Interesting Temporary Buoyancy Issue

[DiveClimbRide]

It sounds like you don't have much experience wearing lots of neoprene. Your bcd is there to counteract the effects of suit compression at depth. Ie When you descend your wetsuit compresses. This makes you sink (less buoyant) so you add air into your bcd to make you neutrally buoyant again. When you ascend you RELEASE air from your bcd because the air inside is expanding as well as your wetsuit expanding making you positively buoyant.
If you release too much air then you add air until you are neutrally buoyant again then resume your ascent.

Makes sense. I just thought air in your BC was mostly for the surface.

 

[johndiver999]

Pretty much ALL the accomplished scuba divers I know swim down head first. Generally they don't worry about getting every last fart out of the BC; just invert, stick a leg or two out of the water, exhale, kick hard and then don't inhale until like 15 feet and keep pumping the legs. If they are doing a back roll entry then they just straighten out after hitting the water and start kicking straight down. They are gone in 10 seconds.

This assumes a diver who is competent and can equalize easily and the visibility is such that you can see where you are going. There are always pockets of air trapped in the wetsuit and if you just power down to a depth of 15 feet, you should be sinking and you can ease up on the kicking and stay vertical with head down. This is the most efficient use of your gas, your time and your effort.

This also assumes that you have your weighting correct. The feet first descent is best for beginners or people with "problems". If you are in no hurry to get to the bottom and air supply and time are not an issue, then the efficiency of your descent is inconsequential.

The idea that you are going to dive deep with a thick wetsuit and NOT add air to the BC is nonsensical. Don't be afraid to add air, you will need a lot at 100 ft.

 

[ibj40]

Pretty much ALL the accomplished scuba divers I know swim down head first. Generally they don't worry about getting every last fart out of the BC; just invert, stick a leg or two out of the water, exhale, kick hard and then don't inhale until like 15 feet and keep pumping the legs. If they are doing a back roll entry then they just straighten out after hitting the water and start kicking straight down. They are gone in 10 seconds.

This assumes a diver who is competent and can equalize easily and the visibility is such that you can see where you are going. There are always pockets of air trapped in the wetsuit and if you just power down to a depth of 15 feet, you should be sinking and you can ease up on the kicking and stay vertical with head down. This is the most efficient use of your gas, your time and your effort.

This also assumes that you have your weighting correct. The feet first descent is best for beginners or people with "problems". If you are in no hurry to get to the bottom and air supply and time are not an issue, then the efficiency of your descent is inconsequential.

The idea that you are going to dive deep with a thick wetsuit and NOT add air to the BC is nonsensical. Don't be afraid to add air, you will need a lot at 100 ft.

My exact circumstances, and very well described. Feet down for the first three to five feet, but once fully under, head down, right hand to the bottom BCD dump, and fin to the bottom.

All the while watching my dive buddy (wife) as she profiles down in the classic feet first fashion.

Very sensitive to ear clearing, and left hand is on my nose the full way.

Once I am at around 30 feet or so, revert to feet down, and stabilize.

I'm in a 5mm BARE Velocity, using 20 pounds total weight (two three's in trim pockets on tank strap, two four's and two three's in a pocket weight belt).

Yes, I might appear to be a little over-weighted, but at 5' 10" and around 170 pounds, I'm also a little overweight as a person.

This configuration allows me to add a little bit of air at depth, and be neutral at the Safety Stop. Anything less, following a decade of experimentation, makes me a cork at the SS.

The OP needs to allow themself to experiment to what they are comfortable with.

In my world, a couple of extra pounds can be compensated for throughout the dive, a couple of fewer pounds creates frustration at the SS that is unnecessary.

 

[Ayisha]

At least to me it is...

Hello All,

I did an ocean dive last Saturday with 18 lbs. on my weight belt. I was wearing a 1.5 mil wetsuit, a 2/5 mil hooded vest, and a 5 mil wetsuit over all of it. Warm as toast, by the way, with 50 degree water.

I had enough trouble descending that my dive buddy suggested that I needed a little more weight. However, once I worked at it and was submerged for a while, I found 18 lbs. to actually be a bit too much. When fully exhaling, I could lay on the bottom and stay there while breathing. So I don't want more weight.

What seemed to do the trick was giving up on a vertical descent and just diving down, headfirst, with a few fin kicks. Releasing air from my BC was also easier because I used the release valve at the bottom of the vest since it was then closest to the surface. That method seemed more foolproof than getting the hose extended just right and hoping no air was trapped someplace.

My guess is that I had all kinds of air trapped in my suits which was eventually squeezed out by the pressure. I also assume my breathing got better as I relaxed and that helped too, but I suspect air in the suits was the main culprit.

Is getting down with a few fin kicks and head down frowned upon? Is there a better solution you would recommend, given my situation? I am open to any and all suggestions...except for add more weight.

Thank you all.

Once you check your ability to easily maintain a safety stop with a nearly empty tank at the end of a dive, then you'll know if you're underweighted or not. 1000 psi isn't really low enough to tell, but it isn't bad. Also after the safety stop, you should still be able to make a controlled ascent, not pop to the surface.

It might just be nervousness or technique. I didn't see you writing that you exhaled fully rather than just breathing as you descended. Try exhaling really slow and exaggerated, then breathe in sharper and quicker, then exhale slowly and longer again. You should be descending by then.

Whichever dump you use, make sure that you roll that side of your body/BCD slightly toward the surface so the gas isn't trapped.

If you're going feet first, hold the deflator hose high. You can even put both hands up. Keep your fins closer together. Make yourself streamlined. Your "head is your rudder", so look down.

These will help if some nervousness is keeping you from venting appropriately.

I prefer to descend horizontally. You just roll your left (or dump) side up slightly to vent from the rear dump, exhale fully and slowly, and stay almost neutral through the entire descent, adding some gas as you get deeper. My ears don't equalize quickly and this gives me time to equalize as well as control my descent.

Hope you figure it out so you'll be more comfortable.

 

[Shasta_man]

the original problem was likely moving your feet ever so slightly which keeps pushing you up so try crossing your feet when starting descent. that suit will also be a little buoyant until you get a little pressure compression on it. Sometimes people have a tendency to hold their breath a little too which is enough

Tursiops is right about needing just a short blip of air once at depth to be neutrally buoyant. After that, it should be rare to have to add any air. If you find that you have to add a little air, then vent a little air here and there, you are not weighted quite right. A pound or two makes a surprising difference. At the end of the dive, I always dump all my air when going to the surface though not everyone does that, but you will become more buoyant as you get closer to the surface and pressure is reduced.

When safety stop is over which you should be able to maintain the depth at with no effort, very slowly ascend, as in kick, pause...kick, pause....kick as the pressure differential is greatest in the last 33 feet to the surface.

 

[drk5036]

The BCD plays a much bigger role as you increase the amount of neoprene you have on.

Let's try an example.

In a 3 mm wetsuit and BP/W, I need 4 pounds of weight to be neutrally buoyant.
Descending to x meters depth, results in my wetsuit compressing, and only containing 2 pounds of buoyancy now, so I'm ~2 pounds negative. I can counteract this by adding a squirt of air into the BCD, or just using my lungs.

In a 7/8 mm semidry wetsuit and BP/W, I need 16 pounds of weight to be neutrally buoyant.
Descending to the same x meters of depth again results in my wetsuit compressing by half, and now it only has 8 pounds of buoyancy. This leaves me ~8 pounds negative, which is going to be extremely noticeable.

This is inherent to the extra neoprene, and nothing you can do about it. In my case, I've learned with my semidry it compresses significantly near the surface, and a lot of expansion doesn't occur until the 2/3 meter mark. So I use less weight and swim down so that I'm less negative at depth. At my 5 meter safety stop I can hold it without any external effort. At the 2-3 meter point, I can feel the suit wanting to go up, but I can actively prevent it if I want to. Given we do generally 30-35 minute dives with an average depth of 10-12 meters or so, I'm not worried about the slightly faster ascent rate from 5 meters to the surface.

tl;dr: more neoprene causes a bigger drop in buoyancy at depth that requires more counter-acting.

Hope this helps.

 

[Patrick Star]

Hope this helps.

Definitely. Thank you. After spending significant time on ScubaBoard, I was fixated on avoiding overweighting and counteracting with too much air in my BC. To a fault. Using the minimum amount of air to counteract suit compression, and no more, appears to be the right balance.

With any luck, our weather will cooperate and I will be back in this weekend for more experimentation.

Thank you all.

 

[CanadaDan]

A buoyancy course wouldn't be the worst idea. Helps you get your rig dialed in and teaches you more thorough theory and practices. I'd also advise a different instructor than your first course (and one with lots of experience and dives) just to diversify your input.

 

[tursiops]

Using the minimum amount of air to counteract suit compression, and no more, appears to be the right balance.

Not quite. You've also got to compensate for the extra buoyancy at the end of the dive due to air used out of the tank.

 

[Patrick Star]

Not quite. You've also got to compensate for the extra buoyancy at the end of the dive due to air used out of the tank.

Yes. That too.

A buoyancy course wouldn't be the worst idea. Helps you get your rig dialed in and teaches you more thorough theory and practices. I'd also advise a different instructor than your first course (and one with lots of experience and dives) just to diversify your input.

That is a given. Thank you. I am just fooling around a bit on my own and with ScubaBoard guidance so that when I take the course, I will get the most out of it. I would like to get a few more dives under my belt just to get the rust off, then I plan to take AOW, of which buoyancy will be a definite component.