Considering small doubles (~50's)

[Stoo]

Undrwater, I only dive doubles, unless I'm on vacation. I'd do it there too if it was easy to do. The ONLY reason not to dive doubles is that you are schlepping some additional weight. If you are shore diving, that might be an issue. I do literally one shore dive a year, and damn near kill myself in the process. And you are right, they sit nicely on your back and feel kinda nice back there. My big tanks are HP steel 130s. They're a bother on land, but in the water, they are great. Most of the time however, I dive LP steel 80s, and I also have LP steel 95s. Your choice of steel vs aluminum might be guided by the buoyancy characteristics of each. If you're diving wet, then the steels might be a bit much. (And then there's the corrosion issue in your salty water.) We all dive dry here, so twin steels double as additional "sinkage"..

As someone who dives solo most of the time, I wear doubles for the redundancy. IMHO, doubles without an isolator defeats much of the benefit, but it's not the end of the world. You will need to develop the flexibility to do shutdowns. Freeze-up can be an issue up here, so that's especially important.

As for the size of the tank, one advantage of going to larger tanks, is that it saves trips to the shop. On many of my dives, I comfortably sneak two in on a fill. (My 2400 psi tanks routinely get filled to about 2800 - 3000 psi). The HPs get filled to 3600 and I can dive for a month on those babies... or do one "big" dive. (I rarely dive shallower than 110'... rarely more than 190'.)

One other thing to consider is sidemount. I'm looking at it because (a) my wongs are about shot and need to be replaced anyway, (b) it solves that weight thing I was complaining about to a large extent and (c) it gets the "moving parts" in your face...

Feel free to PM if you want to chat more about this...

Regarding some comments in here, I will only say that Nemrod's opinions are well-known in the solo forum and I'll leave it at that. ;-)

 

[dumpsterDiver]

My SAC here in S Cal diving is approx 0.7 cu ft/min , mildly stressed it's 1cu ft/min so that's what I'll use, forgetting time to sort out problem.

from 100 ft it takes 100/30= 3.33 mins of ascent time which uses (50/33+1) * 1cu ft/min * 3.33 min= 8.4 cu ft
Safety stop takes (15/33+1)*1cu ft/min *3min = 4.4 cu ft

so that gives 8.4+4.4=12.8 cu ft as a minimum. It would require sucking out a 13 bottle empty, and as you point out we often start with partial fills. You also may be more than mildly stressed and require several seconds to get yourself together, so more realistic pony for those depths is a 19.
Personally when solo I use a 13 for depths up to 60 ft or so and 19 for deeper dives.

I was surprised Dumpster would recommend a 6 as usually he gives good advice.

If someone chooses to use a 6 cu-ft pony in 100 feet, they have to assume that they do not have time to sort out a problem on the bottom. They probably are going to assume an ascent rate of over 30 feet per minutes for at least the initial portion of the ascent. They are going to have to assume that they can keep their breathing rate moderated and to me that means, NOT kicking and using their BC to "get the ball rolling". I am comfortable with all of those issues enough to allow my 14 yr old son to dive to about 90 feet with a 6 cu-ft bottle.

I don't think my advice is "bad ", but you certainly can criticize my assumptions as being too aggressive. If someone does the math and actually thinks about what they want and need in an emergency, then I am glad that I motivated them to do the analysis themselves.

have you ever seen my stupid pony bottle video?

[video=youtube_share;r-L9extLDZQ]http://youtu.be/r-L9extLDZQ[/video]