Holding a safety stop - the effect of breathing/weighting

[boulderjohn]

This is intended to be a (hopefully) useful illustration in support of BlueTrin's post #30.

In teaching OW students in a 12 foot swimming pool, I am always a few pounds overweighted so that I can submerge quickly if needed. Even so, I am able to do the following demonstration with ease. I dump all air out of the BCD so I settle to the floor. I then add a random shot of air to the BCD and inhale. I react to what happens (not much, slow ascent, fast ascent--it's random) by adjusting that inhalation. If not much happens, I add a touch more air to the BCD and try again. The goal is to have enough air in the BCD so that I can ascend with the additional help of my lung volume. I then inhale and exhale so that I very slowly ascend until my head breaks the surface. Then I make a sharp exhale so that I begin to descend, and I then inhale and exhale so that I slowly descend until the midpoint, at which point I hover for about a minute. Then I exhale and drop to the bottom. The point is that even with a few extra pounds (and the BCD air needed to compensate), I have total control of my buoyancy over that entire 12 foot range using nothing but my lungs to overcome the changing volume of the BCD and the wetsuit.

Now here is what happens if I decide to do some tech practice wearing steel doubles. In that case, I am very much overweighted, and my wing must have significantly more air in it to compensate. In that case, there is no way my lungs can control my depth over that full 12 foot range. The air in that BCD is expanding and contracting far too much, so if I go up or down more than a few feet, I have to remove or add air to the wing to compensate. Once I get the right amount of air for that depth range, I again have a few feet either way where my lungs can take over.

 

[RainPilot]

The math doesn't make sense in my case until I start diving lots more.

SP Hydros $945
Mk11/C370 $510
SPG $60 (used)
SP Seawing Nova 2 $210
Mask and Snorkel ~$70
SP Shorts w/pockets $60
Titanium dive knife $45
DSMB ~$60
Boots $20

These prices don't even include the import duties they charge here in Colombia.

I'm doing a 5 tank dive this weekend here in Colombia that goes for $236; the guy took $14 off that price because I'm bringing my own stuff. But regardless I'm really happy to be popping the cherry on the BCD, regulator and even the shorts and just having my own gear.

You can pick up a decent BP/W setup for your use at way below 500 USD, the Deep6 one is under 200:

Out of stock right now but they should have in before the end of the month. Generally you can get a BP/w for single tank recreational use under 300$ new, if you hunt eBay a bit you can get used ones even less. $945 for a BCD, that you're going to end up replacing when you go to tech diving, seems counterproductive.

Dive knives, I used to carry all sorts, a trilobite has become my go-to and they are cheap and cheerful.

 

[ScubaWithTurk]

The math doesn't make sense in my case until I start diving lots more.

SP Hydros $945
Mk11/C370 $510
SPG $60 (used)
SP Seawing Nova 2 $210
Mask and Snorkel ~$70
SP Shorts w/pockets $60
Titanium dive knife $45
DSMB ~$60
Boots $20

These prices don't even include the import duties they charge here in Colombia.

I'm doing a 5 tank dive this weekend here in Colombia that goes for $236; the guy took $14 off that price because I'm bringing my own stuff. But regardless I'm really happy to be popping the cherry on the BCD, regulator and even the shorts and just having my own gear.

If you are looking into going the tech path, I would take the Hydros and the Seawing Novas off that list. I find the Hydros to be a very expensive piece of kit that can easily be replaced with a much cheaper BP/W that works better than the Hydros.

The Novas are popular among recreational divers but I would never use them for tech dives.

Good luck and remember, just breath normally. No need for big inhales and exhales. Keep working at it and it will come to you.

 

[KenGordon]

Just for discussion's sake and to improve my knowledge....so exhaling N2 does not contribute to the gradient that facilitates diffusion of the N2 out of the tissue and into the blood? I would have thought that if the blood had a lower N2 load vis-a-vis exhalation that it would contribute to a gradient that would facilitate further circulation of N2 out of the body.

I am now compelled to learn more about this.

And this is why I put that caveat at the bottom of that post.

-Z

Consider how much of a difference in partial pressure of n2 will happen between breathing once a minute vs 5 times a minute, it would be whatever the N2 off gassed in about 45 seconds diluted into 3 to 5l of gas, even an extremely bent fizzy person doesn’t bloat up with off gassed N2, it is a very small quantity of free gas.

 

[NothingClever]

The Novas are popular among recreational divers but I would never use them for tech dives.

What Turk said.

I couldn’t get along with frog kicks with the Novas. I changed to a more traditional fin and my frog kick improved immediately. The Novas are just too easily overpowered, IMO.

 

[boulderjohn]

The Novas are popular among recreational divers but I would never use them for tech dives.

I couldn’t get along with frog kicks with the Novas. I changed to a more traditional fin and my frog kick improved immediately. The Novas are just too easily overpowered, IMO.

It is not just the frog kick. Pretty much every one of the kicks associated with technical diving is more difficult with them. There are two different techniques to doing back kicks and helicopter turns, and the one I prefer and teach cannot be done with those fins.

 

[halocline]

My most recent dive where this was an issue I was in a 3mm shorty, 4kg (9lbs) weight on a belt, jacket style BCD and a 12L/200 bar steel tank.

That sounds like a lot of weight to me for a 3mil shorty. Your tank is probably 3-4 lbs negative (with a regulator) which should pretty much compensate for the thin wetsuit. I don't understand why you would need 9 lbs more, unless you are naturally very buoyant. Are you, how do I put this delicately... on the large size?

 

[Rickk]

.....
Free hovering a stop without a visual reference is HARD in the beginning. It's a very good skill to practice. Look at particles in the water to tell if you're moving, don't hyperfocus on your depth gauge or computer. Practice holding position, then practice doing tasks (like a mask flood or DSMB deploy) while holding position and it's amazing how much your buoyancy will improve. Dive a lot! Yes you will get to a point where you don't move at all, even while preforming tasks.

....

On a recent dive I was in a up current. During initial decent the particles in the water were going up, I could feel the flow of water and felt negative buoyant. I should have been descending about 15 m per minute.

However the rocks were not moving.

Good hovering practice is at the end of a night dive, have everyone turn off their lights and see who can hold position best. Worst performance buys first round.

 

[lowwall]

.Good hovering practice is at the end of a night dive, have everyone turn off their lights and see who can hold position best. Worst performance buys first round.

If you ever find him.

 

[60plus]

Most point raised have already been covered. Do not breathe deeply, inhale gently for 2 seconds then out for 4. If you have a shot line to hold (as is usual in decompression diving) adjust your buoyancy so you can you can easily hold the shot line with one hand. If there is no shot line I find it best to be slightly negatively buoyant and use finning to hold my depth. Alternative swim round in horizontal circles. My wetsuit seems to undergo its most rapid change in buoyancy between 3 and 5 m.