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Using SMB For Backup Flotation

Discussion in 'Advanced Scuba Discussions' started by rstofer, Nov 7, 2008.

  1. rstofer

    rstofer Loggerhead Turtle

    # of Dives: 100 - 199
    Location: Northern California
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    On review using the Buoyancy Calculator spreadsheet over in Equipment -> BCs, it is evident that with a steel HP 100 and a wetsuit a complete loss of the wing will result in being about 12# negative even after ditching the weights. This condition occurs at the beginning of the dive, with a full tank, when the wetsuit is compressed at depth.

    So, how to provide backup lift?

    I am looking at the OMS SMB which provides about 50# of lift OMS Lift Bags & Surface Marker Buoys http://www.OMSdive.com. This seems like a reasonable approach in that it provides plenty of lift and the desirable SMB function.

    Has anyone actually tried to get off the bottom using only an SMB? Is the web strap at the base of the OMS SMB actually capable of staying attached (not just tearing out) while providing the full lift capability?

    Are there other methods (not including redundant bladders) that I should consider?

    Thanks!
    Richard
     
  2. Noboundaries

    Noboundaries Manta Ray

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    Hmmm, wing failure. Has it happened to anyone on the board?

    Interesting question about using an SMB to provide lift.
     
  3. Whatgoesdown

    Whatgoesdown Instructor, Scuba

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    Well, the SMB looks to be made well enough to lift 50lbs so I wouldn't worry about that (worst case you only want to lift 12lb). The downside is control, I don't see a dump valve at the top and a runaway ascent could easily happen. If you haven't practised ascending with a lift bag I wouldn't recommend it and definitely do not clip it to yourself and then inflate. That said, if all else fails I would give it a go.
     
  4. Dive-aholic

    Dive-aholic Dive Shop

    # of Dives:
    Location: North Florida - Marianna area
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    Whatgoesdown makes an excellent point. The OMS lift bag has the dump valve located on the lower section. Not a great location for dumping air. I would consider the Oxycheq lift bag with the dump valve on the upper section. It will allow you better control in dumping the air as it expands while you are ascending. And yes, these bags will withstand 50 lbs. I've used my lift bags to actually lift objects off the bottom and had no integrity issues. When I dive wet, I carry a lift bag for this very reason. I dive sidemounted 108s and would be very negative without my wing. I have not had my wing fail on me, but I do know a couple of people that have. Were it not for their dry suits, they would have had a rough time.
     
  5. battles2a5

    battles2a5 Divemaster

    # of Dives: 200 - 499
    Location: Charlotte, North Carolina
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    As others have stated, you need something with the dump valve on the top of the bag, so you can control the ascent. You do NOT want the thing attached to you. Instead, puff a little air into it to stand it up then slide your arm through the straps, then you can exhale into the bag pretty easily to add lift (imagine an upside-down purse). This way, you can easily add lift, pull the dump to vent gas as you ascend, and you can let it go if you needed to. This method is easier with a lift bag vs. a SMB but you can do it with either one. With a lift bag, you can reach the top of the bag and pull it down to vent in case the dump can't keep up.

    Another option for redundant bouyancy (aside from dual-bladders) is a drysuit. If this did happen when the tank was full you could just sit on the bottom and use up some of the tank until you can kick the rig up. This would give you plenty of time to think about your decision to dive w/o redundancy :)
     
  6. Cheetah223

    Cheetah223 Solo Diver

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    This is assuming there's a solid bottom and not a wall dive where the time spent thinking is time spent drifting toward the 300' bottom with a single HP100 :D

    I've tried using my SMB for buoyancy control, not so much for ascents, just playing around on the dive. It's an art to be sure, if you plan to use your SMB as your backup, make sure you practice it often enough that you feel comfortable that you can, without a doubt, make a controlled ascent, maintain neutrality for any safety/deco stops, and bleed air out in a controlled enough manner if you need to avoid overhead obstacles for any reason (thinking more along the lines of speeding boats than a cave overhead..)
     
  7. Pug211

    Pug211 Angel Fish

    # of Dives: 25 - 49
    Location: Maryville, TN
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    If you had it attatched to a reel instead of yourself you could use it to ascend, but I would be afraid of hitting the surface like missle if it was attatched to my body.
     
  8. rstofer

    rstofer Loggerhead Turtle

    # of Dives: 100 - 199
    Location: Northern California
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    I too was concerned that the dump valve on most SMBs seems to be at the bottom. I suppose one way to use it is to shoot it to the surface and climb an attached line. Unfortunately, the type of line that is normally used might be difficult to grasp with gloves. But it's only 12#, how hard can it be? It'll certainly be easy to control the speed of ascent!

    The OxyCheq lift bag (among other bags, not SMBs) says it has the overpressure dump valve on the upper section but I can't see it in the images. I have no idea whether the valve can be manually operated. I'm assuming from Dive-aholic's reply that it can. You certainly wouldn't want to apply 50# of lift to a 10# object so a controllable valve would seem to be a necessity. But I like pictures...

    I don't know that a complete wing failure is a realistic possibility. I haven't done enough research to find someone who says "MY wing failed and here's how...". One can imagine a number of failure modes including an errant shark biting a hole in it. At some point, redundancy becomes pointless! But it is true that the entire system depends on that cheap plastic elbow, a corrugated hose and a couple of Tye Wraps.

    I think the lift bags hold more promise than the SMBs. That's too bad because I would rather carry just one gadget.

    The other possibility is to concede that steel tanks are evil when used with a wetsuit and go back to my Al 80's. With this configuration, I would be positive at depth by just ditching the weights.

    Or, replace the SS backplate with something that is essentially neutral. Then I would be just 4# negative at depth.

    I wonder if I am overanalyzing again?

    Richard
     
  9. Noboundaries

    Noboundaries Manta Ray

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    Confu-noboundaries say The road to the simpliest answers winds through the forest of complications.

    Good discussion.
     
  10. rstofer

    rstofer Loggerhead Turtle

    # of Dives: 100 - 199
    Location: Northern California
    2,199
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    I guess if it is a wall dive and you lose control of the descent, you grab the wall! The 'reef police' will get upset but, oh well...

    I have done a couple of wall dives and you're right, the bottom was a long way down. Several thousand feet. But, we were dropped on top of the reef, not out in an open column of water miles from anything to grab.

    I have always known that there was something under me before I started the dive. The only exception was a reef dive about 20 miles from nowhere. There was this little pimple sticking up from the ocean floor. Right in the middle of nowhere.

    There can certainly be exceptions to the types of dives I have made.

    I am also considering making my inner-tube float a standard part of beach dives. I still have to work out a better anchoring scheme but the anchor line will be more than adequate for ascending and the inner tube probably provides a couple of hundred pounds of flotation.

    Richard
     

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