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There is no perfect pony it all depends on the individual dive profile. That said, anything smaller than a 19 cu. is too small as a backup gas supply. It’s best use may be a separate drysuit inflation system. You may want to consider slinging a 50 or 60 cu. At times you may need to consider deep stops and emergency Deco stops.
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You need to do the math, SAC (Surface Air Consumption) and assume you will be breathing faster because of something that made you go to the pony in the first place times how deep are these wrecks you’ll be diving = what to buy.
I carry a 12cu ft and add a 19 cu ft deco bottle on deep dives but I can’t use that deco mix deeper than 60’ so the 12 needs to get me there.
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Here are the limits for the dives during this certification:
Wreck penetration limits.
The overhead environment poses three limits beyond the normal open
water constraints of depth, air supply and no-decompression limits:
1. Edge of light zone — You should never penetrate a wreck past
the point where you can see the natural light of the entrance.
For this reason, you do not make penetration dives at night or
in water so deep and murky that there is little or no natural
light visible from inside the wreck.
2. Linear distance of 40 metres/130 feet — The maximum total
distance you enter a wreck should not exceed 40 metres/130
feet from the surface, even if you’re still in the light zone.
3. One third of air supply — Wreck penetration uses the rule of
thirds for air planning. Use 1/3 of your air to penetrate (which
starts when you descend), 1/3 to exit and keep 1/3 in reserve.
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The pony bottle is listed as ’recommended’ equipment, not mandatory:
Pony bottle — Although the pony bottle is not generally considered
mandatory for all penetration dives, its use is highly
recommended in addition to an alternate-air-source second
stage or inflator regulator. A pony bottle is a totally independent
air source and serves to provide an extra margin of
safety in an overhead environment. It is easier for you to exit
the wreck using your own pony bottle than sharing another
diver’s air. The deeper you dive, the more useful it may be.
On some wrecks, local divers may consider the pony bottle (or
other redundant, independent air source) normal equipment.
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Required Equipment for the certification:
Wreck penetration equipment: Specific equipment is required
for all penetration dives. This equipment, as well as special training,
is necessary to safely offset the potential hazards of being
inside a wreck:
1. Dive lights
2. Penetration line and reel
3. Slate
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The whole reason for a pony bottle is to have air for an
OOA situation or equipment failure, end of story!
It’s NOT as I have seen to be used to extend bottom time
or to compensate for bad SAC or smoking.
In the old days we would leave a 72 on the sand or deck
of a wreck for emergency’s, the problem with that plan in a panic is the OOA
diver would swim right past it and bolt to the surface which rarely ends well. Hence
the need for a smaller bottle you could keep with you.
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From LatitudeAdjustment: You need to do the math, SAC (Surface Air Consumption) and assume you will be breathing faster because of something that made you go to the pony in the first place times how deep are these wrecks you’ll be diving = what to buy. Ok, you’re going to make me sharpen my pencil here, let me give this a try!
If I’m 130 feet down, to reach the surface it will take me at least 260 seconds, but lets just round that up to 300 seconds even. I need to add in a 3 minute safety stop, so that’s 180 seconds, added to my 300 second ascent, makes it 480 seconds. Divide 480 by 60, gives 8 minutes. If my SAC is 2.0 cu ft per minute (more than double my normal rate due to the added stress of an out-of-air situation), 8 min times 2.0 cu/ft per minute SAC means I need at least a 16 Cu ft tank.
So, by going with the 19 cu ft pony bottle, I should have enough air to reach the surface safely. Does this sound reasonable?
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You are missing the time and gas need for the horizontal movement if you have penetrated the wreck that amount can vary significantly.
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Since your example was a deep dive and you are figuring a 3 minute safety stop you should also add in a deep stop for added safety.
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From RAWalker: You are missing the time and gas need for the horizontal movement if you have penetrated the wreck that amount can vary significantly. Nope, it’s in there! Per the dive requirement #2:
2. Linear distance of 40 metres/130 feet — The maximum total
distance you enter a wreck should not exceed 40 metres/130
feet from the surface, even if you’re still in the light zone.
So my calculation did included that distance. The 130 feet includes both the depth plus the linear distance into the wreck.
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From RAWalker: Since your example was a deep dive and you are figuring a 3 minute safety stop you should also add in a deep stop for added safety. I’m hoping I’m able to just complete even a safety stop under these circumstances, but I add in an additional 60 sec for a deep stop. So that increases my volume requirement by 1 cu ft, raising that to 17 cu ft capacity, so the 19 cu foot tank still covers that!
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Unless I have missed somthing here. Where are you calculating the air for at depth? 1 extra atmosphere 33ft double the air to fill your lungs. 2atm 66ft triple, 3atm 99ft 4 times, 5atm 132 ft 5 times the SAC. Not shure of the formula to calculate this?
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I always carry a AL40. It doesn’t really get in the way and has plenty of gas for deep dives whether I’m using it for deco on a tech dive or just as a redundancy on a rec dive. I like having it. If I am spending any time at the 200’ range I sling an AL80 and an AL40. If you don’t like the length of a 40, a 30 is a good option as well.
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I don’t know what the actual depth of the wreck is that will be used for this certification, haven’t gotten that far into it yet, just looking at what gear I’ll need to get. So until that’s defined, I guess I’ll just have to hold off on getting that pony bottle. The 130 linear foot max requirement leaves a lot of options, could be a wreck 120 feet down and we’ll only penetrate 10 feet into it, or it could be a wreck 30 feet down and we can penetrate 100 feet into it ( as long as that 100 ft is in the sunlight zone). All I know is that there aren’t any wrecks in Arizona, so it will be on a trip somewhere else!
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ks54 - 9/17/2013 7:59 AM 
From Aikidiver: Here are the limits for the dives during this certification: Wreck penetration limits. The overhead environment poses three limits beyond the normal open water constraints of depth, air supply and no-decompression limits: 1. Edge of light zone — You should never penetrate a wreck past the point where you can see the natural light of the entrance. For this reason, you do not make penetration dives at night or in water so deep and murky that there is little or no natural light visible from inside ...
The pony bottle is not used for breathing. (AS GAS PLANNING GOES) It is a replaement for a failed primary gas supply.
You refer to rule of thirds. So worst case will be you are 1/3 in ,at the turn point, and you have a failure of your porimary gas supply, You now have no supply to breath. You switch to your pony and exit the wreck. Your pony will now have to have a min of 1/3 the supply of the original gas supply. If you are using a al s80 then you have now lost 2/3 of 77 cuft. so you need a min of 1/3 of 77 or 23 cuft. Now that leaves you with a min pony size of a 30. Your ascent TO THE SURFACE can not include the safety stop when thinking of a pony with 23 cuft. Remember a safety stop is a deco stop that is not a required one. This is why the no deco limit was placed.
Other things to consider.
23 cu ft is technally 1/3 of your back gas but if you have switched to it you are most probably stressed and your body will respond as such. Respiration will increase to 1.5 or 2 times the consumption untill you FULLY calm down. 1/3 in may be 23 cuft of gas but out on a pony may use 30 or more because of stress related issues. This makes the al40 the appropriate size of a pony.
The degree of stress incurred may cause more problems like breaking your line and not knowing your way out. Loss of visibility (RULE #1) from silting is a common one to deal with. Silted water blocks natural sunlight from penitrating your location and increases the stress level. It is easy to vilote rule #1 and not know it. No matter how good you are at not silting, the silting is the result of the entire group not just you. Lastly a sudden cloud cover can reduce the light level to near nil and it only takes one of the group to silt things up in responce to the sudden low light.
There is one rule to remember. The only emergency underwater is an out of air. All other issues are irritating problems allowing you to stop, think, plan, and act. So a sudden loss of visibility is not an emercency. stop and lay stationary for a few minutes let the silt settle and exit when vis improves via the nearest exit. If those few minutes require 10 cuft then once again the 40 is the way to go.
Anything over the al 40 is normally not needed as everyone in the group will be toting a al40 as a pony. A 40 fits very nicely snap bolted to your rig as opposed to bottles less than 30. been there done that.
General rules for pony’s are that they be al tanks for the bouyancy charactoristics. neutral at half filled. Tank valve is shut till needed to guarentee a supply is there if needed. Possibly opened upon entrance to wreck.
Things that can cause a primary gas failure: in an overhead environment these examples are not that uncommon.
regulator failure. primary secondary or cut hose
blown tank o-ring
shered burst disk
tank valve rolled off.
Arguments for using less than a al40 for a pony:
Rule#2 you hit turn point long before you hit the 1/3 rule.
Rule #3 may be based on the primary air supply available at wreck entry point
More reasons to use a al40.
You can use this size as a deco bottle in the future
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ks54 - 9/17/2013 8:16 AM
A couple of flaws here. A safety stop is not needed. In an emergency plan on skipping it. Do it if you feel your gas supply can handle it. remember Rule # what ever (DIVE WITH IN THE NDL) That means NO STOP IS REQUIRED. The safety stop is and always has been an OPTIONAL stop, not a MANDITORY one.
Next you example of being 130 ft allows zero penitration. Rule # what ever 130 linear feet to the surface. If you are at 50 ft youare allowed 80 ft penitration. If at 100 ft you are allowed 30 ft penitration.
Next if at 130 ft and using a 30 ft/min ascent that is apx 4.5 min.
A 19 is still not reasonable. Always plan for murphy showing up. You may need to stay on the reg while on the surface for some time till picked up. A current could have drifted you far from the boat. Your bc will need 1 cu ft to fill and suspend you on the surface. The boat may have nad to relocate to pick up another diver.
From Aikidiver: Ok, you’re going to make me sharpen my pencil here, let me give this a try! If I’m 130 feet down, to reach the surface it will take me at least 260 seconds, but lets just round that up to 300 seconds even. I need to add in a 3 minute safety stop, so that’s 180 seconds, added to my 300 second ascent, makes it 480 seconds. Divide 480 by 60, gives 8 minutes. If my SAC is 2.0 cu ft per minute (more than double my normal rate due to the added stress of an out-of-air situation), 8 min times 2.0 cu/ft ... From Aikidiver: Ok, you’re going to make me sharpen my pencil here, let me give this a try! If I’m 130 feet down, to reach the surface it will take me at least 260 seconds, but lets just round that up to 300 seconds even. I need to add in a 3 minute safety stop, so that’s 180 seconds, added to my 300 second ascent, makes it 480 seconds. Divide 480 by 60, gives 8 minutes. If my SAC is 2.0 cu ft per minute (more than double my normal rate due to the added stress of an out-of-air situation), 8 min times 2.0 cu/ft ...
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Thank you ks54 & GoDeeper, looks like the AL40 is the way to go. If I have to carry a pony bottle, I want it to be the right one, or why bother if it doesn’t get me to the surface!
And also thanks for the other insights & philosophies for wreck diving ks54, exactly the info I’m looking for!
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From ks54: The pony bottle is not used for breathing. (AS GAS PLANNING GOES) It is a replaement for a failed primary gas supply.
Amen to that. So many divers don’t get the difference between stage tank and pony bottle.
There is also another alternative or maybe addition (very cheap and handy) - SpareAir bottle. Main draw back it’s only 1 c.f. but since I bought it I never dove without it (wreck, no wreck). Very good for your "instant buddys" when diving on vacations so often found so far away, or for the photographers too busy to check the air while waiting for this one great shot or just simply for added redundancy and will at least get you to either you buddy or in most cases to the surface.
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Wow lots of opinions here mine is 19cf it is only for out of air emergencies to get you to the surface or to your buddys octo! plain & simple! if you get the 40 & I am not saying dont do it but you will find it too much to take with you & you wont bother most of the time! A 19 is small enough to have attached to your primary tank all the time on every dive you do regardless of depth its a nice back up that you will always use. Just my 2 cents for what ever its worth.
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I’m just going to go with ks54’s recommendation of a 40 cu ft pony bottle, makes the most sense with the rule of 1/3’s.
I’ll be using a 100 cu ft steel tank for these dives. At worst case I’d need to switch over to the pony bottle right at my turn around point, so it makes since that I need at least 1/3rd of my primary tank to get me back to the surface from that point. So that means I need a minimum of 34 cu ft, which means I’ll round up to the 40 cu ft tank.
The extra 6 cu ft gives me a bit of a safety factor, which is not much, but better than nothing!
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Aikidiver....When are you taking the course? and where are you doing it? It seems like you really enjoy diving and I bet you will have a lot of fun in your class!
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You should never plan a dive where you count your pony as part of your air to make it to the surface!!!!! it is only for emercency You should get a larger primary tank or think about doubles. Any good instructor is not going to approve of you using your pony to plan your dive!!!
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From tshark: You should never plan a dive where you count your pony as part of your air to make it to the surface!!!!! tshark, please re-read the post that made you reply with this comment, because it’s never even been suggested that the pony bottle should be used as part of your air supply! The entire thread has been about using a pony bottle as an EMERGENCY/back-up air supply only, in the event of your primary air source failing!
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ks54 - 4/04/2014 1:30 PM
Akidiver
One more thing i did not consider on the tank size is this.
As stated by others the pony is not part of your gas planning. It is for OOA.
Now add this for your 130 ft dive example. You have a 5 min ndl dive planned at 130 ft wreck you are accessing from the shore. In the wreck, you get tangled at the 4 minute point. Now you need to cut your way out. You are still on back gas. You are now a min of 4 in into deco. you increased SAC has burned all but 600# (if your spg is correct) of back gas. You will have to stop for say 5 min of deco and you have 2 min of air. You will need the pony for this now created OOA upcoming event because of the deco you will have to do. ONce you are on the surface you may still need air depending on waves ect.
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I just got the 30cuft. Go figure!!!
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ks54 - 4/06/2014 2:21 PM
Akidiver and others. The al40 is not going to give you 40 cu ft. It may hold 40 but that is going form 3000 psi to 0 psi. So to get to the surface with 500 psi (??? a faulty high reading gage) you will have only about 34 cubes available for use. So now you are almost back to your 30. Peter there is nothing wrong with with a 30. You just have to understand the limits imposed by the gas available in the pony.
Now Aki earlier had a post that basically went Let me get this straight I am down 130 ft.......
You need to remember that the 130 or 120 rule is depth pluss penitration. at that depth you dont enter a cavern or cave with the certification being referenced. so lets change the dive to a 70 ft dive and a 50 ft penitration. That will remove 2 atm from the problem. Your pony gas willl now last longer. A 30 will probably sufice but leaves little margin for problems when trying getting out on the 25 cuft avail in a 30.
This has been a good discusion
Regards to all
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ks54 I bow to you, in total awesome thoughts. wow, you are smart!! your knowledge is what makes others, like me get ahead. thanks. I am trying to get my checklist of equipment for technical diving to 180 feet deep.
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ks54 - 4/26/2014 12:29 AM
peter
If you are going to 180 ft you wil probably be taking many tanks. Probably an extra 80 of trimix to get you to 70 ft and a 50% for deco. or a 80 with trimix >20% o2 and a 40 or more of o2 for deco.
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