Optimal Pony Bottle Size for Failure at 100ft?

[mattvish]

1650 Liters=58 cubic feet.

But your assumptions do not match the OPs situation of no deco and solo ascent. Using your initial ascent rate and SCR I get a Total Ascent Time of 7.6 minutes at an average depth of 2.16 ATA (1 minute at 30m, 2.6 minutes to 4m for the safety stop, 3 minutes at 4 meters, 1 minute to surface).

7.6 x 2.16 x 30 = 492.5 liters or 17.4 cubic feet

Ok, might be 58 cubic feet and not 50... I'm not sure because of this Z factor/true capacity thing.
I just gave an example of a minimum gas calculation how I learned and approach it. Minimum deco means safety stop, the 1-2-3 method is just more conservative. Diving is not an exact science, it's easier and totally valid to use an average DCR of 2.5 ATA for a 30m ascent. Even I would not use a pony I guess it's fine to consider it for a single diver's emergency ascent... still better then to search for the buddy in an out of gas situation.

 

[mattvish]

LOL. double tanks are declared optimal. There ain't no optimal when you consider the variability in environments and divers. The same tank(s) are best for a 90 lb woman as a 250 lb man? The same tank(s) should be used in a high energy beach dive or one that requires scrambling over rocks as a dead still quarry with a dock and a ladder?

It takes 50 cu-ft to ascend from 100 feet in an emergency? More silliness!

The OP has a good handle on it, he is specifying his assumptions and shows the math. Perhaps it is best to take pony bottle advice from people who actually use pony bottles?

As always, the discussion morphs into ridiculousness when this topic comes up.

I thought a forum is there to express opinions, having discussions and listen how others approach it. Doubles and sidemount are the safer configurations IMHO, sure they can be cumbersome and heavy in some situations and it's not for everyone. But you can't blame someone to make his point of preferred configuration (unless it's an unsafe practice). Yes, i have no clue about pony tanks but I'm interested and think it's a totally valid way to add some safety.
On the 50 cu ft... Technical diving is more conservative, I will reserve 130bar on a single AL80 for the ascent from 30m (for two divers). Don't think that's silly.

 

[johndiver999]

I thought a forum is there to express opinions, having discussions and listen how others approach it. Doubles and sidemount are the safer configurations IMHO, sure they can be cumbersome and heavy in some situations and it's not for everyone. But you can't blame someone to make his point of preferred configuration (unless it's an unsafe practice). Yes, i have no clue about pony tanks but I'm interested and think it's a totally valid way to add some safety.
On the 50 cu ft... Technical diving is more conservative, I will reserve 130bar on a single AL80 for the ascent from 30m (for two divers). Don't think that's silly.

Reserving air in the primary tank(s) to facilitate the sharing of air between two divers in a technical setting is not the same as recreational diving and making an ascent with a pony bottle. We don't plan to share the air in a pony bottle; that would be ridiculously conservative.

The OP never asked about side mount or doubles or technical diving or what is the optimal way to dive, it is a 100-ft nodeco dive.

Single 80 for tech dive huh?

 

[wetb4igetinthewater]

I thought a forum is there to express opinions, having discussions and listen how others approach it. Doubles and sidemount are the safer configurations IMHO, sure they can be cumbersome and heavy in some situations and it's not for everyone. But you can't blame someone to make his point of preferred configuration (unless it's an unsafe practice). Yes, i have no clue about pony tanks but I'm interested and think it's a totally valid way to add some safety.
On the 50 cu ft... Technical diving is more conservative, I will reserve 130bar on a single AL80 for the ascent from 30m (for two divers). Don't think that's silly.

You must be new here. Search for previous discussions on pony bottles and draw your own conclusions

 

[mattvish]

Single 80 for tech dive huh?

should have said I would have to reserve... I converted to doubles a year ago, and not going back (if I don't have to)... I did plenty of 30m+ dives on single 80 and often violated the recreational reserves or turning pressure, so do many... but I'm getting older and just not comfortable anymore with unnecessary risks.

 

[jborg]

LOL. double tanks are declared optimal. There ain't no optimal when you consider the variability in environments and divers. The same tank(s) are best for a 90 lb woman as a 250 lb man? The same tank(s) should be used in a high energy beach dive or one that requires scrambling over rocks as a dead still quarry with a dock and a ladder?

"double tanks" doesn't necessarily mean two of the biggest, heaviest tanks you can imagine. They come in different sizes for different people and purposes. I'd much rather scramble over rocks with a pair of small, balanced doubles strapped to my back than an AL80 plus a slung AL40 swinging around.

 

[SlugLife]

If I had an unlimited budget for tanks, and my own compressor, I'd dive 2x 50cu sidemount all day!

The reason I think OP might consider independent doubles or sidemount (not "need" doubles), because most of his dives are at 100ft and his calculations on redundant gas are VERY conservative. He's already not that far off of doubles with an 80cu and 40cu. The advantage being he could use 2 standard tanks as a "dive extender," while also having lots of spare gas if either air-source failed.

For most other divers, I might advocate for a 19cu, or bigger if you have the discipline to carry bigger consistently.

 

[Gone for diving]

One thing, about a 40 cu ft tank, is that its handy for other messing around.

Like a small monkey dive where you don't want all your gear. Fixing something in a pool, filling tires, running your brad nailer. Etc.
Big enough to be useful.
30 cu ft is ok. But a 40 just that much more useful.....

How many times did you do a dive with an 80 and still have 1500 psi left....?

 

[broncobowsher]

Lets try a different approach. What's your turn pressure? What is your surface pressure? That was done on an AL80, right?

Remember the safety stop is optional, so the numbers already have a safety factor baked in.

Work with some real data instead of theoretical data.

I was laughing when the number kept going up and up. At one point it was up to 58 cubic feet, for a pony. I was looking at that and thinking, if he started with an 80 that leaves maybe 2 seconds of bottom time at 100' according to the bloated math. It is worse than internet advise on wiring gauge needed. Reading car forums and see people insist on running 10 gauge wire and a relay for a low wattage LED light that can safely be ran without a relay on 18 gauge wire and have the exact same output. But the armchair warriors keep stacking another safety factor on top of the last one. Quoting marine wire gauge charts and not willing to accept any voltage drop. Wiring up LED lights that have driver modules that will put out the exact same output over a huge voltage window. Oh, just go up another wire gauge to be safe, on top of being safe, on top of the baked in safety margin, which is a step above the original wiring, you know, just to be safe.

 

[NW Dive Dawg]

Ya know....... I guess we can all argue all of the what-if's to death....... and if I have an emergency at 100ft then my preference would be to have 1000 cf of back-up gas and all day to figure it out. But the fact is that I am way more likely to actually carry my 19cf pony and am happy and confident knowing that I have that additional 5 minutes to deal with the situation !