Another buoyancy discussion

[Progen]

From Tips for Wearing Less Weight While Diving | Sport Diver Magazine.

The ballast weight you carry doesn't change during a dive, but it's often the biggest problem for divers who struggle with perfecting neutral buoyancy. Many divers are overweighted for the type of diving they do, carrying more lead than they need. That makes buoyancy control more difficult because every extra pound of lead has to be balanced with an extra pound of buoyancy.

But because the air in your BC expands and contracts with depth changes, you have to be constantly adding or subtracting air from your BC. So extra lead means extra thrust up or down when you change depth, and requires extra fiddling with your BC valve controls. Sometimes it means nearly constant fiddling.

Here are our tips for taking off that extra weight.


Just do it. Take off two pounds before your next dive. Can't get below the surface? Before you reach for the lead again, make sure you really need it. Getting below the surface, especially on the first dive of the day, can be surprisingly difficult and can trick you into carrying more lead than you really need.


Be patient. The plush lining of a dry wetsuit can trap a surprising amount of air, and therefore buoyancy, in its fibers, and it takes a minute or so to get fully wet.


Reach up. Hold the inflator hose over your head and stretch it upward a little so its attachment point to your BC is highest. At the same time, says Linda Van Velson, a PADI course director, "dip your right shoulder and squeeze the BC against your chest with your right arm." This maneuver encourages the last few bubbles to find the exit.


Rock backward a little. Many BCs trap a bubble of air just behind your head. Rocking backward as if you are in a La-Z-Boy recliner moves the exhaust hose over the bubble and lets it escape.


Relax. Many of us move our hands and feet more than we realize, especially at the beginning of the dive. To counteract that, hold your right arm still at your side (your left is holding up your exhaust hose), extend your legs and point your fins straight down so they have the least resistance to sinking.


Exhale. Another tendency is to hold your breath, and a lungful of air adds as much as 10 pounds of buoyancy. Exhale and hold it until you start sinking, then take shallow inhales until you get below five feet.


Force it. Another option is to use your body weight to generate some downward momentum by lifting part of it out of the water, then letting it fall back. Lying on your face, jackknife your upper body downward, then lift one leg, then another, out of the water. The weight of your legs will drive you downward, and once your fins are in the water you can kick down.

Use the line. If the dive boat is tied off at a mooring, use the line to help pull yourself down. This trick also works for divers who need to go down slowly to equalize.


Get a little help from your friends. Ask your buddy to gently tug your fin and pull you down for the first few feet of your descent. Usually, by the time you're in 10 to 15 feet of water, you should sink without help.

This is coming from a beginner so perhaps the experienced ones will forgive my audacity in saying that the ones I've highlighted in red are very misleading to beginners in the sense that they may become entrenched in their minds and become something they will pass on to other beginners.

If you can't sink after you've gotten all apparent air in the bladder out, forcing yourself to descend with your head down and feet up or getting others to pull you down will probably spell more problems as the AL80s which is widely used worldwide get buoyant and we all know that it's harder to tame an unplanned ascent than descent.

I have spent a few seawater dives with perfect weighting at the surface but needed to get all air, right down to the last bubble, before I could sink. Then I ran into the problem of becoming a little too buoyant when at the safety stop but that's nothing some controlled breathing couldn't handle but at least I could sink without assistance from other divers or having to forcefully make a head first descent.

ps. Before anyone takes this as a PADI bashing thread, PADI's the only agency I'm certified with and being their diving society member, they're the only articles I get sent to my email.

 

[chrpai]

Regarding "reach up" I find that a Backplate and Wing is far less likely to trap hidden air then a vest jacket.

Regarding "force it" and "get a little help" these sound like variations on "duck diving". Some people like to weight themselves a little light. Just enough weight to hold a safety stop at 15' with say 500psi but as they slowly ascend from there they actually because a little more positive then neutral at the surface. At the beginning of the dive you should still be negative by a few lbs but it's so minimal that you might need a little help getting down those first few feet until the suit compresses and you become more negative.

 

[Progen]

... At the beginning of the dive you should still be negative by a few lbs but it's so minimal that you might need a little help getting down those first few feet until the suit compresses and you become more negative.

Precisely my point. Since you're supposed to be negative at the beginning of the dive, if a little bit (assuming that it's not an unnoticed mountain of air at the uppermost part of the bladder) of air in the hose or bladder which results in a few tiny small bubbles when forced out, can keep you up there, you're already under-weighted and for a beginner will result in difficult staying at the safety stop although after spending time paying attention to equipment and skills, at 5 metres, the pressure at that depth and some controlled breathing will keep me down with no difficulty even if I'd run into the God forbidden zones of 200 - 300 PSI by then but I still need to squeeze everything out at the beginning before I can descend.

 

[chrpai]

That's one heck of a run on sentence. I'm having a hard time parsing it.

 

[Craig66]

Having someone pull your fins down is horrible advice. It's all fun until someone blows an ear drum.

The one part I do agree with is the relax part. I think a lot of new divers believe they have exhaled but in reality due to nervousness are doing significant air trapping and when they realize and relax it can make a huge difference.

 

[chrpai]

Having someone pull your fins down is horrible advice. It's all fun until someone blows an ear drum.

The article only said to gently pull down "a few feet". While I wouldn't do this, you are going to get a blown ear drum going from 1ATA to 1.1ATA. I'm usually down to atleast 8'-10' before I feel the need to do my first equalization.

 

[Craig66]

The article only said to gently pull down "a few feet". While I wouldn't do this, you are going to get a blown ear drum going from 1ATA to 1.1ATA. I'm usually down to atleast 8'-10' before I feel the need to do my first equalization.

That's you. Many people experience issues long before 10 feet. Furthermore if the person is having an issue due to nervousness having an over zealous newbie partner pulling a newbie diver down is only going to add to the anxiety - just not a good idea.

The blown ear drum specifically was tongue in cheek but the idea is not.

 

[Progen]

That's one heck of a run on sentence. I'm having a hard time parsing it.

Sorry, I thought I had inserted enough periods.

 

[spectrum]

Reach up. Hold the inflator hose over your head and stretch it upward a little so its attachment point to your BC is highest. At the same time, says Linda Van Velson, a PADI course director, "dip your right shoulder and squeeze the BC against your chest with your right arm." This maneuver encourages the last few bubbles to find the exit.
Spectrum; Many BC's do take something of a trick for getting them to fully vent. Creating that high point is essential physics. Sometimes it's easy and sometimes it takes parlor tricks. if you need to squish the thing you have a lousy BC.

Force it. Another option is to use your body weight to generate some downward momentum by lifting part of it out of the water, then letting it fall back. Lying on your face, jackknife your upper body downward, then lift one leg, then another, out of the water. The weight of your legs will drive you downward, and once your fins are in the water you can kick down.
Spectrum: Jack knifing as a skin diver would is OK IF and only IF you are correctly weighted to end your dive neutral in the shallows with an empty BC. Some suits and gear have significant stowaway bubbles / buoyancy that will disipate a few minutes into the dive. You may also be in rolling seas that want to pitch you upward making a gentle submersion difficult. Normally the weight of your air fill should make you just negative enough to let you relax, exhale and slip below the surface. You do not want to jack knife because you are underweighted as this promisses an uncontrolled ascent at the end of the dive.

Use the line. If the dive boat is tied off at a mooring, use the line to help pull yourself down. This trick also works for divers who need to go down slowly to equalize.
Spectrum: Only as in the case I described for jacknifing. If you are really pulling yourself under something is wrong. It's fine to go down the rope to hold location in a heavy current. Using the rope to manage your descent rate while equalizing is OK a few times, just like training wheels are.

Get a little help from your friends. Ask your buddy to gently tug your fin and pull you down for the first few feet of your descent. Usually, by the time you're in 10 to 15 feet of water, you should sink without help.
Spectrum: That's just silly. If jack knifing won't do it then you need remedial attention.

The basics of eliminating ths nonsense is in correct weighting, read more here. The real same of this sidebar is that it never suggests an objective weight check... Just do it???????????

Pete

 

[Bob DBF]

Sorry, I thought I had inserted enough periods.

You could randomly toss in a few more.



Bob
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I may be old, but I'm not dead yet.