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.
Nitrox mix in pony bottle
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ianr33
Contributor
I would use 21%.
If you are doing NDL dives you dont need a deco mix and making a direct ascent to the surface,even with a safety stop,you will be on the high O2 mix for only a few minutes,not really long enough to make any difference.
pO2 of 40% nitrox at 130 is just under 2.0, much higher than I would ever use by choice.
If you are doing NDL dives you dont need a deco mix and making a direct ascent to the surface,even with a safety stop,you will be on the high O2 mix for only a few minutes,not really long enough to make any difference.
pO2 of 40% nitrox at 130 is just under 2.0, much higher than I would ever use by choice.
ChrisA
Contributor
USE AIR. Maybe I should say it louder.
The only reason you have a pony is for bail out should you be so unlucky as to run out of air or have a total equipment failure. The last thing you need is to be below the MOD on a bailout bottle.
Don't worry one bit about abuot nitrogen exposure. That is the least of your problems. You will only being using the pony for a few minutes and most of that time will be well above the depth you started from. Even with air will will off gas on the way up. and in any case you'd not be diving again that day.
Remember the ONLY reason you have the pony is bailout.
Had you said "stage bottle" and "planned decompression" different story
I would also counsel the use of air. If you need to use a redundant source of gas, the dive is over and you're on your way to the surface.
But there is at least one other scenario in which bail-out bottles have been used: they've been handed off to another diver. Many recreational regs have short hoses, which can be problematic when accompanying an OOA diver to the surface. Handing off a bail-out bottle has been known to occur. In such cases its likely better for all concerned if the bail-out is filled with air.
[This is not to say that any of this bozonity is endorsed, recommended, or otherwise sanctioned, but if you're going to carry one in the first place you're likely safer for lots of reasons if its filled with Air.]
YMMV,
Doc
But there is at least one other scenario in which bail-out bottles have been used: they've been handed off to another diver. Many recreational regs have short hoses, which can be problematic when accompanying an OOA diver to the surface. Handing off a bail-out bottle has been known to occur. In such cases its likely better for all concerned if the bail-out is filled with air.
[This is not to say that any of this bozonity is endorsed, recommended, or otherwise sanctioned, but if you're going to carry one in the first place you're likely safer for lots of reasons if its filled with Air.]
YMMV,
Doc
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If you're using a pony as a redundant gas source, ie a bailout it HAS to contain a mix that is safely breathable at your maximum depth. Air is good for this - its safe in ppO2 terms at any depth you're likely to hit so wont need refilling for the next dive. Dont spike the ppO2s - what if for any reason you're delayed at depth a minute or 2 for example?
If you're using it as a rich mix for the stop then fine but then put 100% or 80% in it and DONT treat it as a redundant gas source.
If you're using it as a rich mix for the stop then fine but then put 100% or 80% in it and DONT treat it as a redundant gas source.
I know this is against the popular opinion here... but I agree with you. A 19 CF is a bailout situation, and obviously if you need to change to it briefly (using 40% at a PO2 of 1.97 isn't recommended for a dive, but to bailout, and ascend to above 99 feet (to get the PO2 below 1.6) wouldn't take but 30 seconds) - Then DO...
Ox Tox is a function of both TIME and EXPOSURE. So a brief encounter at high PO2's will MOST LIKELY not cause you to tox.
However... You could use 32% nitrox, and achieve the same results (that you ask about in part 2 of your question) and still have that margin of safety by not exceeding the dreaded 1.6 PO2.
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Part II of your question... Yes... If you did switch to the 40% on your safety stop... you'd be accelerating your decompression (IF YOU WERE DIVING AIR or a LEANER MIX for your bottom gas), and by doing a 3 minute hang on an NDL dive and fairly rich nitrox mixture would add a "buffer" of safety.
Blackwood
Contributor
I suspect it is pretty much a wash between 40% and 20% on a "safety stop."
Is 40% better for deco? Yah, technically. Is if enough better to make a difference? I doubt it. Especially for only 3 minutes. Maybe if he sucked down the whole bottle, but given a pony-involved ascent, I imagine he's trying to get out of the water ASAP. 3 minutes isn't even one cycle of the circulatory system.
The question the OP has to ask him/herself is: Is the (what I claim to be) negligible benefit of using 40% over air worth the CNS risk (which I agree is likely fairly small)?
In addition to all the good advice above, I'd just chip in: depending upon what you are doing, an immediate ascent may not be possible. If you are in a cave or a wreck below 100' you don't want to be swimming sideways on a mix that it is too rich in a stressed emergency situation.
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?
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?
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