When to adjust BC?

[ClayJar]

IIRC, the NAUI materials state that with a completely empty BC, a full breath of air, and a full tank, you should float at the surface at eye-level. Personally, I haven't found that to be a very useful way to fine tune, although it should give an indication of being grossly overweight.

*Ideally*, if there's a training platform about 5m / 15ft down, that would be where you want to dial in your weight. You bring down extra weights, and it's convenient to have some twos and a three (to allow you to easily go up or down a pound at a time). Lie down on the platform and remove a weight. If you dont float up with a full breath, you're still too heavy -- set down another weight.

Once you're close, you can just hold a weight or two in your hands while you inhale and exhale deeply. You should slightly float up when you inhale a big, deep breath, and you should sink when you do a big exhale. Pick up and set down different weights until you find the point where you're neutral mid-breath and able to lift off with a bit more inhale and sink with a bit more exhale.

Once you're at this neutral point, you're *mostly* done. I'm assuming you're using a good old "normal" Aluminum-80. Take a look at your SPG. If it's at 500psi, you're set -- the weight you have is the weight you need. If you've got more than 500psi, add one pound for each 500psi: For 1000psi, add 1 pound. For 1500psi, add two pounds. 3 pounds for 2000psi, 4 pounds for 2500psi, and 5 pounds for 3000psi. Now, write down the final weight, along with the suit you're wearing and the tank you're using. You're dialed in.

You'll need to repeat this procedure for each different suit you use. If you dive different tanks, on the other hand, you can look up the buoyancy changes from one to another and just add or subtract as necessary.

If you're changing from fresh to salt (or vice versa), you can easily do that, *BUT* you need one more piece of information. With everything dry, put on everything you had on when you dialed in your weight, including your tank (you can hold some of it, too, if that's easier). Now step on a scale and get your total dry weight. To go from freshwater quarry weighting to saltwater weighting, multiply this total dry weight by 0.025 (i.e. 2.5%). That is how much weight you need to add to be neutral in saltwater with this same set of gear. To go from saltwater weighting to fresh water, take your total dry weight in that set of gear and multiply by 0.025 and remove that much weight.

(The thing to remember about going fresh to salt or salt to fresh is that the 2.5% change must be done with "absolute weight", i.e. your weight belt weight plus everything else that is you and your gear, not the "relative weight" of just your weight belt, just like the gas laws require absolute pressure and absolute temperature.)

(Since I typed this, I figured I might as well second Charlie99, and maybe some of it's lagniappe. )

 

[Jasonmh]

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.

Yes, definitely common, it just takes alittle practice an patience, and you can overcome it. The word "slight" it not very exact, but I don't really consider a change in depth from 50 feet to 30 feet to be slight. Two things that helped me with this (and helped me learn how to better control my bouyancy with my lungs is:
- Find something you can use as a visual marker at a certain depth, like a rock, formation, or platform. Get your self neutral and horizontal in front of your reference, then exhale fully and watch yourself drop a few feet, then inhale fully until you rise back to your original depth. Then inhale fully and watch yourself rise above your reference a few feet, then exhale fully to come back down to your original location. This will help you get used to using your lungs to make bouyancy adjustments, and will help you understand your personal limits of how much depth change you can overcome using just your lungs. Personally, I know that I can drop or rise 5-6 feet, and return to my original depth using just my lungs. If I go much beyond that, like 10-15 feet, I have to use my BC or something to control my bouyancy.
-Practice acsending and decending in 10 foot increments. So, when you are descending, stop and 10, 20, and 30, stopping to get neutral at each before moving on. Then do the reverse, back to the surface from 30, 20, 10, stopping for a minute at each to get neutral.

I know that for me, if I am at 30 and relatively neutral, if I then find myself at 45, I am not going to be able to get back up to 30 and stay there on breathing alone. So I would need to either kick back up to 30 and attempt to get neutral agian, or use my BC (which would just mean that I would have to dump even more air as I got back closer to 30).

... If I were to make my descent adjustment on a full breath, wouldn't I ultimately end up slightly positive when I fully exhale?

Actually the opposite would happen, if you get yourself neutral on a full breath, when you exhale you will be negative.

As others have said, being overweighted can make the problem worse. Try real hard to stay horizontal while working on this. Alot of new divers will go vertical to get from 50 to 30, and that can be harder to control. Also, it is easier to stop your ascent at 30 if you are horizontal by kicking down slightly while you adjust your BC. If you are vertical, you don't have much of a way to arrest your acsent other than dumping an ton of air, and then you yo-yo up and down.
Good luck, i'm sure you'll get it.

 

[cork2win]

- Find something you can use as a visual marker at a certain depth, like a rock, formation, or platform. Get your self neutral and horizontal in front of your reference, then exhale fully and watch yourself drop a few feet, then inhale fully until you rise back to your original depth. Then inhale fully and watch yourself rise above your reference a few feet, then exhale fully to come back down to your original location. This will help you get used to using your lungs to make bouyancy adjustments, and will help you understand your personal limits of how much depth change you can overcome using just your lungs. Personally, I know that I can drop or rise 5-6 feet, and return to my original depth using just my lungs. If I go much beyond that, like 10-15 feet, I have to use my BC or something to control my bouyancy.

Ugh, lost my whole response... will try again.

This reminds me of a question I've been meaning to ask. I understand this test. I will perform this test. Assuming I am properly weighted, here is my question. During this test the following happens: I am neutral and eye level with the platform. I fully exhale and descend 5 feet. I immediately fully inhale and ascend 5 feet, bringing me back to eye level with the platform. Now, let's assume that I want to descend those 5 feet and remain there to swim under the platform. Is a BC adjustment necessary due to the increased pressure or to offset the inevitable inhale I have to make? If I were to inhale at this point, wouldn't I just ascend 5 feet finding myself pinned to the bottom of the platform? I understand that I could make a partial inhale to lessen the depth change and prevent my collision with the underside of the platform but is that proper "form"? Is that what I'm supposed to do?

What seems to be happening to me is that I exhale and descend these five feet (or some number of feet) and because of the pressure change, instead of remaining neutral at this new depth I continue to descend due to the decreased buoyancy of my wetsuit and general increase in pressure, so I have to add air to my BC to stop my descent. This has occasionally worked just fine and I can cruise along at my new, lower depth fairly neutral. When I need to go up a few feet to say swim over a line, an inhale is all that is required. However, sometimes my inhale seems to make me ascend too much and I need to quickly bleed off some air before losing control. Has that been because I was overweighted during these dives and probably shouldn't have had to add air to my BC to hold my neutral buoyancy to begin with?

Is there a number, a fixed depth change that I can keep in mind that tells me, ok, you've descended X feet, a short burst of air is now required to keep you neutral vs you can control your descent with a simple inhale? Or ok, you've ascended X feet and now need to bleed off some air to remain neutral vs just exhaling to control my ascent?

This drives me crazy!!! I need to understand!!

I really want to thank everyone for their answers so far. I know you all must get really tired of answering these questions over and over again.

 

[ClayJar]

We never tire... or at least not a mere 13 posts in.

If you're properly weighted, the volume change of the air in your BC for a few feet up or down at depth is going to be small enough that you can offset it easily by merely breathing a bit more toward the top or bottom of your lungs, i.e. if you've descended five feet, you just breathe a little deeper on the inhale and exhale a little less, and if you've ascended, you just exhale a little more and don't inhale quite as fully. You don't actually change the amount of air you're breathing with each breath, just the amount of reserve air you have above or below that breath. You only need to add or remove BC air when you run out of room on the inhale (to compensate for moving deeper) or exhale (to compensate for getting shallower), and then you only add or remove a small amount -- just enough that you "center your breathing", i.e. you have plenty of room to inhale more to ascend or exhale more to descend.

When you're overweighted, the volume change of the air in your BC can overwhelm the size of your lungs, making it much harder (if not impossible) to stay neutral by merely shifting your breathing to allow more or less reserve in your lungs. Once you're better weighted, it becomes quite easy to pick up, and in fact, soon you might not even realize you're using your breathing to compensate.

(There's no fixed feet-per-burst, as that changes based on where in the water column you are. The deeper you go, the less the change of BC air volume per foot of depth. When you're down on a wreck at 70 feet, you may not have to reach for your inflator for almost the whole dive, while if you're on a shore dive at some jetties, you may have to work hard to control depth with breathing alone.)

 

[Charlie99]

Just a couple more comments ---- as long as you haven't gone up or down so far that your wetsuit or air-in-the-BCD has changed buoyancy a lot, you can exhale a bit extra, sink down and then when you breathe normally again you will stay at the same depth.

It's only when making large changes in depth that you need to fiddle with the BCD.

"Large" depends a things like whether or not you are overweighted, whether it is early in the dive and you are 6 pounds heavy from the air in your tank, and how much neoprene you are wearing.

As Clayjar notes, your depth also makes a big difference. A 60' to 50' depth change isn't very much. A 20' to 10' depth change is pretty large. What counts is the relative pressure change (think back to that balloon in our OW course and Boyle's law).

At some point, you just gotta go do it. Then think about it a bit and go do it again.

At some point after doing it again and again and again, this part of diving really does become automatic, much like breathing is pretty automatic.

Just remember that breathing and air in the BCD don't directly affect your DEPTH --- they affect how fast and what direction you CHANGE depth. Rather than figuring out X number of feet that you can change, what you need to be sensitive to is how far off of neutral you can get before having to fiddle with the BCD. If you have full lungs and you are descending, you are waaaaay off. If you empty your lungs and you are still ascending you are waaaaay off.

Doing a motionless safety stop, starting at 20' and slowly, slowly ascending to 10' over a 3 minute period is an excellent way of practicing buoyancy control on every dive. You'll quickly find that the extra vertical drag of being horizontal helps in doing this.

 

[Jasonmh]

...let's assume that I want to descend those 5 feet and remain there to swim under the platform. Is a BC adjustment necessary due to the increased pressure or to offset the inevitable inhale I have to make? If I were to inhale at this point, wouldn't I just ascend 5 feet finding myself pinned to the bottom of the platform? I understand that I could make a partial inhale to lessen the depth change and prevent my collision with the underside of the platform but is that proper "form"? Is that what I'm supposed to do?

That is a good question, the definative answer is "it depends". It depends on alot of things, like your weighting, equipment, lung volume, etc. I like to be comfortable, so if I get to the point where I am working too hard to keep my bouyancy with my lungs, then that is a signal to me that I need to adjust the air in my BC. If it is a small platform, I can move down 5 feet or so by exhaling, take a few smaller breaths while swimming under the platform, then one big breath when I get to the other side to rise again. That is pretty comfortable for me, but I can't answer for anyone else. You may find it to be a far bigger effort than I do, or you may find it easier than I do. I try to not make any adjustments to my BC if I know that the depth change is only going to be a small amount, and especially when I know that depth change is only temporary.

...What seems to be happening to me is that I exhale and descend these five feet (or some number of feet) and because of the pressure change, instead of remaining neutral at this new depth I continue to descend due to the decreased buoyancy of my wetsuit and general increase in pressure, so I have to add air to my BC to stop my descent. This has occasionally worked just fine and I can cruise along at my new, lower depth fairly neutral. When I need to go up a few feet to say swim over a line, an inhale is all that is required. However, sometimes my inhale seems to make me ascend too much and I need to quickly bleed off some air before losing control. ?

For this example, if I am at 30 feet and neutral, just swimming along, and I run into a line that is also at 30 feet that I have to go over, I would try to inhale, rise and swim over it, and then exhale on the other side to return to 30 feet. If I ended up rising too much, for example if I go up 10 feet instead of 5, I would exhale and swim down slightly, knowing that since I was neutral at 30 feet, and I haven't added/subtracted air from my BC, then I will be neutral again when I return to 30 feet. If you find that you have risen too far, and try to dump a bunch of air, first you will have to dump alot to slow then stop then reverse your acent, and second you will have to add a bunch of air when you get back to 30 feet, since you now don't have enough air in your BC to be neutral at 30 anymore. Then you get in the cycle of adding and subtracting, and slowly yo-yoing up and down.
Now if my depth change was not temporary, as in I rise to get over the line and decide that I want to just stay at 20 feet and not return to 30, then I definitely adjust my BC, knowing that if I don't I will be struggling to maintain at 20, and that is no fun.

... Has that been because I was overweighted during these dives and probably shouldn't have had to add air to my BC to hold my neutral buoyancy to begin with?

It sure could be. Being overweighted makes the whole process much more difficult, and as Charlie, ClayJar, and others have said, should be something that you try to correct asap, since it is going to make a big difference.

... Is there a number, a fixed depth change that I can keep in mind that tells me, ok, you've descended X feet, a short burst of air is now required to keep you neutral vs you can control your descent with a simple inhale? Or ok, you've ascended X feet and now need to bleed off some air to remain neutral vs just exhaling to control my ascent?

Unfortunately there is no number, it can be different for everyone. The good thing is that once you are aware of it, and working on it, if is something that you can learn very quickly. This is not something you have to have hundreds of dives to get down, it just takes a concentrated effort, and the reward is worth the practice time.
I found that the other drill I talked about, pausing at 10, 20, and 30 both going up and down, helped me to figure out what works for me. If you do this, you are going to get into a pattern where you know that if you are ascending to 20 feet, that you need to give a half-second tug on your deflator at 24 feet will get you to park right at 20 and be neutral. Or maybe for you it is a full second tug at 25 feet, it just takes some pratice. It also helps keep your descents under control, by knowing how much air you have to add to your BC to be able to stop your descent, without crashing in to the bottom.

...This drives me crazy!!! I need to understand!!

I really want to thank everyone for their answers so far. I know you all must get really tired of answering these questions over and over again.

It can be difficult to explain in writing, controlling your breathing, but ClayJar gave a pretty good description. If you are having to take full deep breaths and hold them for longer than normal to keep from sinking... you are too negative, and need to adjust, because noone wants to spend a dive struggling to maintain depth like that. You should be able to breathe close to your normal tidal lung volume, not taking HUGE breaths, and not having to exhale constantly. If you are at 30 feet, and negative but compensating by taking huge breaths and holding them to keep from sinking, then how are you going to be able to rise above that line at 30 feet? You can't, because your lungs are already full at 30, and there is no "extra" room left to make that adjustment with your lungs. I hope that makes sense.

 

[scubafool]

Excellent, excellent thread, everyone. Really good explanations, some of the better I have seen.

One thing I would add is this. When breathing out deeply to descend, as soon as you start descending, you need to start inhaling to slow the process down. If you stay on the very bottom of your breathing cycle for too long after the descent starts, it will be much harder to stop the descent as your wetsuit and the air in your BC starts to compress, and oftentimes you will have to add air to your BC to correct, as things get too far gone to correct with breathing. It takes a moment or so for changes in volume of your lungs to affect your bouyancy, so you need to be proactive with your lungs just as you do with your BC.

 

[CJ Waid]

Get so you float eye level with no air in your BC, like you were probably taught, and use your lungs from then on.

Try going out alone one time in shallow water, like 10-20 feet, and do a toe pivot to check your buoyancy, once you raise up with a breath and fall back down when you exhale, swim along the bottom bor a while using your lungs to go up and down, anticipate the acent and decent when you inhale and exhale, if you are prepared for it, it helps to stay in control.

When you are neutral for the first time, you will know it. It feels like you are in outer space or something. You stay where ever you are in the water, and the only way you go up and down is by breathing.

You should check out the PADI Peak Performance Buoyancy course. My instructor wrote it, I was neutrally buoyant within my first few dives of being certified, I mean like almost mastered buoyancy control, it is all in the lungs. But, my first couple dives I was stirring up the bottom or floating to the top.

 

[Jasonmh]

Get so you float eye level with no air in your BC, like you were probably taught, and use your lungs from then on.

Try going out alone one time in shallow water, like 10-20 feet,

With no air in your bc you're going to be negative, and why in the world would you want to practice this "alone"? Not trying to be rude, but telling a new diver to dive alone is not a good answer, ever.

 

[CJ Waid]

I am honestly sharing my opinion. I started diving alone right after I got certified in OW, I would dive a few dives with my buddy during the day, then when he had to go home, I would go burn off tanks by myself in shallow water. I learned a lot more diving in shallow water on my own than I did trying figure things out when I had to consider other people in the water too. Come on, 10 feet deep is not going to do anything, except maybe make someone more comfortable in the water.