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sampling good bourbons while waiting for the ice to melt...
Join Date
Feb 2011
Location
"La Grande Ile"
Posts
1,084
Dives
100 - 199
gas mixes for the pony....
Just working through an interesting thought on gas for a pony cylinder... the usual statement applies that the gas there is only a redundant source, and all gas management are based on the main supply....
couple scenarios here:
1. "Air" dive, recreational limits with hard bottom - pony is also "air". Would it beneficial to instead carry an enriched mixture (say a EAN28) as a "safety cushion"? I chose EAN28 for this discussion as the MOD works for the diving I would be doing....
2. Enriched air dive - I thinking if you are diving EAN, you should then be carrying the same or richer (MOD limited) for the dive in the pony, correct (rather than "air")?
It seems that if your dive is following an EAN schedule, and that emergency showed up, you wouldn't want to be switching to air due to the potential of being out past the NDL range of the gas.
If not, I guess I also make the statement that the NDL should be based on the emergency gas too, rather than the main cylinder gas.....
If using the gas for backup, keep the same gas in the pony as you would in your back gas. If you are using the pony for deco, put whatever the best deco gas you are certed to dive with in it. IMO, get rid of the pony for backup and just dive doubles
There are tons of variables, but “ideally” I would choose as rich a Nitrox mix as max depth allows — shooting for a 1.6 PPO2 in a no or soft overhead environment. The logic being that switching to a pony would be a leave-bottom-immediately event and minimizing decompression is desirable. In less demanding no-decompression situations, I would choose a mix for the maximum depth you normally dive so you don’t need to change gases.
I think you should use the same gas unless you are trained and comfortable in selecting a gas mix. In which case you would be planning a gas switch.
I dont see you getting that much benefit from switching to EAN28 in a "normal" air dive situation, if you are only using it as a backup/ reserve, and in an emergency situation, switching to a different gas just complicates your situation by adding in MOD, among other things, to your already complicated situation.
Emergencies 'tend' to be linked with faster than desired ascent profiles - there is some logic to off-setting that risk by adding some nitrox. But then, you've got to be absolutely sure that the mix will be breathable at any potential depth you might reach on the dive. What if it was a wall dive, and the emergency was preceded by some need/situation that took you deeper than planned?
Likewise, emergencies 'tend' to be linked with elevated respiration and exertion. For that reason, I would disagree with Akimbo's suggestion to plan for a 1.6 (or even 1.4) PPO2. There's no point adding risk when selecting emergency equipment - that defeats the purpose.
If this is an emergency/redundant cylinder, then you've got to assume that if/when you need it you'll already be in a physically and psychologically stressed state. For that reason, there is a lot of sense in keeping it simple. When poop hits oscillator... simplicity is beautiful. Having the same gas in main and redundant cylinders is as simple as it gets...
Even when stressed, you can knock off the USN 10 min. missed deco procedure on 30 cuft. So if in any doubt, do the procedure and skate back home happy.
-as an aside, 500 psi in my double 100's is enough to do the procedure also.
My "ponies" have either air or O2 and no possibility of confusing them. I totally agree with keeping important things simple.
Even when stressed, you can knock off the USN 10 min. missed deco procedure on 30 cuft. So if in any doubt, do the procedure and skate back home happy.
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Originally Posted by Superlyte27
