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Pony Bottle redundancy to 99 feet
SpacemanCraig - 9/29/2015 5:10 PM
Category: Equipment
Replies: 9

I am looking into getting my Self-Reliant (Solo) Diver Certification and one of the items needed to safely dive by yourself is a redundant air source, which got me wondering how large of a pony bottle i would need as well as doing the math for an emergency accent from a depth of at least 99ft. The following is based on:

- Redundancy to max depth of 99ft
- 2 minutes at 99ft to collect myself, solve any problems, and/or move to the accent location (if needed)
- Accent Rate of 30ft per minute
- 3 minute safety stop at 15ft
- Surface Air Consumption (SAC) of at least 0.750 cu ft per minute (Giving me 150% my average SAC of 0.500 cu ft per minute in case I am stressed)


2 minutes on the bottom, 99ft -or- 4 Atmospheres X SAC of 0.750- = 6.0 cubic feet will be consumed on the bottom

During ascent to 15 feet your average depth is 58ft -or- 2.75 ATM, taking roughly 3 minutes X SAC of 0.750 = 6.2 cubic Feet

At your safety stop your depth is 15ft -or- 1.5 ATM for 3 minutes X SAC of .750 = 3.3 cubic feet

A leisurely minute to surface in a minute, with average depth of 8ft -or- 1.25 ATM X 1 minute x SAC of 0.750 = 1.0 cubic feet

With all 4 components of a safe accent to the surface I would consume around 16.5 cu ft of air, which told me that for this and most scenarios (99ft depth or less) I would need at least a 19 cu ft pony bottle. I found this pretty interesting and quickly realized that knowing this information can be very important even outside of a solo diver setting. Have you ever consider how much air you would need to get you and your buddy safely to the surface? Even with a 80 cu ft tank this same scenario of 99ft, 16.5 cu ft of air per person (33 cu ft -or- 1250psi) is an awfully lot when you think about it... hope this got you thinking.

Dive safe everybody!
Curtis - 9/29/2015 6:25 PM
Isolation manifolded doubles, or independent doubles with proper gas management procedures.

Next topic. :-)
SpacemanCraig - 9/29/2015 6:41 PM
I’m not talking about proper gas management, this is for an emergency situation so who knows how it’d play out. Do you really want to plan your redundant air source amount on your best SAC values or give yourself some leeway?
Curtis - 9/29/2015 7:44 PM
If responding to me, proper gas management is a qualifier to independent doubles, hence the placement of a single comma between the two alternatives to having a pony. Either solves by default the problem of sizing a pony, and the amount of reserve gas is going to be greater.
DiveBuddyChgo - 9/30/2015 3:49 AM
30 cu/ft pony or better yet a set of dbles. I have dove with a pony for many dives. Excellent choice for buddy diving. But dbles is the only way I dive except for vacation diving.
Agojo - 9/30/2015 4:59 AM
Living in FWB where do you plan to solo dive to 99’? Only from a boat will you reach a 99’ dive in this area and then there is a whole new set of things you should consider. If no one is in the boat while you are diving you need to consider anchor slip, anchor rope break, can’t find the anchor, dive plan filed with someone topside including gps location with dive start and stop times, safety sausage with 100’ reel, etc. I’ve doubles, gave them up and settled for a buddy or a 19 pony or a depth I can CESA from.
DiveBuddyChgo - 10/01/2015 7:44 AM
Returning back to Chgo. on Friday the 2nd. I don’t dive in Destin. But I do enjoy the beach and view from our place down here. The vis is just so so in the gulf. I’ll be in town to get together during the year anyways. Eating breakfast on the patio. The ocean is calm and the vis looks great today.
MDW - 10/08/2015 8:51 AM
When calculating for an actual emergency (in which you WILL be at least a little stressed) you should use a much higher SAC than your normal rate for calculation purposes. I’d say if your normal SAC is .75 you should figure on your emergency SAC being at least 1.0 or even 1.5 at least until you get to the safety stop where you may have time to calm down as you are now "almost home"

In practical terms, that means figure on needing 1.3 to 2.0 times as much emergency reserve as you calculated above. An AL30 or AL40 will give you that extra margin of safety you will want just in case of heavy breathing under stress.

So if you do choose to solo dive with "just" a pony instead of a fully redundant gas supply such as isolation manifold doubles or 2 full size side mount tanks, I would recommend slinging a larger pony such as a 30 or 40 cf tank. They are only marginally more expensive than a 19 and not that much more weight or drag to carry and have 1.5 to 2 times the gas in case you need it.
DaveT - 2/13/2016 2:25 PM
MDW is pointing you in the correct direction. No one can tell you what amount of breathing gas you might need at99 feet without knowing how much gas you breathe. The class should educate you on gas management and calculating gas requirements. The Instructor should be able to then help you decide what redundant gas you should be using for specific dives. Along with the bottle, the method of carry comes into play.... once again ask your Instructor, that is what you are paying them for.