I'm considering buying a BP/wing system and am confronted with judging whether a wing rated at 30# will provide enough lift. Here's the analysis I've done. I'd appreciate your taking a look and telling me if I've gone astray and where.
Here's what I think is my worst case scenario. I'm in saltwater in a 7/5 wetsuit that has 24# of bouyancy, as measured by a sink-it-in-the-bathtub test. I'm diving a full Al80 tank, which is about 2# negative, and the regulators add another 2# negative. The BP is about 5# negative. My body is essentially neutral.
Before adding any lead, my net positive buoyancy is about 24 - 2 - 2 - 5 = 15#. So to get neutral at the start of the dive I'd need to offset this with 15# of lead. I'd have to add 5# more lead for the Al80, to be sure to be negative at the end of the dive, making a total of 20# of lead. I'd wear this in two 10# ditchable pockets.
So at the start of the dive, I'd be net negative, to the tune of 5#. That's no problem for a 30# wing to support on the surface.
Suppose I had to abort without doing the dive, and that for whatever reason I had to doff the BP/wing rig. Its net buoyancy is 9# negative (BP plus regs plus Al80), which again would be no problem for the wing. Interestingly, my 24# positive wet suit ought to float me and the 20# of lead without my having to ditch anything.
OK, now let's dive the rig to 66 feet in seawater, where the pressure is 3 ATA, and let's check out the buoyancy. The tiny air cells that provide the wetsuit's buoyancy will be compressed to 1/3 their original size at this depth. So the suit's buoyancy is reduced from 24# to 8#, for a loss of 16#. Add to this the extra 5# I'm wearing for the Al80 and I find I'm 21# negative at this depth. If I had to get to the surface, the 30# wing would lift this with capacity to spare. [By the way, I'd only become an additional 2# negative if I were at 99 feet--still not a problem.]
Bottom line: I seems to me that the 30# wing should work out fine, with enough extra capacity to take care of details like hoods (positive), lights (negative), seawater density, and whatnot. I'd really appreciate your critiquing my analysis and giving me a sanity check.
By the way, here's an interesting safety observation. If the wing were to fail just as I reached my 66 foot depth, I'd have to swim the 21# negative weight to the surface (probably too heavy) or ditch some lead, either 10# or all 20#. Ditching would become a safety problem because as I ascend the wetsuit would become more and more buoyant, and I'd probably be looking at an uncontrolled ascent. Am I right about this?
Edit: Another "by the way:" If I repeat this analysis for an LP85 steel tank, it basically moves about 5# from my ditchable weights into the tank itself, leaving me with 15# of ditchable weight. I'd still be 21# negative at 66 feet. So, in an emergency, after ditching all that's ditchable, I'd remain 6# negative. That would problably be hard to swim up. I think I've discovered why people say that wetsuits with steel tanks can be a problem.
Here's what I think is my worst case scenario. I'm in saltwater in a 7/5 wetsuit that has 24# of bouyancy, as measured by a sink-it-in-the-bathtub test. I'm diving a full Al80 tank, which is about 2# negative, and the regulators add another 2# negative. The BP is about 5# negative. My body is essentially neutral.
Before adding any lead, my net positive buoyancy is about 24 - 2 - 2 - 5 = 15#. So to get neutral at the start of the dive I'd need to offset this with 15# of lead. I'd have to add 5# more lead for the Al80, to be sure to be negative at the end of the dive, making a total of 20# of lead. I'd wear this in two 10# ditchable pockets.
So at the start of the dive, I'd be net negative, to the tune of 5#. That's no problem for a 30# wing to support on the surface.
Suppose I had to abort without doing the dive, and that for whatever reason I had to doff the BP/wing rig. Its net buoyancy is 9# negative (BP plus regs plus Al80), which again would be no problem for the wing. Interestingly, my 24# positive wet suit ought to float me and the 20# of lead without my having to ditch anything.
OK, now let's dive the rig to 66 feet in seawater, where the pressure is 3 ATA, and let's check out the buoyancy. The tiny air cells that provide the wetsuit's buoyancy will be compressed to 1/3 their original size at this depth. So the suit's buoyancy is reduced from 24# to 8#, for a loss of 16#. Add to this the extra 5# I'm wearing for the Al80 and I find I'm 21# negative at this depth. If I had to get to the surface, the 30# wing would lift this with capacity to spare. [By the way, I'd only become an additional 2# negative if I were at 99 feet--still not a problem.]
Bottom line: I seems to me that the 30# wing should work out fine, with enough extra capacity to take care of details like hoods (positive), lights (negative), seawater density, and whatnot. I'd really appreciate your critiquing my analysis and giving me a sanity check.
By the way, here's an interesting safety observation. If the wing were to fail just as I reached my 66 foot depth, I'd have to swim the 21# negative weight to the surface (probably too heavy) or ditch some lead, either 10# or all 20#. Ditching would become a safety problem because as I ascend the wetsuit would become more and more buoyant, and I'd probably be looking at an uncontrolled ascent. Am I right about this?
Edit: Another "by the way:" If I repeat this analysis for an LP85 steel tank, it basically moves about 5# from my ditchable weights into the tank itself, leaving me with 15# of ditchable weight. I'd still be 21# negative at 66 feet. So, in an emergency, after ditching all that's ditchable, I'd remain 6# negative. That would problably be hard to swim up. I think I've discovered why people say that wetsuits with steel tanks can be a problem.
