Using SMB For Backup Flotation

[Splitlip]

Oh, I understand the crossing of terms, but this is what OP said.

"I am looking at the OMS SMB which provides about 50# of lift OMS Lift Bags & Surface Marker Buoys OMS Official Site - Technical SCUBA Equipment http://www.OMSdive.com. This seems like a reasonable approach in that it provides plenty of lift and the desirable SMB function."
The link is a DSMB, tube.

In reading the entire thread, I see that Web Monkey probably was in fact speaking of a lift bag.

It was a great thread by the way.

 

[dumpsterDiver]

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.
Richard

A few years ago i had a total BC failure while wearing a steel 125 cu-ft tank and a skin and no ditchable lead on a solo dive to 180 feet. The over-pressure releif spring failed completely so my BC held zero air. I delayed my ascent (to shoot a couple of fish as I crawled around on top of the wreck in 160 ft) and had to do some deco and it was a strenuous ascent and deco stop and I arrived at the surface pretty tired and with very little air to spare. The spring was corroded and worked fine the week before, but just decided to fail that day.

 

[Splitlip]

Question for the anybody.

Mempilot puts a 3mm full suit at +1# at 170ft. Is there a "formula" for thickness of neoprene and depth compression?

 

[deepstops]

Question for the anybody.

Mempilot puts a 3mm full suit at +1# at 170ft. Is there a "formula" for thickness of neoprene and depth compression?

There's a boatload of variables: open-cell versus closed-cell neoprene, material density and hardness, and how many times have you compressed the crap out of it by taking it deep. I've often wondered if taking my wetsuit for a chamber ride would rejuvenate it

 

[Scared Silly]

Yes there is a formula:

1. Diver
2. Wet suit
3. Pool
4. Lead weights

Place 1 in 2 then in 3 then add 4 until submerged. This gives starting buoyancy. Then figure 0 at depth below 2 atm.

 

[mempilot]

Question for the anybody.

Mempilot puts a 3mm full suit at +1# at 170ft. Is there a "formula" for thickness of neoprene and depth compression?

As Brian states, it depends on the wetsuit. My Mares 3mm becomes nothing more than a skin at 170, but keeps me plenty warm in the summer months. Karen's Pinnacle 3mm is of much better quality neoprene, and needs a lot more weight to conteract the bouyancy it provides. I'm sure if I stripped out of my Mares at depth, it would still start to float up slowly. I'll probably take a look at a new wetsuit next spring when the drysuit goes back in the locker for the summer.

 

[Splitlip]

Yes there is a formula:

1. Diver
2. Wet suit
3. Pool
4. Lead weights

Place 1 in 2 then in 3 then add 4 until submerged. This gives starting buoyancy. Then figure 0 at depth below 2 atm.

LOl!

Any jerk can do that! In fact I do.

 

[mempilot]

Yes there is a formula:

1. Diver
2. Wet suit
3. Pool
4. Lead weights

Place 1 in 2 then in 3 then add 4 until submerged. This gives starting buoyancy. Then figure 0 at depth below 2 atm.

Figuring on zero below 2 ATA is probably the smart way to figure it for a 3mm. I gave it +1 for grins just to take the edge off the negative.

 

[Lucy's Diver]

Brian,

Neutral at the end of the dive has nothing to do with it. It is the negative bouyancy at the most negative part of the dive that matters, which would be on descent, where a run away descent scenario is most likely to happen. This is especially important on blue water wall dives, and there are accidents documented where this has happened. Swimming up empty AL80's would not require a whole lot of effort, and isn't a pratical example. And since you'd like to call me out on this, let's go do the Lowrance together, descend, put in maybe 10 minutes of BT to accrue some decompression obligation, empty your wing, and watch you come up from the sand and do your stops without a form of backup bouyancy, since you consider your rig to be balanced. When you have a leg cramp from kicking vertically during your ascent and stops, would you like some help? Or, would your cavalier attitude get you through? I can hear the bubbles forming in your joints as I type this.

You presume either a total lack of redundant buoyancy or an inability to use it. I dive redundant bladders with a 20 lb SMB and a 50 lb lift bag with an OPR valve designed to be operated during an ascent. This is with a drysuit and twin steel 120s. The drysuit as my only buoyancy is just not realistic. The rig (with the other safety items I need, such as reels and lights) is so heavy at full pressure that air would burp out the suit seals when attempting to lift it. The tank buoyancy does swing enough that when I get under 1000 PSI (from a starting PSI of about 3700) I am need very little in the wing to be neutral (so the drysuit would lift me at that point), but at the start of a dive I need to add a lot of air (a lot) to the wing when I get to depth. I do not hook an LP hose to the redundant power inflator, I was taught to orally inflate it.
For the problem you envision to happen to this sort of set up when diving wet or with heavy enough stuff to make the drysuit irrelevant, the following would have to happen: total primary bladder failure, total backup bladder failure (or operator error), an inability to get to the anchor line, an inability to use the lift bag, and inability to shoot the SMB and reel up to it, and a dive buddy with insufficient lift or the ability to help.


When balanced against cutting my air supply by a thrid (if I were to go to doubled 80s) I would rather have the air as a safety measure to address issues as they happen.