How much air in your BC?

[Deefstes]

...and that only 2 lbs more negative was readily noticeable to me (that was 16lbs compaired to the 14lbs that I normally have)

Yeah, that's quite interesting. I recently test-dived my new chicken vest with hoodie which, along with my regular 5mm wetsuit, accounted for a fair amount of neoprene. For the first time I could really tell a significant loss of buoyancy as I descended and the neoprene compressed.

I'm enjoying diving more and more as I start to really get my buoyancy down pat, sort out my trim, be aware of minor changes like these etc.

 

[gr8fuldiverNC]

neutral at 15 feet with 500 psi and no air in the bc is proper weighting. add about 4.5 pounds for the air in a full 80 and youve got it. that means just a coupla liters in your bc at depth with a full tank (depending on wetsuit compression of course). anything more is too much, requiring a lot of adjustments for depth changes, making proper trim harder, and increasing your cross section, which means more energy (=more air consumption) to swim.

 

[The Chairman]

I find that my breathing helps to compensate for any variance in buoyancy. If I am a tad light during my safety stop which is usual, I just pause my breathing cycle on the exhale rather than after inhaling. People thought I was nuts for diving Bonne Terre mines with only 14 pounds with my drysuit and a 20 pound wing, but the 16 pounds was far harder to manage. This was with a steel 120.

As a rule of thumb, I try to weight myself at the beginning of the dive so that with my arms and legs folded or crossed and absolutely no air in my BC, the top of my head just barely touches the surface! I add about a pound for every inch that sticks above the surface.

 

[gypsyjim]

I'm enjoying diving more and more as I start to really get my buoyancy down pat, sort out my trim, be aware of minor changes like these etc.

Proper buoyancy allows you to focus on the dive itself, and enjoy it more. In the pre BCD days it was much harder, buoyancy was always touchy, and for me seldom right

 

[D_B]

Yeah, that's quite interesting. I recently test-dived my new chicken vest with Hood which, along with my regular 5mm wetsuit, accounted for a fair amount of neoprene. For the first time I could really tell a significant loss of buoyancy as I descended and the neoprene compressed.

I'm enjoying diving more and more as I start to really get my buoyancy down pat, sort out my trim, be aware of minor changes like these etc.

Yeah, I was really bummed after that dive, I thought my buoyancy control had gone south, I at first thought it was because there are long times between dives for me ... was very happy after I discovered those 2 extra 1lb weights I was looking for
It kinda impressed on me that just a 1lb or two over can be very noticeable

 

[elan]

Makes sense. I forgot to add that we rarely dive AL cylinders but mostly steel and there is very little difference in buoyancy between the full and empty cylinder.

I believe this statement is incorrect. The material of the cylinder has nothing to do with change of buoyancy... I will be the same whether you dive steel or aluminum and the change will be equal to the weight of air you breath out during the dive

 

[Deefstes]

The material of the cylinder has nothing to do with change of buoyancy...

Sure it does. First of all, steel is heavier than aluminium and secondly it is stronger meaning that the cylinder can be smaller and still hold the same amount of air. Seeing as buoyancy is a function of the amount of water displaced (size of the cylinder) and the total weight of the cylinder + air, it will affect the change of buoyancy.

I've not given it much thought as to all the factors that cause steel cylinders to have more constant buoyancy but I don't think the fact that it does can be disputed. I've always just assumed that it was because the same amount of air can fit into a smaller space and that more of the overall weight of a filled steel cylinder is made up by the steel, meaning that if you take the air away you take a smaller fraction of the total weight away than with an aluminium cylinder.

 

[Walter]

Sure it does. First of all, steel is heavier than aluminium and secondly it is stronger meaning that the cylinder can be smaller and still hold the same amount of air. Seeing as buoyancy is a function of the amount of water displaced (size of the cylinder) and the total weight of the cylinder + air, it will affect the change of buoyancy.

Your facts are correct (mostly). Your conclusion is not.

Assume you have an AL tank that holds 6 lbs of air. This tank is 4 lbs negative when full and 2 lbs postive when empty.

Assume you also have a ST tank that hold the exact same amount of air. This tank is 8 lbs negative when full and 2 lbs negative when empty.

There is a 6 lb shift in buoyancy with either tank.

The reason I said your facts are mostly correct is when you said, "First of all, steel is heavier than aluminium." That is true, but perhaps a tad misleading. An AL tank is usually heavier than a ST tank of the same volume (assuming we're talking cubic feet and not liquid volume).

 

[TSandM]

Walter is right (as usual). As you exhaust gas into the water, you get lighter. It doesn't matter what kind of tank the gas is in. If you are underweighted in a steel tank, you will be just as overly buoyant at the end of a dive as you will be in an aluminum tank.

My personal preference is never to be light at the end of a dive. I don't dive in a heavy wetsuit, so I don't have the "incomplete rebound" of compressed neoprene to help me, so I need to be properly weighted for neutral on ascent. I prefer to weight myself neutral at the surface with no air in the BC and only a comfortable amount in the suit -- this means I carry more weight, but ensures I can do a very slow and completely controlled ascent all the way to the end.

You CAN do a lot of compensation with your lungs (especially if you are a six foot man -- less if you're my size) but it's also a good way to build up CO2, if you overuse your breath for buoyancy control. I have also learned that it's a good way to use your gas faster. When I first started running line into caves, I would use my breath to compensate, so I would stay nicely neutral while putting in ties. I also watched my SAC rate go up by 30 to 50%, and thought it was just the stress of line running. A friend pointed out that I should adjust my wing or suit instead, and when I started doing that, my SAC rate dropped significantly.

One of the things I have learned from starting to do some tech-type diving is that overweighting is overstressed as a diving problem. It is a big problem for new divers, but tech divers routinely go into the water extremely heavy from the large amounts of gas they are carrying. You learn to manage the bubble in the wing and suit by anticipating buoyancy changes well ahead of time. Once I realized that absolutely minimizing the weight I carried wasn't that critical, it allowed me to carry a bit more and run my suit looser and stay warmer.

Coping with an uncomfortable ascent -- or being unable to control your ascent rate well in the last ten feet -- seems to me to be an unacceptable price to pay, just to have your BC empty during the dive.

 

[GrumpyOldGuy]

Deefstes

Your volume air weighs the same, AL or Steel, it really does not matter what it is in or ar how much pressure.

What some people forget it with a 7M W/S, you get compression at depth that does not spring back instantly when you return to SS depth. Also, air sometimes gets trapped before you decend and works it way out at depth. You can play with these in your favor to be a little bouyant or neutral the start of the dive and still be neutral at 15' at the end of the dive despite the loss of the gas weight. This is highly dependent on your gear and your max depth, so its a trial and error thing that changes in a non linear manner and won't work the same some someone else or for someone in difference thickness / size of wetsuit.