Rock bottom, 500 PSI, or something else?

[Jamesdclxxv]

anyone know of a recreational course taught by one of the large agencies where calculating your sac rate is actually required?

calculating sac is part of the padi aow material

 

[tbone1004]

calculating sac is part of the padi aow material

is it required on all of the dives though? I've never actually seen it done in the real world *i.e. recording sac rates for an entire dive, or doing a sac rate swim*. It is not in my AOW manual, granted that one is a bit old.

Being in the manual, and being required portions of dives are completely different though.

 

[boulderjohn]

Actually, the person that you quoted and posed the question to, above, as well as the other person who responded, both stated "to half your maximum depth" or "to 50% of max depth", not a "straight 10 FPM ascent". See quotes below:

See below.

I don't know where the GUE min deco ascent profile came from, but there's a study quoted in Deco for Divers that tracked bubble scores from divers that did various ascent profiles from direct ascent, 3(?) minute safety stop, 5(?) minute safety stop, and staged ascent. I don't remember the exact name of the study or the specific times/scores listed since my copy is in a box somewhere, but I distinctly remembering that the group that did the staged ascent had lower bubble scores.

I believe you are referring to the Marroni study in 2004. (A deep stop during decompression from 82 fsw (25 m) significantly reduces bubbles and fast tissue gas tensions. - PubMed - NCBI)

It concluded that a single deep stop plus a single shallow stop was advantageous.

It also compared 3 different ascent rates--10, 33, and 60 FPM, and it observed that "The highest bubbles scores (8.78/9.97), using the Spencer Scale (SS) and Extended Spencer Scale (ESS) respectively, were with the slowest ascent rate [10 FPM]."

Since then, another study found different results, and the Marroni study is now not highly regarded--it is still the only study ever done showing a benefit to deep stops in recreational diving. But it does show that the 10 FPM ascent rate was inferior even to a 60 FPM ascent rate.

So, that is NOT a min deco schedule. To my knowledge, there is no study showing a benefit to such an ascent profile, and it is the only study I know that looked at ascending at 10 FPM--which it found to be disadvantageous. Now, I am NOT by a means an expert, so I will be happy to be shown the research that indicates that this strategy for recreational divers is better than the one that has been used by everyone else.

 

[Jamesdclxxv]

I haven't actually done the dives yet but I assume they will be just the bare minimum basics. I get your point but someone who learns the material properly will have a basic understanding of sac and how to do the calculations.

 

[seeker242]

I will be happy to be shown the research that indicates that this strategy for recreational divers is better than the one that has been used by everyone else.

Did someone claim it’s better from a decompression standpoint?

 

[NAM001]

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?

I do my deep portion of the dive at the start. if anything goes wrong you will have enough air to do what is needed. I dont do 100 ft till 800 psi perhaps to 2000 psi then i go shallow as available. on a wreck that may be 70 ft.. That depth extends you ndl and i go up prior to hitting ndl. If bored i go to 40 and hang and watch others and practice hovering, for a while while i burn ndl time. Math is math and caution is just that. I am not going to pay 300$ for a two tank dive and get 20 min total dive time out of it, returning to the boat with half a tank each time. This would perhaps be a good thing for the OW's that are still raw to do so they do ot overload their A$$ by not checking their gages often enough.

 

[boulderjohn]

Did someone claim it’s better from a decompression standpoint?

If it isn't better, why do it?

 

[seeker242]

If it isn't better, why do it?

I enjoy doing it that way because it’s good practice for hitting and keeping stops.

 

[Lorenzoid]

Did someone claim it’s better from a decompression standpoint?

It wasn't made clear to me in my GUE training whether they really believe it's better from a decompression standpoint or whether it's more along the lines of: for recreational dives to less than a maximum depth of 100 feet, regardless of whether it's better, it's probably not worse to any significant degree, and besides, it's a good skill to develop.

 

[Lobzilla]

I enjoy doing it that way because it’s good practice for hitting and keeping stops.

It wasn't made clear to me in my GUE training whether they really believe it's better from a decompression standpoint or whether it's more along the lines of: for recreational dives to less than a maximum depth of 100 feet, regardless of whether it's better, it's probably not worse to any significant degree, and besides, it's a good skill to develop.

Exactly. This ascend pattern is introduced in GUE Fundies and the next level is T1 where you have to precisely control ascent rates, make stops within a foot or so, perform other task, and resolve failures - on a precise time schedule. (Here is an older T1 class report to give anyone an idea of the skills required to pass that class.)

To get to that level, one needs to start somewhere. So, doing similar things in an abbreviated fashion in open water NDL diving is a good idea IMO.

@boulderjohn Someone who can perform the GUE Fundies ascent or the "old" GUE T1 RD schedule under massive stress can perform any other deco procedure just as easily. Yes, the current scientific evidence favors other deco patterns but these "new" patterns also require the skills to adhere to planned stops and timing.

I have seen too many "technical" divers who lack these skills and just bumble up the water column letting their Shearwater fix the problems they create by deviating from their schedule. Since we have a discussion about gas quantities here, It should be clear that a lackadaisical attitude toward ascent precision could invalidate someones minimum gas calculation to the point of creating an OOG emergency.