New BPW setup - worry about lift capacity?

[inquis]

There was a recent thread where the poster wished he bought a bigger wing because he got into photography needed more lift because of the camera.

Why was the camera not neutral?

 

[L13]

A wing is advertised with 30lbs of lift, what exactly does that mean?

The change in displacement from full to empty is ~13.64 Liters.

Is that in freshwater or saltwater? How was lift measured? Did they add weight to a naked wing until it sinks and call it a day? or Did they measure it with a plate attached?

All of that is N/A to measuring a BCD/Wing's lift.

 

[ams511]

Why was the camera not neutral?

I have no clue, you need to ask the guy with the camera.

 

[ams511]

The change in displacement from full to empty is ~13.64 Liters.


All of that is N/A to measuring a BCD/Wing's lift.

I believe you are incorrect.

First, the weight of salt water is not 1 Kg but 1.02 Kg which will affect the lift. So 30lb of lift in fresh water is actually around 31lb in salt water. I used Aqua-Calc.com to get this figure. My understanding is ocean salinity is not uniform and can vary from area to area so YMMV.

Second, the lift of a wing comes from the water it displaces. Depending on the shape of the wing, the plate and/or tank can restrict it from expanding. This will reduce lift because it displaces less water. As I mentioned in my prior post there were threads on SB discussing this.

My point is that the advertised lift may not equal the actual lift once you add a plate and tank. So divers that want to use the minimum size wing may be unpleasantly surprised if their rig sinks because the actual buoyancy is not the same as advertised.

 

[L13]

I believe you are incorrect.

First, the weight of salt water is not 1 Kg but 1.02 Kg which will affect the lift. So 30lb of lift in fresh water is actually around 31lb in salt water. I used Aqua-Calc.com to get this figure. My understanding is ocean salinity is not uniform and can vary from area to area so YMMV.

Second, the lift of a wing comes from the water it displaces. Depending on the shape of the wing, the plate and/or tank can restrict it from expanding. This will reduce lift because it displaces less water. As I mentioned in my prior post there were threads on SB discussing this.

My point is that the advertised lift may not equal the actual lift once you add a plate and tank. So divers that want to use the minimum size wing may be unpleasantly surprised if their rig sinks because the actual buoyancy is not the same as advertised.

The "Lift Rating" of a BCD or wing, is the "Buoyancy" it can "Correct" for, and is defined relative to a density of 1.0 not fresh or salt water. Yes, its actual buoyancy compensation range will be higher when diving in denser water(and lower in fresh water above 25C which will be less than 1.0).

The positive or negative buoyancy of anything you attached to the wing (back plate, harness, d-rings, lead, etc), or even the empty wing itself, is not part of the lift rating. All of that stuff does effect your total buoyancy, but not your buoyancy compensation capability.

 

[inquis]

I have no clue, you need to ask the guy with the camera.

You used that to support your opinion that larger wings are better, but a neutral camera (as should be the case) does not increase the lift requirement. One could claim more lift is better if it floods, but honestly, if it's that big then you should have redundant lift, not bigger lift.

 

[ams511]

The positive or negative buoyancy of anything you attached to the wing (back plate, harness, d-rings, lead, etc), or even the empty wing itself, is not part of the lift rating. All of that stuff does effect your total buoyancy, but not your buoyancy compensation capability.

You are missing the point. The plate and the tank can prevent the wing from expanding fully, thus you may not get the advertised lift, which does affect your buoyancy compensation capability. I am not talking about the weight of the backplate, tank, wing, or any other component of the rig.

You used that to support your opinion that larger wings are better, but a neutral camera (as should be the case) does not increase the lift requirement. One could claim more lift is better if it floods, but honestly, if it's that big then you should have redundant lift, not bigger lift.

These people seem to disagree with you.

" Most of the Backscatter staff set our camera systems between 0.5-pounds and 1.5-pounds negative. We find a slight bit of negative buoyancy allows us to maintain smooth camera moves but also allows us to set the camera on the bottom without it drifting away when we need to use our hands to assist another diver or deal with a situation."

So now that we know why a camera may not be neutral, what about a metal detector or a bag of lobsters? I also used those as examples. You can add a speargun to the list also.

I doubt a flooded case of a DSLR camera is going to cause much of a problem. However a camera is ditchable weight (expensive but not worth a life), so no redundant buoyancy is required.

 

[Doc Harry]

You'll know if the wing is large enough when you're on the surface at the end of the dive.

You have to lay on your back on the surface with a BP/W, and if you're head is not sufficiently out of the water then you need a larger wing.

I dive in 3mm wetsuit with a 30-pound wing and 8# of lead. My wing is barely adequate, especially if I have my camera. I wish I had a 35# wing. I carry a 3 meter SMB that I can use for additional flotation on the surface if necessary.

With a 7mm wetsuit I think you'll have enough flotation on the surface with that wing.

A wing that is oversized is almost as problematic as an undersized wing.

 

[jellycatsdad]

You'll know if the wing is large enough when you're on the surface at the end of the dive.

You have to lay on your back on the surface with a BP/W, and if you're head is not sufficiently out of the water then you need a larger wing.

With a 7mm wetsuit I think you'll have enough flotation on the surface with that wing.

A wing that is oversized is almost as problematic as an undersized wing.

I haven’t done that test - I’ll do it next dive. But, I was on the surface with an empty wing, 500psi in my tank with a Al80 and 24lbs of lead(20 on a belt, 4 on cam band) and successfully did a buoyancy check.

 

[Caversluis]

Just read both the articles. I was using Optimal Buoyancy Computer and it recommended to move any non-trim weight to my body since it said my rig is too heavy when I ran the numbers. But…


Looks like I’ll also be fine using droppable weight pockets on my waist webbing and I can drop my weight in an emergency and really need to do an a buoyant ascent. I don’t plan on finding myself in “extreme”, techy dives beyond my current and planned certs. I’ve been diving with a thick wetsuit and a rental BPW for the last 3 dives - so far, I’ve found myself in control of my buoyancy underwater, no major changes from a “hybrid” BCD(Hydros Pro).

Note that you can use a DSMB for extra buoyancy. If I remember correctly, the Optimal Buoyancy Computer also mentions that.