Couple questions on a pony bottle for bail out

[nimoh]

Yes, but wouldn't the better strategy for a recreational diver be to dive conservatively instead of pushing up against the NDL's and depending on the fractional content of O2 in emergency gas to ward off impending DCS? Deco limits being somewhat theoretical in the first place. When I hear people using their choice of pony gas as an anti DCS strategy I tend to think the horse is already out of the gate.

yes, I agree completely

 

[dumpsterDiver]

I was a little curious after reading this discussion of using air for ascent, so I ran a scenario through vplanner. I chose a dive profile that is at about the NDL of EAN32, specifically 29 minutes at 100 feet.

- doing this dive completely on 32% and with an ascent rate of 30 ft/min gives a deco stop of 0:20 at 20 feet

- doing this dive and switching to air at 95 feet with an ascent rate of 30 ft/min gives a deco stop of 3:20 at 20 feet

- doing this dive and switching to air at 95 feet with an ascent rate of 60 ft/min gives a deco stop of 2:40 at 20 feet

I wouldn't call that an insignificant difference, but wouldn't really call it dangerous either, if the dive is well planned taking this into consideration. However, personally, if I were putting 32% on my back, I would put 32% in my pony as well, as it makes the planning easier.

Thanks! Can you run one more.. say a dive with NO-DECO and show me the difference in ascent time. I suspect that most recreational divers will be a little freaked if they actually have to use a pony to ascend and have zero air available from their primary tank (or a buddy) so I would think their ascent rate would be closer to 60 feet per minute, than 30 -especially for the fist 30 or 40 feet. Also, the deco times that you generated are LESS than the recommended safety stop for a recreational dive.. so those deco stop times don't seem too onerous.

 

[nimoh]

Thanks! Can you run one more.. say a dive with NO-DECO and show me the difference in ascent time.

not sure what you mean, I did run my previous example contrasting a 30 ft/min rate with a 60 ft/min rate and the faster ascent rate knocked off some deco time. The example was also specifically intended to be on the edge of no-deco. If the profile were a few minutes less, then there would be no deco stop if using air for the ascent. Calculating residual nitrogen loading for subsequent dives might be an issue, but that's a different conversation.

I suspect that most recreational divers will be a little freaked if they actually have to use a pony to ascend and have zero air available from their primary tank (or a buddy) so I would think their ascent rate would be closer to 60 feet per minute, than 30 -especially for the fist 30 or 40 feet.

I would also suggest that a diver that gets freaked out by having to use a pony bottle has no business pushing the NDL in the first place

Also, the deco times that you generated are LESS than the recommended safety stop for a recreational dive.. so those deco stop times don't seem too onerous.

I was thinking the same thing, although it is important to note that the safety stop would become a required deco stop when using air for the ascent (for a dive profile that is already pushing the NDL)

 

[John C. Ratliff]

That's funny .. about the Dacor Pacer regulator. i have been using one for my octopus second stage now for maybe 15 year (or 25)???? I like it because it is rugged, solid metal. Any other plastic second stage would have been cracked and gone after all the abuse I have put the second stage through.

This is the first I have heard that it is a good choice for this application for reasons other than robustness...in fact I always assume an adjustable second stage might make more sense, because you could tighten it up making it less likely to freeflow and then when you deploy it, you could crank the knob and make it easier to breath? But of course those plastic knobs would get snapped off ever 5 or dozen years with my abuse...

DumpsterDiver,

Here's part of the Dacor PACER regulator ad which shows how the anti-free flow device works. This is the same for the Dacor PACER (Positive Air Controlled Equalizing Regulator) and for the Dacor Olympic metal second stages. I've tested it in the Clackamas River, and it works well on both second stages.

SeaRat

 

[dumpsterDiver]

DumpsterDiver,

Here's part of the Dacor PACER regulator ad which shows how the anti-free flow device works. This is the same for the Dacor PACER (Positive Air Controlled Equalizing Regulator) and for the Dacor Olympic metal second stages. I've tested it in the Clackamas River, and it works well on both second stages.

SeaRat

Thanks! I learned something today!

Sometimes in diving, I follow certain protocols, habits or rigging etc. and I don't even really remember why... Then when I try to change something or improve it, a problem pops up that I had forgotten or never realized. Those second stages just seem to work for me.

---------- Post added July 23rd, 2014 at 09:25 PM ----------

not sure what you mean, I did run my previous example contrasting a 30 ft/min rate with a 60 ft/min rate and the faster ascent rate knocked off some deco time. The example was also specifically intended to be on the edge of no-deco. If the profile were a few minutes less, then there would be no deco stop if using air for the ascent. Calculating residual nitrogen loading for subsequent dives might be an issue, but that's a different conversation.



I would also suggest that a diver that gets freaked out by having to use a pony bottle has no business pushing the NDL in the first place



I was thinking the same thing, although it is important to note that the safety stop would become a required deco stop when using air for the ascent (for a dive profile that is already pushing the NDL)

Sorry I was unclear, the argument has been made by others that switch to air is going to load you up with nitrogen on the ascent. If the diver is NOT in deco on nitrox (but right at the knife edge).. what are the consequences of using air for the ascent? Does it still kick you into deco? From your original runs, I suspect that it does...I guess that is why you used a profile that was 20-30 seconds of deco on nitrox...

 

[DaleC]

What can be accomplished with a single and a pony: index

 

[John C. Ratliff]

What can be accomplished with a single and a pony: index

Everyone should follow this link Dale provided and take a look--it is spectacular.

SeaRat

---------- Post added July 23rd, 2014 at 09:14 PM ----------

 

[mrdre]

Running "Deco Planner".... Dive plan: EAN32, 120 feet, 24 minutes..... Catastrophic tank failure happens after spending 24 minutes at 120'. The diver switches to 19 cuft pony bottle.

a) the pony bottle has EAN32, 30 ft/minute ascend + 7 minutes stop at 20 feet (which is actually part of the dive plan, if the failure didn't happen then the diver would continue breathing the main gas).
b) the pony bottle has air, 30 ft/minute ascend + 10 minutes stop at 20 feet.

So with air in the pony bottle in the very worst case scenario the diver just needs to stay 3 extra minutes at 20 feet. What's the big deal?

 

[John C. Ratliff]

Running "Deco Planner".... Dive plan: EAN32, 120 feet, 24 minutes..... Catastrophic tank failure happens after spending 24 minutes at 120'. The diver switches to 19 cuft pony bottle.

a) the pony bottle has EAN32, 30 ft/minute ascend + 7 minutes stop at 20 feet (which is actually part of the dive plan, if the failure didn't happen then the diver would continue breathing the main gas).
b) the pony bottle has air, 30 ft/minute ascend + 10 minutes stop at 20 feet.

So with air in the pony bottle in the very worst case scenario the diver just needs to stay 3 extra minutes at 20 feet. What's the big deal?

I'm going to add some discussion here, and the first is to point out that this dive, at 120 feet on EAN32 (32% oxygen in the breathing mix) is over the U.S. Navy Diving Manual's Table 10-1 limit for diving EAN. That limit is set for 1.4 atm oxygen partial pressure. The limit using this table is 100 feet, which is an Air Equivalent of 90 feet on regular breathing air.

One of the problems I have with people using nitrox is the lack of knowledge some have about the limitations of this mixture. Unless someone has gone into a chamber, as I have (albeit in 1967) for an oxygen tolerance test, going beyond the line specified in the U.S. Navy Diving Manual is asking for trouble. As I recall, that test was going down to a simulated chamber depth of 66 fsw for half an hour on pure oxygen, while being monitored by a diving medical technician. Most passed, but a couple did not. I would not want to be even close now, at age 68.

SeaRat

 

[mrdre]

I'm going to add some discussion here, and the first is to point out that this dive, at 120 feet on EAN32 (32% oxygen in the breathing mix) is over the U.S. Navy Diving Manual's Table 10-1 limit for diving EAN. That limit is set for 1.4 atm oxygen partial pressure. The limit using this table is 100 feet, which is an Air Equivalent of 90 feet on regular breathing air. ...

OK, fair enough. Let's not push the limits. I assumed 1.6 partial pressure. Let's move to the 100 feet. Rerunning the program...

Dive plan: EAN32, 100 feet, 30 minutes..... Catastrophic tank failure happens after spending 30 minutes at 100'. The diver switches to 19 cuft pony bottle.

a) the pony bottle has EAN32, 30 ft/minute ascend + 5 minutes stop at 20 feet (which is actually part of the dive plan, if the failure didn't happen then the diver would continue breathing the main gas).
b) the pony bottle has air, 30 ft/minute ascend + 7 minutes stop at 20 feet.

So with air in the pony bottle in the very worst case scenario the diver just needs to stay 2 extra minutes at 20 feet. What's the big deal?