Buoyancy with drysuit?

[Seville]

I only inflate the dry suit enough to keep off the squeeze of the water pressure.

If you use the dry suit for buoyancy control you will find the "bubble" inside the suit moves with your trim & swim attitude, further messing up your trim and swim attitude. I never understood why anyone teaches primary buoyancy control with the dry suit vs the BCD.

My instructor was telling us that we can hold a bubble in your upper back and slowly vent it when going up. I never felt it and it seemed complicated to me. Honestly the drysuit scares me a lot more than using a wetsuit. My biggest fear is shooting up to the surface feet first.

 

[Jcp2]

My instructor was telling us that we can hold a bubble in your upper back and slowly vent it when going up. I never felt it and it seemed complicated to me. Honestly the drysuit scares me a lot more than using a wetsuit. My biggest fear is shooting up to the surface feet first.

I’m going through the same process now and have had a couple of unexpected ascents. I’m just doing shallower half hour dives in the 30-40 foot range so that nitrogen loading isn’t a big issue. It’s like learning how to dive all over again, but I’m appreciating getting undressed mostly dry and having a chance to extend my dive season by diving locally.

 

[Subcooled]

I just took my PADI drysuit class. They say to keep the drysuit valve completely open and let all he air out of your BC then only control your buoyancy with your drysuit only by adding and letting air out. A guy I dive with says all the other agencies tell people to adjust buoyancy with the BC and only add air to the drysuit for comfort as solely using the drysuit for buoyancy can cause excessive air and result in a higher chance of a run away acent if air gets in your legs. What do you recommend for buoyancy control for a beginner with a drysuit?

You have two buoyancy compensators:
#1 the BCD
#2 the drysuit
You had better to learn to use both of them. One at a time.

When you take a drysuit class and you need to learn how to _control_ a drysuit, you will benefit from using drysuit inflation/exhaust only. It is simpler this way. You will learn faster.

If you dive deep however (lots of gas/stages/etc) then maybe you want to
#1 add enough air to the suit to avoid squeeze
#2 add air to the suit for precise adjustment of buoyancy
#3 if you feel that there is a wandering air bubble in your suit (hands<-->feet) then you will want to place some of that air into a wing instead
#4 if water is cold, though (arctic diving) then you will want to add more air (and lead!) fo your suit to stay warm!

 

[jtsfour]

My instructor was telling us that we can hold a bubble in your upper back and slowly vent it when going up. I never felt it and it seemed complicated to me. Honestly the drysuit scares me a lot more than using a wetsuit. My biggest fear is shooting up to the surface feet first.

I'm still new to drysuit diving but it gets a LOT easier the more you dive it. One drill/trick my instructor showed me was how to emergency neck vent. (I'm pretty sure it is not standard but I don't know.) It's pretty easy for me (I have a neoprene neck seal no idea how other materials would work.) Get head up and purposely open the neck seal with your hand to rapidly vent gas. However, I'm typically in water that wouldn't put me at risk of hypothermia if my suit flooded... In Ice cold water that might be dangerous.

Knowing I can do that it greatly reduces my worry of uncontrolled feet first ascent. I've practiced it occasionally in shallow water. I can arrest a light-moderate feet first ascent in 5-10 feet.

I would also make sure to practice removing you drysuit inflator hose in case of an emergency.

 

[Markus Kircher]

BC = compensate changes in buoyancy, specifically contained and shaped air bubble, primary compensation for depth and gear changes
Drysuit = just enough gas to loft your undergarments, big bubble of places air can be

Sometimes a couple clicks tighter than all the way open keeps the drysuit from venting more often than you’d like or, in the case of the crappy low profile apeks valves, keeps them from leaking so often.

In short, PADI methodology an overwhelmingly poor choice.

So true!

 

[Bowers]

If you’re weighted correctly then keeping the suit comfy is also keeping you neutral. The bc is to compensate for large gas volume changes in your tanks which is a bigger deal when diving with twinsets and such. I teach to start with the valve open then as you learn your suit and undergarment characteristics, you can dial it tighter a bit to get it perfect.

 

[Gareth J]

if I am diving a single tank, the amount of air I need to offset squeeze is enough to get neutral.

If you are correctly weighted with a single cylinder, by the time you have added enough air to take the squeeze off, you are already neutral.

In a membrane, you need to remember that you need enough air in the suit to make the thermal layers work. They need air in the fibres to get them to ‘loft’. If the thermal layers are compressed they don’t provide much in the way of thermal protection/benefit. It’s the air between the fibres and individual layers that provide the thermal protection.
When on CCR, I tend to use the wing a little more because I don’t want to vent (waste) dil. I have to be careful, because you can be noticeably colder if there is insufficient gas in the suit. (That’s even with the positive effect of warm damp air in the breathing loop.)

Generally, you want the CVD closed down about 4 clicks. Then the valve works effectively, you have just enough gas in the suit to get the thermal advantage, but not so much that you’ve got a big bubble of gas to manage. That immediately makes most people neutral with a single steel12.

Things do tend to get more complicated when you move to twin 12’s and stages. But using aluminium stages reduces some of the issues anyway (it’s a long time since we used steel stages).

One issue with the CVD being fully open is people are more likely to get wet. If the pressure in the suit is negative you are more likely to siphon water into the suit through the CVD. The CVD is only a spring and diaphragm.

 

[tbone1004]

If you are correctly weighted with a single cylinder, by the time you have added enough air to take the squeeze off, you are already neutral.

That doesn't make sense. An AL80 has ~6lbs of gas in it, so you have more gas at the start than you do at the end. For a single tank it's normally not an issue, and we are talking about 1/10th of a cubic foot, but it's still there.

 

[Gareth J]

That doesn't make sense. An AL80 has ~6lbs of gas in it, so you have more gas at the start than you do at the end. For a single tank it's normally not an issue, and we are talking about 1/10th of a cubic foot, but it's still there.

@tbone1004

I would never dive an aluminium cylinder on a standard rig unless I was on holiday somewhere tropical and (ideally) in a wetsuit. Horrible cylinders to dive on a traditional rig with a drysuit because they 'pop' - they go positive. Lovely things to dive as stages, because they are neutral when full, and go positive when empty.

Aluminium cylinders have been traditionally rare (as main cylinders), in the UK. The saga over them cracking and requiring extra testing killed off the few that where in circulation.
There are a lot of ALi80's in the UK - used as stages, where the way they behave is a benefit.
It's also noticeable, that they are Ali80's. If you want an European spec aluminium cylinder, it would normally be a 7 litre, or smaller.

We tend to dive steel cylinders, even the rentals are steel. The only time we see aluminium cylinders normally is as stages, or when on holiday, where you are likely to be diving in a wetsuit.

One nice thing about a steel 12, on a standard rig or wing, with a drysuit, is the buoyancy control is really easy. Once the CVD is set, you don't need to do anything, other than add gas as you get deeper. The CVD does all the work, all you need to do is roll right a fraction if you do feel too positive.

 

[rongoodman]

But the weight of the extra gas is the same whether contained in steel or aluminum, which is the issue at hand.