divegeek:
With the 7lbs of my non-ditchable backplate (and STA), I want to get something. I have a small safety sausage (3'). I'm leaning towards getting the OMS 50Lb SMB, but was wondering if a lift bag would be a better emergency lift device.
Whoa. Let's run through some numbers for a moment...
I have no idea what sort of exposure protection you're using, but if you're in New York, it's likely a 7mil or a drysuit. I have no idea what sort of buoyancy you have to counteract with that suit, but let's assume for the sake of argument that it's 10 pounds.
If it's a drysuit, then the problem's solved... THAT is your redundant bouyancy source.
If not, then consider this:
At 100 feet, depending on the brand, type, etc... A loss of 85% bouyancy is probably 'bout accurate in most wetsuits. That means that your suit goes from 10 pounds positive to 1.5 pounds positive at depth.
If you're diving AL80's like most people, then they change buoyancy about six pounds from full to empty.
Thus... Your total bouyancy change should be about 14.5 pounds, assuming all of the above, and assuming a single. Please step in here and correct my numbers for your situation.
...So your buoyancy change - what you need to compensate for from the most buoyant (at the surface, empty tank) to the most negative (at the bottom, full tank) - will be about 14.5 pounds. It's more if you're wearing doubles, but you aren't if you're wearing a STA.
A BC is designed to counteract this. Notice that even a Pioneer 27 is plenty enough to compensate for 14.5 pounds. There is no need for huge wings or bladders, and are actually a hinderance in most cases... They allow air bubbles to move around, shifting the buoyant forces. They also cause a lot of undue drag in the water.
In your above example, 7 pounds is nonditchable weight. That means that you should take about 7.5 pounds of ditchable, assuming that your body is neutral.
So... You should be diving with 7.5 pounds ditchable. Maybe a little more, or a little less, depending on how much YOU float.
In a total loss of buoyancy (like you tear a big hole in your wing), if your tanks are FULL and your suit is totally compressed, you'll be 14.5 pounds negative.
Here's the answer: Swim that up. You can probably swim up 14.5 pounds. Remember, you've got a full tank.
If you can't swim up 14.5 pounds, then ditch. That'll leave you 7 pounds negative. You can definitely swim that up... And remember, that's worst case scenario. If you've got less gas in your tank or are at a shallower depth or are diving in salt water or have a thinner suit on... You're in even better shape.
By the time you get to the surface, your suit will have expanded and you'll be buoyant... Even without a BC. If you have to stay at the surface for days (lost at sea) then you can ditch your backplate and wing. 7 mil floats you really nicely.
Of course, a "catastrophic failure" in your wing isn't likely to happen anyway... They get a hole, leak, or otherwise fail... But rarely to they completely let go.
...Now, if you're diving a whole bunch more weight than that, then you may be overweighted... But even then, that should still be ditchable, so it shouldn't cause you too much angst in an emergency situation.
Something tells me that you've been catching junk from a buddy who's misinformed and is telling you that you're gonna die from a backplate... No?
...Anyway, with all of that said, if you're diving a bp/wing, there's some great news about a lift bag... Halcyon makes one that attaches to your plate. Pricey, but very nice... Semi-closed mouth, 61 lbs of lift... And completely invisible when stowed. Very sweet. It also makes for a great SMB - it's emergency orange, and is huge by comparison.
I have one for light salvage and rescue work, and it's with me on every ocean dive.