Air in BC question

[Tortuga James]

I just read the "Scary first deep dive" thread and was going to ask this question there, but it was very long thread.

There was a lot of talk about when to dump air from your BCD. I always dump all of it at the surface before descent, and if there is any left at depth, finish dumping at the beginning of the dive. I usually never add air until back at the surface, and then only if necessary for flotation.

I usually dive at 60-70 feet, but when I did my AOW dives the past summer on the U352 at 115', I used the same program.

Am I doing something wrong? It seems to work just fine for me and I have really fine tuned my weighting system and easily maintain neutral buoyancy and make adjustments only with my breathing.

My buddy commented the other day that he never sees me add or dump air.

 

[PerroneFord]

If that works for you, awesome. Question though. How do you know you are neutrally buoyant? Meaning, can you stop for 30 seconds to look at something without finning?

 

[jviehe]

I just read the "Scary first deep dive" thread and was going to ask this question there, but it was very long thread.

There was a lot of talk about when to dump air from your BCD. I always dump all of it at the surface before descent, and if there is any left at depth, finish dumping at the beginning of the dive. I usually never add air until back at the surface, and then only if necessary for flotation.

I usually dive at 60-70 feet, but when I did my AOW dives the past summer on the U352 at 115', I used the same program.

Am I doing something wrong? It seems to work just fine for me and I have really fine tuned my weighting system and easily maintain neutral buoyancy and make adjustments only with my breathing.

My buddy commented the other day that he never sees me add or dump air.

Keep in mind, you will gain bouyancy as you dive due to draining your tank, and that you will lose bouyancy as you go deeper due to compression of the wetsuit (and gain it back as you ascend)

 

[rjsimp]

If that procedure works for you and your gear set up that's great! Since you say you are neutral at depth, that's all that matters (and that you don't balloon up at the end of the dive).

There are a lot of variables and everyone is different. A BC's function at depth is its namesake.. "Boyancy Compensator". So, if I am wearing a wet suit that looses boyancy at depth because it compresses, then I need to add air to compensate for that loss of boyancy..

Just like some people use 6 pounds and someone with similar body weight uses 12 pounds to dive, its up to the personal preference, physical makeup, and abilities. For example, my body is much less positive than my wife's body and she uses more weight than I do.

Whatever works best.. but IF you do have air in your BC and you are starting your ascent, it is time to start dumping it and (unless you are overweighted) eventually dump all the air on your way up.

 

[Tortuga James]

If that works for you, awesome. Question though. How do you know you are neutrally buoyant? Meaning, can you stop for 30 seconds to look at something without finning?

That is something I have worked on and have been able to do the last 15 dives or so since my AOW class where my instructor really helped me fine tune my weighting system. I have noticed that I get a little positive towards the end and on my ascent, but can still get negative by dumping all the air in my lungs, even in the last 15 feet or so.

But what I am hearing, and the answer to my question is: I am not missing anything that could jeopardize my safety by NOT having any air in my BC, as long as I can maintain good control throughout the dive and at all depths, right?

 

[herman]

I release maybe 90% of my air at the start of the dive, enough to make me slightly positive but enough I can start a slow controlled decent using breath control. Depending on the depth and wetsuit I may add a little at various depths on the way down and finally trim out at the bottom. Remember, if you are properly weighted you are starting the dive roughtly 5lbs overweight (80 cf tank) and your wetsuit is going to compress along with the air that is remaining in your BC so you will have to add some air at depth to establish a neutral state. During the dive I will add tiny amounts of air. If you are are truly neutral you should have to add some as the dive progresses to compensate for the weight of the air used from your tank. On accent, again depending on depth and wetsuit I may have to release some to compensate for wetsuit and bladder air expansion on the way up. On accent I prefer to stay slightly negative and either swim up or use breath control to control the rate of my accent. This avoids the possibility of an uncontrolled accent. On a deeper dive with a wetsuit you will have to vent some air if you intend to do a safety stop, something I always do reguardless of depth. The objective is to maintain a neutral state throughout the entire dive, due to air use and wetsuit expansion and compression adding and releasing air is necessary to maintain true neutral bouyancy. By dumping all your air at the beginning of the dive you have the potential for a runaway decent, you start decending fast, your wetsuit and BC start to compress adding to your negative bouyancy so you speed up and things keep getting worse. By not releasing air on the accent you have the potential to set up an uncontrolled accent. I don't like to use the term dump. It implies a quick and large release of air. Doing so by a new diver, esp one overweighted like so many are is a great way to set up an uncontrolled decent. Air adjustments should always be in a controlled fashion. Adding or releasing large amounts are OK if you are prepaired to deal with them but many divers are not, slow and controlled is always better.

 

[jviehe]

Right, ideally you should be weighted such that you are neutral at your safety stop with an empty tank and an empty BC. This means you will be negative at the bottom at the start of the dive and have to have some air in your BC. If you have 0 air in your BC at depth with a full tank, you should be unable to hold a safety stop.

 

[jbd]

There was a lot of talk about when to dump air from your BCD. I always dump all of it at the surface before descent, and if there is any left at depth, finish dumping at the beginning of the dive. I usually never add air until back at the surface, and then only if necessary for flotation.

I usually dive at 60-70 feet, but when I did my AOW dives the past summer on the U352 at 115', I used the same program.

Am I doing something wrong? It seems to work just fine for me and I have really fine tuned my weighting system and easily maintain neutral buoyancy and make adjustments only with my breathing.

What is the rest of your initial descent technique? When you dump the air from your BC and exhale, do you slowly sink, motionless? Or do you have to force yourself UW by either executing a skindiving technique or pull yourself down a line?

I would add to PerroneFords question by asking, can you hold a depth motionless for 1 to 3 minutes?

Going back to my questions;

If you have to force yourself UW then you are positively bouyant at the begining of the dive and there will be a depth at which you will be neutrally bouyant. Beyond that point you will be negatively bouyant. Depending on your lung volume you may have quite a depth range that you can compensate for. Generally speaking you would be essentially "flying" yourself through the water to maintain a given depth in this case. At the end of the dive you will become positively bouyant at a depth greater than when you first became neutral at the begining of the dive. This may make it quite difficult to hold a stop unless you are holding on to an ascent line.

If on the otherhand you sink slowly and motionless from the surface then you are starting the dive negatively bouyant and this will only increase as you descend due to all the compression taking place in gear and the body. In shallow water you would have some ability to control this by breathing. There will be a depth at which the only way to maintain that depth you must swim to generate lift, again "flying" yourself through the water.

In this case, judicious use of the BC will allow you to hover motionless in the water column for as long as you like be it 30 seconds or 20 minutes or whatever.

 

[jbd]

Right, ideally you should be weighted such that you are neutral at your safety stop with an empty tank and an empty BC.

I'll muddy the waters here just a bit by disagreeing(respectfully) with this line of thought. While this is a commonly held concept I tend to agree more with donacheson, who has posted several times over the years his idea on this. The depth to be neutrally bouyant with the empty BC and the tank empty(generally taken to be 500 psi) is just beneath the surface. This way the diver has full control of the bouyancy state throughout the dive. From my experience it makes it much easier to hover in shallow(5-10 feet) water when there are choppy surface conditions while one is waiting to re-board the boat.

With the BC and tank empty at 15 feet and neutrally bouyant the diver will become positive as soon as he/she moves up to 14 feet and this trend towards positve will only increase as the ascent continues to the surface.

 

[Tortuga James]

What is the rest of your initial descent technique? When you dump the air from your BC and exhale, do you slowly sink, motionless? Or do you have to force yourself UW by either executing a skindiving technique or pull yourself down a line?

I would add to PerroneFords question by asking, can you hold a depth motionless for 1 to 3 minutes?

Going back to my questions;

If you have to force yourself UW then you are positively bouyant at the begining of the dive and there will be a depth at which you will be neutrally bouyant. Beyond that point you will be negatively bouyant. Depending on your lung volume you may have quite a depth range that you can compensate for. Generally speaking you would be essentially "flying" yourself through the water to maintain a given depth in this case. At the end of the dive you will become positively bouyant at a depth greater than when you first became neutral at the begining of the dive. This may make it quite difficult to hold a stop unless you are holding on to an ascent line.

I am going to have to test that before I can answer the question. Normally, since I mostly dive from a private boat, we descend by pulling ourselves down the anchor line, or empty BC and lungs to sink far enough to turn and kick to the bottom. My last dive (last week) I made a point of testing whether I could hang at 15' without a line for 5 minutes at the end of the dive with 600psi. I could move up and down a foot or so with my breathing and no air in my BC. I will also test even further my assertion that I am neutral at 60' (the bottom where we hunt) during the dive. It is possible that I am slightly negative and breath with my lungs half full to full. I will report on my findings after my next dive. Thanks a million for the food for thought. This is a skill I want to master.

Herman, glad to see you can get online at the beach. Is the weather straightening out yet?