How much air to surface with?

[Dhboner]

Just dive thirds.

 

[Adobo]

Is it pretty universal and standard to expect everyone to "get back on the boat with 500psi" even if they are diving bigger tanks or doubles? Or would it actually be considered correct to plan for a specific volume of usable gas on exit, rather than a specific psi that is used without regard to the capacity of the tank being used?

No. On a rec boat dive a couple of weeks ago, I ended a dive with 400 psi on my doubles.

On boat dives in Hawaii, I have on many occasions returned to the boat with less than 500psi.

If they are checking to see how much gas is in your tank(s) at the end of the dive and they have a rule about it, you kind of have no choice. Their boat, their rules. If you are on a boat where they expect you to conduct yourself as a mature/experienced diver, its possible and in my case, very likely that on one will check how much gas you have left when you get back to the boat.

Now, whether or not it is wise to be returning back to the boat with less than 500psi, that's an entirely different topic altogether.

 

[oldschoolto]

200 psi is good for me in a al80.. But im a old school j-valve diver...

Jim...

 

[scubadada]

Just dive thirds.

That's pretty inflexible and is totally inflexible for drift dives

 

[Superlyte27]

I hate drift dives. I'm trusting my life to the guy driving the boat that doesn't even scuba dive. But that's another topic.

 

[guyharrisonphoto]

t

As far as rock bottom, whoever told you that Rock Bottom is for technical diving only vs. recreational diving is an idiot. It's far more important for recreational diving. That said, how do you know when to start your ascent if you aren't calculating Rock Bottom? You can choose whether or not to pad the SAC rates up to 1cfm, and whether or not to factor it in for 2 divers, but padding SAC rates and calculating for 2 divers is far more important on rec dives than tec dives. Think about it. Recreational divers are generally less experienced, generally have higher SAC rates, generally take longer to get things corrected before being able to make a safe ascent, are more likely to need to borrow your gas due to not paying attention to their SPG, and may be more prone to having equipment failures due to rental gear *this is debatable, but I've seen more hose leaks and o-rings burst on rental gear than personal gear*. All that factored in means planning for a 2cfm gas requirement for ascent is much higher on my priority list in a recreational dive than it is on a technical dive. 80ft dive on an AL80 means start your ascent with 1400psi, 1100 on your 120. That gets you to the surface with 200psi which is a reasonable buffer.

This is why rock bottom is not going to catch on with rec divers. Most would be very surprised to know they have to begin their ascent after using 1/2 of their tank and reaching the surface with 1000 psi or more, still remaining. This means, for nitrox divers with good consumption, not even getting close to their NDL time underwater and giving up a big chunk of their dive, every time. They will sacrifice many, many minutes and eventually many hours of time underwater by anticipating the very worst situation that can happen once in 100,000 times, if even that often, based on exaggerated gas use assumptions that are not necessarily needed in rec NDL diving. I don't think rec divers are going to do this when it means sacrificing 30% of their normal dive time.

Put simply, if I have a buddy with a low/out of air situation underwater, he is getting my primary and we are heading for the surface, pronto. We are not going to worry about ascent rates, and are certainly not going to do a safety stop. A real OOA emergency calls for the surface, as fast as we can short of a full CESA. Formal "rock bottom" planning would be far more gas than needed to accomplish this. In fact, the "surface with 500 psi" guide means leaving the bottom with 700-800 at 80' (in my experience). If it takes me 300 to do a slow ascent and safety stop, even doubling that still leaves 200 left (close, but this is emergency planning). In reality, we would not use that much gas with a faster ascent and no stop. Remember, we are NDL diving so stops are optional and we have the safety margin of the tables or computer algorithm to help offset the fast ascent. I choose to plan this way, considering the very remote likelihood of the event even happening and the remote risk of injury with my plan, rather than sacrifice what, by now in my life, would have been days of underwater time as well as lots of unused gas.

Rock bottom is crucial if you have mandatory deco. At that point, you do have to calculate based on slow/proper ascent rates and the mandatory stops. But otherwise, I just don't see the crusade becoming widespread.

 

[scubadada]

I hate drift dives. I'm trusting my life to the guy driving the boat that doesn't even scuba dive. But that's another topic.

That's nearly all I do, my Capt, is equisitely familiar with all of his dive sites

 

[Dhboner]

That's pretty inflexible and is totally inflexible for drift dives

Maybe but you are SO less likely to ever get into trouble if you stay with thirds.

 

[Superlyte27]

Read the article in DAN this month where the captain drove off to go fishing. Lol

 

[gopbroek]

I am probably just rationalizing personnel behavior but I believe that different diving conditions have different requirements and there is no one size fits all gas planning. Cave, Tech / planned DECO and low vis cold water diving would IMHO require much more stringent gas planning and reserve requirements over basic warm water drift diving in good vis like I do in Cozumel (AKA rec Bunny dives). I try to be up to 60' by around 500 PSIG and complete the SS requirement by 250 PSIG. At that point I may hang out until the boat comes around or until I reach the IP crack point on my 1st stage. I have always checked my regs in shallow water to see how they react to low pressure and with balanced regs have found them to draw to the end of the bottle. I want to know how they feel and flow at less than IP so that as a last fail safe I know when an OOA situation is immanent based upon feel and 'draw', in the event I was silly enough to not monitor one of my two gauges.