Rock bottom, 500 PSI, or something else?

[2airishuman]

A thinking, conservative diver is able to construct a gas plan that has a "rock bottom" pressure, that is, the minimum SPG reading at which it is still possible to respond to another diver's out-of-gas emergency and reach safety.

Typical rock bottom calculations assume an elevated SAC for both divers due to stress, allow some amount of problem-solving time at depth, and allow an orderly ascent (usually 30 fpm). In some cases a 3-minute safety stop and a surface reserve is included, and in some cases there is an allowance for unusable gas (where the cylinder pressure is too low to allow the regulator to deliver enough gas volume for two divers).

@NWGratefulDiver has a web page with an example calculation here. His example includes a 3 minute safety stop and a combined reserve for surface use and unusable gas of 200 PSI. The example concludes that rock-bottom pressure for a 65' dive using AL80s is 1603 PSI. Using the "rock bottom" gas planning strategy, a diver would start the ascent at this pressure (or sooner). While this is just an example, it is fairly typical of the conservatism encouraged when using this gas planning strategy.

But it is my experience that very few recreational divers actually run their dives this conservatively. Even DAN recommends that divers exit the water with 500 PSI remaining on the SPG. For a 65' dive matching NWGratefulDiver's example, this would mean the ascent would start no later than about 750 PSI for a diver with a SAC of 0.6.

My ongoing, well, fixation on pony/stage cylinders, twinsets, and larger cylinders is motivated in large measure by the fact that I've always performed rock bottom calculations and never, ever want to get caught trying to help an OOG diver when I don't have enough reserve to do it.

Let's frame the discussion around deeper dives. On shallow dives, less than about 30' or so, the calculated rock bottom pressure will typically be the same or less than the "exit the water with 500 PSI" ascent pressure.

I think there are two things going on here. One is that those who advocate thorough gas planning calculate "rock bottom" using unrealistically conservative assumptions for the amount of SAC elevation and the amount of time spent on problem solving at depth. The second is that the "500 PSI back on board" divers are, without realizing it, diving a gas plan that does not allow for reasonably foreseeable failures.

A fact to consider is that there are, nonetheless, very, very few accidents that occur because a buddy did not have sufficient air to share.

So, how do you conduct your gas planning for deeper recreational dives? Why? Do you think you'd be able to bring an OOG diver to the surface at the very end of your dive?

 

[Doc]

I think your sig line says volumes...

Never ascribe to wisdom that which is adequately explained by good luck

 

[Bowers]

i wont get super specific, but i will point out some factors that i take into account.

depth of dive
this one is obvious. if im diving a 20' training quarry, very little reserve is necessary.

how secluded is the dive site?
i play it safer when im further from potential aide. also other bottles around a popular site alow for in water decompression.

how many dives in are we?
on a first dive well within NDL, even the more extreme "blow and go" is far less likely to get you bent.

also in OOG situations on NDL dives, the safety stop is allowed to be forfeit.

 

[Lorenzoid]

Understanding the rock bottom (or "minimum gas" as some refer to it) calculation was a milestone in my training. After that, I reconsidered some of the deeper dives I had been doing on an Al80. For a deep dive with a square profile, I will go through the rock bottom calculation using elevated rates (i.e., full conservatism) to give myself an understanding of the worst-case scenario. Then I may take into account, as you allude to, the likelihood of one of us actually experiencing a total loss of gas and dial down the conservatism a bit. With the full conservatism, you'd be back on the boat with something like 1000 psi every dive, and most people consider that pretty darn conservative. If it's a multi-level dive, I may memorize or write some numbers down on a slate, indicating what minimum pressure I should have at what depth, and based on that, decide if it may be prudent to ascend to a shallower depth. I can work my way up to the top of a reef that way. I'm not a slave to it on my recreational dives, but it's good to understand rock bottom and go through the calculation to get a ballpark understanding before making a judgment.

 

[tomfcrist]

The rock bottom calculations on NWGreatfuldivers page is an accurate representation of what is really needed. That being said, if you carry the bailout gas needed in a redundant system...it makes life easier.

The 500psi thing has less to do with safety and a lot to do with preservation of functional breathing systems.

 

[tbone1004]

I try to avoid going deeper than 1ft/cf in general, and generally go 15cf/ata. I follow "guidelines" instead of getting super picky with it. I know that I will not breathe at 1.0cfm unless something SERIOUS is going on. It takes a lot for me to breathe more than 0.7, but I still use 1.0/diver as an average. I don't bother with a PSI reserve because with rock bottom you assume NDL diving and if you cut your safety stop short, then oh well. Breathe as long as you can.

My "rules" for al80 diving
0-40ft=500psi
40-70ft=1000psi
70ft-100ft=1500psi

Obviously this changes based on tank size, but it's my "rule". That 15cf/ata means that I do not believe any pony bottles less than 30cf are useful, and prefer 40's if people are going to be adamant about using them. The reality is most people are going to react poorly and not take time to take their time so they'll get up, it just may not be pretty

 

[KeithG]

There is a whole pile of recreational vacation divers that "gas plan" using the rule of halves: when you get down to half your starting pressure, ascend to half your depth and head back towards the boat. Once you reach the boat start your final ascent (or play around under the boat if you still have lots of gas left). This assumes a simple out and back 2 level dive profile.

I have not looked at the numbers but it would be interesting to see at what type of depths this approach starts to violate rock bottom guidelines. From the 65 foot example above 1603psi is the ascend pressure. Seems to me the rule of halves violates this badly as 1500psi is always the halfway turning point regardless of depth. Both 100 feet at 1500psi and 40 feet at 1500psi are valid turning points. One just happens much sooner than the other.

Thoughts?

 

[tbone1004]

@giffenk
100psi is pretty inconsequential IMO

 

[2airishuman]

It is very easy to inflate the actual gas requirements beyond what is, realistically, needed---by increasing the "problem solving time" and using overly elevated SAC rates. I think this produces implausible results that lead people to discard the whole methodology in some cases.

There isn't much data (perhaps not any data) on how far SAC rates increase in actual emergencies, or on the actual "problem solving time" required and so everyone has to make assumptions that seem reasonable for them.

Trying to arrive at a realistic reserve that covers 95% or 99% of OOG emergencies, I think it's probably enough to allow 10 seconds for problem solving. It will seem like 30 seconds because of the psychological time dilation effect of the emergency. I figure my SAC rate will kick up 50% (from .60 to .90) if I'm helping another diver and allow for 1.0 for the other diver (who may not be my buddy). Even these may be too high since both divers are going to calm down as the ascent progresses and it becomes clear that the emergency is under control. I use a 1 minute surface reserve and 100 PSI of unusable gas. I include a safety stop since there are some dives where such a stop is truly necessary for safety. For a 65 foot dive using these assumptions, and an AL80, rock bottom pressure is 813 PSI. A normal ascent for me from 65' would calculate out to 202 PSI used out of an AL80, so if I were following a "back aboard with 500" planning strategy then my start-of-ascent pressure would be 702 PSI.

I would be confident of my ability to get another diver to the surface, from 65', with 813 PSI, because I think these figures are realistic, and because I'm willing to accept that in the 1% or 5% or whatever of the worst cases I might have to skip part of the safety stop. The same set of assumptions applied to a solo pony bottle ascent gives me 5.2 cf of gas used from 65 feet, and 17 cf of gas used from 130 feet.

 

[tbone1004]

@2airishuman have you ever dove on any wrecks with any current? If the OOA happens on the wrong side of the wreck, the last thing most divers should be doing is making a blue water ascent. You have to get to the up-line so you don't end up in the middle of nowhere