Weekend OW class. Pool

[Stirling]

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

We're probably starting with very different assumptions about how negative you might be at 80'. I'm talking about warm water diving without a lot of neoprene to be compressed, and without a lot of weight to compensate for the neoprene, so it takes very little effort. I wouldn't make the same assumptions if diving with a lot of neoprene and steel doubles.

As for why, remember that this started as a discussion about how new divers were being taught to ascend. Avoiding blown safety stops, letting the positive buoyancy get away from them, plus not actually feeling the changes in buoyancy that occur as they ascend are all reasons why I would encourage new divers to lay off the BC power inflator when ascending.

 

[Warren_L]

We're probably starting with very different assumptions about how negative you might be at 80'. I'm talking about warm water diving without a lot of neoprene to be compressed, and without a lot of weight to compensate for the neoprene, so it takes very little effort. I wouldn't make the same assumptions if diving with a lot of neoprene and steel doubles.

As for why, remember that this started as a discussion about how new divers were being taught to ascend. Avoiding blown safety stops, letting the positive buoyancy get away from them, plus not actually feeling the changes in buoyancy that occur as they ascend are all reasons why I would encourage new divers to lay off the BC power inflator when ascending.

Yes, you are right - the environment we teach open water in is typically fairly cold and students wear full 7mm wetsuits. Students are not taught not to use the power inflator altogether, but rather to use it to deflate on the ascent to control buoyancy and rate of ascent.

 

[jeffreyd]

Ok so there are many ways to ascend when neutral.

Vertical light fin up.
Deep breath and ensuing ascent (vertical or horizontal).
Tap power inflator. (wastes gas)

All while venting excess air during ascent.

One question on acending horizontally. Whats above you? Once during the pool session the Instructor was at the surface helping a student and I was right underneath them and got kicked in the face. (we were done training and just using the tanks down swimming around) I just assumed nothing was above me because it wasn't above me before. I know you wouldn't need to worry about the surface because of the saftey stop at 3m but...

Thanks.

 

[Stirling]

One question on acending horizontally. Whats above you? ...

Propellers. If you ascend to the surface horizontally without being able to observe the surface 360 degrees around you, the answer is invariably propellers.

 

[jbd]

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.

Stirling,
Very nice explanation of your method. You are clearly dialed into the method you are using. I may give it a try in the next few months.

To all the other readers, especially new divers or those without lots of experience, can you explain how Archimedes principle applies to Stirling's method of ascent?

 

[jbd]

Ok so there are many ways to ascend when neutral.

Vertical light fin up.
Deep breath and ensuing ascent (vertical or horizontal).
Tap power inflator. (wastes gas)

All while venting excess air during ascent.

One question on acending horizontally. Whats above you? Once during the pool session the Instructor was at the surface helping a student and I was right underneath them and got kicked in the face. (we were done training and just using the tanks down swimming around) I just assumed nothing was above me because it wasn't above me before. I know you wouldn't need to worry about the surface because of the saftey stop at 3m but...

Thanks.

This is one place to use the helicopter turn. You can pivot 360 degrees and with occasional, slight rolls to the side you can look above you. If I'm diving a lake and there are boats around or some bonehead on a PWC is using my float/flag for "target practice" then I wil stop about 6 feet from the surface and go into a vertical hover and spin 360 degrees to check the area. Of course in most of the lakes around here you need to rely on your perception of sound ot determine if there is anything close or coming closer because you certainly won't see them until they are right on top of you.

With time and experience, you start to develope an awareness of whats around you.

 

[jeffreyd]

Just a note. I'm not trying to foul you up or get you mad (JBD) but I play devils advocate and only take what makes it through. And every post you've made has answered my questions.

But my theory on Archimedes principle for Stirlings ascent would be that he displaces enough water to be neutral at 20' with the BC. Coupled with boyles law, if he descends below 20' he will become negative.

Assuming he is using an AL80 he will become +4lbs at the end of the dive which would require him to displace less water and still remain neutral. This now explains why he can now have no air in his BC and still be neutral at 20' where before he needed some air in his BC.

 

[Warren_L]

Assuming he is using an AL80 he will become +4lbs at the end of the dive which would require him to displace less water and still remain neutral. This now explains why he can now have no air in his BC and still be neutral at 20' where before he needed some air in his BC.

Actually, the type of tank being used has very little to do with buoyancy shift during a dive. AL, steel, it's all the same. It depends on how much air is drawn down. 80 cu ft of air will have about a 6 pound negative effect on buoyancy, but since you generally don't draw the tank down to zero PSI, you would have about a 5 pound shift. The more positive buoyancy of an AL80 versus a steel 80 is compensated by weighting in other areas, so the overall change in buoyancy (assuming the same amount of air used) is the same in either type of tank. However, the weight distribution, which affects trim, is a whole different issue.

 

[jbd]

Just a note. I'm not trying to foul you up or get you mad (JBD) but I play devils advocate and only take what makes it through. And every post you've made has answered my questions.

But my theory on Archimedes principle for Stirlings ascent would be that he displaces enough water to be neutral at 20' with the BC. Coupled with boyles law, if he descends below 20' he will become negative.

Assuming he is using an AL80 he will become +4lbs at the end of the dive which would require him to displace less water and still remain neutral. This now explains why he can now have no air in his BC and still be neutral at 20' where before he needed some air in his BC.

No "offense" taken. You have asked legitimate questions. Such questions help flesh out the discussion, and either broaden or increase our depth of understanding of the topic. An excellent example of this is provieded by Warren L's post #38.

Back to Stirlings ascent--what will happen if he ascends above 20'? If his BC is empty at 20' how will he control the ascent from 20' to the surface?

 

[jeffreyd]

Back to Stirlings ascent--what will happen if he ascends above 20'? If his BC is empty at 20' how will he control the ascent from 20' to the surface?

With breathing. If he is truly neutral and the only device that can change volume are his lungs, then on every inhale he would ascend a little. This because if there is no trapped air (as inside a BC) no air will expand, per Boyles law, therefore he will not displace more water during ascent, thus leaving him neutral through the entire ascent.

Or of course swimming. But that doesn't require formulas.

Hmm... I would have never thought of that.