BCD, weights, and buoyancy control questions

[Lake Hickory Scuba]

Thanks for sharing that video, Bryan. Just one follow up question for you.

I'm not clear how you briefed this before the demo. It's quite clear to me what you're doing but what did you tell the students they were supposed to learn from this? Was it about the effect of trim and buoyancy or was it about the effect of breathing and finning? ... or maybe all of that in one?

You're pulling a lot of different elements into one fairly technical demo, particularly if we're talking about novice divers. What is clear is that you were demonstrating that you can control your position in the water column to a pretty wide degree without using the BCD. I think that's a very valuable lesson and something that, in particular, novice divers need to become aware of.

However, you did it using a stabilizing kick that is normally used for station holding and as a starting point for teaching reverse frog kicking. I have to admit that I've had few OW students over the years who could have reproduced that kick with enough control to do what you were doing. Was it your expectation that the students were to reproduce the result or were you just trying to get the point across about how important breathing is to your buoyancy control?

I'd like to hear your thoughts on this. I like to see stuff like this because there's always something I can "borrow" to improve my own methods (if you don't mind).

R..

You're very welcome, and feel free to use this anytime you see fit. Introducing skills like this to new students, as great as they are, at least in concept, can be difficult if an instructor puts the expectations on the student to match his abilities (in the Open Water Course). At least from the stand point of a shop instructor who is on a time crunch to get them certified. Yet taking just a few extra minutes to demonstrate what can be achieved by a diver who is properly weighted and properly trimmed, can be beneficial to both the student and the instructor. The student can use this learning experience to set goals for himself or herself to reach, and the instructor can use it as a selling point to sell and teach a Buoyancy class. Either way it is a win win for both the student and instructor. The simplest way to introduce it to a student, is to make it part of the teaching routine, by having students perform it during the proper ascent and descent skill sets. In PADI terms, the 5 point ascent and 5 point descent. In regards to the finning technique, I use it during every Buoyancy class I teach. Though I still teach the Frog Kick in Open Water, along with the Flutter, and Dolphin Kick, I can really have students focus and master the frog kick, and its variants during this skill, during the Buoyancy course (PADI's Peak Performance Buoyancy and SSI's Perfect Buoyancy). Another purpose I put into this skill, is to show the student that the equipment they choose doesn't really matter, if they are properly weighted and trimmed, they can still perform the skill. Doubles, Sidemount, BPW, Jacket, Back Inflate, or any and all combinations this skill can be perfected by anyone. Lastly, I believe it helps show the students that proper breathing does work hand in hand with being properly weighted and trimmed, and even the slightest change in breathing can change the diver's buoyancy. I rarely ever have a student that can match this during the Open Water Course, and I would never fail a student for not being able to repeat this skill, but I believe there is great value at the same time, by sharing it with the students. Hopefully this answers your question.

 

[Triad]

Going in to my third week, thanks to the advice you guys have given me and the video, I'm doing pretty good with buouancy save for a bit of a struggle with my legs being heavier and sinking. But my instructors are working with me to help me learn how to deal with that, so it's going very well. Thanks for all the input!

 

[Triad]

And my closed water dives are done. I made a ton of progress today, again thanks to you fine folks. I was even able to address the problem of my legs wanting to sink without adding trim weight! It seems so simple now that I know how to do it. I found that I could keep my legs up by slanting my back forward and bending a bit at the hips, and my legs just kind of naturally settled in to a horizontal position. It was a good feeling to be neutral and trimmed right, and that led to a sudden ability to swim up and down and all around without touching the bottom or breaking the surface.

I found that I could change the angle of my body up or down to ascend and descend, rather than trying to rely on messing with my BCD or breathing to do it. It all just clicked! By the end of it, I could be floating at the surface with a BCD full of air, deflate and make a good controlled descent, then level off and add just the right amount of air near the bottom to get myself neutral very quickly.

Thanks again, everyone, for all the different perspectives and thoughts. Now on to my open water dives in a couple of weeks in a cold, cold quarry!

 

[Neilwood]

And my closed water dives are done. I made a ton of progress today, again thanks to you fine folks. I was even able to address the problem of my legs wanting to sink without adding trim weight! It seems so simple now that I know how to do it. I found that I could keep my legs up by slanting my back forward and bending a bit at the hips, and my legs just kind of naturally settled in to a horizontal position. It was a good feeling to be neutral and trimmed right, and that led to a sudden ability to swim up and down and all around without touching the bottom or breaking the surface.

I found that I could change the angle of my body up or down to ascend and descend, rather than trying to rely on messing with my BCD or breathing to do it. It all just clicked! By the end of it, I could be floating at the surface with a BCD full of air, deflate and make a good controlled descent, then level off and add just the right amount of air near the bottom to get myself neutral very quickly.

Thanks again, everyone, for all the different perspectives and thoughts. Now on to my open water dives in a couple of weeks in a cold, cold quarry!

It is often difficult to explain but when it all clicks in to place, it is an amazing feeling.

 

[AZNetEng]

At the end of your dive you should be able to hold a 10ft stop without much if any air in your BCD. So that sounds good to me.

What @KevinNM said. If you have to add air to the BCD at the end of a dive you are likely carrying too much weight.

Buoyancy and trim are both something you will work on for a bit after getting certified. It will change, potentially alot, as you polish your skills and get more comfortable.

 

[yle]

So I've adapted a freediving weighting philosophy to all my diving. I am weighted so that I am neutral with a full breath, empty bcd and empty tank at 15 ft/5m. This makes me just barely positive on the surface with a full tank. I use my freediving duck dive to descend and once I break about 10ft/3m I am negative with a full tank.

Trying to wrap my mind around the logic here. You start a dive weighted so that you would be neutral holding a full breath and with an empty tank (your description.) I assume that at the start of the dive the tank is actually full, though. And the air in the tank weighs about 6 pounds. A full breath also provides buoyancy of 4 to 6 pounds (depends on the person.)

Which means if you would be neutral with the tank empty and lungs full, at the start of the dive you should be about 10 pounds negative with the tank full and if you exhale.

Why would you need to "duck dive"? If you're weighted to be neutral at the end of the dive, you should be negative at the beginning of the dive. Wetsuit compression between the surface and 5m shouldn't make that much of a difference.

Edit: after reading your other posts, you seem to imply that the compression of your 3mm wetsuit makes a difference of about 5 lb in buoyancy between the surface and 15 ft depth. Seems like a lot, but even if that was the case, you should be able to descend at the beginning of your dive by just exhaling (i.e. what you've described, even considering the wetsuit compression, would make you a few pounds negative at the beginning of the dive when you exhale.)

 

[Saniflush]

your 3mm wetsuit makes a difference of about 5 lb in buoyancy between the surface and 15 ft depth.

Can't speak for his but mine is worth about 3# of buoyancy at the surface.

 

[yle]

Can't speak for his but mine is worth about 3# of buoyancy at the surface.

But it's not the buoyancy of the suit itself; the issue is the difference between the buoyancy of the suit at the surface vs the buoyancy of the suit when it's at 15 ft depth. That is, by how much volume is the neoprene compressed due to the additional pressure of the 15 ft of water. For a 3mm suit (heck, even for my 7mm suit...) it shouldn't be very much, and certainly not 5 lbs.

 

[RainPilot]

Wetsuit compression can lose up to 80% of its buoyancy. For a 7mm suit that can be a lot of lead. For a 3mm, not so much.