Pony tank

[Divin'Papaw]

Primary Gas = (back mounted cylinders, or side slung cylinder(s)) gas used during the dive. Part of the gas calculation.
Stage Cylinder = (generally side slung cylinder(s)) gas used to extend the range of the Primary gas. Part of the gas calculation.
Travel gas = (generally carried side slung), used as a transitioning gas from the bottom gas to the decompression gas. Part of the decompression phase of the dive. Part of the gas calculation.
Decompression gas = (generally carried side slung), used for the decompression phase of the dive. Part of the gas calculation.
Pony Cylinder = (back mounted or side slung), a small cylinder (3 litre typical), carried as an emergency bailout supply for the diver. NOT part of the gas calculation for the dive. (Although sensible to know how much/how long it will last). Generally carried for no deco or light deco dives due to minimal available gas.
Bailout Cylinder = (generally side slung). Carried primarily by CCR divers. Part of the emergency supply in the event of unit failure. Specifically part of the bailout gas calculations - may well be more than one cylinder of differing gas mixes to enable safe ascent and decompression.

Pony cylinders are NOT used as part of a normal dive plan / profile. They are an emergency self help supply of gas, with the specific intent that when in use the diver is aborting the dive.

Generally;
Primary Gas and Stage cylinder gas follows rule of thirds.
Decompression gas & travel gas, rule of half's.
A Pony cylinder should have sufficient gas to get THE diver from the bottom to the surface and cover any required decompression.
Bailout Cylinder(s), should ensure the CCR diver can safely get from the furthest extent of the dive to the surface, completing all decompression. On extreme dives this may well require shared bailout protocols - to be avoided if possible. Calculations may need to be adjusted if it also intended to support OC divers/buddies.

DO NOT use the pony to extend a normal dive.
Do practice, ensure you are proficient with your pony if you have one.

Fantastic post. Couldn't have summarized it any better. Three cheers!!

 

[beldridg]

Travel gas = (generally carried side slung), used as a transitioning gas from the bottom gas to the decompression gas. Part of the decompression phase of the dive. Part of the gas calculation.

Good summary, thanks.

Just a note that some people (incorrectly?) ALSO refer to Travel Gas as the gas to breathe at the surface and on descent until you get to a depth suitable to breath a hypoxic primary gas.


- brett

 

[johndiver999]

I think the issue is more complex than people who say: "never this" or "never that". A more useful conversation would center upon exactly what types of contingencies you wish to address.

If a diver is without a pony bottle (and diving with a buddy), then he SHOULD be leaving the bottom with enough gas to get him and his buddy to the surface in a safe and sound manner.

If a diver is very sure they are diving alone (i.e., solo with no pony) then the amount of gas required in the main tank to make a safe ascent is somewhere close to only half the volume required above (possibly less if they are confident their emergency RMV will be controlled).

Now what about diving with a pony bottle and a buddy? I think things get a little more complicated.

IF a diver assumes there will only be ONE failure on a recreational dive, then he only needs to preserve enough gas in the primary tank to get ONE person to the surface. He also needs to preserve only enough gas in the pony to get one person to the surface,

In this "one-failure" scenario, there are only two potential situations. The key issue is that the diver ASSUMES that only one of these two potential situations will occur (in other words no simultaneous failure).

The two situations are

1. a failure (or loss) of the primary or pony bottle or...
2. a buddy runs out of air and there is a need to donate.

Under this one failure scenario assumption, then you only need to save enough air to get one person to the surface in each tank.

On the other hand, if you plan for TWO failures, meaning the diver experiences a scuba failure AND his buddy simultaneously has to receive air, then you need to preserve enough air in each tank to get both people to the surface.

Is this second "double simultaneous failure" really something a recreational diver is planning for? I would think not. But some people might like to be extra conservative and plan for that situation. The worst case would be two people on one pony bottle and that presents a problem in that buddy breathing would be required (i.e., share one second stage). I don't see that being hugely practical because so few people are now trained to buddy breath.

So it makes sense to me how a person may assume a "single failure" and thus needs to preserve only HALF the normal amount of gas in both the pony and the primary. Under this "single failure scenario" the presence of the pony bottle clearly allows the diver to safely drain down the main tank to a much lower pressure.

Defining the relevant emergency situation(s) being planned for seems far more useful than playing around with semantics like "pony bottle", "travel gas" ,"stage bottle" or "bail out bottle".

 

[Divin'Papaw]

I think the issue is more complex than people who say: "never this" or "never that". A more useful conversation would center upon exactly what types of contingencies you wish to address.

If a diver is without a pony bottle (and diving with a buddy), then he SHOULD be leaving the bottom with enough gas to get him and his buddy to the surface in a safe and sound manner.

If a diver is very sure they are diving alone (i.e., solo with no pony) then the amount of gas required in the main tank to make a safe ascent is somewhere close to only half the volume required above (possibly less if they are confident their emergency RMV will be controlled).

Now what about diving with a pony bottle and a buddy? I think things get a little more complicated.

IF a diver assumes there will only be ONE failure on a recreational dive, then he only needs to preserve enough gas in the primary tank to get ONE person to the surface. He also needs to preserve only enough gas in the pony to get one person to the surface,

In this "one-failure" scenario, there are only two potential situations. The key issue is that the diver ASSUMES that only one of these two potential situations will occur (in other words no simultaneous failure).

The two situations are

1. a failure (or loss) of the primary or pony bottle or...
2. a buddy runs out of air and there is a need to donate.

Under this one failure scenario assumption, then you only need to save enough air to get one person to the surface in each tank.

On the other hand, if you plan for TWO failures, meaning the diver experiences a scuba failure AND his buddy simultaneously has to receive air, then you need to preserve enough air in each tank to get both people to the surface.

Is this second "double simultaneous failure" really something a recreational diver is planning for? I would think not. But some people might like to be extra conservative and plan for that situation. The worst case would be two people on one pony bottle and that presents a problem in that buddy breathing would be required (i.e., share one second stage). I don't see that being hugely practical because so few people are now trained to buddy breath.

So it makes sense to me how a person may assume a "single failure" and thus needs to preserve only HALF the normal amount of gas in both the pony and the primary. Under this "single failure scenario" the presence of the pony bottle clearly allows the diver to safely drain down the main tank to a much lower pressure.

Defining the relevant emergency situation(s) being planned for seems far more useful than playing around with semantics like "pony bottle", "travel gas" ,"stage bottle" or "bail out bottle".

But working through these scenarios is exactly where these definitions and guidelines came from. Someone didn't just make them up. They have become standard definitions with best practices taught by all of the training agencies for a reason.

 

[johndiver999]

But working through these scenarios is exactly where these definitions and guidelines came from. Someone didn't just make them up. They have become standard definitions with best practices taught by all of the training agencies for a reason.

LOL...the relevant issue is to define your emergency and figure out how to get out of it, not semantics.

The people who actually understand all these semantics, probably see little benefit to this discussion.

The recreational diver who is trying to figure out how big of a pony they need and how much air to preserve in a single tank before starting the ascent need to go through this in a thoughtful and rational manner, rather than simply delegate their responsibility to strangers on the internet who say "never".

 

[Divin'Papaw]

LOL...the relevant issue is to define your emergency and figure out how to get out of it, not semantics.

The people who actually understand all these semantics, probably see little benefit to this discussion.

The recreational diver who is trying to figure out how big of a pony they need and how much air to preserve in a single tank before starting the ascent need to go through this in a thoughtful and rational manner, rather than simply delegate their responsibility to strangers on the internet who say "never".

... or seek out proper training to help them make the proper decision instead of simply delegating their responsibility to strangers on the internet.

Many of us on this thread are not just bantering about semantics or saying "never". We are trying to share from our experience and training how the industry has learned to safely approach these decisions. Some don't seem to like our answers but none of them seem to be people with an adequate level of training to backup their opinions.

 

[gamon]

Do you dive with a buddy?

The gas in a pony is only accessible to one diver.

I mostly dive solo and I almost never will accept a dive with an instabuddy, but I do occasionally dive with a known buddy and yes, you're right if they had an OOA emergency at the end of the dive when I've depleted most of my primary tank gas we'd be forced to share a regulator off the pony bottle as we ascended.

The 900psi min. tank pressure required upon ascending represents 26cuft of gas for two divers to use in the event of emergency.
Ascending with 500psi in an 80 and a full 13cuft pony is essentially the same.

Exactly my point, thanks for helping to clarify it. For the record, I use a 30cf pony bottle so I've got an even larger reserve.

 

[johndiver999]

I mostly dive solo and I almost never will accept a dive with an instabuddy, but I do occasionally dive with a known buddy and yes, you're right if they had an OOA emergency at the end of the dive when I've depleted most of my primary tank gas we'd be forced to share a regulator off the pony bottle as we ascended.

I think that is a somewhat "aggressive" assumption. Why not just save enough in the primary tank to get your instabuddy to the surface and you use the pony? Isn't that a far safer assumption than relying on sharing a single second stage?

 

[Scraps]

The reserve in my tank and the air in my pony are for two separate contingencies.

The reserve in my tank is for my buddy in case he has an OOA emergency. I begin my ascent with enough air in my tank for both of us to breathe from it and reach the surface at a proper ascent rate with a safety stop.

The air in my pony is for me in the event I have an OOA emergency and my buddy is not immediately at hand. There is enough air in my 19 ft^3 tank for me to ascend safely.

-----------------------------
Gamon--BTW, you may want to reconsider your practice of keeping your pony valve open during your dive. Before the dive, I open then close the valve to the pony to put some air in the hose. That way, I have air for the couple breaths I take while reaching back to open the pony valve, and I eliminate the risk of losing my pony air to a free flow incident during the dive.

 

[scubadada]

...BTW, you may want to reconsider your practice of keeping your pony valve open during your dive. Before the dive, I open then close the valve to the pony to put some air in the hose. That way, I have air for the couple breaths I take while reaching back to open the pony valve, and I eliminate the risk of losing my pony air to a free flow incident during the dive.

I sling my pony and leave the valve on. The valve, regulator, and SPG are right in front of me. I would be immediately aware of any gas loss.