How much air in BDC at the start of a dive?

[MountainManCO]

I am still new (8 dives) and am about to go on a drive trip again after about 18months since last dive. I am just reviewing all my learning materials and wanted some clarification. How much air should one have in their BDC at the very beginning of the dive. If I recall correctly what I did last time was that I had next to none in the BDC and was weighted enough (4lbs) so that at full exhale I started to descend, and at full inhale I would start to ascend. Is that the right approach?

thanks...

 

[IXΘYΣ]

If you are perfectly weighted, you should have enough air in your BC at the beginning of the dive to balance the weight of the air in your tank(s). So, for an AL 80, you should have about 6 pounds positive buoyancy (about 0.1CF, or about 3 liters) in your BC to counter the six pounds of air in the tank. That way, at the end of the dive, when your tank is five or six pounds lighter than at the beginning of the dive (and therefore you are five or six pounds more buoyant than at the beginning of the dive) , you'll still be able to maintain neutral buoyancy all the way to the surface by dumping all the air from your BC.
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[Dhboner]

Most new divers (especially when boat diving) will inflate their BC enough to float them at the surface then release air to begin the descent. Once your descent has begun you can control your rate of descent by adding air. As you get deeper your wetsuit will compress and you will be less buoyant so you may need to add small burst of air to compensate. Don't overdo it or you might get caught in the "yo-yo" effect.

 

[Indian Valley Scuba]

Good to see you getting back into the water there, MountainManCO.

This is a topic that can be argued till the cows come home, but ideally, if you are weighted to be neutrally buoyant at the end of a dive (near empty cylinder) at your safety stop, then you'll be a couple of pounds overweighted when the cylinder is full, due to the actual weight of the gas in the tank. So at the surface, every pint of air equates to approx one pound of lift, so a couple of short bursts should be good.

But, and the big but here, if you are entering the water from above, i.e. a giant stride off a boat or dock, then you want to have sufficient buoyancy in your BC to bring you back to the surface and not start your dive prematurely. So, a half-full BC should be a good start, and then just vent air until you can begin your descent. No more after that!

Have a good time diving!

 

[MountainManCO]

that makes sense because i remember having trouble at the end of my dive holding my safety stops and i had no air in the BCD, so i should start with some...

 

[rlynch]

actually that boyancy test you are thinking about is valid for a nearly empty tank (~500 psi). At the begining of the dive your tank will be heavier ( there is an apx 5lb swing from Negatively boyant to positively boyant with an AL80 cylinder)
if you are neutral at the start of the dive you will be postive 1/2 way into the dive (~ apx 2.5lbs +) and +5lbs at the end of the dive.

you should weight your rig with full exposure protection on with a tank at 500psi in a controlled enviornment prior to heading out for your trip.

Normally upon initial decent you would deflate your BC completly and then add air to becme neutral at your target depth. I generally deflate fully to begin the decent and will add air to by BC at 10ft, 15ft, 20ish and come to a stop at my target depth. Once you have achieved neutral boyancy you generally don't need to adjust your BC for that depth range. Get it set and then you stop fiddling with it.
You will need to vent as you rise (say following a reef, etc) or add as you decend on a multi-level dive.

Remember always plan your dive & don't just fly the computer

hope that helps.

 

[IXΘYΣ]

that makes sense because i remember having trouble at the end of my dive holding my safety stops and i had no air in the BCD, so i should start with some...

Right... in your example you were underweighted for the dive... if you needed to dump all the air from your BC at the beginning of the dive to get down, then you need to add the weight of the air you're going to use over the course of the dive to your weightbelt (weight system) and compensate for that weight with air in the BC at the beginning of the dive. For years the "neutral with no air in the BC at the safety stop with 500psi" has been taught - it was even in the NAUI advanced book and elsewhere - but I maintain that "neutral at the surface with an empty tank" is the way to go, since the most critical part of the ascent is that portion closest to the surface, and if you're neutral at the safety stop with an empty BC at the end of a dive then you'll be positive - and likely out of control - from the safety stop to the surface, especially if you've used more air than you planned to use.
It's no big deal to carry the weight you need to eliminate one more hassle (being unable to achieve neutral buoyancy for the final part of the ascent) when things have turned to poop and you're coming up on fumes.
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[MountainManCO]

excellent, thanks

 

[DivemasterDennis]

The previous responses cover the area pretty well. Now its time for you to start noting things in your dive log (hopefully you have started doing so already, so that you can use it as a reference for proper weighting. Among the things I log for every dive are : my weight, exact equipment used /worn including wetsuit gauge, fins, bc, octo and weight carried. I also note the location, water temperature, and other relevant conditions. Note any issues on air consumption (seemed fast, seemed good) as well as any feeling of being overweighted or difficulty maintaining buoyancy at a safety stop. As you review your log from time to time (something I think people should do more often) you can correlate the various factors and it will be a big help in proper weighting. Do remember that you will be more buoyant at the end of the dive than at the beginning due to air consumption, but also remember that there is no "rule of thumb" for weighting that works for everyone. We are all different. There is no substitute for a weight check at the time of diving, unless you have a log of diving with he same equipment under the same or very similar conditions when you yourself were in the same shape. If you have that data in your log, then you can use the same weight with a high degree of confidence.
DivemasterDennis

 

[TSandM]

The answer to your question comes out of the name of the device: It is a buoyancy COMPENSATOR. So, if it is compensating, what is it compensating for?

The answer is two-fold. One, it is compensating for the weight of the gas you are carrying at the beginning of the dive, which you are going to exhaust into the water by the end of it. With relatively small tanks, this isn't much -- about five pounds with an Al80 (which is actually, by historical standards, a rather LARGE tank). Before buoyancy compensators were developed, divers compensated for this weight swing with their breathing. Nowadays, we use the air bladder in the BC to do it, but if you realize it can be done with the lungs, you'll recognize that the amount of air required is fairly small.

A bigger consideration is the compression of exposure protection. Neoprene floats, because it insulates by having tiny gas bubbles trapped in the rubber. As you descend, these compress, and the neoprene gets less "floaty". If you were perfectly neutral in five feet of water, you will be negative in ten, and more negative in 100. By how much is determined by how thick the neoprene is. So you can see that, in cold water, you will need much more air in the BC at depth than you do in warm water, where you may be using no exposure protection at all, or a suit which is very thin.

Most dives will have either a square or a check-marked profile -- you descend fairly steadily to your deepest point, and either remain there, or work your way slowly shallower. In either case, the greatest amount of air in the BC will be at that early, deep point, where you are compensating for both gas in the tank and exposure protection compression. As the dive proceeds and you exhaust gas, and move shallower, you will slowly but steadily vent air from the BC. Proper weighting means that at the very end of the dive, in very shallow water, you will no longer have ANY air in the BC at all.

Does that help?