How much BC lift do I require?

[rescuediver009]

If you are using AL80's you would have to be a little overweight at the beginning of the dive, because the tanks become annoyingly positively bouyant at the end of the dive. I am 185 lbs. and 5'11" and I wear the same as you (as far as suits) and I use 20 lbs. with a steel 80 (when not using doubles).

 

[operabuffa]

Thanks for everyones input.
Has anyone ever measured how much lift an empty Al 80 has at various depth and temperature?

 

[Don Burke]

operabuffa once bubbled...
Thanks for everyones input.
Has anyone ever measured how much lift an empty Al 80 has at various depth and temperature?

The buoyancy of the tank doesn't change enough with depth or temperature to be noticed.

The tables will tell you that an AL 80 is about 4.1 to 4.4 pounds positive when empty. My experience is that an AL 80 with 500 psig of air, a first stage, a second stage, a short LP hose, a few feet of nylon rope, and two big boltsnaps is just positive enough to be a pain in the butt. I would expect some variation from tank to tank.

Your original question was about BC lift.

Here's what your BC needs to do:

When you get into the water, you are weighted to hold your shallowest stop with minimum air. Most agencies seem to be teaching 15 feet at 500psig.

When you get to depth, you are carrying more air than you're weighted for and your wetsuit isn't as buoyant as it was at 15 feet. The BC needs to be able to pick up the difference in both.

One fine day, I went to a pool, put my neoprene in a mesh bag and figured out how bouyant the stuff was at various depths. It turns out a Bare one piece 3/5, boots, gloves, and hood are about 16 pounds positive at the surface. Your outfit would be a bit more.

I lose a few pounds of that to get to 15 feet, so going to depth involves a loss of something like 10-12 pounds.

An 80 cubic foot tank weighs about 5 pounds more full than at 500psig, no matter what the tank is made of.

So, in theory, my BC should only need about 18 pounds of lift. According to the same computations, your BC should need about 30 pounds due to more neoprene.

What the manufacturers don't tell you is that the stated lift of a given BC assumes that the bladder will be able to expand unrestricted. In a jacket BC, there is a human body in the way. With backinflate, the tank may be in the way (maybe not).

In the real world, the 40-45 will probably serve you well. It allows for the discrepancies in BC lift, getting yourself a little higher out of the water at times, and the possibility you might want to use a really big tank like a 131.

There is a danger in going too big in that a stuck inflator will get more air in the bladder before you can stop it. That could make you look like Shamu jumping for the tourists. It could also embolize you or drive a boat keel through your skull. The people diving 100 pound lift BCs face that danger.

 

[Cinjodi]

Ok, in all the figuring of BC lift needs I'm getting lost. Please forgive a newbie. I guess maybe I should read some manuals and figure this all out, but I don't know that I would get the actual human answer.

I keep reading about BCs needing to contribute to a "controlled ascent". And how it is supposed to "lift" , etc. etc.

I've only dove to around 45 ft in salt water on all the dives I've done. I have very little, if any, air in my BC during the dive. I start with air in my BC and then essentially dump it all to go down to my dive depth. I use my breathing to rise and drop over the reef as needed. When I want to go up, I just point up and give a small kick and I start ascending slowly. When I reach the surface, I fill the BC and relax.

So all this talk about "controlled ascents" scares me. What am I doing wrong? Is it because I haven't gone deeper? Will I become heavier if I go to 65 or 70' and I will need air in my BC to maintain my dive depth?

Scary, isn't it? I guess this is something I'm supposed to know as a function of my OW classes, right? Sorry, but I don't know. And so I don't know about this "controlled ascent" and "lift" that I'm supposed to have.

Help?

 

[Don Burke]

Cinjodi once bubbled... Ok, in all the figuring of BC lift needs I'm getting lost. Please forgive a newbie. I guess maybe I should read some manuals and figure this all out, but I don't know that I would get the actual human answer.

We can try.

Cinjodi once bubbled... I keep reading about BCs needing to contribute to a "controlled ascent". And how it is supposed to "lift" , etc. etc.

Actually, "lift" may not be the best term to describe what a BC does. Some divers use a BC as an elevator control, inflating to ascend and venting to descend.

A BC should be used to compensate for buoyancy changes. They fall into two categories:

- things that get used up, like gas

- things that change buoyancy with depth, like neoprene and air bubbles.

A BC should require venting as gas gets used from your tank. This is something like five pounds spread out over the length of the dive, so it isn't obvious.

A BC should require venting to ascend to stay neutral as neoprene and the bubble in the BC expand.

A BC should require inflating to descend to stay neutral as neoprene and the bubble in the BC contract.

Cinjodi once bubbled... I've only dove to around 45 ft in salt water on all the dives I've done. I have very little, if any, air in my BC during the dive. I start with air in my BC and then essentially dump it all to go down to my dive depth. I use my breathing to rise and drop over the reef as needed. When I want to go up, I just point up and give a small kick and I start ascending slowly. When I reach the surface, I fill the BC and relax..

It looks like you've got it pretty much whipped. I'm guessing you don't wear much neoprene. You may find the air in your BC can be fine tuned to make it a little easier on your lungs.

Cinjodi once bubbled... So all this talk about "controlled ascents" scares me. What am I doing wrong? Is it because I haven't gone deeper? Will I become heavier if I go to 65 or 70' and I will need air in my BC to maintain my dive depth?

You are probably not doing anything wrong.

As you go deeper, the compressible stuff displaces less water, so more air is needed in your BC to stay neutral. This is most pronounced shallow, so if you've handled it so far, it shouldn't be a problem.

Cinjodi once bubbled... Scary, isn't it? I guess this is something I'm supposed to know as a function of my OW classes, right? Sorry, but I don't know. And so I don't know about this "controlled ascent" and "lift" that I'm supposed to have.

If you're diving properly weighted without neoprene, you may not have seen the problem. You may never.

If a diver is ascending with a bubble in his BC, he needs to be venting it off to reduce lift. If he doesn't he will accelerate and may head for the surface uncontrolled.

If he vents properly, he will perform a controlled ascent.

Cinjodi once bubbled... Help?

I hope I did.

 

[Cinjodi]

Yes, you were helpful in your answers.

But I can't "vent" what I don't have. When I ascend from the dive, there is no air in my BC to "vent" because I don't use it during the dive. I only use it at surface.
All of my air is essentially already vented so that I could sink down to my dive depth.

Should I be using more weight so that I can keep more air in my BC so that I have some air to vent off in my ascent??

I wear a 2/3mm shortie in the salt, with 14lbs lead, which is about 10% of my weight.

Everything has worked fine in previous dives, but I hope to start making deeper dives and I am now wondering if I should be carrying more weight (with corresponding air in BC) so that I can make the "controlled ascents" I keep hearing about. I don't want to rocket to the surface because I have no air to vent off and am relying only on natural ascention.

I bring this up in this thread only because I'm in the market to purchase a BC and want to make sure that I buy something that will have the proper "lift". I just haven't had to use "lift" before (or it's opposite!) so I'm not sure what I need.

 

[Don Burke]

Cinjodi once bubbled...
Yes, you were helpful in your answers.

But I can't "vent" what I don't have. When I ascend from the dive, there is no air in my BC to "vent" because I don't use it during the dive. I only use it at surface.
All of my air is essentially already vented so that I could sink down to my dive depth.

Should I be using more weight so that I can keep more air in my BC so that I have some air to vent off in my ascent??

I wear a 2/3mm shortie in the salt, with 14lbs lead, which is about 10% of my weight.

Everything has worked fine in previous dives, but I hope to start making deeper dives and I am now wondering if I should be carrying more weight (with corresponding air in BC) so that I can make the "controlled ascents" I keep hearing about. I don't want to rocket to the surface because I have no air to vent off and am relying only on natural ascention.

I bring this up in this thread only because I'm in the market to purchase a BC and want to make sure that I buy something that will have the proper "lift". I just haven't had to use "lift" before (or it's opposite!) so I'm not sure what I need.

Actually, you've got me wondering about how you are weighted.

Are you neutral at 500psi and 15 feet with no air in your BC?

During the dive, you are using up air, which means you get lighter. The weight of the air in an 80 foot tank changes by about five pounds from 3000 to 500psig. I find it hard to believe that isn't showing up somewhere.

That 14 pounds sounds like a bunch. How it doesn't result in air in your BC is another puzzle.

10% is a gross estimation to get you started. 500psig at 15 feet really tells the tale.

I've never heard of "natural ascention".

Is your OW instructor available so you can run these questions by him/her?

 

[jplacson]

I'm having a similar dilemna cuz I just purchased a Halcyon 27 MC system...and I'm trying to figure out if I should go for the AL or the SS BP

Prior to this, I only dove with rentals... whatever was available. With some crappy rental jacket BC, I could pretty much go down with just 4#... considering that it was not a great BC which probably trapped air... I'm diving pretty light.

Now, the Halcyon is a much more efficient system. I'm trying to figure out how I should weigh myself with this new rig. Now of course I'll probably end up doing the whole trial-and-error thing this weekend when I first try it out...but I'd like to get a big chunk out at surface and just fiddle with trimming under water.

Here's what I've figured so far:
The regulator (-1 or -2) and the Pioneer 27 wing (empty; about +1 or +2) should negate each other and be almost neutral together.

The AL80 tank is +4 when empty

The AL BP is -2

Which means that to make my new rig neutral... all I need to add is about 2# more... right? I hope my math on this is more or less correct.

Now, all I need to do is compensate for my buoyancy right? I just have to adjust to float at eye level with a full breath without my rig...since with my calculations above, my rig should be about neutral already.

I know I'll only get to really figure it out when I use the thing... but like I said, I'd like to get things approx figured out before I dive.

I'm trying to break up the calculations to (what I need to be neutral without my rig) + (what my rig need to be neutral without me) so that if I ever change rigs, or change wetsuits... I can just add the appropriate amount of trim weights.

 

[Don Burke]

jplacson once bubbled...
I'm having a similar dilemna cuz I just purchased a Halcyon 27 MC system...and I'm trying to figure out if I should go for the AL or the SS BP

Prior to this, I only dove with rentals... whatever was available. With some crappy rental jacket BC, I could pretty much go down with just 4#... considering that it was not a great BC which probably trapped air... I'm diving pretty light.

Now, the Halcyon is a much more efficient system. I'm trying to figure out how I should weigh myself with this new rig. Now of course I'll probably end up doing the whole trial-and-error thing this weekend when I first try it out...but I'd like to get a big chunk out at surface and just fiddle with trimming under water.

Here's what I've figured so far:
The regulator (-1 or -2) and the Pioneer 27 wing (empty; about +1 or +2) should negate each other and be almost neutral together.

The AL80 tank is +4 when empty

The AL BP is -2

Which means that to make my new rig neutral... all I need to add is about 2# more... right? I hope my math on this is more or less correct.

Now, all I need to do is compensate for my buoyancy right? I just have to adjust to float at eye level with a full breath without my rig...since with my calculations above, my rig should be about neutral already.

I know I'll only get to really figure it out when I use the thing... but like I said, I'd like to get things approx figured out before I dive.

I'm trying to break up the calculations to (what I need to be neutral without my rig) + (what my rig need to be neutral without me) so that if I ever change rigs, or change wetsuits... I can just add the appropriate amount of trim weights.

If the tank and regulator are the same, there should be no weighting change due to that.

It appears aluminum would be your best choice.

Since the BC is being changed, about two pounds removed from the four pounds you have been wearing looks like a good call.

I'd wear two or three pounds if it was me. If my weighting is going to be off, I'd much rather be a pound or two heavy than one pound light.

The floating at eye level thing is just an approximation. The true test is your shallowest stop at minimum air pressure. Most agencies teach 15 feet at 500psig. I use 10 feet at 400psig. Just pick one and use it.

 

[jplacson]

Thanks! Actually, I'm worried about being OVERWEIGHT... the 6lb SS BP felt a bit too heavy and I figure it was easier to add ballast, than have 6lb that I can't trim anymore.

If, for some reason, I find that I can dive with just the AL BP...then well, and good! My total rig's weight would be heaven!

without any gear, and just testing my own body's buoyancy, I can float at eye level with a full breath...so I think it's safe to say, that I'm pretty neutral as much as I can get.

My fins and knife probably offset any buoyancy (if any at all) that my fleece suit has.

I'm trying to trim off as much lead as I possibly can.

I've recently learned that for safety, you should be able to fin yourself to the surface with full gear, and a full tank... empty bladder... in an emergency... you're rig shouldn't be an anchor, in case your BC fails.