Question re. how much lift.

[Kharon]

No, I'm not going to ask how much lift I need.

What I would like to know is - if you are properly weighted to be neutral with 500 psi at 15 feet why do you need more than nominal lift. As you rise from 15 feet to the surface you should become slightly positively buoyant, so why the need for more than a few pounds of lift?

 

[scubadada]

Perhaps to float your rig if you must remove it in the water?

 

[tursiops]

Perhaps to float your rig if you must remove it in the water?

Especially at the beginning of a dive when the tank+air is 5lbs heavier than at the safety stop.
Also, whether and how much buoyancy you gain from 15ft to the surface depends on your exposure suit.

 

[tbone1004]

No, I'm not going to ask how much lift I need.

What I would like to know is - if you are properly weighted to be neutral with 500 psi at 15 feet why do you need more than nominal lift. As you rise from 15 feet to the surface you should become slightly positively buoyant, so why the need for more than a few pounds of lift?

@cool_hardware52 has explained this in depth several times and we have a slight difference of opinion on the last point I'll make which stems mainly from his side of the woods with thick wetsuits and drysuits.
Point 1-your wing needs to compensate for the mass of the gas in your tank
Point 2-your wing needs to compensate for the compression of your wetsuit
Point 3-your wing needs to be able to float your entire rig at the surface. More lift required if you put all of your ballast in pockets, less if you wear a weight belt
Point 4-your wing needs to be able to float you, in your entire rig, comfortably at the surface

Now, the difference of opinion that Tobin and I have is how much extra lift you need for point 4. If you are balanced at 15ft in a thick wetsuit, there is a good bit of compression that goes on that is often enough to keep your head above water. I weight myself for neutral at the surface since I teach and always need to be able to comfortably go down and I often dive in thin/no exposure protection. I need to be able to comfortably stay at the surface at the beginning of the dive and that means more lift than the minimum.

So for me in normal single tank diving
LP121- 12ish lbs of gas
3mm wetsuit-6lbs ish of compression
6lbs of lead in the ocean to be neutral, usually on a weight belt
15lbs of my head.

I don't own or dive with less than 30lb wings personally because of that. If I regularly dove AL80's in no exposure protection, I'd consider a smaller wing, but the difference in drag between my LCD30 from Deep Sea Supply and my VDH 35lb wing is negligible since they are long and narrow wings.

 

[Kharon]

@tbone1004 Thanks for your answer. Sorry I didn't see previous discussions and didn't use the right search parameters to find them. I'll try again using cool_hardware52 as part of the search.

It now makes some sense to me. Though I'm still a bit puzzled. If I'm weighted with the least possible amount of weight, and with an empty wing/BC, I can barely sink. It seems to me that at any later part of the dive I should be more positively buoyant at the surface. So, at least at the surface, I should need very little air to stay on the surface.

Sorry if I'm being dense. I dive almost entirely from shore so this isn't something that's been an issue for me. It's also never been a problem diving from boats - so far. But I've got a trip coming up where I will be diving with no bottom in sight so I figured it's better know than to assume.

 

[tbone1004]

@Kharon it depends on how thick your exposure protection is. Since you weighted yourself for 15ft, there is a good bit of suit compression that has to happen for you to descent. If in a 7mm or thicker wetsuit, that is quite a bit of buoyancy, in a drysuit, 3mm or thinner, not so much. At the end of the dive, it will be very difficult to get back down once you surface without any sort of inertia *i.e. a pencil surface dive*.
Again though, this is all about getting to the true minimum weight requirements and minimum wing lift requirements of a balanced rig.
You may choose that for your diving, you need to be out of balance. It is unsafe IMO for an instructor to dive a balanced rig, they need to always be at minimum 4lbs overweighted to be able to arrest an uncontrolled ascent. If doing hot-drops onto wrecks, you typically want to be at least 4lbs negative so you don't float at the surface.
You may choose for the wing that you need a wing with 10ish lbs more lift than you have calculated because you have to do long surface hangs waiting for a boat after drift dives, or debriefing students, etc. and want something that can keep more of your head above water.

What we talk about with balanced rigs and wing sizes is to get you to the minimum ballast, and minimum lift that you need to do that dive safely. It is on you to figure out if that is safe and smart for you.

 

[Aviyes]

Another thought, is that the excess lift could potentially be needed for rescue of your buddy should their wing fail and you need to provide some extra buoyancy while sorting out their rig/weights.

 

[Bob DBF]

I don't own or dive with less than 30lb wings personally because of that.

And one other factor; the buoyancy of a wing is measured seperatly from the rig for its rating, once put on on your rig you may find, as I did, that the tank does not let my donut wing expand as far as it does when off, reducing its effective buoyancy.

My approach to choosing a wing was low tech, compared to the help available on ScubaBoard, I emailed the jacket manufacturer of the one I was using and asked for its buoyancy. Since I'd been diving the jacket for years and had no buoyancy issues, I figured it should work. And it has worked well for over a decade now, 40# may be a bit on the high side, but it empties easily and completely for fine buoyancy control, even when quite shallow.



Bob

 

[tbone1004]

@Bob DBF the wings that I have are rated with 8" tanks on them, but yes, a lot of wings will lose capacity once you put a bottle on there.

 

[Kharon]

... At the end of the dive, it will be very difficult to get back down once you surface ...
... It is unsafe IMO for an instructor to dive a balanced rig, they need to always be at minimum 4lbs overweighted to be able to arrest an uncontrolled ascent. If doing hot-drops onto wrecks, you typically want to be at least 4lbs negative so you don't float at the surface.

What we talk about with balanced rigs and wing sizes is to get you to the minimum ballast, and minimum lift that you need to do that dive safely. It is on you to figure out if that is safe and smart for you.

Once again, thanks for the assistance and especially your patience.

At the end of a dive I'm not going back down, so if it would be difficult, that is what I'm aiming for.

Not ever considering ever attempting being an instructor (though I do see the necessity of what you are saying). I almost exclusively dive solo. So needing to hold someone down or up doesn't factor in for me. I also shun current and have no interest in wrecks so ...

Thanks to your input I think I've got a balanced rig & after this discussion I think I can trust it to dive safely, and whenever I dive I always accept that it's on me.

Thank you for clearing up a connundrum.