Buoyancy AHHH!!

[JTH2711]

I am having some issue with my buoyancy. The beginning of the dive through about to 1500 psi in my tank I do great, give or take some minor adjustments here and there. However once I get below that magic number of 1500 my buoyancy really suffers it is poor, i'm constantly inflating or deflating my bc. I just can't seem to hit a sweet spot like I can at the beginning of the dive.

Most of my dives as of late have been relatively shallow (max depth of about 30-35 feet sometimes 20 feet). We have just been moving about the quarry checking out fish and the like. Very simple dives in an effort to get some experience. Any help would appreciated

 

[blackvans1234]

I would be willing to bet that you are overweighted!
As you breathe the tank, you become lighter
Rinse and repeat!

It may just be inexperience with buoyancy control?

You should try to do a proper, unrushed weight check
If that doesnt work, you could take a peak buoyancy class.

The more experienced forum users will give you more advice too, but I think my suggestion is spot on

 

[matthewmark_john]

Above: "I would be willing to bet that you are overweighted!
As you breathe the tank, you become lighter, and your excess weight really starts pulling you down (so you add air)
This is only compounded if you have to change depth, as the air in your bc will expand, and you will become positively buoyant, and probably dump too much air"

This is a misunderstanding -- it is not weight that makes you sink in water. You sink because of density, i.e. you have to be denser than the freshwater or seawater or whatever you're diving in. If you think of it in terms of weight, you'll get it backward.

It's true that you become lighter as your tank gets depleted. But your buoyancy is affected because this changes your density. Density is simply a proportion of weight (more precisely, mass) to volume. Because your weight decreases, but your tank stays exactly the same volume whether empty or full, your overall density decreases.

Therefore, you need the lead (for mass) more at the END of the dive than the beginning. Just barely enough weight to achieve negative buoyancy at the beginning of the dive will become not enough as you breathe out the mass in your tank.

 

[Codiak]

I would be willing to bet that you are overweighted!
As you breathe the tank, you become lighter, and your excess weight really starts pulling you down (so you add air)

It's like a weighting issue but I bet there's more at play.

As the tank is drained of air, it gets lighter that's true... but this means you go up not down, so you tend to dump air or pop to the top like a cork.
The trick is to be weighted for the end of the dive, not the beginning... and that means your slightly over-weighted to start.

1500psi is only a half empty tank and is likely closer to neutral then positive bouyancy, if your noticing a real "lift" this soon in the dive try adding a 1 pound on your next dive.

As I said it's likely more than just weight...
1) There's a tendency to use over correct with the power inflator and dumps, a 100 small corrections is far better than 1 big one followed by another big one... and so on...
in this case unless your intentionally changing depths, you should only have to dump a tiny bit of air....
as you dive more you may find you're constantly dumping a little as you go...
2) Expectations...
At this point you seem to expect to have an issue at 1500psi and it becomes self-fulfilling.
Make sure you exhale fully and relax before making adjustments next time.
When I started out I found I was burping hard back on the boat after dives.... once I started breathing easier buoyancy improved.

 

[DivemasterDennis]

Lets make this a diving discussion, not a physics discussion. First- I agree you are overweighted. As you deplete your air you will be less overweighted, not more. I believe your problem is constant, you just notice it more later in the dive because of depth, depth varition, or some other reason. When you are overwighted you have more air in yor bcd than you otherwise would. As you vary in depth, that air is compressed or expandsaccordingly, affecting you buoyancy to a greater extent than if there was less air in your bcd. So here is my prescription for finding a solution. It will take a little trial and error, and a few dives.

1) On your next dive, try using 2 pounds less of weight. In addition, add and vent air from you bcd in SMALL LITTLE PUFFS- THE SMALLEST YOU CAN MANAGE.
2) If there is still a problem , and you were able to descend with 2 pounds less, then drop another pound on your next dive. Do this 1 pound more each dive until the problem is clearly less pronounced.
3) If you cannot descend with less weight, then focus on using the bcd with the smallest increments of air in and air out possible. Chances are you are adding until you feel movement and venting until you feel movement and that is just too much air at a time. Also, be aware of changes in depth and adjust arccordingly.

Keep diving, there is no subsittute for bottom time in developing peak buoyancy skills.
DivemasterDennis

 

[JTH2711]

Thanks
You are right about the adding air till I see movement. Some dives are better than others in terms of buoyancy. Last week I tried my level best to control my buoyancy with my breath and not mess with the inflator till it was a absolute necessity. I had some success with that. I figure the real issue here is lack of experience and weighting. Thanks for the input and I will try your weighting method because that seems to make more sense than going overboard the other way.

 

[spectrum]

As you breathe the tank, you become lighter, and your excess weight really starts pulling you down (so you add air)

You have it backwards... The lighter cylinder weight would make the diver more buoyant, requiring less BC air and improving bubble stability.

This could be the point where the OP is getting underweighted and strugling on the edge.

It could also be that the diver is correctly weighted but trim is breaking down. The light cylinder could be reefing up on the upper body with a sensation of light overall buoyancy leading to the inflater dancce. Try moving 2-4 pounds higher on the body in trim pockets or on cam bands.

Pete

 

[JTH2711]

Okay so maybe I need you guys more info. Here is the deal, long and drawn out. I dive a 7/5 wetsuit typically with hood and gloves when I go deeper than 35 feet (i dive quarries mostly for experience and proximity) I rent a BC typically Back inflate(Zeagle Scout). The best dives have been when I used AL80 tanks but I always burn though my air. Recently I started using a Steel LP 112 and I shave off 5 pounds of what I carry when I use that tank. I usually wear 21-24 pounds with the above configuration (The 21 is for when I don't wear a hood and Gloves). I carry 8 pounds on the back pockets and the rest on the front.

 

[101recon]

Hey bud.
Here's my take on it from observation.
1.You haven't dove the same set up enough to get the "feel" right yet.You dove the same BC a couple of times but also switched from AL80s to 112s in the process.(If I recall correctly)
2.Don't let your air consumption rate bother you.It WILL drop with more dives and it really isn't that bad for the #of dives you have.
3.Use small adjustments when adding or venting air from your BC.When you get movement up or down you have already gone too far.
4.Last week was a shallow dive which makes control more difficult.So cut yourself a little slack.
I think you're doing pretty good so far.Next dive we can spend some time on weighting and trim if you like.

 

[spectrum]

It's probably some combination of weighting and trim configuration. Nothing finite can be said without being in the water with you. However since you usually use the same model BC, vary the neoprene accessories and are experimenting with cylinders wile having less than 25 dives I'll say that you don't have a stable dialed in configuration. You need to make something work and stay with it or get really good with puts and takes.

Lots has been written on how to make that journey. Some of it is here.

Pete