Changing gasses mid-dive???

[emtdan]

Hey,
I usually dive nitrox, but sometimes am on air. I am looking at buying a pony bottle and was wondering if i can switch breathiing gasses, mid dive, if i have to.... Have my pony be nitrox, but for the dive be breathing air? or change the o2 level of my nitrox half way through....

thanks a lot :06: :06:

 

[wedivebc]

Hey,
I usually dive nitrox, but sometimes am on air. I am looking at buying a pony bottle and was wondering if i can switch breathiing gasses, mid dive, if i have to.... Have my pony be nitrox, but for the dive be breathing air? or change the o2 level of my nitrox half way through....

thanks a lot :06: :06:

That is exactly what many divers do to accelerate decompression.
You should get training if this is an avenue you wish to persue. May I suggest a course such as Advanced Nitrox/Deco Procedures http://sditdi.com/ or Technical Safeair Diver http://www.andihq.com/consumer.htm
There are other agencies as well that teach this discipline. This is not to be taken lightly. Many divers had paid dearly for small mistakes underwater and proper training will help you learn how to avoid these mistakes and enjoy a safe dive.

cheers,

 

[MechDiver]

Hey,
I usually dive nitrox, but sometimes am on air. I am looking at buying a pony bottle and was wondering if i can switch breathiing gasses, mid dive, if i have to.... Have my pony be nitrox, but for the dive be breathing air? or change the o2 level of my nitrox half way through....

thanks a lot :06: :06:

Yes, that's done quite frequently for decompression diving. As long as you don't exceed the MOD of the EAN you're changing to, its fine.
Keep in mind that changing from EAN to air will effect your NDL time.

MD

 

[Snowbear]

I am looking at buying a pony bottle and was wondering if i can switch breathiing gasses, mid dive, if i have to....

Why would you have to? You're not planning to run out of gas in your main tank are you? And if you "have to" switch, it will no longer be mid-dive, but end-of-dive, correct?

A good buddy wisely said something to the effect of, "Gas management by pony bottle is a bad idea."

Oh - to answer your question as to switching gasses - as long as you are above the MOD for the gas, switching for an ascent should not be a problem.

 

[Don Burke]

Hey,
I usually dive nitrox, but sometimes am on air. I am looking at buying a pony bottle and was wondering if i can switch breathiing gasses, mid dive, if i have to.... Have my pony be nitrox, but for the dive be breathing air? or change the o2 level of my nitrox half way through....

I'd go for air in the pony and nitrox in the backgas. If you need to go on the pony, the extra nitrogen won't be much of an issue since the time will be pretty short. I'd allow some extra stop time for it anyway.

I looked at pony tanks and decided doubles fit my needs better.

 

[emtdan]

Yeah, i would put nitrox in the pony. And no, i would never want to use my pony. I have never ended a dive under 1000 psi.
I would always be moving from air to nitrox or nitrox to nitrox, but not nitrox to air.
I am now looking into one of those advanced courses...

thanks a lot

 

[miketsp]

I have never ended a dive under 1000 psi.

thanks a lot

Don't worry about it, you'll get there, keep practicing.

That reminds me of the joke about the glass of water 50% full.
To the optimist it's half full, to the pessimist, it's half empty, and to the engineer it's over dimensioned.
Sounds like your cylinder is over dimensioned....

 

[sharkmasterbc]

I used to put 40% in my pony to "pad" my ascent and speed offgassing....but the point of my pony/sling bottle is redundancy so when I'm diving to 80ft + that 40% in my sling really does me more harm than good if I had a main system blowout...so I had to "rethink" my choices. I agree that Air in the sling is better sometimes (depending of the dive in question) cause you can swithch to it at any recreational depth without worrying about OxTox. Lately I have been using 2 sling tanks one with a hot eanx mix in it to "pad" my ascent and the other for redundancy (air). Some say its overkill ....one day I would like to venture into the relm of technical diving and I think that I may be a bit more prepared than someone who has never done a gas switch or carried stage/sling bottles before...I plan for rock bottom so that I am absolutly sure I can get me and my buddy from whatever depth we planned to go to our safety stop and then to the surface.


My 2 bubbles

 

[wedivebc]

I used to put 40% in my pony to "pad" my ascent and speed offgassing....but the point of my pony/sling bottle is redundancy so when I'm diving to 80ft + that 40% in my sling really does me more harm than good if I had a main system blowout...so I had to "rethink" my choices. I agree that Air in the sling is better sometimes (depending of the dive in question) cause you can swithch to it at any recreational depth without worrying about OxTox. Lately I have been using 2 sling tanks one with a hot eanx mix in it to "pad" my ascent and the other for redundancy (air). Some say its overkill ....one day I would like to venture into the relm of technical diving and I think that I may be a bit more prepared than someone who has never done a gas switch or carried stage/sling bottles before...I plan for rock bottom so that I am absolutly sure I can get me and my buddy from whatever depth we planned to go to our safety stop and then to the surface.


My 2 bubbles

I agree, the purpose of the pony bottle has to be determined. A deco bottle can not do double duty as a bailout. You would be better off doing a 100ft dive on 32% and keeping the pony full of air than doing the same dive on air then going to 40% on ascent.

Another advantage of having an air bailout is if you needed to exceed your MOD in an emergency you have a gas that could get you deeper. It will screw up your deco profile though and that has to be considered but using air as a bailout will only slightly alter your N2 level and I would think for a no deco dive it would be a non issue.

cheers,

 

[mempilot]

If you do a gas switch and don't use deco tables or a multi-gas cx, then you won't have any idea what pressure group your in. You'll end up not being able to use your cx for the second dive since it won't know you did the switch, or if your on tables, you'll have to use the pressure group as if you didn't do the switch. Your not really buying yourself anything doing it that way, other than a bigger safety margin on the NDL. If you dive within the limits, this is fixing a problem that doesn't really exist for recreational diving and you introduce additional complexity, including MOD for the second gas. If your deco trained, no problem. If not, then get the training and see why this doesn't make a whole lot of sense unless you plan it as a deco dive. Yes, I know all dives are deco dives.

I bring a deco bottle of ean50 on all my dives below 100'. The MOD at 1.4 is 60'. I do the switch, but I have the training. Not that it's hard or voodoo, but doing this on shallow dives with ean40 for the switch doesn't really buy you a whole lot other than complexity.