YD Scuba Diving Forums banner

1 - 2 of 2 Posts

2,839 Posts
Discussion Starter #1
Imported post

As b4, any feedback is good.


So how much lift do I need?
A common question amongst beginner divers is how much buoyancy their BCD should be capable of supplying.
It's an important question: Too much lift is bad, it creates additional drag and can lead to other problems. Too little can result in you being unable to get back to the surface.
It's important to find the right balance: You must have enough, but you don't want too much.
However, there's no simple answer to the question. Or there is, but it doesn't help much. The simple answer is: You need enough lift to ensure that, at any point in the dive, you can return to the surface without needing to fin upwards.
But that doesn't give you a number to plan around.
So let's consider a diver. We'll start with the absolute minimalist approach, and assume that nothing at all can go wrong on a dive. How little can our diver get by with?
Assuming that he is neutrally buoyant with no air in his BCD, minimal air in his drysuit, and empty cylinders, then the answer is, he needs enough buoyancy to counter the weight of gas he starts the dive with. In a 12L cylinder, he'll have around 3kg of air. So, he needs a BCD with 3kg of lift.
However, it's not that simple. His hood, maybe his gloves, maybe even his suit are neoprene. If he's got a significant amount of neoprene, then he needs to take into account the fact that he'll need more lift at depth than he does near the surface. Only empirical testing can determine how significant depth compression is to him - you can't work this out on paper.
The next thing to worry about is, what if something goes wrong? If his drysuit springs a major puncture, it'll loose all its air. That means a reduction in buoyancy. How much lift does he need to counter this?
It's fairly simple: If the air in his drysuit bubbles away, he needs to be able to put that volume of air elsewhere. So, he needs to have the same amount of lift available in his BCD as he has in his drysuit. Right?
Wrong. The worst case scenario is for our diver to suffer his drysuit problem right at the start of the dive - when he still has all his gas.
That means he needs to have enough lift to counter the loss of his drysuit, PLUS enough lift to counter his cylinders.
So, sounds pretty simple, really, doesn't it?
But there's a few other things to consider:
Height above water: This is unfortunately not something that you can put a number on. Different BCDs will give you a different height of mouth above water. Usually not a problem, but if your drysuit does develop its rip right at the start of the dive, having a BCD with a bare minimum of lift will only get you TO the surface, it won't get you ABOVE it - anything neutral or positively buoyant in water becomes negative when it is in air. The best you can do is work out how much lift you need, then make sure you get a bit more than that: A few kilos more lift than you need won't cause any problems.
Finally, floating your rig. If you dekit prior to getting out of the water, it's nice to have a BCD that can keep your rig afloat without you and your floaty drysuit. It should be well able, but if you've got a weight-integrated BCD and lots of gadgets, then you could be a bit borderline - aqualungs lost from sinking aren't unheard of.
So, when you want to work out your minimum lift requirements, the questions you ask are:
What's my buoyancy like in the pool in a swimsuit with empty cylinders?
Positive: The difference in lead needed between the swimsuit and drysuit
Neutral: The amount of lead needed for a drysuit
Negative: The amount of lead needed for a drysuit, plus however negative you are in the pool.

How much does the air in my cylinder weigh?
10L 232bar: 2.8kg
12L 232bar: 3.4kg
12L 300bar: 4.4kg
15L 232bar: 4.2kg

How much lift does my suit loose during a dive?
Membrane drysuit: None
Compressed neoprene drysuit: Unlikely to be significant
Neoprene drysuit: Could be significant
Neoprene semidry: Could be significant
Wetsuit: Unlikely to be significant
Skin: None

Add up all the above figures, and you have your golden minimum number. You absolutely must have at least this much lift. A few extra wouldn't go amiss, to keep you from being borderline. You don't want to be heavily in excess of it though.
P.S. Don't forget to take into account the extra lead you'll need in salt water!

2,839 Posts
Discussion Starter #2
Imported post


You  can tell it's a Friday afternoon

The topic should be "Work out your LIFT"

1 - 2 of 2 Posts