Breathing and buoyancy

[*dave*]

I started using this video during my last class and found it generated some good discussion, so thought I'd post it here.

I run the video prior to the fourth pool session and set it up by asking the class what would happen to a neutrally buoyant diver were they to have 8# of ballast added to them. Secondly, I ask what would happen to a neutrally buoyant diver if they were to lose 8# of ballast.

 

[Vt diving]

It appears that the person in the vid holds their breath when picking up the weights? So they would be more buoyant and no depth change. If you are neutral and have weight added you are going to sink unless more buoyancy is added by bc inflate or lung expansion? Or am i missing something?(I am a very new diver...)

 

[herman]

A nice demo of the value of your lungs to control buoyancy.

VT, you have it. An average male has around 10-12 lbs of buoyancy they can vary using their lungs alone. If dead on neutral with his/her lungs mid range, 4 or 5 lbs either way is fairly easy to deal with, however more than 4 is a pain to have to deal with over an extended time, for me anyway.

 

[JohnE]

Also a good lead in to how someone who is not using their bcd effectively subconciously uses breathing to compensate. Nice teaching aid!

 

[DiveNav]

nice
how much weight are those yellow weights?

the guy in the video has lungs working quite well ..... but I challenge him to do the demo vice-versa (remove weights from belt) and keep diving for 30 minutes .... for what I remember the tidal volume is not perfectly centered in the total lung capacity.

 

[herman]

nice
how much weight are those yellow weights?

the guy in the video has lungs working quite well ..... but I challenge him to do the demo vice-versa (remove weights from belt) and keep diving for 30 minutes .... for what I remember the tidal volume is not perfectly centered in the total lung capacity.

Vintage divers basically do it all the time. We deal with gas use and wetsuit compression with lung volume alone. While I prefer to weight such that I only have to deal with half of the gas loss from the tank at a time (4 lbs for an 80 which I balance for dead on neutral at 1500psi giving me a -2 to +2 lb swing during the dive), I can handle +/-5 lbs with little trouble, much more it starts to be a pain. I would agree that 8lbs would be a PITA and more than I could comfortably do. For 4 lb (+/-2 lbs) swings I routinely do 75+ minute dives with no BC or wetsuit and really don't notice the 4 lb shift now that I am accustomed to it.

 

[Scott]

Nice *dave*. I don't use a video to show it, but we do a similar drill during one of the sessions.
Students will hover and start with two pounds and then work their way up to six in one pound increments. After they have that down I'll have them hover and hand different weights off to each other.

 

[*dave*]

It appears that the person in the vid holds their breath when picking up the weights?

Just a clarification, but the diver is actually breathing continuously. I use the video in OW class, so it would be awkward to explain why the diver would be holding his breath. Aside from that, you're spot on.

Also a good lead in to how someone who is not using their bcd effectively subconciously uses breathing to compensate. Nice teaching aid!

Also a pretty good aid in describing what sucks about rental gear.

nice
how much weight are those yellow weights?

Four pounds each.

the guy in the video has lungs working quite well ..... but I challenge him to do the demo vice-versa (remove weights from belt) and keep diving for 30 minutes .... for what I remember the tidal volume is not perfectly centered in the total lung capacity.

The breathing pattern required to hold buoyancy after an eight pound weight shift isn't very efficient and at some point CO2 levels would spike and O2 levels would drop.

Still a great short term tool to get over or under an obstacle or slow an ascent or descent.

 

[*dave*]

Nice *dave*. I don't use a video to show it, but we do a similar drill during one of the sessions.
Students will hover and start with two pounds and then work their way up to six in one pound increments. After they have that down I'll have them hover and hand different weights off to each other.

Great stuff.

I demo things, but since I started introducing the concepts with video, students seem to get a lot more from the demos. It's also a lot easier to have discussions.