Buoyancy question

[herman]

Do you use slightly more air if you have to inhale more to control the buoyancy?

There is a lot more to it than that.
Breathing depth does come into play but it's more about breathing cadence and how full your lungs are at any given time. An average adult male has somewhere between 10 and 15 lbs of lift between fully deflated and inflated lungs or a more useful way of looking at it, around 5 to 7 lbs of swing above and below mid breath, in no BC diving you learn to take advantage of this. Early in the dive, you tend to "breath off the top of your lungs" which means you do not exhale quite as deeply as normal and at the same time inhale slightly more than "normal" on the inhale. This is not excessive amount, if you are properly weighted you are only around 2 lbs overweighed at the beginning of the dive. No more gas is used, just the average point of lung inflation changes. At the same time you change the cadence of your breathing, you inhale longer and slower and exhale quicker and shallower which will give you an overall bit of positive buoyancy. Average lung inflation and cadence combine to give you the slight amount of positive buoyancy you need at the beginning of the dive to compensate for the slight excess weight you have one. How much you vary the 2 give you control for accents and descents, going over a coral head for example. As the dive progresses your breathing slowly shifts to a normal rhythm and closer to the end of the dive it will shift to a lower average inflation of your lungs with longer slower exhales followed by shorter, quicker inhales, that gives you the slight negative overall buoyancy you need. This may sound like a lot but in reality, once you become use to it, it’s pretty much automatic. I compare it to learning to drive a stick shift car. At first you have to think about how much to let out the clutch and how much gas to give it but after doing it a while, it becomes natural and you don’t think about it, it becomes more feel than thought. As for overall air consumption, mine goes way down. I am not dragging along a flapping wing that creates drag, nor am I hauling around excess weight I have to move.

 

[redacted]

I log the buoyancy of each piece of neoprene making weight adjustments as I change exposure protection quite easy. But I am a bit surprised at the low marginal difference of 2 lb between a 3mm and 5mm suit that some report. I have two different 5mm suits (XXLs) and each is 13 lb positive. My 3mm is 8 lb and my 3/2mm is 7 lb. My wife's 5mm suit (Med) is 10 lb and her 3mm is 5lb.

 

[hroark2112]

Reading this thread has made me realize that I need to log more information on my dives.

 

[Fatterboy]

I can't swim, so I wear a BC to float once I swim back to the top. I dive with 0-2 pounds in fresh water with AL80 and usually 8 pounds with 3mil shorty in Na H20. In the quarry and pool I also am a fan of an aluminum backplate harness (just something to strap a tank to) and no weight. My best friend is an old diver and gave me one that resembles a turtle shell, there is so much freedom in not having all that "extra" gear on, tho I do keep my octo and SPG on.

 

[herman]

That is a Sportways Waterlung back pack.. I have one with orange straps.

 

[miketsp]

..snip..
But I am a bit surprised at the low marginal difference of 2 lb between a 3mm and 5mm suit that some report. I have two different 5mm suits (XXLs) and each is 13 lb positive. My 3mm is 8 lb and my 3/2mm is 7 lb. My wife's 5mm suit (Med) is 10 lb and her 3mm is 5lb.

Yes, 2lb is not realistic based on the calculations I did above. However it could be possible if the 3mm is a loose fit so the 3mm is true (no stretching) and there is some air entrapment while the 5mm is a tight/stretch fit or already worn and partially compressed due to repeated dives. Also if the 5mm is a tight fit it could be inhibiting lung expansion much more than the 3mm so the "at rest" lung volume could easily be different by 1 liter = 1kg.

 

[Paladin]

Two of my wetsuits are both NeoSport made by Henderson. One is a 3mm and the other is a 5mm. Using an AL80 with the 3mm, I need 16 pounds of lead in fresh water. Any less and I can't sink. Any more and I feel too heavy below 6 feet. Using the 5mm, 18 pounds is just right. 16 is too light and 20 feels to heavy at depth. Both suits are the same size.

 

[miketsp]

Two of my wetsuits are both NeoSport made by Henderson. One is a 3mm and the other is a 5mm. Using an AL80 with the 3mm, I need 16 pounds of lead in fresh water. Any less and I can't sink. Any more and I feel too heavy below 6 feet. Using the 5mm, 18 pounds is just right. 16 is too light and 20 feels to heavy at depth. Both suits are the same size.

Ah. That explains it. I was assuming that you were using a standard weighting check - almost empty tank at the surface, no air in BC and checking your flotation level at some point on your face. Comparing like with like.
Assume you are at a depth where you have 50% neoprene compression. 50% of 3mm is 1.5mm. 50% of 5mm is 2.5mm. Difference between the suits is now only 1mm. So the weight difference at depth will be half the difference that you would find at a surface buoyancy check.
I don't care what the weight difference is at depth. I do my weighting adjustment to be slightly negative at 3m with a near empty tank so I can guarantee to hold a deco or safety stop.

 

[Paladin]

Ah. That explains it. I was assuming that you were using a standard weighting check - almost empty tank at the surface, no air in BC and checking your flotation level at some point on your face. Comparing like with like.
Assume you are at a depth where you have 50% neoprene compression. 50% of 3mm is 1.5mm. 50% of 5mm is 2.5mm. Difference between the suits is now only 1mm. So the weight difference at depth will be half the difference that you would find at a surface buoyancy check.
I don't care what the weight difference is at depth. I do my weighting adjustment to be slightly negative at 3m with a near empty tank so I can guarantee to hold a deco or safety stop.

Doing it your way, I would be too heavy at depth, at least for my taste. My way, I am about neutral at 15 feet with a near empty tank and slightly positive on the surface. Please remember that my diving style was developed back in the 1960s. I rarely put air in a BC underwater and use it primarily for surface flotation.

But what you say makes a lot of sense and explains the marginal weighting difference between the 3mm and 5mm. Thanks for answering my question.

 

[windapp]

Ah. That explains it. I was assuming that you were using a standard weighting check - almost empty tank at the surface, no air in BC and checking your flotation level at some point on your face. Comparing like with like.
Assume you are at a depth where you have 50% neoprene compression. 50% of 3mm is 1.5mm. 50% of 5mm is 2.5mm. Difference between the suits is now only 1mm. So the weight difference at depth will be half the difference that you would find at a surface buoyancy check.
I don't care what the weight difference is at depth. I do my weighting adjustment to be slightly negative at 3m with a near empty tank so I can guarantee to hold a deco or safety stop.

Of course, you need to trim for planned depth. The more air required in your BCD, the higher up the weight will need to be on your body.