Can you sink by blowing up your BC below 40m?
- Thread starter Boyan
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Bottom Surveyor
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In short, you got told some crazy information.
I think what you should consider is how the volume of air changes over a certain span of feet, relative to how deep you are.
That is to say, at the surface, the pressure is said to be "one atmosphere," because the weight of all of the air in the entire atmosphere, is about 14.7 psi. Because water is much denser than air, you only need to travel 33 feet down before you are at 2 atmospheres of pressure. So in those 33 feet you had 100% pressure change.
Alternatively, if you had a balloon and filled it with 1 cubic feet of air at 33 feet deep, and let it go up to the surface, it would double in volume even though it contained the same amount of molecules of air.
But if you are at 66 feet and go up to 33 feet, you are going from 3 atmospheres to 2 atmospheres, which is only 50% increase--your 1 cubic foot balloon at 66 feet would be 1.5 cubic feet big at 33 feet, and 3 cubic feet at 0 feet.
So, the deeper you are, the more time it seems to take to add air to your BC to make a "significant" volume change, but if you continued to the surface without dumping any air, your volume will increase slowly at first, and much more rapidly at the end.
This comes into play in many ways, but it might make it seem like you are sinking even when adding air, because you need to add air for more time in order to make a volume change that offsets you well, assuming that the same number of molecules of air are coming out of the hose always.
Now, whether the diver was misunderstood, or just pulling your leg, remains to be known. Or perhaps he jumped in and sank to 40 meters, and was adding air but not noticing the sudden volume change he was used to on the surface, and he continued to swim upwards while adding air, and at 40 meters mark he evened out...and to him it seemed that since he was adding air the entire time, that the air below 40 meters was making him sink, when in reality he just needed to be more patient.
I think what you should consider is how the volume of air changes over a certain span of feet, relative to how deep you are.
That is to say, at the surface, the pressure is said to be "one atmosphere," because the weight of all of the air in the entire atmosphere, is about 14.7 psi. Because water is much denser than air, you only need to travel 33 feet down before you are at 2 atmospheres of pressure. So in those 33 feet you had 100% pressure change.
Alternatively, if you had a balloon and filled it with 1 cubic feet of air at 33 feet deep, and let it go up to the surface, it would double in volume even though it contained the same amount of molecules of air.
But if you are at 66 feet and go up to 33 feet, you are going from 3 atmospheres to 2 atmospheres, which is only 50% increase--your 1 cubic foot balloon at 66 feet would be 1.5 cubic feet big at 33 feet, and 3 cubic feet at 0 feet.
So, the deeper you are, the more time it seems to take to add air to your BC to make a "significant" volume change, but if you continued to the surface without dumping any air, your volume will increase slowly at first, and much more rapidly at the end.
This comes into play in many ways, but it might make it seem like you are sinking even when adding air, because you need to add air for more time in order to make a volume change that offsets you well, assuming that the same number of molecules of air are coming out of the hose always.
Now, whether the diver was misunderstood, or just pulling your leg, remains to be known. Or perhaps he jumped in and sank to 40 meters, and was adding air but not noticing the sudden volume change he was used to on the surface, and he continued to swim upwards while adding air, and at 40 meters mark he evened out...and to him it seemed that since he was adding air the entire time, that the air below 40 meters was making him sink, when in reality he just needed to be more patient.
kotik
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Steelyeyes
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Theoretically it's possible. The trick is learning to blow the negative bubbles.
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@Boyan, JahJahwarrior made a nice explanation. I will try to to help out from a slightly different approach.
Buoyancy is all about a relationship of weight and volume. If the weight of an object is the same as the weight of that same volume of water, it is neutrally buoyant. If it is lighter than that volume of water, it will float. If it is heavier, it will sink. That is why a massive ship floats--as heavy as it is, it is still lighter than an equal volume of water.
When diving with a BCD, we adjust our buoyancy during the dive by adjusting our total volume. We can make ourselves bigger by inhaling or by adding air to the BCD. When we add air to our lungs or the BCD, we are adding volume while adding almost no weight. We do not become lighter (in fact, we are slightly heavier), but because we are now bigger than we were before, we are now lighter in comparison to the weight of that new volume of water.
The volume we need to be neutrally buoyant is the same at any depth, because that volume of water is the same weight at any depth. However, as we go deeper, the water pressure squeezes the BCD, compressing the air space and making it smaller. That means we have to add more and more compressing air to the BCD to achieve the same volume. Let's say you needed 1 million molecules of air at the surface to be neutrally buoyant. At 10 meters, you would need 2 million molecules to have that same volume. At 20 meters you would need 3 million molecules, and at 30 meters you would need 4 million molecules. The deeper you go, the longer you have to push your inflator button to get the same result.
There is no difference in the air at any depth--it's just that the deeper you go, the more air you need to add to your BCD to get the same volume you got when you were shallower.
Buoyancy is all about a relationship of weight and volume. If the weight of an object is the same as the weight of that same volume of water, it is neutrally buoyant. If it is lighter than that volume of water, it will float. If it is heavier, it will sink. That is why a massive ship floats--as heavy as it is, it is still lighter than an equal volume of water.
When diving with a BCD, we adjust our buoyancy during the dive by adjusting our total volume. We can make ourselves bigger by inhaling or by adding air to the BCD. When we add air to our lungs or the BCD, we are adding volume while adding almost no weight. We do not become lighter (in fact, we are slightly heavier), but because we are now bigger than we were before, we are now lighter in comparison to the weight of that new volume of water.
The volume we need to be neutrally buoyant is the same at any depth, because that volume of water is the same weight at any depth. However, as we go deeper, the water pressure squeezes the BCD, compressing the air space and making it smaller. That means we have to add more and more compressing air to the BCD to achieve the same volume. Let's say you needed 1 million molecules of air at the surface to be neutrally buoyant. At 10 meters, you would need 2 million molecules to have that same volume. At 20 meters you would need 3 million molecules, and at 30 meters you would need 4 million molecules. The deeper you go, the longer you have to push your inflator button to get the same result.
There is no difference in the air at any depth--it's just that the deeper you go, the more air you need to add to your BCD to get the same volume you got when you were shallower.
I dive in feet instead of meters so I won't have this problem.
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