Buoyancy with drysuit?

[Jim Lapenta]

Suit for warmth. BC for buoyancy control but there are caveats to this that all depend on whether or not you are weighted properly, how much experience you have, how good your undergarments are, and what type of cylinders you are using and how many. This is the problem with many drysuit classes when they go by the book and the instructor has never tried, taught, or seen any other way.
I use the suit for warmth and the BC for buoyancy. However, if I am diving a single tank, the amount of air I need to offset squeeze is enough to get neutral. To a certain depth. Then the BC takes over because I don't need any more air in the suit.
The argument that needing to manage two sources is too hard is utter BS if the student is taught correctly.
It's harder to manage a large bubble in the suit in the event the diver come up too fast when using the suit only. It takes longer to dump and air may get trapped not be able to to dumped.
Also, as we found out when I was writing the SDI Drysuit course, manufacturers do not say the suit is anything but exposure protection.
The PADI course is also based on 30 yr old ideas and does not take into account the advances in drysuit undergarments.

 

[inquis]

The fact is even when perfectly weighted, the BC or suit will have to compensate for at least 5 lbs (the weight of the air you'll breath), which is equivalent to over 2 liters of air. If you're a couple pounds heavy (as a fair few prefer to be), that's another liter of air to add.

If you put it in the suit, the location of this extra air must be managed by you, so trim is critical. It also cannot be vented very rapidly should the need arise or from a feet up orientation. This is reason enough that I would not suggest that a beginner do it this way.

In contrast, if you put it in the BC/wing, it stays there and can be vented rapidly when either head up or down.

Until you develop the skill to get the exhaust valve at the highest point -- the only place it works automatically -- I think it's good to have a means to immediately arrest a buoyant ascent.

 

[Wibble]

This is the problem with many drysuit classes when they go by the book and the instructor has never tried, taught, or seen any other way.

^^^ this.

Most people quickly realise after a few dives that their instructor's words need taking with a large pinch of salt.

There’s massive considerations depending on the dive conditions and the equipment you’re using. For example if you’re overweighted (e.g. carrying a heavy load of gas or scallops), in which case the BCD/wing does the heavy lifting, or if you’re cold when you may prioritise more gas in the suit.

 

[happy-diver]

So what's it to be, are you feeling lucky Seville!

 

[Edward3c]

This is the problem with many drysuit classes when they go by the book and the instructor has never tried, taught, or seen any other way.

Totally agree.

 

[tbone1004]

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?

We can thank Dick Long for that recommendation that like many things in scuba has been twisted away from its origin.

When you dive with a LP72 or AL80, in a bathing suit, you don't need a BC if you are weighted properly. Your lungs are more than capable of handling the 6lbs of gas in those bottles without needing a BC. You do need one to handle the suit compression of a wetsuit, but not for the mass of gas in the bottle. 6.4lbs is ~3L of volume which is about the normal inspiratory capacity of an adult *little less than a normal male, little more than a normal female, but close enough that we dove single LP72's without any wings for 30 years before the advent of the BC*
When you change that thick wetsuit for a drysuit though, you SHOULD be diving the drysuit with a constant volume, i.e. no buoyancy shift that needs to be compensated for. IF you do in fact dive with the drysuit at a constant volume, then you do not need to use your buoyancy compensator because the buoyancy shift is inconsequential. 6.4lbs is less than 1 gallon of water and when spread around your entire body in the drysuit is an irrelevant amount of volume.

The issue with teaching that thinking from day 1 is Thorndike's Learning Law of Primacy. Your brain will want to default to not using the BC and while that is certainly fine when dealing with an AL80 or LP72, is not so fine when you deal with a HP130. With a tank that big, you now have over 10lbs of gas that needs to be compensated for and that's well over the comfortable vital capacity of normal adults. When you have that much gas moving around, it is best to put it in the wing and when you start to dive doubles it is critical that you are just using the suit for comfort and anything else needs to go in the wing. Doesn't take much extra effort.

 

[drakcheslav]

I was taught to use both. To balance the amount of gas in my drysuit (prevent squeeze) and use mu BC for stability.

I start deflated, drop to 5m, little gas in the suit to prevent squeeze, and add or vent the gas to have that minimal amount of loft.

And use the BC as i would use it when diving a thin wetsuit. I.e. compensate for large compressions when going below 20m.

I learned to dive with an open dump, and control it using my arm position, and so far I didn't have excessive dumpage.

Taking my tech class now, drysuit should be used only as an emergency instead of a DSMB if your BC fails. and only then.

However, I did have a couple of times where I had to disconnect my BC and use only my drysuit for buoyancy as a practice run. And It's much less comfortable than it seems.

 

[Capt Jim Wyatt]

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.

 

[lexvil]

I’ve never understood why people dive dry so grossly over weighted or choose to use such poorly fitting drysuits that they have an unwieldy “bubble” to deal with.

 

[Seville]

Seems like a waste i paid for the class lol! My suit is a little oversized in the upper torso area. but it is a dui 50/50 is nice and tight in my legs

We can thank Dick Long for that recommendation that like many things in scuba has been twisted away from its origin.

When you dive with a LP72 or AL80, in a bathing suit, you don't need a BC if you are weighted properly. Your lungs are more than capable of handling the 6lbs of gas in those bottles without needing a BC. You do need one to handle the suit compression of a wetsuit, but not for the mass of gas in the bottle. 6.4lbs is ~3L of volume which is about the normal inspiratory capacity of an adult *little less than a normal male, little more than a normal female, but close enough that we dove single LP72's without any wings for 30 years before the advent of the BC*
When you change that thick wetsuit for a drysuit though, you SHOULD be diving the drysuit with a constant volume, i.e. no buoyancy shift that needs to be compensated for. IF you do in fact dive with the drysuit at a constant volume, then you do not need to use your buoyancy compensator because the buoyancy shift is inconsequential. 6.4lbs is less than 1 gallon of water and when spread around your entire body in the drysuit is an irrelevant amount of volume.

The issue with teaching that thinking from day 1 is Thorndike's Learning Law of Primacy. Your brain will want to default to not using the BC and while that is certainly fine when dealing with an AL80 or LP72, is not so fine when you deal with a HP130. With a tank that big, you now have over 10lbs of gas that needs to be compensated for and that's well over the comfortable vital capacity of normal adults. When you have that much gas moving around, it is best to put it in the wing and when you start to dive doubles it is critical that you are just using the suit for comfort and anything else needs to go in the wing. Doesn't take much extra effort.

I dive with a hp 120 tank so you recommend mainly using the BC then?