Nitrox mix in pony bottle

[HowardE]

Yeah... I should have quantified mine with "In an open water - non-overhead environment"

 

[JeffG]

However, I would be interested to hear the views of more experienced people on the board what they do if they are going below 200' carrying a pony bottle. Would you normally put a hypoxic mix in the pony? Or live with toxicity risk of deep air?

Wouldn't be taking a pony bottle.

 

[ScubaSteve]

........also at the 15' safty stop I could switch from my dive gas to the pony bottle with the 40% nitrox to increase safety margins for NDC limits.

I think that this statement has got most of the posters here confused. Some of the replies have been based on the fact that you will be able to switch between two different air sources. Obvious problem with this logic? YES!!! What makes you think that if you run out of air at 130', you will magically have air in your primary cylinder at 15' or at 70'? OOA is typically a black or white thing....if you have air then you are not out and there is not need to use your pony (I also reference the solo comment and am ignoring the single breath or two which might come available as the air expands in your tank as you ascend).

The proper answer IMO is as was stated by another....the redundant air source should contain gas which is safe to breathe at your maximum depth. You cannot plan on switching between sources because that means you are not OOA and are simply trying to extend your bottom time.....NOT what a pony is for. If you are OOA, your dive is over, use your pony (filled with 21%) to safely ascend to the surface and do as much of a safety stop as you can given the air remaining and your other circumstances.

 

[Rick Murchison]

I disagree with all of you! (imagine that )
If redundant gas is the object, then the gas in the bailout/redundant bottle should be the same as what's in the back gas.
If you're diving 32 then the pony ought to have 32 in it.
If you're diving air, then air...
etc...

Rick

 

[Reg Braithwaite]

I use a 19 cft pony when diving solo or deeper than 100' (max 130'), my question to the Tech Divers is what percent nitrox mix should I use. My present mix is 40%, I know it exceeds the 1.4 or 1.6 ppo but my reasoning is that in an emergency I would be using this mixture for a short period of time, basically from a max of 130' to 70' my exposure time would be two to three minutes, also at the 15' safty stop I could switch from my dive gas to the pony bottle with the 40% nitrox to increase safety margins for NDC limits.

With great respect, it sounds to me like:

1. you want to do accelerated deco by breathing 40% O2 at your safety stop
2. you do not posess the appropriate certification (such as advanced nitrox) that would give you access to 50% or 100% O2
3. you do not wish to invest in a set of doubles for deep or solo diving, nor do you wish to carry a deco bottle with your rich mix plus a bailout bottle for dealing with an OOA at depth

These things put together lead to your blending a number of different ideas together, to the detriment of each (IMO).

For starters, if you wish to do accelerated deco, I think you need to go out and get the appropriate technical certification, period. Find out why some divers start accelerated deco with 50% on their deep stop at 70', why some use 100% O2 at 20', and why some do both. Then work that into your dive planning rather than bolting it on top.

Second, your bailout strategy should be independent of your deco strategy, period. The optimum bailout strategy for you is going to be doubles filled with your bottom mix and/or a pony bottle filled with your bottom mix and/or a reliable buddy and/or a Y valve or an H valve, depending.

I cannot see any benefit to trying to carry a 40% mix as your bailout. As mentioned repeatedly, you are adding a small risk of instant death to the situation that needs the greatest care--an OOA situation under 100'--while gaining very little deco advantage when doing a safety stop at 5 meters with 40%.

Think about it this way: if you are stressed at depth and hyperventilate your 40% mix, causing a tox, you will spit out your reg and you will probably drown even if your buddy attempts a rescue. If you tox while soloing you are dead. Now, if you breathe air instead of 40% at your safety stop, what is the risk? That you will get bent?? Under what circumstances would breathing air at your safety cost you your life?

When you compare risks, there is no decision. If you are only going to carry one bottle, it should be a bailout with your bottom mix. If you also wish to accelerate deco, by all means obtain the appropriate training, and then set up the correct deco mix or mixes for the dive you wish to conduct, and put them in the appropriate deco bottle.

I am open to correction on this, but I would be surprised if--after researching deco thoroughly--you choose a 40% mix for your shallow stop under any circumstance other than diving a 40% mix.

Please forgive the direct language, I am neither an expert nor am I trying to browbeat you. But I do want to caution you against taking on the risk of an unpleasant and unnecessary death.

 

[Blackwood]

I disagree with all of you! (imagine that )
If redundant gas is the object, then the gas in the bailout/redundant bottle should be the same as what's in the back gas.
If you're diving 32 then the pony ought to have 32 in it.
If you're diving air, then air...
etc...

Rick

Ideally, I agree. Practically, that would involve re-filling the pony for each dive on a different gas. That may or may not be a problem.

Example, I'm going for a couple dives on the SS Hypothetical. They do fills, but no nitrox (air only). I know I want to dive nitrox, so I bring two large tanks and a bottle of 32%. The S hits the F on the first dive, and I'm forced to donate the pony to my instabuddy. Since there is no blending station, they top off the pony with air. Now, for the second dive, I have 32% on my back and something like 23% in my pony.

Is that a problem? Probably not. For a generic recreational dive, dropping the FO₂ for the last few minutes won't likely have an appreciable adverse effect on deco. And if you are really worried about it you can plan your dive for the lowest FO₂ you're carrying (in this case, the pony).

 

[Rick Murchison]

Ideally, I agree. Practically, that would involve re-filling the pony for each dive on a different gas. That may or may not be a problem.

Example, I'm going for a couple dives on the SS Hypothetical. They do fills, but no nitrox (air only). I know I want to dive nitrox, so I bring two large tanks and a bottle of 32%. The S hits the F on the first dive, and I'm forced to donate the pony to my instabuddy. Since there is no blending station, they top off the pony with air. Now, for the second dive, I have 32% on my back and something like 23% in my pony.

Is that a problem? Probably not. For a generic recreational dive, dropping the FO₂ for the last few minutes won't likely have an appreciable adverse effect on deco. And if you are really worried about it you can plan your dive for the lowest FO₂ you're carrying (in this case, the pony).

Yeah, and if you refilled your backgas tank you'd have something closer to your pony, too!

And if... and if... there's always an "if."
But start the day with the same gas in both. Adaptation for "if's" can be made as they come up.
Rick

 

[Blackwood]

I am open to correction on this, but I would be surprised if--after researching deco thoroughly--you choose a 40% mix for your shallow stop under any circumstance other than diving a 40% mix.

Maybe I'm misreading, but... say what? Thorough research into deco leads one to choose the same gas for a shallow stop as for bottom mix?

 

[Reg Braithwaite]

Maybe I'm misreading, but... say what? Thorough research into deco leads one to choose the same gas for a shallow stop as for bottom mix?

Consider the case where a diver does his research into deco procedures and mixes, then goes out for a simple reef dive on a single tank, well within PADI/NAUI NDL. He is going to do his safety stop on his back gas, which could be 40% should he choose to do that dive on 40% EANx.

I may not have expressed myself well, but what I was trying to say was that IF we find an experienced diver breathing 40% at 15', THEN we expect that they are doing an NDL dive with 40% as their bottom mix. And by implication, we do not expect that such a diver chooses to dive less than 40% for their bottom mix and chooses to breathe 40% at 10' or 15' or 20'. They are either breathing their back gas on a simple NDL dive, or they are breathing something richer on a more technical dive.

All I was trying to suggest is that it is not a fabulous idea to do shallow stops on 40%, excepting for the 'trivial' case where 40% is your back gas. There is not enough reduction in risk to justify toxing if you use 40% in your pony at depth, there really isn't much of a deco benefit if you are doing NDL dives, and if you do the math and want to optimize your dive, you're going to want a protocol with a richer mix for deco.

 

[String]

However, I would be interested to hear the views of more experienced people on the board what they do if they are going below 200' carrying a pony bottle. Would you normally put a hypoxic mix in the pony? Or live with toxicity risk of deep air?

I suspect the more experienced people DONT go to anything like that depth with a pony cylinder.

A pony is only useful if you know its limitations - if you KNOW your gas consumption and a panic factor usage so you can calculate what sort of depth its likely to be useful for.
For most people this is 30-35m (100-115ft) with about a 3min stop for a normal size 3l pony. Any deeper or any deco and it is NOT going to get you out.

Too many people blindly strap a pony on without doing the math and do dives where if it all went wrong they'd discover the pony wasn't sufficient. They dive with a dangerous false sense of security.