How much air in your BC?

This question probably deals with weighting as much as it does with BC/Wings usage. There has been some discussion recently on how divers often overweight themselves and also in a recent thread some discussion on how much air you have in your BC.

So basically what I'd like to know is this. What do you consider the ideal balance between weights and air in your BC/Wing? I've fiddled around with my weights until I managed to get neutrally buoyant on the reef (we mostly dive to around 20m) without having any air in my BC. I just don't like having to compensate for being too heavy on the reef. This means however that I'm a little positively buoyant on my safety stop.

The difference is really minimal though and I don't always even have to fin much to stay on my safety stops. On some dives, when I have something extra in my BC pocket or whatever, I do have to inflate my BC a little when I'm on the reef but I try to weight myself so that is not necessary. I'm starting to think now that this is not the best way though.

In a recent thread another diver mentioned that he prefers to be weighted such that he will be neutral on his safety stop with no air in his BC/Wing and add some air when at depth. This sounds like a better approach to me.

Any thoughts?

The amount of air in your BC should be the quantity required to offset the weight of gas in your cylinder(s) at the start of the dive if you are wearing a drysuit.

If wearing a wetsuit it should be whatever you need to stay neutral, but the amount of lead that you carry should be the amount that you need to be neutral at 10 feet with an empty cylinder(s)

I agree with the second approach, I do the same... weight is adjusted such that in the safety stop with empty tank I am neutral with no air in my BC and at depth I add some air to the BC.

Thalassmania hit it on the head. At the end of your next dive while you are on your safety stop, try dumping your BC and decreasing the gas left in your tank down to 500 psi. When I did this originally, I put my thumb and forefinger in a ring around the line and passed weights back and forth with a buddy until my fingers didn't move up or down the line. You will be happier at the end of your dive with a pound or two more. Besides, if you don't put air in your BC to compensate at the start of a dive, then you may as well sell it and use a harness.

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. Either way, I do think that you are all correct and that I should rather adjust my weighting to be suitable for a neutral safety stop rather than a neutral bottom time.

smellzlikefish:

Besides, if you don't put air in your BC to compensate at the start of a dive, then you may as well sell it and use a harness.

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You make it sound like a bad thing. This is what I was hoping to achieve, a point where I have a BC but don't really need a BC - and I think I've been getting close.

I want to be negative (slightly) even at 2 feet depth. I do not ever want to rise from my safety stop to the surface at any rate faster than ultra-slow.

I'm also a fan of getting to 100 feet as fast as possible, so while I don't grossly overweight myself anymore, I do have more weight on me than it would take to be neutrally buoyant anywhere in the water column.

Deefstes:

Any thoughts?

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There is no shame in having air in your BC, that is what it's there for.

The ideal point has your BC empty when you are neutrally buoyant, with a light (500 PSI+-) cylinder at the end of the dive.

Other than that you have neoprene compression and the weight of air to compensate for. With limited neoprene you can manage a lot of this with lung volume but you need to decide how much of this you want to deal with. The question is no different be it BP&W, jacket or horse collar.

For instance if I make a dive in a shorty (limited noprene) with an 80 CF cylinder and no BC I can weight my self to make the dive. For the first 1000 PSI or so I need to keep my lungs fairly full most of the time. The second 1000 PSI is pure bliss with no need to do anything but relax and swim in 3 dimensions. The tail of the dive has me needing to emphasize deep exhales to lower my average lung volume and resulting buoyancy. You also need to swim the dive a little to manage the buoyancy changes as the cylinder empties. As you add neoprene or dive higher capacity cylinders you eventually exceed the buoyancy swings that your lungs can manage. I prefer to let the BC manage the swings of tank weight and neoprene behavior. I use lung volume to manage the contours of the site, allowing a 10-15 range oh hands off swimming.

You are trying to quantify how much air you would ideally carry in your bladder and you can visualize this in English measure quite easily and of course in metric as well. A gallon of water weighs about 8 pounds so a pint of air is equal to 1 pound of lift.

Let's say you're diving an 80 CF cylinder and plan to dive about 62 CF of the air. At .08 LB per cf that represents 5 pounds to compensate for with all things being equal. That equates to starting the dive wanting 5 pints of air or a little over 2 liters. Remember that this is a slippery slope. The air that gave you 5 pounds of lift at 20 feet will compress at a deeper depth and you will need to add air to maintain displacement (lift/buoyancy). This is the real reason to cut your weights close to minimum. The actual lift of your BC will change significantly as the bubble expands and contracts with depth. The less air in the BC the more stable you will be.

The diver you cite at the end of your post has it right.

Pete

All good points. There is a simple way. Do a buoyancy check. When you have that right, add 4 lbs if you're diving an aluminum 80 (probably less with a steel tank-depending on the make and model- but you'll have to play with it). You should be neutral at the safety stop with an empty BC.

Thal, Fish, Spectrum, Gorilla .. are correct .... no air in BC at 10ft with 500/400psi


funny, I noticed on one dive recently that my buoyancy was "touchy" ... just the slightest depth change would cause what I thought was too much fiddling with dumping/adding air to my BC ... I found that I had left 2 extra lbs left over from an earlier experiment and forgot to remove them from my pouches ... glad that I found the cause of my new buoyancy instability, and that only 2 lbs more negative was readily noticeable to me (that was 16lbs compaired to the 14lbs that I normally have)

Deefstes:

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. .

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keep in mind the difference in buoyancy is still equal to the weight of the air used, whether the cylinder is steel or aluminum.