Weekend OW class. Pool

[Warren_L]

In the end, when you want to start your ascent (assuming you are neutral at the time), whatever it takes to begin that ascent, be it adding a little air to the BC, taking a larger breath in your lungs, or kicking up, shouldn't really be a big issue. All that you are really looking to do is to initiate the ascent. Once initiated, gas from the BC will be vented to control that descent. That said, I personally do not like to teach adding air to the BC to begin ascent. It can be easier for a student to forget that it is only a little air that is required and not used as an elevator ride. When new divers are just getting a feel for their buoyancy, it can be difficult to judge how much air in the BC has what effect and how quickly those effects take place. Taking a bigger breath - sure that works too and I have no problem with that. Kicking up - no confusion about adding too much air or continuing to add air after ascent begins. I think any of these methods are acceptable in practice, but for teaching purposes I prefer the kicking method (I teach PADI) and the increased lung volume method.

 

[BabyDuck]

also, isn't it true that going from 10-15 feet to surface is harder to control than going from, say, 40 feet to 25-30? i know that control at 15ft is of utmost importance, but if that is the total pool depth, and you aren't lying on the bottom, you're of course shallower than that. the pool i learned in was 6ft deep, and i had some problems inadvertently popping up that haven't happened in deeper water. cesa was also really tough to grasp... of course, i hope i'm a bit better at this now!

 

[Warren_L]

Yes, you are absolutely correct. The percentage change in pressure for a change in a foot of depth is much greater in the shallower depths such as 15 feet than down deep. Because of this, the air spaces expand at a greater rate and requires more frequent adjustment (whether it be through your BC or breathing - actually likely both) and can be more difficult to manage. If you can do this well in shallower water, the down deeper should be no problem.

 

[jbd]

This and the other posts by JBD are what I was referring to. JBD is apparently a NAUI instructor and Buoyancy control specialist. That's why I put a question mark after NAUI in my earlier post. Not sure if that is a NAUI thing or individual instructor thing. I know that with PADI, I was taught to swim up.

I recognized that in the posts above, it could seem confusing to a new diver and that it was ok to use the BCD like an elevator. That's why I put that it doesn't matter which method you use, as long as you have a controlled, slow ascent.

Perhaps JBD can explain his approach a little more?

I will gladly try to explain a little better. I dive in a horizontal(prone) position. I ascend this way and teach my students to do the same. What I mean by,"The BC is what takes you up"(see post #13) is that the increasing volume of the gas in the BC is what takes you to the surface. There is absolutely no reason to add any air to the BC at all if you are actually neutrally buoyant at any particular depth.

Starting from neutrally buoyant, all that one needs to do is inhale. This will cause the diver to become very slightly positively buoyant, which will start the ascent. At this point the diver can still stop the ascent by simply exhaling to a point where neutral buoyancy is once again established or by exhaling even deeper the diver can become slightly negatively buoyant.

As the ascent continue's, the gas in the BC is expanding according to Boyle's law. To a certain point the diver can control this with breathing; beyond that point the volume of the BC will great enough that the lungs can no longer compensate and the BC will now take over the ascent, which will be picking up speed. The diver controls this by releasing air from the BC at the appropriate times throughout the ascent so that the rate of ascent is controlled.

Boyle's law is in effect whenever we ascend or descend and Archimedes principle applies whenever we get in the water. The key is to learn to use these facts of science to make diving easier, safer and more enjoyable.

I teach this in my OW course and in a Mastering Buoyancy Control seminar that I created.

Does that clear things up at least a little bit?

 

[jbd]

I just read Warren L's post(#21) and I agree that all of the methods he mentions will work. In each case the the basis of the ascent is that the diver has become positively buoyant by changing the volume of the gas in the BC.

One respondent mentioned that if he stops finning he will begin to sink. This, IMO, indicates that the diver is somewhat overweighted.

 

[Stirling]

I just read Warren L's post(#21) and I agree that all of the methods he mentions will work. In each case the the basis of the ascent is that the diver has become positively buoyant by changing the volume of the gas in the BC.

One respondent mentioned that if he stops finning he will begin to sink. This, IMO, indicates that the diver is somewhat overweighted.

I think I'm the person you are referring to. Actually, I have always carried very little weight. I also have very little use for a BC most of the time; I am not one to cycle hundreds of psi through a BC in the course of a dive, inflating and dumping and inflating and dumping air all the time in a constant effort to adjust buoyancy. I can do most of that with my breathing and posture, and will generally add air only to adjust my buoyancy at a depth where I expect to remain for a while.

I was describing how I would ascend from, say 60 or 80 feet to 20 feet. If I am at the end of the dive and will be neutral at 20 feet with no air in the BC, I will be negative most of the way between 80 feet and 20 feet with no air in the BC, and that is how I swim up. Whatever air I have in the BC at 80 feet is going to be dumped just as I am starting up, I get negative, and I swim up. If I stop swimming up at 40' or 50', I will immediately stop ascending and begin to sink again if I stay still. Depending on the amount of air remaining in the tank, I won't get neutral again until I am approaching 20'. If I am going to be neutral at 20' with the BC empty, then I am still going to be slightly negative at 30' with the BC empty, and it's already empty at 30'.

Of course it is possible to leave some air in the BC from 80' and use that to boost your ascent, dumping air from the BC as you ascend to avoid ascending too quickly. I just don't see the point. As I said, it may be a holdover from the days of diving with a horsecollar vest that had to be orally inflated, but I think I have better control over my ascent by using my fins than the people I have seen bobbing around with their hands on their power inflators.

 

[MoonWrasse]

also, isn't it true that going from 10-15 feet to surface is harder to control than going from, say, 40 feet to 25-30? i know that control at 15ft is of utmost importance, but if that is the total pool depth, and you aren't lying on the bottom, you're of course shallower than that. the pool i learned in was 6ft deep, and i had some problems inadvertently popping up that haven't happened in deeper water. cesa was also really tough to grasp... of course, i hope i'm a bit better at this now!

That certainly was true for me during my OW class. I could easily arrive at the 5 meter safety stop on ascent, hang there for 30 seconds or so, and dirft upward even when emptying my lungs. It took me a few dives to fix this problem and fortunately I've only destroyed the brain cells I don't need

 

[Warren_L]

I think I'm the person you are referring to. Actually, I have always carried very little weight. I also have very little use for a BC most of the time; I am not one to cycle hundreds of psi through a BC in the course of a dive, inflating and dumping and inflating and dumping air all the time in a constant effort to adjust buoyancy. I can do most of that with my breathing and posture, and will generally add air only to adjust my buoyancy at a depth where I expect to remain for a while.

I was describing how I would ascend from, say 60 or 80 feet to 20 feet. If I am at the end of the dive and will be neutral at 20 feet with no air in the BC, I will be negative most of the way between 80 feet and 20 feet with no air in the BC, and that is how I swim up. Whatever air I have in the BC at 80 feet is going to be dumped just as I am starting up, I get negative, and I swim up. If I stop swimming up at 40' or 50', I will immediately stop ascending and begin to sink again if I stay still. Depending on the amount of air remaining in the tank, I won't get neutral again until I am approaching 20'. If I am going to be neutral at 20' with the BC empty, then I am still going to be slightly negative at 30' with the BC empty, and it's already empty at 30'.

Of course it is possible to leave some air in the BC from 80' and use that to boost your ascent, dumping air from the BC as you ascend to avoid ascending too quickly. I just don't see the point. As I said, it may be a holdover from the days of diving with a horsecollar vest that had to be orally inflated, but I think I have better control over my ascent by using my fins than the people I have seen bobbing around with their hands on their power inflators.

Sounds like too much work to me. Personally, I don't "cycle through hundreds of psi" through my bc or wing during a dive. Yes, it takes some air, but unless you're diving a saw-tooth pattern, you really don't go through a lot - probably just a few puffs on the inflator as you get down to the deepest part of your dive.

So what do you do when you're cruising around at 80 feet and are several pounds negative if you do not use your BC? Are you trying to compensate by swimming up the whole time?

 

[Stirling]

Sounds like too much work to me. Personally, I don't "cycle through hundreds of psi" through my bc or wing during a dive. Yes, it takes some air, but unless you're diving a saw-tooth pattern, you really don't go through a lot - probably just a few puffs on the inflator as you get down to the deepest part of your dive.

So what do you do when you're cruising around at 80 feet and are several pounds negative if you do not use your BC? Are you trying to compensate by swimming up the whole time?

No, as I said I adjust my buoyancy for the depth where I am going to be for a while. So if I'm going to be cruising around at 80', I am going to be neutral at that depth. I'm talking about a vertical ascent from 80' or 60' to 20'. If I were following a slope to shallow water, I would vent only the air required to remain neutral along the way. And it is very little work at all; in fact, it is very easy to stop and hover at a constant depth for as long as desired, and it takes very little motion to ascend at the desired pace.

 

[Warren_L]

No, as I said I adjust my buoyancy for the depth where I am going to be for a while. So if I'm going to be cruising around at 80', I am going to be neutral at that depth. I'm talking about a vertical ascent from 80' or 60' to 20'. If I were following a slope to shallow water, I would vent only the air required to remain neutral along the way. And it is very little work at all; in fact, it is very easy to stop and hover at a constant depth for as long as desired, and it takes very little motion to ascend at the desired pace.

Sorry, I must have mis-read that part of your post.

I was describing how I would ascend from, say 60 or 80 feet to 20 feet. If I am at the end of the dive and will be neutral at 20 feet with no air in the BC, I will be negative most of the way between 80 feet and 20 feet with no air in the BC, and that is how I swim up. Whatever air I have in the BC at 80 feet is going to be dumped just as I am starting up, I get negative, and I swim up..

However, when you dump all the air out of your BC just as you are starting your ascent as you indicate and are negative, it sounds like more work than it should be. Why not just vent air as required? I'm not sure that I could do that in my double steel 120s.