Using a pony. Mix?

[stuartv]

I'm signed up to take a U-boat Diving course. Part of the recommended equipment is a pony bottle, so I found an AL30 on CL for a good price and bought it.

Now I'm wondering what to fill it with. The obvious answer would seem to be fill it with the same mix that goes in my main tank(s). But, if I understand the intended use correctly, I expect that I will never actually use the gas in the pony bottle. I'm a (new) Rec diver and the U-boat course is a Rec course. So, correct me if I'm wrong, but the pony bottle is ONLY for emergency OOA situations.

In which case, once I get it filled, what do I do for my next dive, in the event I'm using a different gas. E.g., my first dive is relatively shallow, so I get tanks and pony filled with air. I complete my dives without using the pony. Then, next time out, I'm going (Rec) deep and get my tanks filled with EAN32. Would SOP be to have my pony emptied (of air) and refilled with EAN32? Should I leave it alone? Should I just get the pony filled with EAN32, so it will be good for almost any Rec depth and then leave it filled with that until such time as I ever use it and need to get it filled again?

And if I use the pony with a different mix than what's in my tank, should I always program my PDC with a 2nd gas set for whatever the pony is, so that if I end up using it, I can also do a gas switch on my PDC so that it can continue to maintain correct NDL and SI calculations?

I'm particularly concerned about the example scenario where I'm at 110' on EAN32 and have air in the pony. Suppose I am 1 minute from my NDL (for EAN32) and, for whatever reason, I have to switch to the pony (i.e. air). Is that potentially going to put me in a mandatory deco situation? I think it would not. I *think*, in that scenario, when I switched to the pony it would be no different than if I had been diving on air the whole time and gotten to 1 minute from my NDL. I.e. I would do a normal ascent immediately, with a standard safety stop, and be fine.

Of course if Pony Bottle SOP is "always empty/refill with the same gas as your main tanks", then all this is a non-issue.

 

[Dr. Lecter]

Either approach is fine, simply using air is logistically easier/cheaper.

Yes, you might hit a situation where you're riding the EAN NDLs very closely, switch to the pony and air, and because you're not ascending quickly enough your computer will say you now have a mandatory deco hang of a minute or five. So what? First, you shouldn't be riding the EAN NDLs that closely anyway - especially knowing you have air in your pony. Second, you're ascending and have a full AL30 to work with - just do the piddling stop and get on with life.

 

[stuartv]

Thanks, DL!

 

[tbone1004]

This is why I recommend an AN/DP type course as soon as your planning on going deeper than 80ft regularly. You may only be doing deco on backgas, but you don't have to worry about NDL's anymore. As far as SOP you can do one of two things, you can use the pony for your ascent/deco stop which would burn some of the gas out of there, then top off, you could buy or make a transfill whip, buy an O2 analyzer and let the bottles equalize, make sure your computer has two gas mixes and since pony is for you to breathe off of switch to that gas. Better solution is to just dive EAN32 all the time and not worry about it, or start the process of technical training at least for multi gas decompression so you understand the implications of having different gas mixes.

What you are experience above is why few of us recommend pony bottles and instead recommend a straight move to smaller doubles or H-valves for redundancy and O2 for decompression for these types of diving.

In this specific case, this is the best scenario. Run EAN32 exclusively in the pony bottle, you won't be going deeper than EAN32 at PO2 of 1.6, run whatever gas you are planning on diving in your back gas bottle. IF you have to bail out to the pony, you're going to be starting ascent, but don't make the gas switch. This is comparable to diving nitrox on air tables, you won't get the NDL benefit, but you will obviously be loading your tissues less, won't be 100% accurate obviously, but you are erring on the side of safety which is where you want to be, the last thing you want is to have the pony bottle be a leaner mixture than your back gas which would put you into decompression sooner than backgas, and would certainly not be the safe way to go.

 

[dumpsterDiver]

I strongly recommend you fill the pony with AIR. It is cheap (often free), fast, simple and you will never get screwed up with a rich mix for a dive. Also if you get fills with partial pressure blending, then if you use the pony a little then they have to dump the whole freaking thing and fill it again. If you come back from a weekend of diving and the pony is down 500 psi, the shop SHOULD fill it for free when you are getting your nitrox fills.

Trying to match your mix with the main tank adds unnecessary complexity, increases the chance for oxygen toxicity and provides negligible benefit in case of an emergency..

 

[Dr. Lecter]

This is why I recommend an AN/DP type course as soon as your planning on going deeper than 80ft regularly. You may only be doing deco on backgas, but you don't have to worry about NDL's anymore. As far as SOP you can do one of two things, you can use the pony for your ascent/deco stop which would burn some of the gas out of there, then top off, you could buy or make a transfill whip, buy an O2 analyzer and let the bottles equalize, make sure your computer has two gas mixes and since pony is for you to breathe off of switch to that gas. Better solution is to just dive EAN32 all the time and not worry about it, or start the process of technical training at least for multi gas decompression so you understand the implications of having different gas mixes.

What you are experience above is why few of us recommend pony bottles and instead recommend a straight move to smaller doubles or H-valves for redundancy and O2 for decompression for these types of diving.

In this specific case, this is the best scenario. Run EAN32 exclusively in the pony bottle, you won't be going deeper than EAN32 at PO2 of 1.6, run whatever gas you are planning on diving in your back gas bottle. IF you have to bail out to the pony, you're going to be starting ascent, but don't make the gas switch. This is comparable to diving nitrox on air tables, you won't get the NDL benefit, but you will obviously be loading your tissues less, won't be 100% accurate obviously, but you are erring on the side of safety which is where you want to be, the last thing you want is to have the pony bottle be a leaner mixture than your back gas which would put you into decompression sooner than backgas, and would certainly not be the safe way to go.

The added inert gas loading is far less of a safety risk than having an inappropriately rich EAN mix in an emergency gas source that (emergencies being what they are) may wind up getting used under stress/exertion at or below the planned max depth of the dive. Going into deco is not some magical, binary threshold beyond which one may never, ever safely venture without first carrying a specific piece of plastic. Unlike a CNS hit, which is a magical, binary event with an effectively 100% fatality rate and little to tell you it's coming/take preventative measures.

Any crossing of the NDL line caused by going from EAN backgas to air in a pony for the duration of an emergency bailout and ascent is going to present a negligible deco risk. Yes, it should still be treated as a mandatory stop and completed; yes, the dive should be planned such that the diver can do that regardless of what goes wrong; but no, you don't need a ing AN/DP class to figure that much out.

 

[lowviz]

@ the OP. Please, do the numbers. Know for yourself what the real advantage of using the exactly optimal nitrox mix will be in an emergency.

Next, weigh that advantage against the brain dead simplicity of just keeping it topped off with air and never anything else. It is not like you are ever planning to actually use the thing IRL...

 

[TONY CHANEY]

A very good question. My thoughts are to fill it with air and be done with it. It will be good to 218 fsw should anything go wrong. If you have to jump on it then your dive is over and you are heading toward the surface. IMHO, you should never use it to extend your bottom time. Treat it as a one time get out of jail free card, play it and go. Do not worry about the NDL because you should never push the NDL that close to start with. You might want to add a few minutes to your safety stop. No O2 cleaning, gas matching, etc to worry about.

 

[tbone1004]

Lecter, I agree with you, but with 32% you have 111 as MOD for 1.4, 132 at 1.6, and I doubt he'll be diving where the floor is deeper than 132, in that case, then yes you would want a leaner mix on the pony bottle, for recreational depths, 32% in the pony bottle regardless of what is in back gas is safer than having 32% in the back gas, air in the pony, and not having a multigas computer or remembering to make the gas switch. As long as the Pony bottle has an MOD no shallower than the floor, you're fine, especially since he mentioned using 32% for the "deep" dives and using air for shallower dives, if he was using 28% or 30% for the deep dives, then you set the pony bottle there if the MOD is appropriate, but I doubt he's going to get a CNS hit at 60ft going from air to 32% which is when he said he'd be running the leaner mixes.

 

[Dr. Lecter]

He was talking about 32% at 110' - not sure where you got 60'. Realistically, none of this matters at all: a few minutes of deco due to riding the NDLs to the edge and then having an ill-timed emergency is NBD, and the odds of a CNS hit from anything remotely possible during a recreational length/depth dive (incorrect gas analysis aside) are very low. What really matters is having enough of something to breathe to get your ass back to the surface where you can reflect on what you just learned.

If he has a situation like one we have locally ($100 annual membership gets you two tank fills per day for a year of air, 32%, or 36%, all banked and no O2 cleaning required) I'd say do whatever you feel like so long as you're not rolling in the sand at 130' on air with a pony full of 36%. But for a lot of people, the KISS answer of 'fill with air, practice using regularly, top off as needed' is the right answer. Anything that makes it harder for you to dip into the reserves by practicing deployment and use/top it up afterwards...is probably the biggest hazard at issue.