Pony bottle ascent schedule?

[H2O 70]

Fair enough. Now I will assume you have some rational behind this besides seeing a fair amount of others do it. Care to share your logic, I am still at a loss as to the gain for NDL dives?

From an NDL point of view. This is your pony which is there for that oh sh&t moment. Likely that oh sh*t is OOG. If you're OOG you're likely pushing something. Switch to you higher FO₂ gas and make your slow ascent. The higher ppO₂ will help with off gassing.

From a practical point of view, I've done and seen this a fair amount is when were diving off on NC, we're typically diving deep wrecks. Plan your rock bottom gas so you have that covered. We'll more often that not rack up 10 minutes of total stop time. Switch over to EAN40 at 100fsw and start to slowly make your way back up. Often by the time you're in the SS range, you've cleared any stop time. On an AL 30 or 40, you can easily get a day like this and still have enough in reserve for a true oh sh&t moment.

 

[GrumpyOldGuy]

^^ Thanks.
From my perspective, you are using the tank as a stage, not a pony. Will it supply gas in an emergency, of course, but you are optimizing for deco, not for safety.

I realize that it is mostly semantics.

 

[Rainer]

IMO, keeping air in the pony is MUCH smarter than 40%.

First, you don't have to worry about any reasonable MOD limits. I'd much rather chase someone deep (>130') on air than 40% (yes, certainly an unlikely scenario, especially given the people I dive with, but why have to worry). For dive sites without a hard bottom, no telling how long you'd be below MOD.

Second, if you *are* planning an immediate ascent after switching, 40% is not going to offer you any reasonable decompression advantage over air.

Third, and what carries most weight for me (admittedly, someone who doesn't dive a pony bottle), air is cheap and readily available. The reason this matters is because I believe it promotes frequent practice with the pony. If you have air in it, I believe you're much more likely to often practice switching to it than if it has a more expensive and harder to find gas.

 

[H2O 70]

^^ Thanks.
From my perspective, you are using the tank as a stage, not a pony. Will it supply gas in an emergency, of course, but you are optimizing for deco, not for safety.

I realize that it is mostly semantics.

Sure, I'll give you that. It is there, so why not use it. Would I do that with an AL19? Probably not. With an AL30 and especially an AL40 how much are you really going to breath off of it in the example I gave above? 10ft³? 15ft³? I don't want to do the math. Someone else can.

 

[GrumpyOldGuy]

Sure, I'll give you that. It is there, so why not use it. Would I do that with an AL19? Probably not. With an AL30 and especially an AL40 how much are you really going to breath off of it in the example I gave above? 10ft³? 15ft³? I don't want to do the math. Someone else can.

I was questioning more the gas mix (EAN40 at 132') than the amount of gas. You optimized to deco at the cost of an increased oxtox issue if you use it at depth in an emergency. You mitigate this risk by planning to kick up to 100' ASAP.

My plan would be to use pony gas that is safe at all planned depths and plan on a few extra minutes of deco. I can hang out for a long time on an AL30 if needed.

 

[scubadada]

Really?

Inexplicably going OOG at 130' wouldn't cause you to want to take some time to stop and rethink... SOMETHING... before diving again?

:cool2:

Hey, how likely is this to happen, it's a stupid scenario. I would handle it as I proposed and dive again as suggested. This was a no deco situation. Problem with that?

Good diving, Craig

 

[Puffer Fish]

IMO, keeping air in the pony is MUCH smarter than 40%.

First, you don't have to worry about any reasonable MOD limits. I'd much rather chase someone deep (>130') on air than 40% (yes, certainly an unlikely scenario, especially given the people I dive with, but why have to worry). For dive sites without a hard bottom, no telling how long you'd be below MOD.

Second, if you *are* planning an immediate ascent after switching, 40% is not going to offer you any reasonable decompression advantage over air.

Third, and what carries most weight for me (admittedly, someone who doesn't dive a pony bottle), air is cheap and readily available. The reason this matters is because I believe it promotes frequent practice with the pony. If you have air in it, I believe you're much more likely to often practice switching to it than if it has a more expensive and harder to find gas.

Humm.. well, first off... The question was not about loosing all your air and then having to chase someone...not sure why you would be doing that anyway... but it was not the subject.

In the original proposed use of an air pony, our op would have actually gone into deco.(if he was at the end of the no-deco limits), and as you can see by most of the posts here, it would have been extremely difficult to know, and to know how much. Even using software, give the number of stops, it took a while to model.

Lastly, where I dive, it is readily available, and I have several filled at the same time.. for an average cost of around $3.33 a tank.

Had he had 40%.. he does need to mozy on up to a safe depth.. but there is no deco issue, his computer would reflect a very safe dive, and he could go do another dive if he wanted to.

Would he die from using air... doubt it... might he get a DCS hit... maybe, even though he blew off some deco time, the amount is fairly small, and he would most likely be ok..but if he uses Nitrox.. there are no issues...I would vote Nitrox.

 

[dumpsterDiver]

Ignoring the issue of particular enriched air mix and depth combination - you later amended that, and it wasn't the focus of your question anyway, even if it did generate a lot of comment - there is really no reason to fundamentally change your (presumed) previous plans for a normal ascent, just because you are ascending on air instead of enriched air. You have plenty of gas (30cf) to ascend from 130', and the fact that you are breathing air instead of a slightly enriched mix on the ascent is inconsequential, in terms of the actual additional nitrogen loading that would occur during the ascent. You could use the schedule you proposed. Or, you could ascend at your normal rate (let's assume 30 ft/min) to your 15ft S/S (or 20ft, if that is your preference) and hang out there.

What would I do if my backgas became unavailable at 130', and I had a 30cf bottle of air, and was within NDL? I would ascend normally to 15ft, do my safety stop, and surface. If I wanted to be very conservative, I would add 5 min to my S/S. If my backgas becomes compromised, and I have a more limited reserve (e.g. a 13 cf bottle, instead of a 30), my interest would be in getting shallow sooner rather than later - but, again using a safe, normal ascent rate - and doing whatever safety stop I could with the remaining gas. If, for whatever strange reason, my limited reserve ran out (bottle wasn't full to start with, I was breathing hard looking at the tips of the trident sticking out of my tanks, I was using a Spare Air, etc) I could more easily do a CESA and surface from 15 ft than from 70 ft.

Totally agree. The diver needs to think about the probablities in situations like this.

The chance of having to use the pony bottle is really very small (if you dive wisely) and the probability of getting bent from an ascent on a no-deco dive is also small. What is the probablity that a diver will have the total failure AND get bent because he absorbed an inconsequential amount of extra N2 due to breathing air on the ascent on one particular day is ridiculously small. Use air and worry about things that MIGHT get you like heart disease, cancer and car wrecks.

 

[Puffer Fish]

Totally agree. The diver needs to think about the probablities in situations like this.

The chance of having to use the pony bottle is really very small (if you dive wisely) and the probability of getting bent from an ascent on a no-deco dive is also small. What is the probablity that a diver will have the total failure AND get bent because he absorbed an inconsequential amount of extra N2 due to breathing air on the ascent on one particular day is ridiculously small. Use air and worry about things that MIGHT get you like heart disease, cancer and car wrecks.

Problem is, he turned a no-deco dive into a deco dive and then blew off the roughly 5 -8 minutes or so he should have done. That is obviously not a ton of deco, but it would seem to be more than "an inconsequential" amount.

Part of this is switching to air.. but the biggest was his ascent profile. A nice simple, ascent would have made that inconsequential, but it was not what he asked.

 

[TSandM]

PufferFish, did I miss something? I just went back and reread the original post, and he clearly states that he was within his NDLs at the time that Neptune speared his tank. Are you saying that doing the ascent on air would cause him to accrue a 5 to 8 minute decompression obligation? Even if he were just at the edge of his NDLs when the incident occurred, I have an extremely hard time seeing how that would happen. It's going to take him three minutes at 30 fpm to reach 20 feet, and I can't see racking up 8 minutes of deco for three minutes of air breathing, all on ascent.