3 Bottle dives

[Cave Diver]

I'm going to guess that he means that not very many people are doing the dives and it gets boring talking to the same old people.

I'm sure no one ever gets bored talking to you.

 

[johnkendall]

For a dive like that I would use 3 bottles: Bottom Gas, 50% and O2. Probably 1 AL80 and 2 7L cylinders. I'm afraid I can't really comment on backgas size as I don't really understand the US cylinders. I've just about got a handle on 104s (roughly 16.6L) and 80s (roughly 11L). Here in the UK we would do the dive on double 12s. With bigger backgas tanks you could do away with the 3rd bottle. 240ft is around the limit that I would add in a 3rd deco gas. That would probably be 21/35 rather than 35/25 and it would be an AL80. If averaging 220 then I wouldn't worry about the 3rd deco gas.

HTH

John

 

[RTodd]

For a dive like that I would use 3 bottles: Bottom Gas, 50% and O2. Probably 1 AL80 and 2 7L cylinders. I'm afraid I can't really comment on backgas size as I don't really understand the US cylinders. I've just about got a handle on 104s (roughly 16.6L) and 80s (roughly 11L). Here in the UK we would do the dive on double 12s. With bigger backgas tanks you could do away with the 3rd bottle. 240ft is around the limit that I would add in a 3rd deco gas. That would probably be 21/35 rather than 35/25 and it would be an AL80. If averaging 220 then I wouldn't worry about the 3rd deco gas.

HTH

John

That is actually interesting. Almost all of my deep experience is in cave so the stage useage is different. But, my first thought was, if gas was on the margin I might use a 190 bottle (21/35) instead to travel with it to depth since the getting squared away bit tends to burn the most gas and then switch back to it on the ascent. That way I would have the gas cushion without having to deal with an empty AL80 for over half of the bottom portion of the dive. But, that also feels like a bit of a convoluted plan so the reality is I would really crunch the numbers better before doing the dive to decide what to bring and my bias would be more bottom gas than intermediate gas.

 

[lamont]

My idea would be to shut down this stupid DIR Practitioners thing.

If I had the ability to ban people out of the DIR forum, including scubaboard moderators, that would make sense.

 

[johnkendall]

That is actually interesting. Almost all of my deep experience is in cave so the stage useage is different. But, my first thought was, if gas was on the margin I might use a 190 bottle (21/35) instead to travel with it to depth since the getting squared away bit tends to burn the most gas and then switch back to it on the ascent. That way I would have the gas cushion without having to deal with an empty AL80 for over half of the bottom portion of the dive. But, that also feels like a bit of a convoluted plan so the reality is I would really crunch the numbers better before doing the dive to decide what to bring and my bias would be more bottom gas than intermediate gas.

In a cave that is pretty much what I would do (although in that case I might also add an additional bottom stage as well) In the ocean, I wouldn't bother with a travel gas, but would like to have the 21/35 to switch too just as the ascent rate gets slower. It also means you have a closer gas to switch to in the event of a gas issue.

I'm not keen on diving in the ocean with really big backgas tanks. If you have a wing failure it can be very hard to manage if you have very large, and very negative backgas tanks. I prefer to take addtional stages as these can be handed off to team mates if you have a buoyancy problem.

HTH

John

 

[lamont]

As far as the bottom stage goes... I don't wanna deal with it. I have scootered, running line, and switched off the stage with drygloves on and it sucks. Its not that's its dangerous, its just a major hassle at ~midpoint of the dive. At that time, the bottom stage was along mostly because I was using smaller doubles.

I got to listen to an argument between a GUE instructor and a SCRET member about double-130s (SCRET) vs. double-100/80s and a stage (GUE instructor). The argument for the double-130s was that divers tended to fumble the stage switch. The argument in favor of the stage was that anyone fumbling a stage switch shouldn't be diving that deep.

I thought the argument on the whole was pretty poor. Seems to me like the better argument is even if you have the skills to do it, bringing a stage rather than bigger backgas is needlessly complicating the dive. The argument got derailed onto the skills issue, which seemed like a bit of a distraction.

 

[RTodd]

If I had the ability to ban people out of the DIR forum, including scubaboard moderators, that would make sense.

This is why Lamont is the last man standing. You guys really should use the practioners forum as the default place for posting since the problem moderators, and they were the biggest problem, aren't cleared to post in this forum. (Of course, they may be able to do so anyway.) In the DIR forum, they simply unmod anything that the DIR mods try to clean up.

Leave the DIR forum for posts by people that have no DIR training and are asking basic questions.

 

[RTodd]

In a cave that is pretty much what I would do (although in that case I might also add an additional bottom stage as well) In the ocean, I wouldn't bother with a travel gas, but would like to have the 21/35 to switch too just as the ascent rate gets slower. It also means you have a closer gas to switch to in the event of a gas issue.

I'm not keen on diving in the ocean with really big backgas tanks. If you have a wing failure it can be very hard to manage if you have very large, and very negative backgas tanks. I prefer to take addtional stages as these can be handed off to team mates if you have a buoyancy problem.

HTH

John

Yeah, it is a completely different gas issue in a cave. In a cave, I would generally only be breathing bottom stages on a dive like this anyway so saving them for the actual dive portion makes sense because the travel to 220 will take longer. Plus, all of the deco bottles get dropped.

I am not a huge fan of giant tanks in the ocean either. Heck, I avoid them in caves where I can. Of course, I don't have good local dives at this depth range so I have to travel to them. When I do it, I travel to nice warm clear water places which generally dictate al80s. So, my preference would be a bottom stage too.

The people tend to flub the stage switch rationale for a 200' dive scares me a bit. I understand rjack's point that he only has 25 minutes on the bottom and doesn't want to waste time with the switch. But what lamont is discussing seems like a really bad argument for not using stages.

 

[Cave Diver]

I thought the argument on the whole was pretty poor. Seems to me like the better argument is even if you have the skills to do it, bringing a stage rather than bigger backgas is needlessly complicating the dive. The argument got derailed onto the skills issue, which seemed like a bit of a distraction.

That's the place that a lot of arguements get derailed. If backgas of sufficient size can be used to do the dive a bottom gas stage of the same mix seems to be an extra point of failure. If the stage is being carried as a deco mix, that makes more sense.

 

[rjack321]

In a cave (not that I'm cave diving anywhere near this deep) I am definitely going with more bottom gas. The deco gas being dropped anyway.

In the ocean, I am intrigued by using another deco gas, not so much for deco as just the deep stops. Keeping rock bottom lower, not putzing with a stage on the bottom when I might really want to be on the trigger, avoids the largest most awkward backgas tanks, etc. When scootering, maybe with line (maybe not) in drygloves I am reasonably efficient with stage switches, they just happen at truly inopportune times and there's not that much latitude to switch early or late at 7+ata. I suspect dealing with the scooter and the reel while switching is never going to be elegant.

Thanks for some other considerations here.