How much BC lift do I require?

[Cinjodi]

Don Burke once bubbled...

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

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?

I wouldn't know about being neutral at 15 feet with 500psi because I have never ran my tank to 500psi. The boats always say to come back with either 500psi or after one hour whichever comes first. I come back with between 1000 and 1200 after the hour.

I'm a chick, so I guess the 14 pounds is because I'm all blubber no muscle. I've tried less, but it just doesn't work and I have a harder time trying to get down below the surface.

14 lbs with my shortie is just right. I have very little air in my BC during the dive and just breathe deeper to make small climbs along the reef, and exhale more fully to fall a little lower. No problems. No breath holding. Nothing wierd. I don't monkey with the air in the BC. 14 lbs works.

What I mean by "natural ascension" is that I just float very slowly, very creepingly up, up to the surface. No playing with BC air, no weight dropping. So since I'm not manipulating anything, does this mean that I am also not able to "control" the ascent?

My OW class was 4 years ago and I haven't seen my instructor since. This subject never really came up. He bounced me up and down and tossed me in the water (like grownups do to children), predicted a starting weight for fresh water, and it was pretty darn close.

Since the important measure is the 500psi at 15 feet, should I rent a tank and vent it off to 500psi and try it??

 

[Don Burke]

Cinjodi once bubbled...



I wouldn't know about being neutral at 15 feet with 500psi because I have never ran my tank to 500psi. The boats always say to come back with either 500psi or after one hour whichever comes first. I come back with between 1000 and 1200 after the hour.

I'm a chick, so I guess the 14 pounds is because I'm all blubber no muscle. I've tried less, but it just doesn't work and I have a harder time trying to get down below the surface.

14 lbs with my shortie is just right. I have very little air in my BC during the dive and just breathe deeper to make small climbs along the reef, and exhale more fully to fall a little lower. No problems. No breath holding. Nothing wierd. I don't monkey with the air in the BC. 14 lbs works.

What I mean by "natural ascension" is that I just float very slowly, very creepingly up, up to the surface. No playing with BC air, no weight dropping. So since I'm not manipulating anything, does this mean that I am also not able to "control" the ascent?

My OW class was 4 years ago and I haven't seen my instructor since. This subject never really came up. He bounced me up and down and tossed me in the water (like grownups do to children), predicted a starting weight for fresh water, and it was pretty darn close.

Since the important measure is the 500psi at 15 feet, should I rent a tank and vent it off to 500psi and try it??

From that description, it sounds like you're doing fine. You are not having any trouble controlling your ascent, so your weighting is probably within a pound or so. I certainly wouldn't make a special dive to mess with it. You may want to try stopping at 15 feet some time when you are ending a dive and have something around 500psig. Anything below 1000 will give you a pretty good indication.

I'd be willing to bet that you find yourself a bit heavy at the beginning of the dive and have been swimming to carry the weight. There is about a 5 pound weight change in an 80 foot tank from 3000 to 500. Since you are not running your tank down that far, it is probably more like three or four pounds in your case.

While women tend to be a little more positively buoyant than men, the buoyancy doesn't change with pressure.

In many fields, there are people who are so comfortable doing things, they don't realize how they do them. You appear to be that way diving.

From here it looks like you've got controlled ascents down pat.

When you go deeper, you may find yourself shooting a little air in the BC to make up for the wetsuit getting compressed, maybe not. It won't be much if you do.

 

[ClassAction]

Don Burke once bubbled...


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.

I'm also a newbie and I'm in the market for a bp/wings setup. In salt water with a shortie I wear about 8#. Do you think it's a good idea to get the SS BP? I'm concerned that I won't have enough ditchable weight. In other words, is there a safe percentage of your total weight that needs to be ditchable? Is 2 pounds out of 8 safe to dive? Maybe I should just go with the AL BP at 2 pounds?

Thanks1

 

[jplacson]

ClassAction, are you neutral at 15 feet with 500psi in your tank at the end of a dive? Your BC should be completely empty at 15ft with 500psi, and you should NOT sink.

If you are neutral at 15ft with 500psi... and you're using 8# ... the SS plate isn't a bad idea.

There's no real danger in having 8# of non-ditchable weight if you are weighed properly. You can easily fin up even with a deflated bladder.

 

[paulwlee]

jplacson once bubbled...
There's no real danger in having 8# of non-ditchable weight if you are weighed properly. You can easily fin up even with a deflated bladder.

But keep in mind that at the beginning of the dive, there will be an additional 5lbs for a 80cf tank. 13lbs may get a little dicey if you are already tired.

If it were me, I'd get an AL backplate and wear the 6lbs as ditchable. That way, if the wing fails at the beginning of the dive, and I'm having problems, I will be positive on the surface just by ditching the belt instead of the whole rig!

 

[freedive]

Cinjodi, 14 Lbs of lead seems like alot for someone your size. I know everybody has a different natural buoyancy so weight requirements will vary a bit from person to person. I'm 6'1" and 225 Lbs using a 2/3 mm shorty in salt water with AL 80 and I'm using 12 Lbs and still sink a little at 15 ft with 500 PSI with the BC completely deflated. It takes a very small amount of air to get me buoyant at 15 ft and actually this weekend I may try 10 Lbs to see if I can knock off a couple of extra pounds of lead.

 

[richies]

Hi

How much lift does a BC actually need?

If you weight yourself correctly, you would be neutral at about 3m (10ft?) with not much air in your tank and no air in your BC/drysuit.
Obviously you would start your dive overweight by the weight of the gas in your tanks, and the BC would have to compensate for that, but that's never really very much - perhaps 3kg (6lb?) for a 15 litre cylinder (er...104cu ft?) at 232 bar (um...3300 psi?).

In this case, all but about 6lb of lift in the BC is going towards surface support. So how much lift do you need to get your head out of the water comfortably?

cheers

R

 

[SeaJay]

Yep, I'm with you on this one.

I would point out, though, that there's an additional factor here... If you're diving a wetsuit, then there is quite a bit of loss in buoyancy at depth; some say as much as 90% at 100 feet.

...So don't forget to put that into your calculation as well.

There's also the issue if you've got more than one tank, or a tank that has more than a 6 lb swing.

Add a couple of pounds for comfort at the surface, and 25-30 lbs. should do most 3 mil wet, single AL80 divers nicely. In fact, they might be able to get away with as little as 20 lbs... Especially if they forgo the wetsuit.

 

[Don Burke]

You're looking at about 5.9 pounds per 80 cubic feet of air/nitrox.

I think some of the advertised lift is lost by restricting the BC.

In any case, I dive a 45 pound wing with double 80s wet or double 72s dry and I get on the surface just fine.

One time I screwed up my math and went into a quarry 12 pounds overweighted. It was certainly manageable, so I think a 40 pound wing would do as well for me.

I don't dive with more than one sling bottle. Perhaps a couple of 80 foot stage bottles would change my opinion.

 

[Dryglove]

In any case, I dive a 45 pound wing with double 80s wet or double 72s dry and I get on the surface just fine.

Im with Don on this one as i use a OMS 45lb wing with my double steel 72's in my drysuit. It gives me plenty of surface flotation.


Read this about wing sizes

http://www.baue.org/faq/wing_size.html

http://www.baue.org/faq/dynamic_instability.html