BCD requirements for Drysuit

[Jim Lapenta]

Using only the suit for buoyancy is a good way in the beginning to get hurt. I don't know why agencies keep spewing this junk. Once you have a crapload of dives in the suit and have your weighting nailed, you will find that the amount of air you use to offset squeeze is enough to achieve neutral buoyancy. In the beginning though I find it stupid to tell students to ignore a perfectly good bc that vents faster, is familiar as far as controls go, and has a much smaller and more compact bubble to manage. So I refuse to and teach enough air to offset squeeze, distribute weights so that you only drop the smallest amount necessary, and use the bc for buoyancy control.
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[hypertech]

Agencies teach to use the suit to minimize task loading so you are focused on one air space instead of two. They also want you to get experience managing the airspace in the suit.

I can see the motivation to do so as a learning aid. In practice however, inflate the suit to manage squeeze. Use the bcd for bouyancy. After all, that's what it is for.

 

[NE_SSI_Diver]

That is exactly what I thought during my class, but the instructor was adamant that I should use the suit. He said that managing one air bubble was easier than two. I did find it difficult at first to maintain trim, but now with about 10 dry dives I'm getting used to it.

My dad/dive buddy does not like drysuits for the reason that the air is hard to manage. I'll pass on the advice you guys have given and he might start trying drysuits again.

 

[raftingtigger]

I have a BCD which can only lift around 20lb which I'm afraid is not enough. Would it be acceptable/safe to pump the dry suit a bit when surfacing in order to compensate for the lifting capacity of the BCD at least until I get some of the weights handed up to the boat crew?

Going back to the OP's original concern. They wanted to know if they could use the dry suit at the surface to compensate for the additional weight needed to dive dry. This is a solution to what should be a non-problem.

First: The diver/kit combo should be near neutral without air in the BC, so any amount of lift should keep them on the surface.

Second: If the BC has enough lift to use the same rig with no wet or dry suit then it has enough lift to use with a properly weighted dry suit. However if all the extra weight needed is attached to the tank/BC unit then it may not be enough to float the kit without the diver/drysuit attached. Solution: wear the extra weight on a belt. That way the diver and the kit has the same buoyancy as they do when diving without a suit.

 

[antattack]

Going back to the OP's original concern. They wanted to know if they could use the dry suit at the surface to compensate for the additional weight needed to dive dry. This is a solution to what should be a non-problem.

First: The diver/kit combo should be near neutral without air in the BC, so any amount of lift should keep them on the surface.

Second: If the BC has enough lift to use the same rig with no wet or dry suit then it has enough lift to use with a properly weighted dry suit. However if all the extra weight needed is attached to the tank/BC unit then it may not be enough to float the kit without the diver/drysuit attached. Solution: wear the extra weight on a belt. That way the diver and the kit has the same buoyancy as they do when diving without a suit.

Opinion alert:

Clarity on this subject comes when you do rescue drills. When I play the victim, and all my gear is stripped off and handed to an assistant, all my gear needs to be supported by my BCD. In a dry suit, I have 27# of static weight, and my HP100 is about 9# negative when full. My 40# wing needs to support 36#, leaving 4# positive buoyancy. That's pretty close at a balanced rig. All the extra buoyancy in my dry suit for me if I ditch my gear. If I do ditch my gear, I might want to keep all of it, including my weights, floating on my SCUBA unit near by.

When asking the question "does my BCD have enough lift?" the answer is "Can you float all your gear on it?". Using the Dry suit a related question, but doesn't answer the worse case requirement of the BCD.

 

[FoxHound]

That is exactly what I thought during my class, but the instructor was adamant that I should use the suit. He said that managing one air bubble was easier than two. I did find it difficult at first to maintain trim, but now with about 10 dry dives I'm getting used to it.

My dad/dive buddy does not like drysuits for the reason that the air is hard to manage. I'll pass on the advice you guys have given and he might start trying drysuits again.

It is a bit more task loading but in all honesty managing a small bubble in your drysuit and then your BC is WAY easier then trying to manage a gigantic bubble in the drysuit.

I would almost bet that if your dad had just enough gas in the suit to make it comfortable squeeze wise that he would start finding it much easier to use and manage.

Its worth a shot trying both and then picking the way you want to do it. Im not gonna tell ya to go one way or another but my way is easier haha

 

[TSandM]

One of the things about dry suits is that they aren't always . . . dry, that is. All of us who dive dry have had the experience of a failed seal or zipper, or a hole in the suit that ends up with us being pretty darned wet at the end of the dive. And the worst thing about that is that, when you get to the surface, the suit may not hold air well (for example, a failed neck seal) and you are trying to lift your head and shoulders, which are wet, out of the water. The first major dry suit flood I had, my 30 lb wing was barely able to keep my mouth out of the water when we got near shore, and that was really stressful for me.

A wing should have enough lift to float the equipment attached to it, and to compensate for the lost lift of a compressed wet suit, or flooded dry suit. A 20 lb wing in cold water is a major roll of the dice.

 

[raftingtigger]

Foxhound and TSandM have good points above. I can see where there is an advantage to having the BC/Wing able to support all of the negatively buoyant gear.

I'm still having a problem with the side discussion on whether to use the dry suit or the BC/Wing or both for buoyancy control underwater. I agree that keeping excess air out of the suit is good practice and if you need a big bubble you should contain that in the BC/wing. However am having a hard time seeing how a properly weighted diver would need to use a large bubble during the dive. The amount of air needed to prevent squeeze is the same volume regardless of depth (not same the same as the surface amount of air). The only difference during the dive is the weight of the air being used from the tank. That is only about 5# which shouldn't make a very cumbersome bubble....

Light bulb moment! I'm strictly a recreational single tank diver typically using an 80 cf tank (+/- 5 lb). A lot of you are doubles divers often with stage bottles and 120 cf tanks. That is a lot more air weight to compensate for from full to empty (10-20 lbs). Is that the reason? or am I missing something else?

Remember we are only discussing dry suits, not the compression of a wet suit.

 

[elan]

A wing should have enough lift to float the equipment attached to it, and to compensate for the lost lift of a compressed wet suit, or flooded dry suit. A 20 lb wing in cold water is a major roll of the dice.

For the dry suit:
Not if you have the weight on the belt or ditchable on the BC.
Mordern shell suit is slightly negative, neoprene is positive. Flooded undergarment will be neutral. So if you have the ditchable weights on the belt or bc you just ditch it. 20 lb will be more than enough to float the gas in a single tank and the diver.

 

[elan]

Foxhound and TSandM have good points above. I can see where there is an advantage to having the BC/Wing able to support all of the negatively buoyant gear.

I'm still having a problem with the side discussion on whether to use the dry suit or the BC/Wing or both for buoyancy control underwater. I agree that keeping excess air out of the suit is good practice and if you need a big bubble you should contain that in the BC/wing. However am having a hard time seeing how a properly weighted diver would need to use a large bubble during the dive. The amount of air needed to prevent squeeze is the same volume regardless of depth (not same the same as the surface amount of air). The only difference during the dive is the weight of the air being used from the tank. That is only about 5# which shouldn't make a very cumbersome bubble....

Light bulb moment! I'm strictly a recreational single tank diver typically using an 80 cf tank (+/- 5 lb). A lot of you are doubles divers often with stage bottles and 120 cf tanks. That is a lot more air weight to compensate for from full to empty (10-20 lbs). Is that the reason? or am I missing something else?

Remember we are only discussing dry suits, not the compression of a wet suit.

What you said is correct.
If you have a shell suit, dive a smaller size tank and if you have big lungs you might not need to use any inflation for the gas lose compensation and just adjust the suit for the squeeze. You can compensate with your lungs.
If the dry suite is neoprene you will also need to compensate for the buoyancy lose with depth.