Diving in cold water (higher weight system capacity)

[RXTdiver]

I am taking the PADI online eLearning course for open water diving.

In point #3 in the pic below, they say if you're diving in cooler water you will need a weight system with higher capacity. Why is that? Colder water is more dense than warm water, but if you're talking about cold salt water, doesn't the salt content drop (relative to warm salt water), thereby causing less density and therefore less buoyancy? If you have less buoyancy, the last thing you would want is to add more weight right?

Also what is meant by exposure suit? Is that the same as a wet suit or dry suit?

 

[grantmac]

Thicker wetsuit or thicker drysuit undergarments require more weight to become neutral.

 

[JoeTPhilly]

What they are trying to say is you may need a BC with a higher lift capacity. The reason being in cold water you wear more exposure protection so you need to carry more lead and might need more lift to compensate.

 

[inquis]

1. You may need larger/more weight pockets -- if using integrated weights on your BC -- because it's going to take more lead to offset the higher buoyancy of a thicker wetsuit or drysuit. Do note, however, that a weight belt is also an option.

2. IF you put a bunch of lead on your rig (in integrated weight pockets): it still needs to float when you're not in it (and your wetsuit isn't helping float it), which is why a larger BC lift capacity may be required. (You might take off the rig when in the water in some situations, like climbing back into an inflatable RIB boat.)

You may wish to split your lead: put some into integrated pockets and some on your person on a weight belt. Pro: larger BC lift capacity is not required (smaller bladders have less tendency to taco/trap air and less bulk in your suitcase); if you ever have to remove the rig at depth (e.g., entanglement), you and the rig are each close to neutral, so it's very easy to work with and little danger of losing your rig as you float up, up, and away; easier to climb the boat ladder after handing up the weight belt to the boat crew. Con: it's another thing to put on or forget; you may not have the hips for it (but a rubber freediving belt fixes this problem).

 

[RXTdiver]

What they are trying to say is you may need a BC with a higher lift capacity. The reason being in cold water you wear more exposure protection so you need to carry more lead and might need more lift to compensate.

So wearing more clothes increases buoyancy, therefore you need more weight to offset that additional buoyancy. Because you need more weight, you need to offset the additional weight with a BCD that has more buoyancy. Is this correct?

If this is correct, although you need more weight than normal to offset the increased buoyancy from the additional clothes you are wearing, just don't add too much more weight and if you do that, then the buoyancy from the clothes and additional weight you added would be in balance, then you wouldn't even need a BCD with additional buoyancy. Is this true? If so, why not go this route so you don't need to buy an additional BCD with increased lift capacity?

 

[rhwestfall]

If diving wet, the suit compresses, loosing buoyancy. You then need the air bladder to offset the loss at depth...

 

[JoeTPhilly]

So wearing more clothes increases buoyancy, therefore you need more weight to offset that additional buoyancy. Because you need more weight, you need to offset the additional weight with a BCD that has more buoyancy. Is this correct?

Yes, the thicker the wetsuit the more weight needed. You may need a BC with additional lift because you want to be able to float the whole rig without you in it.

If this is correct, although you need more weight than normal to offset the increased buoyancy from the additional clothes you are wearing, just don't add too much more weight and if you do that, then the buoyancy from the clothes and additional weight you added would be in balance, then you wouldn't even need a BCD with additional buoyancy. Is this true? If so, why not go this route so you don't need to buy an additional BCD with increased lift capacity?

You want to dive a balanced rig; which means weighting that allows you to hold your shallowest stop with near empty tank(s) and be being able to swim up with a failed BC when you are most negative at the beginning of the dive.

You should take a look at this great resource:

Info - Optimal Buoyancy Computer

The Optimal Buoyancy Computer A tool to help nail buoyancy and improve safety, before you splash 1) How much lead should I carry with my new wetsuit? 2) How big a wing should I buy? 3) Will my BCD support my lead, both at the surface and when my wetsuit is compressed at depth? 4) Will my BCD...

scubaboard.com

 

[inquis]

Because you need more weight, you need to offset the additional weight with a BCD that has more buoyancy. Is this correct?

Not quite. You need more weight to counter the increased buoyancy from the wetsuit when on the surface. (Otherwise, you could never descend.) The reason you need more lift from the BC is because you lose that wetsuit buoyancy as it compresses during descent. The BC is used to compensate for that missing lift making you neutral at depth.

In general, the BCD (Buoyancy Compensation Device, right!) compensates for wetsuit compression and air you haven't yet breathed (about 5 lbs in the common AL80 tank). My 3 mm wetsuit buoyancy is about 7 lbs. (I know this because I stacked lead bricks on it until it sank in a pool.) To be conservative, I assume all of that goes away at depth. At bare minimum, I'd need 7 lb (wetsuit) + 5 lb (non-reserve air in an AL80 tank) = 12 lb of lift. In practice, I want more so I float higher at the surface and have some margin to help a buddy. That's why warm-water, travel BCs may have 18 or 20 lbs of lift and pack down nice and small. OTOH, my 7mm wetsuit takes 16 lbs of lead to sink. The BC needs 16 lb + 5lb + margin, so a 30 lb lift is more appropriate.

(A similar conclusion is found if you think about taking off the rig for whatever reason. Your wetsuit buoyancy is no longer helping to float the rig, so assuming you're not going to ditch weight, the BC has to pick up the slack.)

 

[RobPNW]

Another consideration for the lift capacity of the BCD is how the weight is split between your kit and let's say your weight belt if you wear one. For example, if you choose to not wear a weight belt, all the additional weight you carry will be on your kit along with the wing, and if you want to be able to float the kit at the surface without ditching weight, you need at least that much lift.

Edit- oops inquis already mentioned this.

 

[inquis]

Another consideration for the lift capacity of the BCD is how the weight is split between your kit and let's say your weight belt if you wear one.

I think it's worth clarifying that this comment (and the similar one I made earlier in the thread, #4) is more relevant for a drysuit scenario since it doesn't normally lose buoyancy at depth. Splitting between the rig and a weight belt can enable use of a smaller lift BCD in that case. It'd be insufficient in a catastrophic suit flood, but ditching addresses that. The smaller wing can be optimal for the 99.99% scenario rather than the 0.01% "just in case" scenario.

Specifically for the wetsuit case, the location of the lead is irrelevant to the BCD lift requirement. I didn't really draw that distinction earlier.