BC or Drysuit for Buoyancy

[CompuDude]

Originally Posted by jj1987

It's also a lot easier to vent air from a front zip drysuit if you're horizontal than a back zip, due to exhaust valve placement.

The make of the drysuit is more important than front zip vs. back zip. Every design, every manufacturer puts the dump valve in slightly different places. It's [nearly] always on your arm, but if there is a difference between two drysuits you've used, it's those drysuits, not a universal truth.

Personally, I'm in the "BC is for buoyancy control, drysuit is for warmth" camp. I understand PADI thinks it's simpler than asking someone to manage an extra thing, but IMO, that's what you're doing when diving dry, no matter what your preference is. Most people learned to dive using their BC to manage buoyancy... why does that have to change? Just use the DS as well, and keep yourself neutral.

That said, depending on the dive, and depending on my undergarments for that dive, there are dives where I end up using the DS primarily, simply because the act of lofting the undergarments to take the squeeze off is all that's needed... no air in BC is needed.

[Kingpatzer]

Originally Posted by jj1987

It's also a lot easier to vent air from a front zip drysuit if you're horizontal than a back zip, due to exhaust valve placement.

You can place your exhaust valve anywhere you want. Or you can have more than one.

My particular thought is that if a new dry suit diver is so task loaded that they can't handle using the Buoyancy Compensating Device to compensate their buoyancy, then they are almost certainly not ready to dive dry. PADI has a real tendency to offer certs to people well before the average diver is ready for that training.

The problem with using the dry suit for buoyancy to me starts first that you have to control your air bubble -- and regardless of how likely it is, the very fact that a diver can find themselves in a feet up uncontrolled ascent to me just shows how bad an idea using the DS for buoyancy is.

[MSilvia]

Originally Posted by SkuaSeptember

If you are wearing a shell suit, the only air space that will compress when you descend is in the lofting of your under garments. Keep the loft consistent and you will maintain neutral buoyancy.

That's only true if you never exhale. Otherwise, your tanks will become increasingly bouyant over the course of your dive as you empty them, and you'll have to add weight to offset that so you don't become buoyant halfway through the dive. Since that will leave a diver slightly overweighted at the beginning of the dive, some volume of additional gas beyond what is required to maintain loft in the undergarment at will be required.

[it_mike]

Originally Posted by thor

I have read in some of the manuals and have also been instructed that the drysuit should be used for buoyancy and that the BC should only be used on the surface.

However, I have also read in some tech manuals that the drysuit should only be inflated enough to insulate and avoid squeeze, but that the BC should be used for buoyancy due to the weight of gear (doubles, backplate, etc.), and that the drysuit would be overinflated if used for buoyancy under most tech conditions.

Which is correct?
Discuss

My turn, but where to begin...

First, I presume we're agreeing that you should be neutrally buoyant (500psi) at the surface. If this requires a little air in your BC due to equipment weight, this is where you manage the air in your BC.

Once neutrally buoyant, the only buoyancy change will be attributable to suit compression, correct? If so, you're adding air to your drysuit to offset compression due to pressure. You're not adding any air to the BC, and you're maintaining buoyancy control via your drysuit.

As you ascend, you still have the same volume of air in your BC as you did in the beginning. Your only buoyancy variant is the air you added to your suit to offset the pressure/compression at depth.

So, effectively, you are managing your buoyancy via your drysuit. If you're adding air to your BC, one of two things has occurred, either;

• Your weight has changed because you added something while under, or
• You've been adding air to your BC to offset buoyancy lost due to suit compression, and are therefore changing your initial dynamics and not retaining thermal protection.

Did I miss anything?

**edit**
If you're using your BC for buoyancy due to the weight of gear (doubles, backplate, etc.), you should reconsider your weighting configuration. You're doubles, backplate, etc. are not removable, and you're diving dry. You're running too much lead if you need your BC to offset it.

[skdvr]

The above would only be true though if you started your dive with 500lbs of air... I dive double HP 120's and there is about 9lbs (weight) of air in each tank. So with my lite undergarment and no lead I am neutral at 15ft when my doubles have 500lbs in them. So at the start of my dive I am 18lbs to heavy. I definately put air in my wing on the way down, as well as in my suit to get rid of the squeeze. Once at the bottom and neutral it is just fine adjustments with my suit as I descend or ascend, but I do not want enough air in my suit to off set that 18lbs of backgas, I would rather have just enough in my suit to get rid of squeeze and the rest is in my wing...

Phil

[Kingpatzer]

Originally Posted by it_mike

Once neutrally buoyant, the only buoyancy change will be attributable to suit compression, correct? If so, you're adding air to your drysuit to offset compression due to pressure. You're not adding any air to the BC, and you're maintaining buoyancy control via your drysuit.

It is wrong to assume that you'll maintain the same loft in your drysuit under water that you do on the surface. After I *burp* my dry suit, I will still need to get down to 40-60' before there is enough squeeze for me to consider needing to add a puff or two of air (actually argon, but that's beside the point).

But let's ask this a different way --

Is the only need for air in your buoyancy compensator due to suit compression? If so, why is it that when diving in the tropics without a suit, starting neutrally buoyant, you'll still need to add air to your bc at depth?

[it_mike]

Originally Posted by Kingpatzer

Is the only need for air in your buoyancy compensator due to suit compression? If so, why is it that when diving in the tropics without a suit, starting neutrally buoyant, you'll still need to add air to your bc at depth?

Without a suit? or without a dry suit? You'd add air to your bc at depth when diving neoprene because it compresses. If you're truly diving 'without a suit', you shouldn't need gas in your bc at depth (unless you're using gas in your bc at the surface to maintain buoyancy control).

[it_mike]

Originally Posted by Kingpatzer

It is wrong to assume that you'll maintain the same loft in your drysuit under water that you do on the surface. After I *burp* my dry suit, I will still need to get down to 40-60' before there is enough squeeze for me to consider needing to add a puff or two of air (actually argon, but that's beside the point).

Let me respond by saying you're gauging it by 'enough squeeze for me to consider needing to add a puff or two' of gas (regardless of type). This isn't the same as saying there wasn't any compression before 40-60'.

I still submit that if you are properly weighted (neutral with 500psi), than the bc is for surface flotation and to offset anything you've added during the dive.

[Splitlip]

Good thread guys. I learned something today.

[cummings66]

Originally Posted by skdvr

but I do not want enough air in my suit to off set that 18lbs of backgas, I would rather have just enough in my suit to get rid of squeeze and the rest is in my wing...

Phil

I agree completely with Phil. The amount of air you'd need in a drysuit diving double steel 120's, or even double steel 100's is more than I like in a drysuit. I can do it, and I often do do it to stay in practice, but it's much nicer to put air in the wing and use the suit as a drysuit only.

I advocate using it as a BC to stay in practice for when something goes wrong, but I do not advocate using a drysuit as a BC while diving as normal procedures. I believe the Wing is what is supposed to be used there.

There are two camps when it comes to this, Padi and the real world. Phil and I belong to the real world but I'm sure because I've dove with him that he's equally at home wherever he may be as am I.