How much BC lift do I require?

[ocpaul]

I don't know why suit compression calculations come into play when determining lift.

I think you answered your own question when you said "You need enough lift so that at the beginning of a dive, you can counter your full tanks and your negative weight at depth"

I still believe that the MINIMUM lift you need is your own personal Factor of Safety (F.O.S.) x (weight of air in tank + loss of bouyancy due to suit compression at depth + excess weight beyond neutral at surface at end of dive).

I agree compression of 7mm suit at depth will be more than 4# - I was just using the figure as an example (might be representative of a 3mm suit).

It all comes back to how you are going to set yourself up to begin with. If you weight yourself so you are neutral at the surface when your tank is empty (or near empty), then when you begin your dive and go to depth you would need to have enough lift for the weight of the air + loss of bouyancy due to suit compression. If your overweighted at the beginning of the dive then you would need to add lift for that overweighting also. Remember I also suggested a F.O.S. of 2.0 - this covers a lot of variables and gives you a little added help at the surface. I would also always recommend a secondary lift source (bag, safety marker, etc. just in case).

 

[Scubaroo]

My beef is with the term "suit compression calculations".

Suit compression would help determine how much weight you need to carry.
How much weight you carry helps determine how much lift you need.

Introducing the concept of suit compression into the lift calculation just over-complicates things - the diver will have a known negative weight figure for gas, weightbelt, backplate, light, etc -> generally all known variables. Suit compression is a big unknown and difficult to measure, and simply is not required to calculate lift, which is what the original question was. It wasn't a weighting question. Keep it simple.

"loss of bouyancy due to suit compression at depth + excess weight beyond neutral at surface at end of dive" as you put it, is simply the sum of the negative weight of your gear! All known values, no need to allocate it to the above two causes. Just add up your gear's negative weight.

 

[ocpaul]

Still sounds like we have a basic difference in how we think about lift requirements.

Do you want to be able to float you rig at the surface without you in it? If so, your lift requirements would always be higher than I would calclulate.

 

[jagfish]

Also,
since being nuetrally bouyant at the surface means floating at eye leve, and many of us would like to float a bit more comfortably that that, this would equate to more lift, right?

JAG

 

[jonnythan]

Uhh, Ben.. you need enough lift to offset suit compression at depth, as well as gas carried. The amount of lead you're carrying should have very little to do with it, as that doesn't tell you anything about how negative you're going to be at say 100 feet. I'm not sure what you're trying to say. For me with a drysuit, the determining factor isn't the 6 to 10 pounds of lift I need at depth to offset the gas in my tanks, it's how much my rig is negatively buoyant on the surface.

Jagfish: You're supposed to be neutrally buoyant on the surface with no air in the BC and no air in the tanks. With a full Al 80, you should be a max of 6 pounds negative so inflating a 27 or 36 pound wing will give you lots of lift out of the water.

 

[jonnythan]

My beef is with the term "suit compression calculations".

Suit compression would help determine how much weight you need to carry.
How much weight you carry helps determine how much lift you need.

Introducing the concept of suit compression into the lift calculation just over-complicates things - the diver will have a known negative weight figure for gas, weightbelt, backplate, light, etc -> generally all known variables. Suit compression is a big unknown and difficult to measure, and simply is not required to calculate lift, which is what the original question was. It wasn't a weighting question. Keep it simple.

So I'm diving a drysuit. I have 6 pounds of gas, 5 pounds of backplate, 8 pounds of channel weight, and 6 pounds on my weightbelt. That's a total of 25 pounds of negative buoyancy, but it has nothing to do with how much lift I need. I'm wearing a drysuit and with all that weight I'm only 6 pounds or so negative at the bottom, so that's how much lift I "need."

"loss of bouyancy due to suit compression at depth + excess weight beyond neutral at surface at end of dive" as you put it, is simply the sum of the negative weight of your gear! All known values, no need to allocate it to the above two causes. Just add up your gear's negative weight.

No, it's not. If I'm wearing a 3mm jumpsuit, I've got 6 pounds of gas on my back, 5 pounds in a backplate, and 5 pounds on a weight belt, that doesn't tell me anything about how much lift I need. That's 16 pounds. If I know that my suit loses 4 pounds at 100 feet, and I have 6 pounds of gas, that tells me I need 10 pounds of lift, and nothing more. Get it? The idea is that at depth, you need enough lift to offset however much you're negative at that depth, and the only two variables there *should* be gas carried and suit compression (plus, of course, any extra weight you might have tossed on beyond what you actually need..)

 

[Scubaroo]

I don't think suit compression applies to the same degree with (trilam) drysuits. They maintain fairly constant lift if you're just keeping enough gas in them to offset squeeze.

My beef is with calculating suit compression - how do you *know* your suit loses 4lb - do you take it to depth by itself on a seperate dive and stick a weight on it on the surface and see how much it takes to sink it, then take it to 100' and stick smaller weights on it? Are you going to go to those lengths before buying a wing, or are you going to go "well I use 10lb of lead with this wetsuit, 5lb backplate, tank is 9lb negative, say 2lb for my light and regulators, that's 26lb of negative weight - so when I'm at the bottom and my suit is compressed and I've lost all of its lift - a 30lb wing should do". Much simplier. Maybe you'll have a few pounds extra lift, but it's not grossly wrong.

re your second point about what I said about compression loss, gas etc adding up to be the same, yeah, I think I was wrong there.

 

[jonnythan]

It's not 26 pounds of negative weight when you factor in the suit and your body. You can't calculate that way unless you just want to guess 10 pounds over for good measure.

 

[Scubaroo]

If you descend to 100-130' a few minutes into your dive, and your wetsuit is fully compressed, it's just your body's natural buoyancy left... I'm proposing it's an acceptable way of calculating lift requirements, without trying to figure out suit compression.

The point I'm making (not sure if it's being missed or not), is HOW do you easily calculate suit loss without actualy getting wet and testing the suit seperately on the surface and at depth. Manufacturers certainly don't publish suit buoyancy statistics, and it can decrease as neoprene ages with use. Why bother if there's a rule of thumb that can be used that's sufficient.

 

[jonnythan]

Point taken