Drysuit or BC for inflation

[DRE]

Before you get into the debate whether to use drysuit or BCD for boyancy, you first have to take a look at your rig, which should be balanced (meaning at your 20ft stop you should be neutral with no gas in the BCD). You wnat to take away the squeeze from the drysuit, but let's look at what happens when you start using the drysuit for boyancy. Nomads has already given most of the reasons why not to use the drysuit for boyancy, but here's another one. When diving you should never be in a feet down position, in fact the feet should be the highest point of your body (either using a frog kick or modified flutter). Whe you use the drysuit for inflation, all the air will go to the feet, thus making it impossible to kick. That's when people start using all this useless contraptions such as ankle weights to keep the feet down, cause they're afraid of a feet first ascent, but that should never happen in the first place. (Has anyone ever tried to do a helicopter spin UW with the head down feet up - you still are not going to make a feet first ascent as long as your boyancy doesn't change) A drysuit never deflates as fast as a wing, that's why it's much asier to control the wing. Just take away the squeeze, and your feet will not balloon, while you'll be able to comfortably remain in a perfect horizontal position (any other position is useless to diving as it is extremely counterproductive to efficient gas transfer in the lungs, thus enhancing your risk of DCS).

[tbrown]

DRE, I agree with you on most points, it's you final conclusion that I differ with you on. Firstly for this discusion, I'm assuming a recreational diving scenerio (single tank not doubles). I agree that you have to weighed properly to start with, although I have my neutal weight at 15 instead of 20 ft. Given that amount of lead, the amount of air that you put into your drysuit to prevent squeeze should be enough to keep you neutral. Works great in a shell suit, think there may be some issues with a neoprine (suit compression in a shell is a non factor, not sure on neoprine). Since your not adding large amounts of air, ankle inflation should not be an issue. I do think that ankle weights are not the way to go, if you have ankle inflation, get gaiters instead.

[beeger]

I only use my BC for buoyancy. My drysuit gets inflated only as much as needed to prevent squeeze. The reasons for this have been outlined by Nomads and DRE above.

But I wanted to address the weighting discussion which seems to be developing. Here's how I think proper weghting should be defined: With full tank(s) determine the amount of weight you require to sink from the surface on a standard exhalation. This is the amount of weight that should be integral to your rig, distributed in such a manner that will provide for good trim in the water. Then, with almost empty tank(s), (maybe 200-300 psi) determine the amount of weight in addition to the above that you require to hover just beneath the surface (10-15 ft). This is the amount of ditchable weight that must be worn (weight belt, pockets, etc.) in addition to the integral weight. Once you have this down then consider the following factor: depending on the exposure suit being worn and the depths you will be diving to, compensation must be made for the compression of the exposure suit. This is done by figuring out how much buoyancy your *suit* loses, independent of the diver, at depth (hint: the buoyancy of the suit will decrease by 1/2 for the first 33 ft). The amount of buoyancy lost will equate to the amount of weight that must be moved from the integral component of your weight to the ditchable component. When diving a shell suit the buoyancy shift of the suit is not really an issue. When diving a foam neoprene wetsuit or drysuit it becomes a big issue. The thicker the suit, the bigger the potential buoyancy loss, the
more weight needs to be moved.

Here's what it comes down to: The worst case scenario is that you have a BC and drysuit failure at the beginning of the dive (when your tanks are heaviest) and at max depth (when your exposure suit has compressed the most). Dropping the ditchable weight will reestablish your neutral buoyancy. And this is where ditchable weight systems that allow you to drop part OR all of your weight are really better than those that dump all your weight (like a weight belt). Given the above scenario, the shallower you are, the less weight you would have to drop. Dropping too much would actually make you overly buoyant.

Maybe this is off topic. Perhaps it belongs in its own thread.

[DRE]

[quote='tbrown link' dateline='1043895460']
DRE, I agree with you on most points, it's you final conclusion that I differ with you on.


Do you mean the DCS component?[/quote]

[freedivernd]

I wonder if the Boyle-Mariotte law comes into play here and it's pressure doubled not depth doubled. ie. 33', 98', 229', etc.

Fred

[tbrown]

Well actually David, I was refering to an out of control feet first ascent. But no I don't think that if you use your drysuit for inflation that you will get DCS.

[Punky]

I found the info on weighting from Beeger very helpful.
I do have a question.
If you are using a neoprene dry suit, as you ascend wouldn't the suit regain buoyancy as pressure decreases?
You would have to be careful how much weight you drop so as not to become overly buoyant as you ascend.
I use a shell suit with underwear, just recently
certified so this is new to me.

[DRE]

You would only drop weight in case of a catastrophic bouyancy loss (as in rupturing your BCD). At that point you're not really concerned anymore with having a rapid ascent in the shallower part of the dive - you just want to get to the surface. If you have any deco obligation, doing proper deep stops and using the proper gasses will diminish the severity of DCS symptoms, but a ride in the chamber would be the probable outcome of such a scenario.

[beeger]

Punky - exactly. That's the major disadvantage with foam neoprene drysuits, or even thick wetsuits. The massive buoyancy shift (I'm guessing around 20 lbs) from surface to, say, 100 ft makes maintaining neutral buoyancy tricky. You are going to have to add weight on the surface and then counteract at depth by adding air to the BC/wing. While this is true with most suits (wet or dry) - the bigger to shift in buoyancy, the more weight you have to add on the surface, and the more air you have to add to the BC at depth. More air means a bigger profile in the water, which means more air to manage when it comes time to remove it on ascents, which means less streamlined, which means more work in moving through the water, which means etc. etc. etc. etc..

(On a related note: foam neoprene drysuits are not all bad. The low cost, durability, inherent insulation, and streamlined fit are all considerable advantages. However, a properly fitted, well built, durable material shell suit worn with proper underwear will always be the better option, all things considered.)