Really NewB question. Why does Lift Matter in a BCD?

[hqduong]

Sorry for the Newb Question. But I always been advised to not even worry about the amount of lift that a BCD Provides.

But why does the amount of lift matter? I mean Theoretically you're suppose to be neutrally buoyant, so all you need is really a pound or two of lift to start ascending? Wouldn't you have to be extremely overweighted if you need that much lift from a bcd?

When would I need to have a BCD with a lot of lift?

 

[cloudflint]

Generally if you are diving in cold water with quite thick exposure protection you will require allot of lift due to compression at depth. If you are in warm water with less exposure protection required the buoyancy swing is likely to be much lower and thus you wont need as much lift to get you up from depth. You also have to take into account the buoyancy swing of your tanks as you breathe them down, large tanks will start out heavier requiring you to have more lift to keep yourself neutral at the start of the dive, while at the same time you need to have lots of lead to stop you being under weighted at the end of the dive.

To give an example one of my instructors likes to give, in the 90's or so there was a dive club who liked to dive in big thick neoprene dry-suits. My instructor advised them that they should really use BC's as well since due to compression their drysuits wouldn't really be enough to properly lift them at depth. They ignored this advice however and a few months later my instructor got a call from their club to say that one of their guys had passed out at about 30 meters and they had been unable to get enough lift to get him off the bottom so the diver subsequently died. This is something of an extreme case but is a good example of when a BC with lots of lift would be needed.

 

[nielsent]

The lift in a BCD refers to the amount of weight it would take to sink the BCD if it was filled to capacity. With standard recreational BCD's it is not a major issue because they usually provide more lift than is required.

Theoretically you are correct, the idea is to be neutrally buoyant at all times, unfortunately, this isn't exactly the case, and there are other small details that you constantly battle with in order to maintain neutral buoyancy.

A BCD should have enough lift to float the entire set-up scuba rig at the surface so that it is possible to remove the rig in the water and have it all still float. The BCD should also provide enough lift to compensate for the loss of buoyancy your exposure suit may have when compressed. People who dive in cold water that have wear a lot of neoprene tend to need a fair bit of weight to get down in the water, but once they reach depth, all the little bubbles in the neoprene compress, and it becomes less buoyant. Or if you wear a drysuit and it leaked really badly, you would lose all the lift from the drysuit and gain extra weight.

BCD's are just a tool for the job. You only need a lot of lift if you are planning on being really negatively buoyant.

I would recommend speaking to an instructor in your area in detail to get a better idea of the factors involved.

Dive Safe.

 

[LakeCountyDiver]

Well who ever told you not to worry about the amount of lift your BC might have been implying that many of the BCs sold today are upwards of 35# to 60# of lift (much more then you need). The reason I look at the amount of lift is that we have to use that lift to compensate for the things that dont float.

We ware led to weight down ourselfs (if we are +) and our wetsuit.
Things like Tanks,regs,reels,and lights are negative in the water

Example: If you have a new 7mm you may need 15#+ to sink it but if you go down to 120ft you now have much more negative weight due to your suit compressing and loseing its +buoyancy.
So if you dive a steel 100HP in a 7mm with hood/gloves with lights you may near the beginning of your dive at 120ft be in and around -20#s so a bc that offers a large amount like the new ones will easily cover it.

Now if you are a harness/wing guy you can shop for a wing size that just fits your needs and does not give you 30# extra lift.

That example is a rough ball park figure to help you understand the method and not to be takin seriously as the amount you would actually need.

 

[lowtech]

Others have mentioned minimum lift, but excess lift also has its problems.
Having more lift then you need will cause unwanted drag and can make the bladder difficult to vent.

You may want to do a search on "balanced rig"

 

[hqduong]

Ah! Thanks everyone that makes a lot more sense. I think I do a lot of things subconsciously to remain neutrally buoyant, I never realized mathematically how wide of a swing those pesky little things make! The Devil is always in the details

 

[fnfalman]

Sorry for the Newb Question. But I always been advised to not even worry about the amount of lift that a BCD Provides.

But why does the amount of lift matter? I mean Theoretically you're suppose to be neutrally buoyant, so all you need is really a pound or two of lift to start ascending? Wouldn't you have to be extremely overweighted if you need that much lift from a bcd?

When would I need to have a BCD with a lot of lift?

You don't need a BCD with a lot of lift. You need a BCD with "enough" lift based on your diving scenario and equipment configuration.

 

[diveprof]

Way too often I see folks purchasing BC with excessive amounts of lift. This results in way too much uncessary drag, clutter, etc. BP/w folks sometimes fall into the trap of trying to get a wing that will work great with singles and doubles. All to often this means a wing with excessive lift and bulk for singles and sometimes not enought for doubles. This compromise is not advised. Get what you need.

 

[nomro]

Also enough lift capacity will keep you buoyant with your head up without squeezing your ribs.

 

[TSandM]

Your BC has to do two things: Float your rig on the surface (for the times when you have to don or doff equipment in the water) and compensate for the gas you intend to waste into the water, and for the loss of buoyancy of compressible exposure protection.

If you are diving in the tropics, your tank is likely an Al80, which is only about 2 lbs negative when full, and you're unlikely to be carrying more than a few pounds of weight. So it doesn't take much to float the rig itself, even if you put all your weight on it. In Puget Sound, it can be very different -- we tend to use big tanks, so you could be 7 to 10 pounds negative from gas alone. Then, with a thick wetsuit or dry suit, you could have 20 or more pounds of ballast. Put that all in a weight-integrated BC, and you need a fairly big air bladder to float that gear on the surface.

Similarly, underwater, you need enough lift to compensate for everything you can lose . . . which, with thick neoprene or a drysuit, could be upwards of 20 lbs of lift at 100 feet. Add the gas, and again, we're looking at the 30 lb area for safety.

These things are much easier in the tropics!