Overweighted at beginning of dive but underweighted at end in shallows

[FreqFlier]

People tend to pause with their lungs full. Why not learn to pause with your lungs empty?

Because it takes time to figure this out and to learn how to do it without brain signals going into panic mode. It's a part of diaphragmatic breathing routine.

 

[hammet]

Also, I found doing a pool dive is great for zeroing your weighting too. Then do the math and compensate for the fresh vs salt water and the air above 500psi remaining in your tank. Try it with all your wetsuits too.

 

[The Chairman]

Because it takes time to figure this out and to learn how to do it without brain signals going into panic mode. It's a part of diaphragmatic breathing routine.

I taught it as part of my OW course. To graduate from the pool to OW, the student had to show proficiency in breathing themselves down and up. Then they had to learn how to add and subtract weight underwater, using only their breathing to accommodate the weight swing. The final was playing underwater Jenga where they pick up and put down weights to build houses underwater. My students never had buoyancy issues when we hit OW. Many times they were thought to have years of experience on their first OW dives.

 

[The Chairman]

Then do the math

I've found, with very few exceptions, that two 2-pound weights will accommodate the fresh to salt swing.

 

[Badger7]

Alternatively, be neutral at the surface with a full AL80 tank, i.e. at the beginning of a dive,and then add 5-6 pounds to compensate for gas that might be used.

This is really good advice, but might need a small disclaimer. This would only work if you actually need 5-7 pounds of lead, which while most people would, if you're diving with a steel backplate you may not, meaning you may not have enough lead to pull out of your pockets to find neutral with a full tank.

You could use .08 pounds per ft^3 of air. So if you are diving an AL80, the math for exactly how much lead you would need to add after you find neutral with a full tank would look like (2500 psi / 3000psi) * 78 ft^3 * .08 lb/ft^3 = 5.2 pounds of gas weight lost breathing down from 3000psi to 500psi.

Similar math for a single faber HP100 (2900 / 3400) * 101 ft^3 * .08 lb/ft^3 = add 6.9 more pounds over what you need to be neutral at 10ft and a full tank

 

[tursiops]

I've found, with very few exceptions, that two 2-pound weights will accommodate the fresh to salt swing.

Since the weight needed to be added is to compensate for the approximately 2.4% density increase from fresh to salt water, and one is compensating for total displacement of water when submerged, then 2-4 pounds corresponds to a total weight of 83 to 166 pounds, which is probably a bit low. 4-7 pounds added probably covers more people; this is typically 4-5 pounds for women, 6-7 pounds for men. In my experience, and as calculated.

 

[The Chairman]

then 2-4 pounds corresponds to a total body weight of 83 to 166 pounds, which is probably a bit low.

Except that most divers are already over-weighted. Most. When I taught in the Keys, giving my students 4 pounds total worked, and they were mostly weighted well. I can only think of one person to suggest they were light at the end of the dive. We added two pounds between dives and they were perfect.

 

[rsingler]

The desire to have full lungs will always be there. Always. It's why anxious or scared divers tend to be floaty because the higher your anxiety, the fuller you want to keep your lungs. However, the more you work at it, the easier it becomes. I no longer have to 'think' of where to pause. After decades of working at it, it's become rather automatic... unless I get anxious.

CAVEAT: again, never hold your breath. Keep your glottis open at all times. This allows bubbles to freely escape as you ascend rather than injuring your lungs.

I believe that a short pause after inhalation is more efficient for gas exchange. This is the opposite to normal breathing pattern on land. Do not hold your breath and keep your epiglottis open.

@rsingler ?

Two competing issues here.
From a buoyancy control standpoint, @The Chairman is spot on, and excessively full lungs can be a big contributor to being "floaty" at the end of a dive with an empty tank. Your working lung volume controls six pounds of buoyancy from moment to moment, which is why proper breath control is so important when cruising in on that fire coral for a photo, or during the safety stop when buoyancy changes rapidly with small changes in depth.

But @scubadada is also correct. During a dive the best way to reduce your SAC and improve dive duration is to optimize elimination of CO2. And the way to do that is to 1) consciously slow your rate of breathing; 2) make a full inhalation; and 3) pause  briefly at end inhalation to allow CO2 exchange in your lungs before...4) a full exhalation to dump that CO2, and a small partial inhalation to resting lung volume.

The whole purpose of this slightly unnatural pattern is to minimize dead space ventilation, which is just a fancy term for what happens when you take small shallow breaths trying to "not use so much gas". Since you are breathing part of the same "dead" air back and forth in your large bronchial passages with those shallow breaths, your CO2 rises because your alveoli are not being effectively ventilated. When CO2 rises, you cannot help but breathe more frequently, and if your shallow pattern continues, you just waste tank gas.

Of course, taking this to an extreme is "skip breathing", where you have plenty of oxygen at depth (since those needed molecules are packed in there double or triple thickness at depth). Since you have plenty of oxygen, you don't pass out. But since you are hypoventilating, you'll end the dive with a screaming headache to complement your nice low SAC. And in an emergency, your already elevated CO2 may kill you when you start making even more with effort.

And, again, ending the dive with a full set of lungs by overdoing the inspiratory pause will bring you right back where you started with too much buoyancy.

So...it's complicated. Just takes practice.

 

[The Chairman]

The most reliable to weight yourself is Dr Bob's weight titration. With arms and legs crossed (to prevent sculling and finning), a full tank, bladder completely empty, and breathing normally, keep adding weight at 1 pound per inch above water, until your head is just awash. Exhaling should start your descent. This allows for all the weight you need at your safety stop.

 

[inquis]

I've found, with very few exceptions, that two 2-pound weights will accommodate the fresh to salt swing.

The math says about 2.4% of the total dry weight (you, tanks, gear, suit, lead, everything). Two 3-lb weights for me checks out, but 4 lb wouldn't be impossible.