When to adjust BC?

[cork2win]

I had some difficulty (as seems common) with buoyancy during my checkout dives. My specific problem was not adjusting the BC quickly enough when descending or ascending, hence crashing to the bottom or shooting to the surface. I don't mean my initial or final ascents/descents, I mean just slight changes in depth during the dive. Like going from 30 feet to 50 feet then back to 30 feet. I had a long discussion with my instructor about it during the SI and he told me to anticipate better and to always make my BC adjustments on a full breath.

I understand the "anticipate" part and I did a better job with that as the dives continued, but I'm struggling with understanding why I should make all of my adjustments on a full breath. That suggestion makes sense to me when considering making adjustments on my ascents, but not my descents. My thinking goes: When ascending I'm trying to avoid becoming overly buoyant, therefore I want to know how much air I need to dump out of my BC in order to remain neutral. Since I am MOST buoyant on a full breath of air, it makes sense to me to take a full breath, see what happens then dump air from my BC to bring me back to neutral, then continue the process as I ascend.

However, the advice doesn't make sense to me for a descent. Again, I'm thinking that as I'm descending I'm becoming less buoyant and my purpose for adjusting the BC is to keep me from sinking. I am LEAST buoyant on empty lungs so it would make sense to me to adjust my BC on a descent on an exhale, not an inhale. On the exhale I would know exactly how far I sink and then could adjust for that by adding a little air to my BC. If I were to make my descent adjustment on a full breath, wouldn't I ultimately end up slightly positive when I fully exhale?

 

[Charlie99]

, but I'm struggling with understanding why I should make all of my adjustments on a full breath.

I'm also struggling to understand why the instructor told that.

It may sound kind of backwards, but one day I realized that what I actually do is to adjust my buoyancy with my lungs / breathing pattern. Then if my breathing gets too skewed towards lungs mostly full or lungs mostly empty, then I add or release air to the BCD to "recenter" my breathing. This wasn't anything conscious -- just one day I realized that *I was changing my breathing automatically to adjust buoyancy and that's what I used as the cue to add or release air.

It helps that I mostly frog kick, with long periods in between kicks that clearly show me my buoyancy.

As you get experience, you'll get a better feel for how negative or positive you are and how much you need to dump or add.

If you want to specifically practice buoyancy, I'd suggest just doing a series of ups and downs in the 30' to 20' depth range while finning just a little or not at all and noticing the interaction between your breathing, the air in your BCD, and your tendency to go up or down.

It may also help a bit to use oral inflation --- that helps to calibrate you as to amounts of air in your lung vs. air in BCD vs. tendency to move up or down.

However, the advice doesn't make sense to me for a descent. Again, I'm thinking that as I'm descending I'm becoming less buoyant and my purpose for adjusting the BC is to keep me from sinking. I am LEAST buoyant on empty lungs so it would make sense to me to adjust my BC on a descent on an exhale, not an inhale.

A high speed initial descent is unusual in that you may puposely choose to be rather negative.

Use your lungs as part of sensing what is going on. If as you descend your lungs are full, but you are still descending, then you are signfiicantly negative. Keep adding air. When you come to a halt, or almost to a halt while your lungs are still full, then you are getting close to enough air. You'll descend a bit more as you breathe out. Add just a tiny bit of air (or orally put a bit of that full lung of air) into the BCD. At that point you should be pretty much back in control and back to normal, small adjustments.

On the ascentyou should always be going up slowly, so you should never get signficantly away from neutral.

 

[Storm]

I had some difficulty (as seems common) with buoyancy during my checkout dives. My specific problem was not adjusting the BC quickly enough when descending or ascending, hence crashing to the bottom or shooting to the surface. I don't mean my initial or final ascents/descents, I mean just slight changes in depth during the dive. Like going from 30 feet to 50 feet then back to 30 feet. I had a long discussion with my instructor about it during the SI and he told me to anticipate better and to always make my BC adjustments on a full breath.

I understand the "anticipate" part and I did a better job with that as the dives continued, but I'm struggling with understanding why I should make all of my adjustments on a full breath. That suggestion makes sense to me when considering making adjustments on my ascents, but not my descents. My thinking goes: When ascending I'm trying to avoid becoming overly buoyant, therefore I want to know how much air I need to dump out of my BC in order to remain neutral. Since I am MOST buoyant on a full breath of air, it makes sense to me to take a full breath, see what happens then dump air from my BC to bring me back to neutral, then continue the process as I ascend.

However, the advice doesn't make sense to me for a descent. Again, I'm thinking that as I'm descending I'm becoming less buoyant and my purpose for adjusting the BC is to keep me from sinking. I am LEAST buoyant on empty lungs so it would make sense to me to adjust my BC on a descent on an exhale, not an inhale. On the exhale I would know exactly how far I sink and then could adjust for that by adding a little air to my BC. If I were to make my descent adjustment on a full breath, wouldn't I ultimately end up slightly positive when I fully exhale?

Another example of a DI who seems more concerned about getting his quota in and not about teaching HOW to dive.

I finally overcame the buoyancy control issues but before I got there I had to realize that my wing is not an elevator. Once you get to depth, and get neutral, leave the infaltor button alone. (Unless of course you have to perfrom some serious depth changes) Use your lungs for controlling your depth.

Of course,once you start your ascent, you will have to bleed off some air from your BC to stay neutral, but your goal is to use the BC only to maintain your a neitral attitude in the water column.

As far as ascent/descents goes, once yoú've got your weighting dialed in;

On the descent, (here's what I do)
empty your wing completey, then as you descend,
Everytime you have to eqauliize add a very small puff to your wing
(I use the need to equalize as a rough guage as to when I hit pressure changes and have to compentate on my wing...this is just a rough estimate,)
Just before you hit my depth,, you switch from a vertical position to a horizontal position.(ths also helps sow the descent and get you ready for the final few puffs that are needed to get me neutral.

As you am swiming along, use a "kick and glide" motion. During the glide you can then feel if you are positive or negative, and make the final few adjustmet to my wing.

If as you are swimming, you are also doing gentle descents, (again this takes some practice) and you notice you're loosing your neutral buoyancy, taks in a breath to see if you can use you lungs to compensate, if you can't stay neutral, add add very small puff to compentate for the pressure change...again I seem to notice that this usually is about the time I have to equalize.

Just rememebr that your wing inflator button is not an ascend and descend button. It's there just to allow you to stay neutral in the water column. If all else fails, find a really GOOD DI this time and take a peak performance buoyancy course....with the right DI it will greatly improve your control...with the wrong DI, you're wasting your money.

 

[jbichsel]

First off, if you are having what sounds like issues of crashing into the bottom, you are probably carrying too much weight. A proper buoyancy check is probably in order.

Second, why add air to your BC on full lungs vs. empty.

Our lungs act as another buoyancy device allowing us to adjust our depth to a certain degree as we swim. Inhale deeply and we ascend, exhale and we descend.

If you add air to your BC when your descending and your lungs are empty, you end up, generally, adding more air to the BC than you need. Then when you inhale, the added buoyancy of your lungs tends to overcompensate and you start to ascend. This then turns into a roller coaster ride of up-down, up-down, with continual adding and releasing of air to the BC.

However, if you are descending and you add air to the BC with a full breath, you have already inflated your internal buoyancy devices and don't need to add as much air to slow your descent. We are not talking about max full lungs, just a normal to slightly more than normal breath, and adding air to the BC in short bursts.

Remember back to the Fin Pivot or other buoyancy skills you learned in the pool. If taught properly, you should have been adding air to the BC with a full breath, and the reasons should have been explained.

But again, back to my first suggestion. It is common for instructors to overweight student divers during training. The idea is to have them heavier than needed in order to reduce the ICBM effect should a student diver panic and clamp down on the power inflator.

It also tends to be easier to just throw on some extra weight to ensure the student diver sinks rather than taking the time to properly weight them and then teach proper buoyancy skills. The tendancy is to let student divers flounder along, fight the buoyancy issue and hope they "master" or al least overcome the issue on someone elses time. This creates a reliance on the equipment that breeds bad habits, poor skills and potential danger.

Ok, I'm off my soap box now.

 

[nolatom]

I would've said get neutral at about mid-breath. That puts you neutral in the middle, or "average" of your normal breathing cycle. Then, breathe in and you're a little positive, breathe out and you're a little negative. That way you can "breathe" your way over and back down minor obstructions, without BC air adjustments, as no doubt you've discovered already.

if you get neutral on a full breath, then you can only "breathe" your way down, not up.

 

[ClayJar]

One thing that I have not heard taught in the classes I've taken (although it is easy to pick up here if you read enough) is that being overweighted can make it significantly harder to control your buoyancy well. With extra weight, you need extra air in your BC, and since that extra air in your BC compresses and expands with changes in depth (i.e pressure), you magnify the changes in buoyancy versus depth. As rate of volume change versus depth increases the shallower you are, the misfortunate standard operating procedures of some instructors (i.e. overweight their students on what will naturally be shallow dives) makes proper control of buoyancy quite a bit more difficult than it need be.

Charlie99's description of only needing to adjust buoyancy to "center your breathing" is spot-on. Nobody taught me that, but by doing lots of diving, I ended up doing precisely that. If I'm having to breathe to the top of my lungs, I add a shot of air, and if I'm more toward bottoming out my lungs, dumping a bit is in order.

By "centering my breathing" and then using my lungs for fine depth control, I end up with only a very few BC air adjustments on a "normal" dive. I'll get myself neutral at the beginning, and I'll dump on ascent, but in the middle, only significant depth changes for a multi-level dive profile tend to make me reach for the inflator or dumps (although I do occasionally need to fine tune a bit... haven't finished my Perfection specialty, yet ).

So, if you can get your weight dialed in, you can make things *much* easier on yourself, and once you're there, you'll be able to use the "centering your breathing" method (which tends to be harder to use when overweighted, as your lungs are only so big).


Anecdote:

I was showing off a quarry to a buddy [AOW, taking Rescue] this weekend, and I brought a weight bag down to the 15' platform at the end of the first dive. Diving her farmer john and top (um, 3- or 5-mil, I forget) in fresh water with an AL80, she had ten pounds of ditchable and a 2.5-pound trim weight. After five minutes of handing weights back and forth, she'd dropped six pounds of ditchable weight, leaving her with four pounds and the trim weight.

The six pounds of weight in fresh water required 2.7 liters of air in her BC to acheive neutral buoyancy. That's approximately the same as the average amount of air in an "average" adult male's lungs while breathing normally. Back when she did her OW class, she had more than twice that much extra weight! Being weighted properly eliminates all that "rogue air", along with the buoyancy changes it creates as you change depth.

After the next dive (with her newly reduced lead load), she commented how it was noticeably easier to control her buoyancy (a fact which was not lost on me, as I was watching to see how often she was reaching for the hose during the dive). Adding in the fact that less dead weight makes swimming easier (not to mention the climb up and down the entry), it was quite a welcome change.

 

[gcbryan]

You want to be in control at all times. Once you start your descent try getting horizontal. Put short bursts in your BC and keep your descent rate rather slow. Once you are close to the bottom kick just a little to dissipate the downward motion at bit and then when you quit kicking if you are negative add a bit more air.

If you add enough to stop the downward motion completely on descent then when you stop you will start to rise again so stop that downward motion before making your final adjustment.

On ascent I find it helps to be horizontal as well as you have more control but even if you are vertical your goal is to remain neutral for control and then get slightly positive to start movement and then get neutral. This is the way I look at it rather than as a continuous upward movement that you have to fight to control.

 

[cork2win]

If you add air to your BC when your descending and your lungs are empty, you end up, generally, adding more air to the BC than you need. Then when you inhale, the added buoyancy of your lungs tends to overcompensate and you start to ascend. This then turns into a roller coaster ride of up-down, up-down, with continual adding and releasing of air to the BC.

Thanks to everyone for your advice thus far, and jbichsel especially for this bit. This explains why I should make my adjustment on a full breath, which I hadn't obviously thought of.

I believe I am definitely overweighted. I never tried it but I seem to recall having a ton of air in my BC at the surface to stay afloat, and when beginning my initial descent I NEVER completely emptied my BC. I sunk like a rock a few times just by bleeding more air than usual and I know there was still air leftover in my BC on those occasions, so I'm guessing I'm definitely overweighted. I feel like if I dumped all my air with my current weighting I'd crash to the bottom which obviously indicates too much weight. I think this may be my biggest problem and thanks to all of you for explaining how overweighting causes such buoyancy problems at depth, ie: requiring so much more adjustment of the BC to compensate, rather than using breathing to compensate.

I'm actually pretty good at using the breathing to adjust minor changes in depth, the problem comes with a significant change in depth where breathing is not enough to keep neutral and changes in the BC would normally be required.

So, since buoyancy takes so much practice, is there advice on what my first step should be? It seems weighting should be my primary focus. I'm planning (after all this reading) on getting to the quarry and getting in with about 6 pounds less than I was carrying last time. I will dump all of the air from my BC and with the full tank that I'll have on (AL80), if properly weighted, on a full breath of air I should be just below the surface, correct? I say just below the surface because this would indicate that with my tank empty (at the end of the dive) I should be just about properly weighted and floating at eye level on an empty BC and full breath, correct? I will continue this exercise until I can accomplish this feat, then see how my underwater buoyancy is affected. Seem right?

 

[Deep_Diver1]

A big key is starting out as close to neutral as you can. There has been mentions to this in other posts and maybe part of what your instructor was trying to point out. At the beginning of your dive do a buoyancy check which is done in water to deep to stand. Totally geared up and reg in your mouth take a normal breath and release all the air in your BC. IF you sink you are over weigthed, no question. You are shooting to stay at eye level with just the top of your head staying dry. If you acheive this, begin to exhale and you should slowly start to sink.

This is a great starting point that will make it much easier for you to get to neutral buoyancy at depth.

 

[Charlie99]

So, since buoyancy takes so much practice, is there advice on what my first step should be? It seems weighting should be my primary focus.

Yes. As others have noted, being weighted correctly makes life a lot easier, as does wearing less neoprene (both air in your BCD and neoprene compress and expand as you descend and ascend). There isn't much you can do about how much neoprene is needed --- us warm water wusses with 5mm or less have it a lot easier than someone wearing double 7mm.

If you are coming up at the end of the dive with a tank with only 500psi or 1000psi you shouldn't have a lot of air in it. If you do, then you are overweighted.

The easiest way to get in the right ballpark is to do it at the beginning of the first dive of the day. Go ahead and use your full tank. (more on that later). Hang out in the water for a few minutes so that your wetsuit gets saturated with water, as does your BCD. Empty all the air from your BCD and see if you can sink down to 5 or 10'. If so, remove some weight and hand it to your buddy. Keep doing this until you can hang out at 5' with an empty BCD, going up and down by changing volume in your lungs. This is the sort of weighting you would like to have at the end of the dive, but at this point you have less than the proper amount of lead, because you of the extra weight of the air in your tank.

Now adjust for the weight of air in your tank. An AL80 has about 6 pounds of air, which nicely works out to about 1 pound for every 500psi. So, if for example you still have 2500psi in your tank, ADD 5 pounds to whatever you have at that point.

Hopefully, this procedure will get you spot on for being neutral at 5' with no air in your BCD and your tank close to empty. If it doesn't, at least you are close enough that you can zero in on the right amount by dropping or adding 1 or 2 pounds each dive.

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Diving is soooooo much more enjoyable when you can just lazily hang there rather than having to fin away to maintain depth.

You will also probably find that your air consumption goes down significantly too.