Ok, a serious question about balanced rigs

[don Francisco]

The problem with the concept of a balanced rig, or trying to balance one is that both the weights and displacements change with depth and time. Your tanks will get lighter as you breathe them down, and your displacement will decrease with depth as your wetsuit compresses.

You need sufficient ballast for the "lightest" situation; shallow depth and empty tank at the end of the dive. Unless you plan to pick up additional weights on the bottom during the dive, you'll be overweighted at the beginning of the dive when you reach maximum depth with an almost full tank .

The difference between the beginning bouyancy and ending bouyancy is what you're compensating with the bouyancy compensator, because no fixed combination of rig and ballast can be neutral throughout the dive.

 

[JeffG]

I have to warn you that some folks thought this to be really stupid, here're the threads:
1

Someone agreed with me on that thread. I like it.

 

[Slonda828]

The usual thought process is to avoid excessively negative tanks if diving wet. Rather than double hp120s which, when combined with a steel backplate and other hardware , can well exceed 20 pounds of negative, a wetsuit diver would be better served by doubling up al 80s on an al or plastic backplate and therefore start off with less than 10 pounds of negative.

Steve brings up a great point. When diving without a BC at all like I do when diving vintage equipment, I calculate which cylinder I am going to use (usually a LP72 or an AL80) depending on which type of exposure protection I plan on using. If I'm diving a thick suit, I dive steel. There's no reason you couldn't have a reasonably balanced doubles rig if you chose cylinders whose buoyancy charecteristics were close to the balance of how much weight you were wearing at a specified depth. This is also why the Cousteau team used buoyancy floats attached to them by lines when they dove heavy triple or double tank setups in warm water.

 

[RJP]

You do not feel the squeeze in a wetsuit because the compression is even whereas with a dry suit it is not, i.e. the folds cause uneven compression - thus the squeeze.

Um, actually you don't feel the "squeeze" in a wetsuit because there isn't any. In order for their to be "squeeze" you need a closed container. A wetsuit is open to the water so it's sort of "self equalizing" if you will. While it is being compressed it's actually being compressed on both sides, so no squeeze to the wearer.

For all intensive purposes...

[peeve]

for all INTENTS AND purposes...

[/peeve]

 

[jeckyll]

My understanding is that a "balanced rig" is is either neutrally buoyant or a little negative, such that you can swim it to the surface if you lose buoyancy... <snip>

In a perfect setup you'll be negative the amount of the gas swing at the beginning of a dive. No way around that.

Next, you have to account for suit compression if diving wet. If the gas weight + loss of buoyancy due to compression exceeds ability to swim up, you need to consider either changing tanks or switching to a drysuit. I wouldn't go trying to reinvent the wheel.

Then again, I'm in Canada, so I dive dry anyways.

Just my $0.02.

 

[Reg Braithwaite]

I'm in Canada, so I dive dry anyways.

I'm also in Canada, and last season being the only wet diver on the boat convinced me to go dry. Then this season it seemed like I was the only dry diver on each charter, and the temps were in the thirties...

All that being said, air is still compressible, so we still need to compensate for depth, no? Just a lot less than neoprene, correct?

 

[dumpsterDiver]

I'm also in Canada, and last season being the only wet diver on the boat convinced me to go dry. Then this season it seemed like I was the only dry diver on each charter, and the temps were in the thirties...

All that being said, air is still compressible, so we still need to compensate for depth, no? Just a lot less than neoprene, correct?

Yes, you also have to compensate for compression of the lungs at depth, just like you do for the air in the dry suit.

 

[JeffG]

All that being said, air is still compressible, so we still need to compensate for depth, no? Just a lot less than neoprene, correct?

With a wetsuit you compensate by adding gas to the wing to offset the compression in the wetsuit.

With a drysuit, you add gas to the drysuit to maintain a ~constant level of buoyancy.

 

[LeadTurn_SD]

Yes, you also have to compensate for compression of the lungs at depth, just like you do for the air in the dry suit.

If you are breath-hold diving your lungs compress..... but not with scuba I hope, where we get to fill our lungs with each breath from those heavy cylindrical do-dads on our backs

 

[Lucy's Diver]

With a wetsuit you compensate by adding gas to the wing to offset the compression in the wetsuit.

With a drysuit, you add gas to the drysuit to maintain a ~constant level of buoyancy.

With havy doubles you end up not controlling nouyancy with the drysuit. YOu'd hve to add too much air. You end up managing bouyancy with both wing and suit, essentially putting enough ait in the suit to just avoid the squeeze.

Which raises the issues of the utility of this concept when diving dry. There are plenty of rigs out there you can't make bouyant with a drysuit. Twin steel 120s with two slung steel 40s (with all the stuff that goes with that) is not going to be swum up no matter you what do with the drysuit.