What to fill a pony bottle with?

[james croft]

Not sure if you post was related to mine..........

But just to clarify, I have never run out of back gas and have never used to pony other than to check it and practice with it..........

When I say top off, that is after a long weekend of diving where due to pressurizing then depressurizing and some practice the bottle is down to 2700-2800 psi.

I just take a breath or two to check that all is OK..........not because I am out of gas

M

No, My post was not directed at what you do. It was more about not putting yourself into a position to need a pony in the first place.

 

[Jorgy]

Thanks, I was not certain........

M

 

[Rhone Man]

It's not an ornament, it isn't "one size fits all".

I kind of take the opposite view - any mix equal to or leaner than EAN36 will usually do for a pony. It just doesn't matter that much.

Pony is emergency backgas to get you to the surface. If you are engaging in decompression diving, you are almost certainly carrying a way to isolate half your backgas in the event of failure, and you are highly likely to be carrying a separate decompression mix. You are not likely to rely on a pony at all, and you are certainly not going to be relying on a pony to accelerate decompression.

If you are diving within NDLs, you use whatever gas gets you to the surface. Air? Fine. EAN36 fine. What about EAN36 if you are diving at 130 feet? You are still fine - you are not continuing your dive - you are getting to the surface. You are going to spend a maximum of one minute breathing with a ppO2 above 1.6 ATA - that is not going give anyone oxygen toxicity.

I don't really buy into the "safety margin" for the safety stop using nitrox. If you just ran out of gas and are breathing on your pony, frankly, you have bigger issues. But even EAN36 only gives you a fractional acceleration of off gassing over air - something you could easily replicate by adding one or two minutes to your safety stop - even the smallest pony has enough gas for that.

I use air because it is cheap and easy and does the job. But I wouldn't worry if for any reason I had to carry nitrox in my pony.

Just my 2 psi.

 

[Jim Lapenta]

I don't. I don't use them at all, I assumed if you were using one something has gone wrong and if there was ever something happening that would get a diver to skip or forget the stop it would be something going wrong with your or your buddy's back gas requiring the use of the pony.




No idea, that's why I ask.



Ok cool.

I have never been below 50' so like water temps (I thought 70 was pretty warm until I jumped in...) I have no practical experience with deep(er) ascents, how much air would be used or any of the rest of it.

And just what instructor gave you a card and all of this was not explained?

 

[deadly_risk]

But even EAN36 only gives you a fractional acceleration of off gassing over air - something you could easily replicate by adding one or two minutes to your safety stop...

Any sources for that? I would like to learn a bit more on the subject.

Not knowing what kind of difference it would make lead me to the reasoning expressed in the OP. Of course I didn't take into account the equipment and top off hassles of carrying a EANx mix...seems obvious now though.

 

[deadly_risk]

And just what instructor gave you a card and all of this was not explained?

All of what? I touched a few topics so I'm not sure what to answer.

Calculating air usage, SAC rate, and similar was explained. I specified no practical experience.

Water drawing heat away 25x faster than air was also explained, and in the reading, but like I said 70 still sounded pretty warm until I had the practical experience. Now I know better.

Edit: Not sure if its appropriate, so I sent you a PM in case it is relevant.

 

[H2Andy]

sorry if someone else has said this but

you want to fill your pony with a mix which you can still use safely at the deepest part of your dive, in case that's where the ssss hits the fan

 

[Rhone Man]

Any sources for that? I would like to learn a bit more on the subject.

I strongly recommend Mark Powell's book, Deco for Divers, on this subject. In Chapter 5 he covers nitrox as a decompression gas. The really short summary is that using nitrox you decompress at the equivalent air depth of the partial pressure of nitrogen (assuming you are not breathing trimix or other exotic gases for the moment). At a 20 foot safety stop with gas enriched by 10% or 15% oxygen, it is not going to be a huge hill of beans.

Amazon.com: Deco for Divers: Decompression Theory and Physiology: Mark Powell: Books

 

[Web Monkey]

With the amount of gas you have in a pony, you could easily hit 1.8 PO2 with no issues... it's PO2 versus time that you need to look at, not only MOD.

Nothing will "probably" happen if you breathe 32% @ 140', however, I prefer to remove "probably" as even a potential issue, since there's absolutely no advantage to an enriched mix and at least a one disadvantage.

If someone wants to dive with a pony full of the Nitrox mix their choosing, that's fine. I'll stick to air.

Terry

 

[deadly_risk]

I strongly recommend Mark Powell's book, Deco for Divers on this subject. In Chapter 5 he covers nitrox as a decompression gas. The really short summary is that using nitrox you decompress at the equivalent air depth of the partial pressure of nitrogen (assuming you are not breathing trimix or other exotic gases for the moment). At a 20 foot safety stop with gas enriched by 10% or 15% oxygen, it is not going to be a huge hill of beans.

Thanks! The SSI Nitrox book I read focused on the effects of nitrox descending and didn't touch at all on what happens when you ascend breathing EANx.