Redundant / Emergency Buoyancy. Lift Bag vs SMB

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CaveSloth

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Calculations say that I may need 25 lbs of redundant buoyancy in case of BCD failure. I already have a redundant bladder, but want a third buoyancy source. I dive in fresh water spring caverns in Florida mainly. I am a DiveRite fanboy. Should I choose the DiveRite SMB or Lift Bag? SMB has 30 lbs lift. Lift Bag has 75 lbs of lift. Both look like they have dump valves from the pictures. Which would be easier to deploy and use in an overhead environment? Comments on technique would also be appreciated. Thanks!

Dive Rite { 5 ft | 1.5 m } Surface Marking Tube w/Built-in Sleeve

Dive Rite Lift Bag {75 lb | 34 kg} w/Built-In Sleeve
 
I would think the tube is easiest to use in a confined space - you could inflate it partially and then place it under your chest either end under the arms like a swimming float.

Seems a rather odd thing in an overhead environment to me. A drysuit would be a better choice IMHO.
 
Personally, I would choose a dsmb that did not have an open bottom. Seems like the open bottom would make it a lot more difficult to manage when using it not-as-intended.
 
Unless the SMB is full, the dump valve will be nowhere near where the air is, so not of much use. Why do you think you need three sources of buoyancy, and if so, why not just dive dry?
 
https://www.shearwater.com/products/perdix-ai/

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