Too heavy after a certain depth ?

[socc]

Hello everyone,

Some days ago I was watching Monty Halls And The Divers' Graveyard. For those who do not know, it is an episode from a small series with dives called "Monty Halls And.." .
What I have noticed, is that they were saying that after a certain depth you can become so heavy that even your BCD cannot provide you with positive buoyancy. I know that as you go deeper your suit and your BCD loose buoyancy but how can it be such that even your BCD cannot compensate that. Of course there were talking about quite deep places such as 60+ meters but still I cannot figure out how this can happen.

If anyone can clarify how this can happen, I would really appreciate it.

Many thanks,

Socc

 

[Steve_C]

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.

 

[socc]

Hello Steve_C,

I get it now. I think you are right, this is probably what they were talking about.

Many thanks for your help !

Socc

 

[herman]

More scuba myth than reality. Remember that after 33ft/10M that you have already lost half of any bouyancy shift you are going to experience from wetsuit compression and at 66ft/20M you lose another half of that. Another way to look at it is at 66ft/30M you have already experienced 75% of the total possible compression of your wetsuit (and any other compressable item you have on) so it can't get much worse. Your BC on the other hand will have the same amount of lift at any depth, assuming that you can inflate it. So, unless you are grossly overweighted, wearing a huge amount of wet suit and using a BC with just barely enough lift at the surface, it's not possible to go deep enough.....within reason....for depth to be so great that your BC can't compensate. The one exception is if you were to go so deep that the ambient pressure is equal to the tank pressure then you can't inflate your BC....but at 6000 feet/2000ish M it really does not matter any longer.

 

[socc]

Hello Herman,

I see what you mean. It makes sense really !

Many thanks for your reply !

Socc

 

[agesilaus]

More scuba myth than reality. Remember that after 33ft/10M that you have already lost half of any bouyancy shift you are going to experience from wetsuit compression and at 66ft/20M you lose another half of that. Another way to look at it is at 66ft/30M you have already experienced 75% of the total possible compression of your wetsuit (and any other compressable item you have on) so it can't get much worse. Your BC on the other hand will have the same amount of lift at any depth, assuming that you can inflate it. So, unless you are grossly overweighted, wearing a huge amount of wet suit and using a BC with just barely enough lift at the surface, it's not possible to go deep enough.....within reason....for depth to be so great that your BC can't compensate. The one exception is if you were to go so deep that the ambient pressure is equal to the tank pressure then you can't inflate your BC....but at 6000 feet/2000ish M it really does not matter any longer.

OK maybe my math is wrong. But going from the surface (1 Bar) to 33 ft (2 Bar) does compress your wetsuit by 50%, but going from there (2 Bars) to 66 ft (3 Bars) only compresses it to 1/3 of surface volume not 1/4. And the air in your BC would also be compressed by the same ratios P1V1=P2V2. Water is essentially uncompressable at recreational diving depths so you lose buoyancy from your BC since it displaces less water as your depth increases.

BK

 

[tursiops]

you lose buoyancy from your BC since it displaces less water as your depth increases.

That's why you put more air into it as you descend....

 

[socc]

Ok thanks guys ! I understand the theory behind it.
I was just not sure how you can become so heavy that you cannot actually compensate and get positive buoyancy.

Thanks everyone for your replies !

Sent from my GT-I9505 using Tapatalk

 

[redacted]

OK maybe my math is wrong. But going from the surface (1 Bar) to 33 ft (2 Bar) does compress your wetsuit by 50%, but going from there (2 Bars) to 66 ft (3 Bars) only compresses it to 1/3 of surface volume not 1/4. And the air in your BC would also be compressed by the same ratios P1V1=P2V2. Water is essentially uncompressable at recreational diving depths so you lose buoyancy from your BC since it displaces less water as your depth increases.

BK

The volume of water displaced by a FULL BCD is constant over depth. Of course, you will have to add air to that BCD as you descend in order to maintain that constant FULL air volume. The main reason you buoyancy changes with depth is your neoprene thermal protection compresses, losing much of its buoyancy. If the total compression of your neoprene exceeds the lift of your BCD you may not be able to achieve positive buoyancy. This could happen with smaller lift BCDs (like under 20 lb) and heavy wet suites like a larger size 7mm 2-piece.

 

[agesilaus]

OK that's what I thought.

BK