SMB as backup lift source.

[-JD-]

It is not clear from your post, but if you intend to send or ride/swim a bag or smb up, it needs to a have either a fully open bottom or an OPV to prevent it popping as the inflation gas expands. "Safety sausages" are for surface use only.

I've tried the beanstalk climb up my spool line with a shot SMB, tedious and a bit tiring if you are respooling as you go - 80', 7mm, & single steel HP100. This season, I'm going to try with an open bottom lift bag to see how well I can control buoyacy by dumping on the way up. It has a dump valve near the top, but I also want to try spilling from the bottom to see if that is an option if the ascent rate gets too high.

Consider the option of a small, partial weight drop if you have ditchable weights, especially if you are early in the dive and gas heavy. There are several threads on it, and @rsingler 's buoyacy calcilator is great for running scenarios on weighting, weight drop, and primary/secondary bladder size.

Step one is getting off the bottom, where, hopefully, suit reexpansion will start to help on the way up. But also consider, what you are going to do at the surface to maintain suitably positive buoyancy. Weight drop, SMB/bag, rig ditch?

 

[doctormike]

Double bladder wing. People hate 'em for some reason, but one situation where I found it helpful was steel doubles and a wetsuit. Much easier to control your ascent than with an SMB.

That having been said, we had to ride an SMB up in training, and it's certainly doable. Don't understand why your instructor is so concerned with the rate of filling, you don't want to jam a huge amount of air into it. So you could use mouth inflation.

But if you are really committed to doing technical diving with steel doubles and a wetsuit, and you want an SMB that you can inflate with a LP hose, then I think I would get an appropriate SMB, rather than trying to DIY one. It might work fine when you tested it, and then have a problem in an emergency. If you really have decided that you need that as your redundant buoyancy, it's worth getting what you want. If you get an open ended one with a baffle, you can use your reg to fill it, inflate by mouth, or an LP hose.

 

[Ryan Mcshane]

If you’re diving wet then swap the steel tanks for Aluminium. Take a minute to read up on balanced rigs too.

 

[Centrals]

Make sure it has a dump valve near the top.

 

[rsingler]

Thanks, @-JD- .
Take a look at this spreadsheet and think about your options in more detail. Extra drysuit lift is limited by seal leak, as @Lorenzoid has pointed out.
DSMB ascents require practice.
Dual bladder is worth considering, in lieu of DSMB as an ascent tool.
Your instructor may not like it, but partial weight ditching is a solution worth some thought. The very subject is fraught with controversy, and a lot of nonsense about runaway ascents, which is why it is frowned upon without thinking it through.
Plug your numbers into this tool, and think about partial weight ditching as a backup solution:
Buoyancy, Balanced Rigs, Failures and Ditching – a comprehensive tool

 

[CaveSloth]

I am looking at the spreadsheet but thinking it can't be right. My rig is as follows:

2 LP85s. Overfilled these should be about -6lbs each.
Aluminum backplate: -2 lbs.
Regs, harness, lights, etc... I am guessing max -5lbs.

Forgetting about wetsuit which would give me some positive buoyancy... I carry no extra / ditchable weight other than my lights.

That puts me at a maximum negative buoyancy of around -20 lbs. Is that correct?

Now that I write it all out it does seem rather heavy. Also seems like the only way to make a "balanced rig" with double steels is with a dry suit or adding a couple of floaty pool noodles to my rig lol.

 

[CaveSloth]

OK, got the spreadsheet working and it now makes sense.

 

[NAM001]

I am looking at the spreadsheet but thinking it can't be right. My rig is as follows:

2 LP85s. Overfilled these should be about -6lbs each.
Aluminum backplate: -2 lbs.
Regs, harness, lights, etc... I am guessing max -5lbs.

Forgetting about wetsuit which would give me some positive buoyancy... I carry no extra / ditchable weight other than my lights.

That puts me at a maximum negative buoyancy of around -20 lbs. Is that correct?

Now that I write it all out it does seem rather heavy. Also seems like the only way to make a "balanced rig" with double steels is with a dry suit or adding a couple of floaty pool noodles to my rig lol.

An lp 85 full shoul be about -9 with the valve. now over fill it and it will go to about -13 each tank. thats a total of over =25 heavy from tanks alone not counting the manifold and back plate.

 

[gr8jab]

Just buy a good DSMB with an open bottom. They inflate quickly and stay sealed and erect on the surface. The one-way open bottom amazes me every time I use it. No way a home-made Velcro closure would work nearly as well.

They are $50 on Amazon. Compared to your other gear, that's cheap.

No comment from me on using it as backup buoyancy.

 

[rongoodman]

The problem with using a DSMB for this is that the OPV is at the bottom and the air will be in the top, so you really have not way of venting it on the way up. Most lift bags have the OPV at the top, so if I was going to try this, that's what I would use.