Need help in determening BCD lift

[dmmike]

I was at my LDS today, and the question of "how do you calulate the actual lift required of a BCD" and "what is the safety margin that should be used for extra lift" came up :11: . What started the conversation was, we were talking about required lift for warm water diving compared to rec. cold water diving ( dry suit and poney bottle) to tech diving ( doubles and stage bottles). Then to make it a little more difficult when we began, we tried to start with a common BCD then changed wet suit thickness, then changed bottle size and material ( steel vs. alum).
Can somebody walk me through the calulations to determine the required minimum lift of a BCD.

As always thanks for all the help

 

[Charlie99]

I'll start with the real basic overview.

During the dive you need enough buoyancy to compensate for

1. the changes in buoyancy of your wetsuit,

2. the change in buoyancy of your tank, plus

3. enough to float any excess lead you have beyond the proper weighting.

The change in buoyancy of the tank is simply the weight of the air that you use. 80 cubic feet is about 6 pounds. Air weighs the same whether it is in an aluminum tank or a steel tank. Obviously, your tank will be heaviest at the beginning of the dive and lightest at the end.

Wetsuits compress and lose buoyancy as you descend. This is the main reason you need to add air to your BCD as you descend. A thicker wetsuit will have more initial buoyancy and will also have a greater change in buoyancy with depth. A XXXL wetuit will of course have a greater change in buoyancy than an small wetsuit.

------------

After or before the dive, there is sometimes a separate and distinct lift requirement --- the ability to float your tank and other gear if you decide to ditch or don it on the surface. During the dive, it doesn't matter if you are using integrated weight BCD or a weightbelt. But if you want your BCD to float your gear on the surface, you need to have enough lift for any integrated weights as well as the full tank.


Charlie

 

[scubascudadoo]

Wetsuits compress and lose buoyancy as you descend. This is the main reason you need to add air to your BCD as you descend. A thicker wetsuit will have more initial buoyancy and will also have a greater change in buoyancy with depth. A XXXL wetuit will of course have a greater change in buoyancy than an small wetsuit.

Actually this is incorrect. Yes you do have to compensate for the compression of wet suit if that is what your using and not a shell drysuit for example.

But the need to add air as you descend deep is more for the fact that as you go deeper the air inside the BCD bladder compresses. 33' 1/3, 66' 1/2, 99' 1/4..etc..

Let's more then anything talk about what weight do you need to break the surface when your tank has 500 psi left. This is the ABSOLUTE min you you should have on exit and only when there is emergency for example. Deco (NON-PLANNED) stop for going beyond your normal time limits for the dive.

As for the actually weight requirements.
- Take in account the weight of air that could be used
- What type of tanks..AL (Catalina and Luxfer tanks differ themseleves) Vs Steel (As these are always negative)
- Exposure suit
- Body Type

The best thing to test your weighting is to ask your LDS if they have pool nights or if you can use the pool when there teaching a class to test what weight you need to use. You test what weight you need as a minium to break the surface with a full tank.

Do not try to use extra just to get down. That would be the weight you should use plus extra weight for 2500 PSI of air wich is around 5 lbs. Can ask the LDS instructor for the actually weight of 2500 psi of air.

 

[scubatoys]

Actually this is incorrect. Yes you do have to compensate for the compression of wet suit if that is what your using and not a shell drysuit for example.

Well, actually, I'm going to agree with Charlie99 on this one. Let's look at a classic case for needing a lot of lift. Everyone has pointed out that a full al80 has about 5 or 6 lbs of air. So let's assume you are wearing a pair of 120's and a thick wetsuit. That means we are starting this dive with about 18 lbs of air on our back. Since we want to have enough weight to do a safety stop, or deco stop at the end of the dive, we must over weight ourselves by 18lbs. To compensate for this 18lbs of extra weight, I would need a BC able to generate at least 18lbs of lift.

Now, in steps the suit. I'm wearing a XXL 7 mil Farmer John suit. That suit takes about 15lbs to sink it... at the surface. But the main thing that a suit is made of is air bubbles. It's the air bubbles trapped in the Neoprene that give us the insulation. So now when we descend with this suit on... it compresses - thus becoming less bouyant.

If we drop down to 100 feet in it.. while not as compressable as a balloon, we have lost a lot of the bouyancy of the suit. Maybe down to 7 lbs of lift. So now I'm an additional 8 lbs heavier. That means I would have to put 18 + 7 lbs into the BC, just to be neutral at that depth. If I had a BC with only 20lbs of lift... I'd be SOL.

Make sense??

Now as I breath out my air, and the tanks get lighter, I'd be ok, or if I come shallower, I'd be ok... but it's really a factor of how much extra lead you carry to compensate for the air you will use, and the compression factor of the suit that you have to look at.

Now with a dry suit, that will remain constant volume throughout the dive as you will add air when descending, so the BC requirement is not as mandatory... unless of course you blow out a dry suit, and then are glued to the bottom with a suit that has no bouyancy and you're wearing a lot of lead because of the amount of air you're carrying.

Hope that answers it!

 

[cool_hardware52]

My assumptions:

Unless your diving a Rebreather, the single element that looses buoyancy is your exposure suit, either by compression of neoprene, or flooding a Dry suit.

Air weighs the same independent of tank material, in other words a 100 cu ft of air weighs ~8 lbs in either a steel tank or Al tank.

For the sake of this discussion "Proper Weighting" is defined as eyelevel at the surface, start of dive with no gas in the BC, wetsuit only single tank, or able to hold 15ft SS end of dive (~500 psi) with minimal air in Drysuit.

Based on the above assumptions in a wetsuit your lift requirement is equal to the maximum buoyancy of your wetsuit (how much lead it takes to sink just your suit, without you in it), or how much lift is required to float your rig at the surface without you in it. Even a large thick suit seldom requires more than 22-24 lbs of lead to sink, ususally much less.


With a Dry Suit you can determine the possible loss of lift by getting in the water in just your suit, with the undies you plan to wear, with minimum air in the suit. Grab a bag o lead and add weigh 'till you are neutral. This weight represents the lift you can loose with a total flood. This number represents minimum lift your BC should have. Again you also need enough lift to float your rig at the surface without you in it.


Now a different case. If you are carrying enough gear, i.e. large Steel doubles, stages, pry bars, cameras scooter etc, that you cannot remain neutral at the surface with no air in your BC, then the lift requirements will increase by this amount.



Hope this helps.


Tobin