Why do you need more lift when cold water diving?

[Freshie1234]

Sorry but that's not correct. If you put all your dive weights on your BC instead of splitting it up between your body and your BC then your BC will need bigger lift to account for all these weights.

Sir, I don't understand your logic. Aren't both your BC and weight belt connected to your body? I don't understand how the separation of the weight from the BC would make difference of it remained on your body.

 

[elan]

The question is not correct in the first place. You do not necessarily need more lift when cold water diving, it depends upon your exposure protection and distribution of your weight.

If you dive a neoprene suit (wet, semi-dry or a dry) you do need more lift as the suit compresses with depth and loses buoyancy, to compensate for that you have the buoyancy compensator.

If you dive a shell dry suit which does not lose the buoyancy and your weight is integrated with the BC your should have more lift so your BC can float that weight on the surface.

If you dive a shell dry suit and all your weight is ditch-able and located on a belt or on the harness then you do not need to have more lift as your BC does not have to float the rig on the surface nor does it need to compensate for the dry suit flood as your weight is ditch able.

Sorry, n00b question, but can't seem to make sense out of it. I understand that you need more weight to compensate for the extra positive buoyancy that comes from wearing a 7mm or drysuit. What I don't understand is why you need more lift in the BC to then further compensate for that extra weight. Whether you use 4 lbs w/ no wetsuit or 22 lbs w/ a 7mm suit, isn't that added weight ultimately giving you a small "net" amount of negative buoyancy (just enough to get you to sink)? Isn't the lift of your BC solely counteracting that "net" negative buoyancy and not the total amount of weight you added to get there? Am I missing something where a 7mm suit actually somehow becomes more positively buoyant at greater depths, so the extra weight is needed to account for that? Thus at the surface, you now need more BC lift to help compensate for that extra-extra weight? Please let me know what I am missing.
Thanks!
bkopec

---------- Post Merged at 03:09 PM ---------- Previous Post was at 03:03 PM ----------

Sir, I don't understand your logic. Aren't both your BC and weight belt connected to your body? I don't understand how the separation of the weight from the BC would make difference of it remained on your body.

I"m not fnfalman though but:

No, not at all times. If your weight is located on the weight belt your BC should not account for that in case your BC has to float your gear on the surface when you do not wear it.

More over if the belt is ditch-able and you dive a dry suit you do not need to have extra lift for emergency when the suit is flooded as you can ditch your weight.

 

[3D diver]

Sorry but that's not correct. If you put all your dive weights on your BC instead of splitting it up between your body and your BC then your BC will need bigger lift to account for all these weights.

This is only correct if you are thinking about the BC & weights as an isolated system, i.e. if you take off the BC and want it to float on its own.

When the BC & weights are all on your body then the important issue is the total wieght and total bouyancy---where it is located only affects trim, not the needed BC lift.

P.S. FWIW I got an A in these classes:

 

[TSandM]

If you put all your weight on your rig, the weight is counterbalanced by your exposure protection, when you are wearing your gear. The minute you take the gear OFF in the water, though, you've removed all the "floatiness", and all you have left is a whole bunch of tank and lead. That's where you need quite a lot of lift to keep the gear at the surface. It is not nearly as much of a problem, as elan notes, if you distribute the weight so that you are wearing part of it independent of your BC and tank.

 

[fnfalman]

Sir, I don't understand your logic. Aren't both your BC and weight belt connected to your body? I don't understand how the separation of the weight from the BC would make difference of it remained on your body.

People like CoolHardware (aka Tobin Georges of Deep Sea Supply) are more eloquent with their descriptions but here's the gist of it:

Let's take my personal case: 7mm one-piece wetsuit, BPW with steel plate, Faber M-series 100cuft tank with no additional weight anywhere. A BPW in steel is about 6-lbs negative. The Faber M-series 100cuft tank is -14.11lbs/6.69lbs at full and empty per the factory chart. My regulator weighs 2lbs (close enough). My one-piece 7mm suit with me in it is about 16lbs positively buoyant. That means if I were to jump into the ocean with a weight belt on, I'd be properly weighted with 16lbs of lead on the weight belt.

At the end of the dive, when I run my tank down to 500-psi or somewhere around there, my rig would weight:

-6lbs (back plate) + -8lbs(tank & 500-psi of gas) + -2lbs (regulator) = -16lbs.

That means I can easily hold a safety stop and do a slow ascend to the surface with no air in my BC, and at the surface I'd float like I would if I were only to have my wetsuit on and a -16lbs weight belt.

Now, let's recalculate the dive weight at the beginning of the dive when my tank is full: -6lbs (bpw) + -14.11 (full tank) + -2lbs (regulator) = -22.11lbs. My wing would only be big enough to lift 22.11lbs if I were to ditch my rig at the surface. So for safety conservatism, I would select a 25lbs lift wing (I have a 30lbs wing because that's the smallest Dive Rite made at the time).

Let's look at a different equipment configuration where I'd have some weight on my weight belt and go with a lighter tank (Faber FX series 100cuft), keep in mind that my dive weight hadn't changed at -16lbs because that's how much it would take to sink me in my exposure suit:

At end of dive with about 500psi in tank: -6lbs (bpw) + -1.59lbs (tank with 500psi of gas) + -2lbs (regulator) + -6lbs of lead = -15.59lbs, close enough for my -16lbs dive weight. A pound or less wouldn't make much of a difference in dive weight though the free divers tend to be very anal about dive weight calculations but that's another story.

Look back at beginning of dive with full tank: -6lbs (bpw) + -8.41 (full tank) + -2lbs (regulator) + -6lbs of lead = -22.41lbs of total weight at the surface before diving.

Now, let's look at how I would carry the -6lbs of lead. Do I carry it on my body with a weight belt or do I carry it on the BC with integrated weight?

On BC with integrated weight: -22.41lbs, which means that I'd go with a minimum of 25lbs lift. If I were to have to ditch the gear at the surface, I'd fully inflate the BC and detach it from me. My wetsuit will float me and the BC will float the rig.

With weight belt on me, that means my rig is now only -6lbs (bpw) + -8.41 (full tank) + -2lbs (regulator) = -16.41lbs. I can get away with using a 20lbs lift BC. The -6lbs of lead is on me if I were to have to ditch the rig. -6lbs of lead isn't going to sink me while I'm in my exposure suit because it takes -16lbs of lead or combination thereof to sink me. I'm floating with -6lbs of lead on my weight belt, and my 20lbs lift BC is floating my rig instead of the 25lbs wing if I were to use a heavier tank with no ditchable weight, or a lighter tank with integrated weight BC.


Once again, BC lift calculation is based on your BC being able to lift the whole rig at the beginning of the dive with a full tank when it is away from your body. So whatever you pile on your BC, the lift requirement goes up. If you were to carry a cannister light and if that thing is dead weight (it tends to be what's with the batteries and all), then you need to take that weight into consideration.

---------- Post Merged at 12:30 PM ---------- Previous Post was at 12:28 PM ----------

This is only correct if you are thinking about the BC & weights as an isolated system, i.e. if you take off the BC and want it to float on its own.

That is how you suppose to calculate for lift.

BTW, you ain't the only one who had taken the below classes.

P.S. FWIW I got an A in these classes:

 

[3D diver]

That is how you suppose to calculate for lift.

That is a philisophical point we'll have to agree to disagree upon.

 

[fnfalman]

That is a philisophical point we'll have to agree to disagree upon.

True enough. You don't have to calculate for lift with the BC rig separate from your body.

If people want to wear BPW with wings 10lbs or more bigger than necessary then by all means, they should do that. Whatever makes them happy.

I simply give another point of view that some of us use to calculate BC lift.

 

[spectrum]

It's not a philosophical point but it's scope creep of the OP's question.

In absolute terms weight is weight and lift is lift and as long as they are married what you see is what you get.

fnfalman extended the discussion to that of a better balanced rig without really saying so in post #8 with statements that in that post do read like a gross misuderstanding. His latter explanation clarified in full color what he was expressing.

Pete

 

[3D diver]

Philisophical in the sense that he asserts

BC lift calculation is based on your BC being able to lift the whole rig at the beginning of the dive with a full tank when it is away from your body.

That is true as far as it goes...you don't want to lose your gear if/when you take it off. But there are other valid configurations and other valid trim criteria not encompassed by that idea, and asserting one is right and another wrong is a matter of opinion/philosophy, IMHO.

And, yes, we do seem to be drifting off topic...

 

[Freshie1234]

Once again, BC lift calculation is based on your BC being able to lift the whole rig at the beginning of the dive with a full tank when it is away from your body. So whatever you pile on your BC, the lift requirement goes up. If you were to carry a cannister light and if that thing is dead weight (it tends to be what's with the batteries and all), then you need to take that weight into consideration.

---------- Post Merged at 12:30 PM ---------- Previous Post was at 12:28 PM ----------

Thanks fnfalman! I get what you were saying now... I guess I was just confused because I never really considered taking off my equipment in deep enough water where that would be a problem.

Here's a question: if you were to have a can light, how do you balance it out so that your trim isn't crazy?

P.s. were you on the Peace in September? I very well could have met you... you had a yellow Halcyon rig... correct?