate extremely high accelerations when suspended in water, but we do not. Increasing the pressure of that water vessel doesn't help, either.

russ_watters
mfb said:
There is no such thing. A pressure is not equivalent to an acceleration.

67 g can lead to serious damage to internal organs. If a human would have a constant density everywhere we would tolerate extremely high accelerations when suspended in water, but we do not. Increasing the pressure of that water vessel doesn't help, either.
The point of increased pressure is to internally support the lungs and other air pockets in the body from being crushed.

I realize that pressure and acceleration are different things, but if you follow why G forces damage flesh you would understand why I'm connecting the two.

When you say we do not tolerate high accelerations when suspended in water, what are you referencing? It's the same basic concept as a G-suit. Are you saying this has been tested, or are you speculating that minor difference in tissue density are enough to cause internal dislocation despite the external support? The current record for G forces sustained was 46 Gs for several seconds with no support.

Tiran said:
Since the diver is buoyant (being the same average density as the water), they are essentially weightless. Not in freefall, but weightless.

Given neutral buoyancy...
That is not correct. Divers do not have neutral buoyancy: their buoyancy decreases with depth, as their lungs get compressed.
Tiran said:
The point of increased pressure is to internally support the lungs and other air pockets in the body from being crushed.

I realize that pressure and acceleration are different things, but if you follow why G forces damage flesh you would understand why I'm connecting the two.

When you say we do not tolerate high accelerations when suspended in water, what are you referencing?
The human body is not uniform in density, so when you apply a high g-force, the various parts of the body will try to re-arrange themselves according to the differences in density. I don't know what would kill the person, but my suspicion is the chest cavity would collapse due to the weight of the ribcage. Due to the difference in density, either the ribcage is insufficiently supported or the lungs are overpressurized.

Tiran said:
The point of increased pressure is to internally support the lungs and other air pockets in the body from being crushed.
You don't support anything with a uniform pressure increase.
Tiran said:
When you say we do not tolerate high accelerations when suspended in water, what are you referencing?
I don't say that. We tolerate higher g-forces, but not as high as we would tolerate if we would have a uniform density matching that of water everywhere.

Every density difference inside the body leads to internal forces, stronger accelerations lead to larger forces.

russ_watters said:
That is not correct. Divers do not have neutral buoyancy: their buoyancy decreases with depth, as their lungs get compressed.

The human body is not uniform in density, so when you apply a high g-force, the various parts of the body will try to re-arrange themselves according to the differences in density. I don't know what would kill the person, but my suspicion is the chest cavity would collapse due to the weight of the ribcage. Due to the difference in density, either the ribcage is insufficiently supported or the lungs are overpressurized.
Not really, since the lungs being supported internally by 1000 psi of air. But that pressure is going to mean the lungs have up to an additional .6 lbs of air, changing the density of the person by less than a half percent. But since we are talking about a controlled environment, the density of the water could be adjusted to whatever density produces net buoyancy since we are talking about a sustained acceleration.

How about we pretend for a moment that I'm asking a legitimate question? For instance, bone density is only 70% higher than muscle, and the muscle is being symmetrically supported in proportion to the force being applied by water pressure, so I wouldn't jump to the conclusion that the forces at work are beyond the tensile strength of muscle. That 70% differential means a typical 9 oz femur is only going to put a total of 17 lbs of downward pressure on the muscle under it at 67 Gs. I think the thigh muscle could handle that, for instance.

mfb said:
I don't say that. We tolerate higher g-forces, but not as high as we would tolerate if we would have a uniform density matching that of water everywhere.
You said that 67 G was specifically too much. Were did you get that limit from?

mfb said:
You don't support anything with a uniform pressure increase.
I don't follow. A person can definitely have 1000 psi in their lungs, but only if the surrounding water pressure is also 1000 psi OR there are no air pockets in the vessel for the water to displace into. At that point, the high pressure in the lungs serve the same roll as increasing the pressure in a tire to match increased load in a truck.

Tiran said:
You said that 67 G was specifically too much. Were did you get that limit from?
I said that it can lead to serious damage.
For the interior organs the external support doesn't matter. Being suspended in water only helps to make the exterior pressure on your skin more uniform. It can lead to damage without water, so it can lead to damage with water as well.
Tiran said:
A person can definitely have 1000 psi in their lungs, but only if the surrounding water pressure is also 1000 psi
Correct but also entirely irrelevant for the discussion.
Tiran said:
At that point, the high pressure in the lungs serve the same roll as increasing the pressure in a tire to match increased load in a truck.
Increasing the pressure in the wheels of the truck while also increasing the external pressure doesn't help the wheels either. Only the pressure difference matters for the wheels.

Tiran said:
am I applying the right basic formula in making High G equivalent to high pressure at 14.7 psi = 1G?
No. There is no equivalence between pressure and acceleration. The units aren’t even right, it would need some conversion with units of mass over area.

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russ_watters said:
That is not correct. Divers do not have neutral buoyancy: their buoyancy decreases with depth, as their lungs get compressed.
That applies to free divers only - great value as you don't need to swim to keep below a few metres of water and can relax on the bottom to gain a few more seconds of that great experience. For divers with breathing equipment, the air (or other mixture) is supplied to them at a tiny fraction above the ambient pressure so it flows into their lungs easily.

Klystron
Tiran said:
Not really, since the lungs being supported internally by 1000 psi of air.
It is not pressure that supports a structure against an acceleration. It is pressure differences. The problem is that at 67 gees (or whatever very large acceleration one imagines), the pressure difference required to support bones is greater than the pressure difference required to support the fleshy stuff (e.g. the heart) which is, in turn, more than the pressure difference required to support the fluffy stuff (e.g. the lungs).

Think of it like a centrifuge. Spin it fast enough and the dense stuff squishes down to the bottom. Bones where your feet and legs should be and air from the lungs expelled up where your head should be.

mfb said:
Increasing the pressure in the wheels of the truck while also increasing the external pressure doesn't help the wheels either. Only the pressure difference matters for the wheels.
You are correct, and the pressure difference that the lungs are designed to work with is 14.7psi, so if you want to support the front of the chest with a pressurized volume of air that is the equivalent of a 2300 foot water column or 67 G, you'll need a starting pressure of around 1000 psi and total lung pressure of 1014.7.

Dale said:
No. There is no equivalence between pressure and acceleration. The units aren’t even right, it would need some conversion with units of mass over area.
Not equivalence in a normal math formula, but you can make a comparison between a water column and acceleration. I was asking if I was using appropriately comparable numbers, and I seem to be.

jbriggs444 said:
Think of it like a centrifuge. Spin it fast enough and the dense stuff squishes down to the bottom. Bones where your feet and legs should be and air from the lungs expelled up where your head should be.
I don't think anyone in this thread has any basis for the claims made so far. Centrifuges have G ranges in the 1000s, not 67.

My idea about water and pressurized air to support flesh at higher G is little different than making an egg drop protector in high school physics class - you can protect the egg best if the impact isn't on the end but is distributed all around the egg. Water under pressure has the property of squeezing the sides and bottom of an object just as hard as the downward force, so increases in G in one direction are mitigated to an extent because the object with similar density will not change shape - the usual way an egg or something else delicate breaks. Certainly, at centrifuge G levels, finer and finer variations in density are going to to overwhelm protein and lipid layer strengths, but my suggestion doesn't seem to be anywhere near that range..I was only suggesting a 50% increase in G over something human beings have survived for short periods with virtually no support.

Tiran said:
You are correct, and the pressure difference that the lungs are designed to work with is 14.7psi
That is the typical absolute ambient pressure at the Earth's surface. It has nothing to do with the pressure difference between top and bottom of the lungs. It is that pressure difference between top and bottom which we refer to as buoyancy. For human tissue not filled with air cavities, the pressure difference is about 62.5 pounds per cubic foot. Or 62.5 pounds per square foot horizontal area per vertical foot.

At 67 gees, multiply those numbers by 67. So you get about 33 psi per vertical foot for water or human tissue.

Figure the lungs at roughly 12 vertical inches for a subject standing upright in water. If you used a pressurized water bath to maintain a pressure gradient around the body so that the air pressure at the bottom of the lungs were equal to the water pressure at that depth (preventing the lungs from collapsing) then the air pressure at the top of the lungs would be high by two atmospheres. The lungs would burst at the top.

Tiran said:
Not equivalence in a normal math formula, but you can make a comparison between a water column and acceleration
Then it is poetry, not physics. Thread closed.

sophiecentaur and jim mcnamara