PADI Deep Diver course- gas management

[steinbil]

3m/min ascent rate is silly slow. Just stay below 18m/min (10m/min is even better) and it is likely you will be just fine.

Maybe. However, with GUE gas planning you're supposed to comfortably make it to the surface in the event of one catastrophic failure, completing the ascent and any planned deco, just like any other dive. We're not planning to have the bare minimum to do a quick ascent and likely be just fine.

By the way, the GUE protocol calls for a 10m/min ascent rate up to half depth (for NDL dives), and a 3m/min ascent from there. Calculating with 3m/min from the bottom just allows for a suboptimal ascent rate (because it might be hard to do a gas sharing ascent with a stressed diver with perfect timing), and/or a higher SAC rate, and/or more time to execute the gas share and stabilize.

So let's say we're calculating with 10m/min. We better use at least 30L/min (1cuft) for a stressed diver, since we have no other conservatism - and there was just a poster saying he had a gas consumption of 1cuft/min as a beginner without any emergencies. And while a safety stop can be skipped in an emergency, it increases the risk of DCS enough that it is "required" on any dive below 100ft on the PADI rdp table, so I see no reason to PLAN to skip it just because of an OOG or loss of gas.

C: 2 divers x 30L/min = 60L/min
A: 2.8 ATA
T: 5 min
= 840L

+ Safety Stop
C: 60L/min
A: 1.5 ATA
T: 3 min
= 270L

Total minimum gas from 36m with safety stop: 1110L / 11.1L = 100 bar (1500 PSI). And that requires a constant ascent speed while sharing gas of 10m/min, without losing control of the ascent, or requires them to stay very calm and breathe less in an emergency. In any case there is not a lot of margin of error in a stressed situation.

If he was on the boat with 800 PSI, I would guess he didn't start the ascent with 1500 PSI, unless he has a very high SAC rate or ascended much slower (which begs the question of how realistic it is to control the ascent speed to hit 10m/min while sharing gas).

By the way,
Even without the safety stop it requires 1100 PSI. So saying 1000 PSI is "more than enough" is quite telling. And I would like to see them do a shared gas ascent from 36m to 5m at 10m/min and make a precise stop at 5m to execute a safety stop.

 

[Blackcrusader]

How is GUE gas planning relevant to a padi course prey tell?

 

[boulderjohn]

Every dive is a "decompression" dive and should be treated as such.

What does that mean, and how does it apply to this thread?

When people use the term "decompression dive," they almost always mean a dive with required decompression stops. Google the question "what is a decompression dive?", and you will quickly get about 100 sites telling you exactly that. That is what the phrase commonly means.

What you mean through imperfect language is that every dive requires some decompression, but that is different from every dive specifically requiting decompression stops.

And the difference is much bigger than it appears. A dive that does not require decompression stops can be completed with an ascent to the surface with no stops whatsoever. A decompression dive requires a decompression stop.

So what do you mean when you say that they are all the same, when they obviously are not?

 

[steinbil]

How is GUE gas planning relevant to a padi course prey tell?

It's relevant because PADI has NO STANDARD for gas planning. Just simple rules of thumb, and the instructor is free to teach (or not) any kind of gas planning.

I even showed an example of an alternative minimum gas calculation using different parameters (Not GUE).

Please show me your minimum gas calculation that shows 1000 PSI of an AL80 is "more than enough" for an OOG emergency ascent with 2 divers from 120ft.

 

[wetb4igetinthewater]

And then I came across the wonderful spreadsheet created by @wetb4igetinthewater which, given the same inputs, gives just about the exact same numbers as mine - even though his work appears to be have been done in Imperial (and seems to enforce a minimum ascent pressure of 700psi) and mine is in metric with a conversion to PSI at the end because that's what the rented SPGs at the dive shop display).

Are you referring to this document? As I covered both imperial and metric.

 

[johndiver999]

The comment of dive and let dive is kinda funny in this context. If you are buddy diving with a buddy, then the reserve in the buddy’s tank is your reserve. If you want to engage in rigorous planning then you have to agree on your buddy’s reserve.

 

[jadairiii]

A dive that does no require decompression stops can be completed with an ascent to the surface with no stops whatsoever. A decompression dive requires a decompression stop.

So what do you mean when you say that they are all the same, when they obviously are not?

This is a tad outside the topic, but, as you should be aware, decompression science is still a bit of an art. We are still using data based on extrapolations from experiments done on goats from 1908. The slow tissue/fast tissue is merely "modeling".

DAN's experiments with doppler ultrasound testing really was a game changer and revealed that our "no decompression" dives produced quite a few bubbles on some divers. Bubbles out of saturation? Sounds like a decompression dive to me? And then of course you have plenty of data on divers getting bent on "no decompression" dives.

So I will agree to disagree based on current data. But, to bring this back on topic, that is the whole reason GUE teaches minimum gas standards in REC1, day one, every dive is a decompression dive and you should, at all costs, have enough gas to surface at a safe rate and do "safety" stops.

 

[boulderjohn]

But, to bring this back on topic, that is the whole reason GUE teaches minimum gas standards in REC1, day one, every dive is a decompression dive and you should, at all costs, have enough gas to surface at a safe rate and do "safety" stops.

A few years ago, I published an article on current thinking on deep stops in decompression diving, with an enormous amount of help from Simon Mitchell. I wanted to do the same thing for ascent profiles in NDL dives, and asked Simon for similar help. He said he couldn't help me, because NDL dives are different, and he himself did not have strong feelings on it.

So I researched myself. As part of that research, I contacted GUE headquarters and asked for an explanation for the thinking behind their min deco philosophy. I got a very thorough explanation, so I am quite sure I understand it.

What I don't understand is what you say here, because that wasn't mentioned. In response to my statement saying that there is a difference between decompression dives and no decompression dives, you reply that it is very important to have enough gas to do your ascent on an NDL dive. I am not clear on the connection.

 

[tursiops]

A few years ago, I published an article on current thinking on deep stops in decompression diving, with an enormous amount of help from Simon Mitchell. I wanted to do the same thing for ascent profiles in NDL dives, and asked Simon for similar help. He said he couldn't help me, because NDL dives are different, and he himself did not have strong feelings on it.

So I researched myself. As part of that research, I contacted GUE headquarters and asked for an explanation for the thinking behind their min deco philosophy. I got a very thorough explanation, so I am quite sure I understand it.

What I don't understand is what you say here, because that wasn't mentioned. In response to my statement saying that there is a difference between decompression dives and no decompression dives, you reply that it is very important to have enough gas to do your ascent on an NDL dive. I am not clear on the connection.

Give it up. You are arguing with people who don't think anymore, and instead just quote GUE, wheter it is applicable or not.

 

[jadairiii]

What I don't understand is what you say here, because that wasn't mentioned. In response to my statement saying that there is a difference between decompression dives and no decompression dives, you reply that it is very important to have enough gas to do your ascent on an NDL dive. I am not clear on the connection.

I can say that it is taught in GUE REC1.

As to current science on that matter, a good look at what David J. Doolette, Ph.D. is doing at the Navy Experimental Diving Unit in Panama City (I think that is the city). Interesting work and dealing with deep stops, HE vs N2, and deep "bounce" diving (what we all do). And, to repeat myself, DAN doppler research.

What I found to be one of the most interesting topics Dr Doolette discussed was the fact that Navy tables past 150' were all all extrapolations and all based on sat diving, not recreational "bounce" diving. That is something he has been studying now for a while.