Spreadsheet for helping pick a pony bottle

[wetb4igetinthewater]

So there has been a bit of talk about redundant air source, with the question of "how big do I need?" There are a number of factors, size of the cylinder of course, ascent rate, and the RMV under stress. So I thought I'd put together a spreadsheet together which will give you the required ascent rates for different sized cylinders for the RMV and stress multipliers you specify.

There has never been a scientific study that addresses the elevated RMV of divers suffering a sudden catastrophic gas loss (obviously as it would be unethical and logistically not possible).

So here's a spreadsheet, set your RMV (both metric and imperial are covered), take a wild guess as to the stress factor you'd experience under stress (this falls a bit under Dunning-Kruger as you don't know what you don't know). Don't think that a test of switching to your backup gas source calmly is the same as a catastrophic gas loss. Your gas consumption rate IS going to go up. No one knows how much.

I hope someone finds this useful.

 

[mi000ke]

I took a slightly different approach when calculating my pony requirements, computing the amount of air needed to ascend from 100’ (which is about as deep as I ever dive).

The average ATA from 100’ (4) to 15’ (1.5) for a safety stop is about 2.75

At 30 fpm ascent, the time to get to 15’ is about 3 min.

Assuming a minute to get sorted at depth, and a stress factor of 2x the air requirements would approximately be
[sorting out] RMV x 4 ATM x 1 min +
[ascend to 15’] RMV x 2.75 ATM x 3 min +
[safety stop] RMV x 1.5 ATM x 3 min +
[Ascend to surface] RMV x 1.5/2 ATM x .5 min

And then double (or whatever factor you like) for stress

So with my normal RMV of about .40 cf/min, my requirement is about
2 x (1.6+ 3.3 + 1.8 + 0.2), or about 13.8

I opted for a 13cf pony, 1) for convenience and 2) recognizing that I could ascend faster and eliminate the safety stop if needed, and 3) if the crap really hit the fan I would have ascended enough to CESA the rest of the way. Alternately, I don’t dive solo so should have plenty of air to get to a buddy (4 to 8 min of air at depth, depending on stress effect).

 

[wetb4igetinthewater]

@mi000ke

Definitely many ways to skin this cat. Hopefully people will find this also useful by modifying it to provide the information that they want. I tried to tackle it from the ascent rate speed given different cylinder sizes. This is a simplification, as one always has to get sorted to ascent, and at depth, that can be significant.

In my dive planning doc that I'm about to release, I do break down the rock bottom gas calculations like you do.

 

[northernone]

So there has been a bit of talk about redundant air source, with the question of "how big do I need?" There are a number of factors, size of the cylinder of course, ascent rate, and the RMV under stress. So I thought I'd put together a spreadsheet together which will give you the required ascent rates for different sized cylinders for the RMV and stress multipliers you specify.

There has never been a scientific study that addresses the elevated RMV of divers suffering a sudden catastrophic gas loss (obviously as it would be unethical and logistically not possible).

So here's a spreadsheet, set your RMV (both metric and imperial are covered), take a wild guess as to the stress factor you'd experience under stress (this falls a bit under Dunning-Kruger as you don't know what you don't know). Don't think that a test of switching to your backup gas source calmly is the same as a catastrophic gas loss. Your gas consumption rate IS going to go up. No one knows how much.

I hope someone finds this useful.

Thanks for putting the calculations together in a nicely laid out spreadsheet! It's interesting to see.

That's direct ascent omitting the optional safety stop, correct? For recreational (no stop) diving as per our basic training? Or for solo diving without a buddy as the redundant air supply?

The calculator suggests 3cf as plenty for 130ft for me... I tend to like a part of an al80 minimum... It can be strapped to my buddy (as per basic scuba training) or on myself when solo diving.

Cameron

 

[Diving Dubai]

I prefer to allow enough gas for a safety stop. My thinking is this:

1. Once you have gas to breath then the “emergency” is over

2. The dive you need the pony, might be on of a number on vacation. So if you’re carrying high residual N2 from relative dives, would “you” want to skip safety stop?

As you say - many ways to skin a cat.

My rule of thumb for increased RMV is add 50% to my normal RMV.

 

[wetb4igetinthewater]

That's direct ascent omitting the optional safety stop, correct? For recreational (no stop) diving as per our basic training, or for solo diving without a buddy as the redundant air

Yes, that is correct. For no safety stop. And for solo divers

 

[wetb4igetinthewater]

2. The dive you need the pony, might be on of a number on vacation. So if you’re carrying high residual N2 from relative dives, would “you” want to skip safety stop?.

That’s all part of the risk you wish to assume when planning your dive. Also, we do decompress as we ascend. It depends on how close you approach your NDL.

Remember how (C)ESAs are taught. No safety stop there.

 

[Diving Dubai]

That’s all part of the risk you wish to assume when planning your dive. Also, we do decompress as we ascend. It depends on how close you approach your NDL.

Remember how (C)ESAs are taught. No safety stop there.

Hey. I was just adding further info to the mix

Do it your way I don’t care

If you think CESA’s are safe fill your boots. It’s a last resort

Personally if I’m dragging a pony I’ll make sure it has more than enough gas rather than the getting the smallest possible. YMMV

 

[wetb4igetinthewater]

Hey. I was just adding further info to the mix

Do it your way I don’t care

If you think CESA’s are safe fill your boots. It’s a last resort

Personally if I’m dragging a pony I’ll make sure it has more than enough gas rather than the getting the smallest possible. YMMV

Not sure where you think I said CESAs are safe. If you have to resort to a CESA, you really screwed up bad. I don't dive ponies anymore as I dive sidemount. But I think that for solo divers who do choose to dive in a single tank plus pony, they should understand that at a projected RMV what their ascent rate will be approximately. I believe in conservatism. I suggest a 40 cu ft pony bottle, as that can be used for tech. That works for local diving. I've only been able to rent 13 cu ft pony bottles on vacation when one was available (one of my motivations for diving sidemount).

Everyone must decide what level of risk they wish to assume.

 

[Kharon]

I complicate my estimation by including depth and how long I expect to be at that depth. If I'm starting at 130' OOA I'd be doing 60'/min till I hit 60'. From there I'd be doing 30'/min. Every minute drops my air consumption by 1 ATM (or 2 for the first 70'). If you are calculating RMV at 130' the whole way up you are vastly overestimating air needed. If you're using surface RMV you are seriously underestimating what you will need.

Also, the whole thing about increased RMV in an OOA situation - shouldn't occur. You have not run out of air. You simply need to switch to a different air source. You should have calculated what you need for the depth of the dive and made sure you have that much independant air. If you don't have enough independant air for a depth - stay above that depth where you do have sufficient air.