Pony bottle - what size?

[derwoodwithasherwood]

I think I mis-judged your original post, sorry... for some reason I had the feeling you were looking for information, rather than simplifying things for others! My apologies.

No apologies necessary. I didn't word things very well originally -- when I look at it again, it could certainly be taken that way. Anyway, I'm just throwing the suggestion out there for discussion.

And yes, I like the idea of a sticky outlining all the pertinent considerations.

 

[Atomic_Diver]

I would use a AL 40 (that would give you a significant reserve for dives between 60-130'),

or an AL 30 is good as well

 

[scubadada]

I'm happy with my 19 cu ft pony for redundancy for no decompression dives within recreational limits. My SAC is about 0.4 cu ft per minute. At twice my SAC, one minute at depth, routine ascent rate, and 3 minute safety stop, I believe I will safely emerge with a bit of air left at the end of the dive. For reference, this would be equivalent to leaving the bottom with an 80 cu ft aluminum tank with about 750 psi.

Good diving, Craig

 

[derwoodwithasherwood]

How about a chart, based on maximum depth and Surface RMV? (I use the term Surface RMV advisedly, since there seems to be some confusion over the term SAC on this board.)

Something like this:

Solo Diver's Pony Bottle Minimum Size Chart:

S. RMV .2 .3 .4 .5 .6 .7 .8 .9 1.0
(cu.ft. /min):
_______
33' 6 6 13 13 13 19 19 19 30
66' 6 13 13 19 19 30 30 30 30
99' 13 13 19 30 30 30 40 40 40


Based on the following caveats:

1. For a single diver only -- no provision for a buddy.
2. Allows for 5 minutes of problem solving at depth before assent.
3. Assumes a direct assent with no overhead.
4. Allows for a safety stop at 15'.
5. No allowance for deco obligations.
6. Surface RMV to be determined by diver's personal experience -- and will increase with work or while under stress.

(Edit: this formatted nicely when originally typed. Anyone know if we can use BBCode to format a table on this board? I can't find it in the FAQ)

 

[AndyNZ]

(Edit: this formatted nicely when originally typed. Anyone know if we can use BBCode to format a table on this board? I can't find it in the FAQ)

How about an Excel spreadsheet? Would allow switching between metric and imperial, plus the option to "play the numbers" a bit more?

I can host one on our website, and get a mod to post a sticky for it.

 

[derwoodwithasherwood]

How about an Excel spreadsheet? Would allow switching between metric and imperial, plus the option to "play the numbers" a bit more?

I can host one on our website, and get a mod to post a sticky for it.

Actually, I used an Excel spreadsheet to generate the table above. If this works right, it'll appear attached to this post.

If someone wants to add a metric table, feel free to modify it at will.

I would suggest that the math be checked before it is hosted elsewhere. Just in case I made an error!

 

[scubadada]

My calculations stand from 130 feet. Rounded up: 4 cu ft at the bottom for a minute, 9 cu ft to ascend, 4 cu ft at safety stop, and half a cu ft to the surface = 17.5 cu ft. I'm sorry, I'm not going to spend 5 minures (20 cu ft) at depth to figure out that I need to ascend on my pony, duh. This is calculated at twice my SAC, I guess if I'm frightened to death it could be more. Maybe I'd have to skip out on some of the safely stop, not too serious on a nonstop dive following a normal ascent.

Good diving, Craig

 

[WD8CDH]

Actually, I used an Excel spreadsheet to generate the table above. If this works right, it'll appear attached to this post.

If someone wants to add a metric table, feel free to modify it at will.

I would suggest that the math be checked before it is hosted elsewhere. Just in case I made an error!

Thanks for the spreadsheet. I already modified it with a row to not round up to standard size bottles.

 

[derwoodwithasherwood]

I'm not going to spend 5 minures (20 cu ft) at depth to figure out that I need to ascend on my pony

Hey, if you can hold your breath for 5 minutes, I'm impressed! <lol>

Seriously, the 5 minutes is a bit of a fudge factor, allowing for some time to deal with whatever the emergency might be. I picked that number based on other's experiences -- stuff posted on Accidents and Incidents, etc. Obviously every emergency is different and ideally rec divers won't get into a situation that does not allow for an immediate assent. And if you are advanced enough to do anything other than a rec dive, you probably don't need any advice on sizing your pony!

However, your post highlights one other assumption that I made (but failed to mention): I allowed for a rather conservative assent rate of 1 atm/minute. To check my math against yours, I plugged 130' with a more agressive assent rate of 2 atm/min into the formula for S.RMV=0.8 and the results are pretty much what you have already calculated for yourself: a 19 cu ft. pony would still get you to the surface if you spent 1 minute at depth after switching to the pony. So I think the basic formula is sound. What's up for debate are the assumptions I made <lol>.

1. Is 5 minutes at depth a reasonable precaution or excessively conservative?

2. Is an assent rate of 30'/minute likely to be followed by someone who's main air supply just failed? For that matter, is it even wise? Would it be better to ascend at 60'/min in case the backup air supply failed too?

 

[WD8CDH]

The assumptions will depend on the dive profile and the emergency. I have been on deep short dives where including the deco obligation, the ascent was over 80 percent of the gas used. Independent doubles with one of them being the "pony" would make sense.:shocked2:

In cases like that, I would let my redundant gas capacity limit the dive planning rather than the main back gas capacity.

The main point is don't out dive your backup.