Too heavy after a certain depth ?

[socc]

I found the following in this article -How To Scuba Dive | Scuba Diving Training & Certification | Scuba Diving - which is related with what you guys were discussing above.

Depth
Whatever the surface buoyancy of your wetsuit, it will change dramatically with depth. Because pressure flattens those thousands of gas bubbles, your wetsuit gets thinner and displaces less water. In effect, it gets heavier. The change is not linear. You lose half of your surface buoyancy in the first 33 feet of your descent and a third in the next 33 feet. Below 66 feet, there's only one-sixth of the original buoyancy left to lose no matter how deep you go. The single larger bubble in your BC behaves the same way.
Buoyancy changes fastest in the first few feet below the surface--three times as fast at one foot as at 60 feet. That's why it's often hard to get submerged, but once you're down five feet or so, you seem to get heavier and sink easily.

 

[TSandM]

There have been deaths associated with the use of heavy DOUBLE steel tanks and thick wetsuits. The problem there is that the lift available from the air bladder used can be less than the total negative force produced by dual heavy tanks, full of heavy gas, and a wetsuit which has lost 20 or more pounds of buoyancy with extreme depth.

 

[kwinter]

There have been deaths associated with the use of heavy DOUBLE steel tanks and thick wetsuits. The problem there is that the lift available from the air bladder used can be less than the total negative force produced by dual heavy tanks, full of heavy gas, and a wetsuit which has lost 20 or more pounds of buoyancy with extreme depth.

Commonly called a "dirt dart".

 

[Scared Silly]

Commonly called a "dirt dart".

Not to be confused with Balkan Dirt Diving.

 

[Subcooled]

This may or may not be what they are talking about but if you delay putting air in your BCD as you drop it is possible to start dropping at a rate that you cannot put air into your BCD fast enough to get buoyant and stop the drop. Thus if you are relying on the BCD alone you will continue down and down. This will not happen on a shallow dive like say 80 ft. If you do put it off until close to 80 you will see that it takes noticably longer to stop with inflation. Tried it a couple times with a soft hard bottom. The ammount of air it takes to get a certain volume of void in your BCD increases with depth. At 100 ft it takes 4 times as much air to create the same void and hence the same lift.

Even if you are not doing a tech dive this is why it is important to keep under control and add air as you go especially when over deep water.

Depth at which this can occur will depend on your reg and weighting and other factors.

The drop results from the fact that lead (and steel) is pulling one down, while the bcd/wing cannot be inflated fast enough to compensate: It compresses faster than it fills.

There is an easy solution, though:

Drop the weights (at least some of it) and you'll become buoyant no matter how fast down you go. The resulting ascent would be quite fast, but better up than down.

Some double tank sets do not have droppable weights. In these cases, the risk may exist. With jacket bcd's there's nothing to worry about, but ones mental state and skill.

People also tend to panic and forget to drop the weights, even if they could...

 

[rambaldi]

I found the following in this article -How To Scuba Dive | Scuba Diving Training & Certification | Scuba Diving - which is related with what you guys were discussing above.

Depth
Whatever the surface buoyancy of your wetsuit, it will change dramatically with depth. Because pressure flattens those thousands of gas bubbles, your wetsuit gets thinner and displaces less water. In effect, it gets heavier. The change is not linear. You lose half of your surface buoyancy in the first 33 feet of your descent and a third in the next 33 feet. Below 66 feet, there's only one-sixth of the original buoyancy left to lose no matter how deep you go. The single larger bubble in your BC behaves the same way.
Buoyancy changes fastest in the first few feet below the surface--three times as fast at one foot as at 60 feet. That's why it's often hard to get submerged, but once you're down five feet or so, you seem to get heavier and sink easily.

This is a bit of a myth (second time posted in this thread) your wetsuit isn't an ideal gas and so won't be half the volume at twice the pressure. There is a fair amount of non compressible rubber in there rather than bubble and those bits won't shrink as much. You do loose buoyancy from your wetsuit but it isn't a balloon full of air that you take down.

 

[Foxfish]

Isaac Newton's equation Resultant Force = Mass x Acceleration predicts what occurs.

If you are neutrally buoyant underwater then there is no movement up or down and no acceleration. ie the resultant force is equal to zero. Assuming you are not finning, the upward buoyant forces balance the downward gravity forces.

If you then let air out of your BCD you begin to descend. Now water resistance begins to oppose your downward movement. If you descend at a uniform speed then the downward force of gravity is opposed by the upward buoyancy forces and water resistance.

As you descend the air in your BCD decreases and your exposure suit compresses. That means you lose buoyancy so the upward forces are reduced. Unless you compensate by adding air to your BCD, you will continue to descent at an ever increasing speed ie accelerate. You may be able to stop the acceleration by inflating your BCD providing it has adequate volume. The faster you are descending when this occurs the further you will descend before you start to ascend. Hence the need to limit your descent speed if in deep water.

On the surface with your BCD fully inflated the upward forces will be greater than the downward forces by some amount which I'll call the net buoyancy. If the loss of buoyancy from your exposure suit as you descend becomes greater than this net buoyancy then the you will descend at an ever increasing speed. The solution would be to fin upwards and or dump weight.

 

[redacted]

This is a bit of a myth (second time posted in this thread) your wetsuit isn't an ideal gas and so won't be half the volume at twice the pressure. There is a fair amount of non compressible rubber in there rather than bubble and those bits won't shrink as much. You do loose buoyancy from your wetsuit but it isn't a balloon full of air that you take down.

The densities of rubber and nylon are in the range of .9 to 1.2. Pretty close to water density. So the compression effects on a wetsuit are pretty close to those predicted assuming an ideal gas. The effect on thickness will have a limiting constant of the thickness of the rubber & nylon materials but the effects on buoyancy is pretty much proportional to the pressure change.

 

[Foxfish]

The densities of rubber and nylon are in the range of .9 to 1.2. Pretty close to water density. So the compression effects on a wetsuit are pretty close to those predicted assuming an ideal gas. The effect on thickness will have a limiting constant of the thickness of the rubber & nylon materials but the effects on buoyancy is pretty much proportional to the pressure change.

You could consider neoprene to be a mix of bubbles in rubber. Then the buoyancy will be from the bubbles if you assume 'rubber' is the same weight as *water*. The buoyant force from the suit in that case is equal to the weight of the water displaced by the bubbles. So if an exposure suit was 30% bubbles at a given depth then the buoyancy force would be some value. If you then descended to a depth that compressed the bubbles to 15 % the buoyancy force of the suit would be halved.

Another approach would be to weigh your suit and determine its volume at a given depth. The buoyant force is then the difference in the weight of the water displaced by the suit and the weight of the suit. The weight of the suit stays constant so as the thickness decreases with increasing depth the buoyancy from the suit reduces.

With a dry suit you would also need to consider the air between suit and the divers body. Many dry suits are not as compressible as neoprene suits.

Of course all of this is academic and I've never come across anyone who bothers with this level of analysis. The main thing is to avoid being grossly over weighted with lead.

 

[Bob DBF]

Isaac Newton's equation Resultant Force = Mass x Acceleration predicts what occurs.

Unless you compensate by adding air to your BCD, you will continue to descent at an ever increasing speed ie accelerate.

A bit of overstatement. In air, say exiting a plane in flight, the terminal velocity for a falling human is about 120mph, so I would assume the terminal velocity in water would be less considering it's density.

I agree that if the diver dosen't change the situation, the previously mentioned dirt dart does come to mind.


Bob
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I may be old, but I'm not dead yet.