A simple enough question....

Note: this thread is not to induce a "normal BC" vs BP/W debate

ok this whole BC lift thing seemed simple enough when I first started thinking about it, but the more I looked into it the more I started to become confuzzled about the answer. (I blame the internet....the more I read the less I know)

I have started to look into purchasing my own gear for diving and my first step was to join Scuba Board to try and pick up as much info from you guys as I could realizing that some info is priceless (lessons learned the hard way)....some info is dated (don't have to search very far to find info on gear that is either no longer used or even made)....and some info will be debated by any and all that have an opinion (read above about BC vs BP/W rodeo)

when it came time to start looking at BCs a common theme I found regardless of style was the "lift" rating. Now at first glance I assumed that more lift would compensate for tank size/weight, single or dual setup, etc. But as I started to look at the different styles and models I begin to start venturing into areas I need some guildance with.

so let's start in the beginning. what exactly is the amount of lift of a bladder used for? pretty much any thread in this area refers to you using weight to adjust bouyance and factor in variables such as salinity of water, weight of wetsuit, how big you are, using aluminum or steel plate, etc..etc. and I know that you add air or release air in bladder to adjust bouyance, etc. so it all ties together in some form or fashion. where the confusion comes in is when I start comparing the different models to buy.

I have found an average of 30-45 lbs of lift for a BC using a single bottle. But it can vary from 18lbs for a OxyCheq travel wing to 100lbs for a Poseidon Besea(yeah, I know...doubles). As I am so far only interested in singles I'll stick with those that are made for that and there is still a variance that maybe you can answer for me.

a random list of BCs:
Zeagle Brigade 35lbs lift (only lift avaliable)
Mares Dragon 44lbs (OLA)
Oceanic Excursion 45lbs (OLA)
Tusa Plantina
27lbs for xs/s
48lbs l/xl
Hollis HD100
35lbs s/m
45lbs l/xl
OxyCheq Mach V Extreme
30lbs or 40lbs
OxyCheq Travel
18lbs
Dive Rite Rec
51lbs
Dive Rite Travel
30lbs


looking at the variations I can't decide if your choice of lift is determined by your size? (Tusa Plantina) or is there another factor come into play that I'm not aware of? Why is o.k. for you to use an 18lb travel wing for flying down to Mexico on a dive trip and upon your return switch out to a 51lb wing for your normal diving? Would it be better to use a smaller bladder and less lead or what? or is this a case of me over thinking an issue?

this issue started for me because I have yet to find a LDS in my area that rents BP/W for me to learn the difference between a 30lb and 45lb wing and since if I go the BP/W route I will probably be buying it online (hello Scuba Toys...giving you fair warning). It would be nice to know if this whole "lift" question is that important or just go with the old "if it feels good, buy it" noobish method

Short answer: You need enough lift in the BC, regardless of type, to offset the loss of bouyancy caused by wetsuit compression at depth, and one that provides enough lift on the surface to float you and your gear at the beginning of a dive when you are the most "negatively bouyant" (due to the weight of the air in your tank(s) ).

The reason for the wide variation in lift ratings is that there is a very big variation in the bouyancy of the diver based on exposure suite type (wet or drysuit) and thickness, equipment used (single aluminum 80 vs double steel tanks, dive lights, other gizmos and gadgets ), and the "personal bouyancy" of the diver (based on their body composition).

Long Answer: I'm sure others will jump in with more detailed and complete explanations!

Safe DIving!

Don't forget, it needs to have sufficient lift to support the rig at the surface without the added buoyancy of the diver and his thermal protection.

the K

The Kraken:

Don't forget, it needs to have sufficient lift to support the rig at the surface without the added buoyancy of the diver and his thermal protection.

the K

Click to expand...

Great point, I forgot to mention that.

RiggedEscape: You are asking the right questions, so you should do fine when you finally decide on which BC is right for you. Keep track of how much weight you need to carry with the exposure suit are using, combined with the type and thickness of the suit, and then you can start getting more precise answers as to how much lift you'll need when you purchase your own BC.

Good Luck!

so basically it doesn't really matter too much as to the lift, based upon different variables I'll just have to compensate with weight. Pretty much as I thought, maybe I was indeed thinking too much about it. But like I said, when I started seeing a huge difference in "lift" ratings on the BCs out there it got me questioning. Thanks for the replies guys. And be safe out there.

RiggedEscape

Good answers so far.

Remember that you are dealing with buoyancies, not weights.

This is is where knowing were you intend to dive is helpful. A Caribbean island is a lot different than Vancouver. This is a thinly veiled suggestion to put some useful information in your profile.

If I am in trunks or even a 2mm shorty and weighted right I can dive without a BC. The only real buoyancy change is the rise of 5 pounds or so during the dive as air mass leaves the cylinder. Planned right I can manage that with lung volume control. On a dive like that 10 pounds of lift would be more than enough to have me high and happy on the surface. It would also be enough to keep my BC with steel cylinder afloat if I get out of my rig.

Now lets say you are in cold water, wearing a 7mm wetsuit with 2X on your core. You may have just added 20-30 pounds of lead just so you can sink and begin the dive. As you descend your wetsuit will compress and loose significant buoyancy. This is where you start to use the lift of the BC during the dive. If you added 25 pounds of lead due to the wetsuit you will need some portion of that to offset compression. The suit will not loose all of it's buoyancy. There is no real good data about how much you will loose at what depth. It will vary with suit, size and the age of the suit among other factors.

When you return to the surface you wetsuit rebounds and becomes buoyant. That same lift that kept you off the bottom can now be used to get your head high.

The bottom line is that you need to be thinking about offsetting changes during different phases of the dive. For most cold water single tank divers something in the low to mid 30 LB range is good. 40+ is usually reserved for divers with doubles or stages. The wings down in the teens and low 20s are for warm water where you have little compression and lead to manage.

Becoming gear savvy is often more difficult than getting certified. Think it through and the light bulb will come on.

Pete

The way it was explained to me was this:

at correct weighting, remember you should be about eye level to the water with a lung full of air and just begin to sink as you exhale with the bc deflated

THUS, the lift determines what "poundage" of your body (in equivalent displaced water) can be held in a stable float ABOVE water with the BC fully inflated. Thus, a travel BC with 18 lbs of lift means that only 18 lbs of you, so to speak, can be held in a float above water at full inflation. The average head weighs, oh, I think 12 pounds give or take. Thus, even for a small person, that degree of lift means that, without struggling, a fully inflated BC will just about keep your shoulders at water level. In calm seas, that's OK, but in waves, it may be unsettling to be chin high to the water even with a full BC. If you have a big fat head, a small BC may not even keep your face out of the water. For pros, this is no big deal (people used to dive, and still do, without BCs at all). But for klutzes like me, the inability to use the BC to keep the head and upper torso safely well out of the water in rough seas or in a cramping/exhausted situation is vital.

A 35 pound lift will thrust you much higher in the water, depending on your size. A thin person will ride higher in the water, obviously. The degree of gear matters, too, and, of course, the nature of the water (salt or fresh) and the type of suit you are wearing.

The LDS people I deal with are leery of travel BCs with little lift, especially for large people or those who are inexperienced.

You've already heard the conceptual answer, which is that the bladder needs to compensate for the loss of lift with wetsuit compression at depth, and that it needs to float your rig on the surface.

Here are a couple of examples that might give you an idea why different amounts of lift might be needed in different places.

When I dive in Maui, I dive a 3 mil wetsuit. I dive an Al80 tank, and I usually use my 5 lb SS BP. I use two pounds on the cambands for perfect weighting. A full Al80 is about 2 lbs negative, and that with my SS BP and 2 lbs of weight makes about 9 pounds negative. A 17 lb wing will easily float the rig with the tank full. I honestly don't know exactly how much lift my 3 mil suit has, but it isn't much -- I doubt it's even ten pounds. Say it is; at 120 feet, I've lost almost all of that, and my tank is still 2 lbs negative, so I need 12 pounds of lift, which I've got.

Now move to Puget Sound. I'm now diving a steel 95, which is almost 2 lbs negative when empty, and has 7 lbs of gas in it -- so 9 lbs negative. I'm using a SS BP with 6 lbs on the cambands, so I have 20 pounds to compensate just to float the rig. The 17 lb wing won't work any more. In addition, I'm diving a dry suit and undergarment which are (I've checked) 28 pounds positive. So, in the worst case, I completely flood the drysuit, lose the 28 lbs of lift, and still have 9 negative pounds . . . This adds up to 37 pounds negative, and my 30 lb wing doesn't have that much lift! (Now, in point of fact, even if I flood the drysuit, I haven't lost all the lift, because it takes time for all the little air cells in the undergarment to get saturated. And I also wear 20 lbs on a belt, so that can be jettisoned in part or whole if necessary. So I'm not in as bad a shape as it sounds ).

But these two examples should give you an idea of why different amounts of lift are needed in different circumstances. It should also give you an idea of the range of lift that is reasonable for single tank diving. What you don't want to buy is a BC that either doesn't have enough lift for the gear configuration you are using, or that has WAY too much, because such a BC may be difficult to vent, and surely will not be streamlined to offer the least resistance to the water.

Good questions, Rigged.

I have 3 wings, which I can attach to either of my 2 backplates.

One is an 18 lbs Oxycheq, for tropical single tank diving.

Another is a 40 lbs Oxycheq, for cold water single tank diving, an oval design.

The third is a 50 lbs Oxycheq, for twin tank diving, a U shaped design.

I went with the 18 lb-er simply because for tropical diving, with an aluminum tank, I am virtually neutrally buoyant at the end of the dive, even with a very light weight belt. For warm water thin-wetsuit diving, you have very little weighting requirements, and therefore an 18 lb wing is plenty.

For cold water single tank diving, 35 to 40 lbs is necessary for me. This is because I dive with a drysuit, therefore I do not need to worry about wetsuit compression at deep depths. And so I can wear a light weight belt of around 8 lbs. The rest of my ballast weight I like to wear on the plate. Thus the wing needs to lift more than the usual rig when divers wear heavy weight belts of 20 lbs or more.

And for twin tanks, 45 to 50. This is what I have found.

I like to be able to toss my rig into the water, and then put it on in the water. Therefore the wing needs to be able to support and float the entire rig by itself.

I also like to be able to inflate my wing and then ditch my rig in the water, such as with an emergency rescue situation. If your wing cannot support the rig when it is off your back, then it will sink, if you do this. And then after the rescue scenario, you will also be doing an underwater grid search for your own rig, after having borrowed someone else's to do so, of course.

hint taken Spectrum, profile updated....my primary interest in scuba is to dive river/coastal looking for artifacts/fossils. Long term goal is to apply graduate program at FSU for underwater archeology. Not sure if I'll ever get there, but in the space in between now and then I hope to dive to satisfy what has always been a secret passion.

South Carolina is the only place I know where the state will allow you obtain a permit that lets you keep what you find, but is officially on loan to you for 6 months. You must submit quarterly reports declaring your finds giving the state a opportunity to examine said finds. If the state doesn't check, or releases your finds then you keep them.

so for the most part I guess you're talking water temp from 50-75 degrees in both fresh and salt, depending on time of year. Depending on my cold tolerance and time duration of dives I'll probably go with either 3mm full to a 5mm full, maybe 7mm (I'm old enough not to care about being a wuss and proving anything...if I'm cold I'm gonna cover up). I am really excited about the Cooper River diving and wouldn't mind getting my feet wet in some of the ponds all around my area in hopes of finding something that has history.

again, thanks for the help. I'll keep the boards updated as to my progress, it should provide ample humor.